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The human brain uses about 25% of the human body's metabolic energy. How are the other 75% spent, in terms of portioning to its various systems?
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Percent of basal metabolic rate by organ (BC Campus Open Education):
- Liver and spleen : 27%
- Brain: 19%
- Skeletal muscle: 18%
- Kidneys: 10%
- Heart: 7%
- Other tissues (lungs, intestine, skin, bone, fat tissue, glands… ): 19%
Basal metabolic rate by 1 kg of specific organ tissue (kcal/kg of organ/day) (Table 5 from American Journal of Clinical Nutrition, 2010):
- Heart: 440 kcal/kg
- Kidneys: 440 kcal/kg
- Brain: 240 kcal/kg
- Liver: 200 kcal/kg
- Skeletal muscle: 13 kcal/kg
- Adipose tissue: 4.5 kcal/kg
- Residual tissues (lungs, intestine, skin, bone… ): 12 kcal/kg
10.4: Human Organs and Organ Systems
- Contributed by Suzanne Wakim & Mandeep Grewal
- Professors (Cell Molecular Biology & Plant Science) at Butte College
You have probably heard this Billy Ray Cyrus song. Heartache, heartbreak. it all has to do with love. Did you ever wonder why the heart is associated with love? The heart was once thought to be the center of all thought processes, as well as the site of all emotions. This notion may have stemmed from very early anatomical dissections that found many nerves can be traced to the region of the heart. The fact that the heart may start racing when one is excited or otherwise emotionally aroused may have contributed to this idea as well. In fact, the heart is not the organ that controls thoughts or emotions. The organ that controls those functions is the brain. In this concept, you&rsquoll be introduced to the heart, brain, and other major organs of the human body.
Figure (PageIndex<1>): Twemoji
Humans transfer and transform energy from the environment into forms useful for human endeavors. Currently, the primary sources of energy used by humans include fuels, like coal, oil, natural gas, uranium, and biomass. All these fuels—except biomass—are nonrenewable. Primary sources of energy also include renewables, such as sunlight, wind, moving water, and geothermal energy.
Fossil fuels contain energy captured millions of years ago from sunlight by living organisms. The energy in fossil fuels such as oil, natural gas, and coal comes from energy that producers (plants and algae) captured from sunlight long ago. Energy stored in these fuels is released by burning them, which also releases carbon dioxide into the atmosphere.
Human demand for energy is increasing.
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Humans harness energy from any available resource and the demand for that energy is ever increasing.
Be efficient and economical with your energy use. This leaves more energy for all—including you—in the future.
Quality of Nutrition
Single nutrient interventions such as fortification of milk with vitamin D, cereal with iron, and table salt with iodine were effective in treating the corresponding nutrient deficiencies . However, when applied to acquired metabolic syndromes that prevail in modern societies, the same approach has yielded inconclusive results [11,12]. For example, decreasing dietary intakes of saturated fatty acid or cholesterol, and increasing the intake of omega-3 polyunsaturated fatty acids do not appear to be effective in reducing the risk of cardiovascular diseases [9,13,14]. The importance of the entire diet that is consumed as a regular practice is being recognized, and an increasing number of studies are analyzing dietary pattern to identify possible causes of under- and over-nutrition. By definition, dietary pattern characterizes the overall diet by the quantities, the proportion, and the variety of foods and beverage as well as the frequency of consumption . The Mediterranean pattern and Western-style pattern are two commonly practiced dietary patterns. The Mediterranean diet contains a high proportion of fruits and vegetables, legumes, whole grains, fish, and poultry with an emphasis on monounsaturated fats and antioxidants, whereas the Western-style diet is generally characterized by energy-dense foods like butter, high-fat dairy products, refined grains, as well as processed and red meat, leaving less space for other nutrients especially those coming from fruits and vegetables. Epidemiological studies have found that the Mediterranean dietary pattern has preventive and protective effects against cardiovascular diseases [16,17], whereas the Western-style dietary pattern is positively associated with dyslipidemia, obesity, hypertension, atherosclerosis, and diabetes [18,19].
Nutrition transition refers to the shift of diet from traditional to modern along with an increase in sedentary behavior, that occur in conjunction with modernization. The concept of nutrition transition was initially proposed by Popkin [20,21] to demonstrate how economic, demographic, and epidemiological changes interact with shifts in dietary consumption and energy expenditure. Interestingly, some countries in East Asia have lower prevalence of certain acquired metabolic syndromes compared to other societies at comparable stages of nutrition transition [2,5,22]. This might be partially due to retention of traditional dietary patterns, which promote consumption of foods with a wider array of nutrients and lower energy density [23-25]. However, the reasons underlying the discrepancy are complex, and extend beyond the chemical composition of food to include social and economic issues. Nevertheless, there is evidence supporting the idea that calorie restriction is beneficial for longevity [26-28], providing support for the potential benefit of low-energy density diets ( Figure 1 A).
The prevailing global increase in the development of acquired metabolic syndromes is associated with nutrition transition [29,30]. One proposed concept that potentially explains the pathogenesis of these syndromes stems from persistent modification of cellular function in response to stress in the endoplasmic reticulum, mitochondria, and other organelles that make up the cellular reticular network . Both nutrient deficiency (undernutrition) and nutrient excess (over-nutrition) cause the loss of nutrient/energy homeostasis and thus trigger cellular stress. Coping response mechanisms, such as the unfolded protein response mechanism [31-33], are activated to resolve stress. In the case of a maladaptive response, programmed cell death is activated to remove malfunctioning cells. On the other hand, persistent adjustment of cellular functions enables cells to cope even with continued exposure to stress inducers. Stress coping response mechanisms initially promote adaptive strategies to recover homeostasis in the short term but become pathogenic in the long term due to long-term modification of cellular functions.
Water is essential for life. From the time that primeval species ventured from the oceans to live on land, a major key to survival has been prevention of dehydration. The critical adaptations cross an array of species, including man. Without water, humans can survive only for days. Water comprises from 75% body weight in infants to 55% in elderly and is essential for cellular homeostasis and life. 1 Nevertheless there are many unanswered questions about this most essential component of our body and our diet. This review attempts to provide some sense of our current knowledge of water including overall patterns of intake and some factors linked with intake, the complex mechanisms behind water homeostasis, the effects of variation in water intake on health and energy intake, weight, and human performance and functioning.
Recent statements on water requirements have been based on retrospective recall of water intake from food and beverages among healthy non-institutionalized individuals. We provide examples of water intake assessment in populations to clarify the need for experimental studies. Beyond these circumstances of dehydration, we do not truly understand how hydration affects health and well-being, even the impact of water intakes on chronic diseases. Recently, Jéquier and Constant addressed this question based on the human physiology. 2 We need to know more about the extent that water intake might be important for disease prevention and health promotion.
As we note later, few countries have developed water requirements and those that do base them on weak population-level measures of water intake and urine osmolality. 3 , 4 The European Food Safety Authority (EFSA) has been recently asked to revise existing recommended intakes of essential substances with a physiological effect including water since this nutrient is essential for life and health. 5
The US Dietary Recommendations for water are based on median water intakes with no use of measurements of dehydration status of the population to assist. One-time collection of blood samples for the analysis of serum osmolality has been used by NHANES. At the population level we have no accepted method of assessing hydration status and one measure some scholars use, hypertonicity, is not even linked with hydration in the same direction for all age groups. 6 Urine indices are used often but reflect recent volume of fluid consumed rather than a state of hydration. 7 Many scholars use urine osmolality to measure recent hydration status. 8 – 12 Deuterium dilution techniques (isotopic dilution with D2O or deuterium oxide) allows measurement of total body water but not water balance status. 13 Currently we feel there are no adequate biomarkers to measure hydration status at the population level.
When we speak of water we are essentially focusing first and foremost on all types of water, be they soft or hard, spring or well, carbonated or distilled water. Furthermore we get water not only directly as a beverage but from food and to a very small extent also from oxidation of macronutrients (metabolic water). The proportion of water that comes from beverages and food varies with the proportion of fruits and vegetables in the diet. We present the ranges of water in various foods ( Table 1 ). In the United States it is estimated that about 22% of water comes from our food intake while it would be much higher in European countries, particularly a country like Greece with its higher intake of fruits and vegetables or South Korea. 3 , 14 , 15 The only in-depth study of water use and water intrinsic to food in the US found a 20.7% contribution from food water 16 , 17 however as we show later, this research was dependent on poor overall assessment of water intake.
The Water Content Range for Selected Foods
|90%||Fat-free milk, cantaloupe, strawberries, watermelon, lettuce, cabbage, celery, spinach, pickles, squash (cooked)|
|80%||Fruit juice, yogurt, apples, grapes, oranges, carrots, broccoli (cooked), pears, pineapple|
|70%||Bananas, avocados, cottage cheese, ricotta cheese, potato (baked), corn (cooked), shrimp|
|60%||Pasta, legumes, salmon, ice cream, chicken breast|
|50%||Ground beef, hot dogs, feta cheese, tenderloin steak (cooked)|
|30%||Cheddar cheese, bagels, bread|
|20%||Pepperoni sausage, cake, biscuits|
|10%||Butter, margarine, raisins|
|1𠄹%||Walnuts, peanuts (dry roasted), chocolate chip cookies, crackers, cereals, pretzels, taco shells, peanut butter|
Source: The USDA National Nutrient Database for Standard Reference, Release 21 provided in Altman. 127
This review considers water requirements in the context of recent efforts to assess water intake in US populations. Relationship of water and calorie intake is explored both for insights into the possible displacement of calories from sweetened beverages by water and also to examine the possibility that water requirements would be better expressed in relation to calorie/energy requirements with the dependence of the latter on age, size, gender, and physical activity level. We review current understanding of the exquisitely complex and sensitive system which protects land animals against dehydration and comment on the complications of acute and chronic dehydration in man against which a better expression of water requirements might complement the physiological control of thirst. Indeed, the fine intrinsic regulation of hydration and water intake in individuals mitigates against prevalent underhydration in populations and effects on function and disease.
Regulation of fluid intake
To prevent dehydration reptiles, birds, vertebrates, and all land animals have evolved an exquisitely sensitive network of physiological controls to maintain body water and fluid intake by thirst. Humans may drink for various reasons, particularly for hedonic ones but most of drinking is due to water deficiency which triggers the so called regulatory or physiological thirst. The mechanism of thirst is quite well understood today and the reason non-regulatory drinking is often encountered is related to the large capacity of kidneys to rapidly eliminate excesses of water or reduce urine secretion to temporarily economize on water. 1 But this excretory process can only postpone the necessity for drinking or for stopping drinking an excess of water. Non regulatory drinking is often confusing, particularly in wealthy societies facing highly palatable drinks or fluids that contain other substance that the drinker seeks. The most common of them are sweeteners or alcohol to which water is served as a vehicle. Drinking these beverages isn’t due to excessive thirst or hyperdipsia as it can be shown by offering pure water instead and finding out that the same drinker is in fact hypodipsic (Characterized by abnormally diminished thirst). 1
Fluid balance of the two compartments
Maintaining a constant water and mineral balance requires the coordination of sensitive detectors at different sites in the body linked by neural pathways with integrative centers in the brain that process this information. These centers are also sensitive to humoral factors (neurohormones) produced for the adjustment of diuresis, natriuresis and blood pressure (angiotensin mineralocorticoids, vasopressin, atrial natriuretic factor). Instructions from the integrative centers to the 𠇎xecutive organs” (kidney, sweat glands and salivary glands) and to the part of the brain responsible for corrective actions such as drinking are conveyed by certain nerves in addition to the above mentioned substances. 1
Most of the components of fluid balance are controlled by homeostatic mechanisms responding to the state of body water. These mechanisms are sensitive and precise, and are activated with deficits or excesses of water amounting to only a few hundred milliliters. A water deficit produces an increase in the ionic concentration of the extracellular compartment, which takes water from the intracellular compartment causing cells to shrink. This shrinkage is detected by two types of brain sensors, one controlling drinking and the other controlling the excretion of urine by sending a message to the kidneys mainly via the antidiuretic hormone vasopressin to produce a smaller volume of more concentrated urine. 18 When the body contains an excess of water, the reverse processes occur: the lower ionic concentration of body fluids allows more water to reach the intracellular compartment. The cells imbibe, drinking is inhibited and the kidneys excrete more water.
The kidneys thus play a key role in regulating fluid balance. As discussed later, the kidneys function more efficiently in the presence of an abundant water supply. If the kidneys economize on water, producing a more concentrated urine, these is a greater cost in energy and more wear on their tissues. This is especially likely to occur when the kidneys are under stress, for example when the diet contains excessive amounts of salt or toxic substances that need to be eliminated. Consequently, drinking enough water helps protect this vital organ.
Most drinking obeys signals of water deficit. Apart from urinary excretion, the other main fluid regulatory process is drinking, mediated through the sensation of thirst. There are two distinct mechanisms of physiological thirst: the intracellular and the extracellular mechanisms. When water alone is lost, ionic concentration increases. As a result, the intracellular space yields some of its water to the extracellular compartment. One again, the resulting shrinkage of cells is detected by brain receptors that send hormonal messages to induce drinking. This association with receptors that govern extracellular volume is therefore accompanied by an enhancement of salt appetite. Thus, people who have been sweating copiously prefer drinks that are relatively rich in Na+ salts rather than pure water. As previously mentioned, it is always important to supplement drinks with additional salt when excessive sweating is experienced.
The brain’s decision to start or stop drinking and to choose the appropriate drink is made before the ingested fluid can reach the intra- and extracellular compartments. The taste buds in the mouth send messages to the brain about the nature, and especially the salt of the ingested fluid, and neuronal responses are triggered as if the incoming water had already reached the bloodstream. These are the so-called anticipatory reflexes: they cannot be entirely phalic reflexes” because they arise from the gut as well as the mouth. 1
The anterior hypothalamus and pre-optic area are equipped with osmo-receptors related to drinking. Neurons in these regions show enhanced firing when the inner milieu gets hyperosmotic. Their firing decreases when water is loaded in the carotid artery that irrigates the neurons. It is remarkable that the same decrease in firing in the same neurons takes place when the water load is applied on the tongue instead of being injected in the carotid artery. This anticipatory drop in firing is due to a mediation neural pathways departing from the mouth and by converging on to the neurons which simultaneously sense of the inner milieu (blood).
Although everyone experiences thirst from time to time, it plays little day-to-day role in the control of water intake in healthy people living in temperate climates. We generally consume fluids not to quench our thirst, but as components of everyday foods (e.g. soup, milk), as beverages used as mild stimulants (tea, coffee) and for pure pleasure. As common example is alcohol consumption which can increase individual pleasure and stimulate social interaction. Drinks are also consumed for their energy content, as in soft drinks and milk, and are used in warm weather for cooling and in cold weather for warming. Such drinking seems also to be mediated through the taste buds, which communicate with the brain in a kind of “reward system” the mechanisms of which are just beginning to be understood. This bias in the way human beings rehydrate themselves may be advantageous because it allows water losses to be replaced before thirst-producing dehydration takes place. Unfortunately, this bias also carries some disadvantages. Drinking fluids other than water can contribute to an intake of caloric nutrients in excess of requirements, or in alcohol consumption that in some people may insidiously bring about dependence. For example, total fluid intake increased from 79 fluid ounces in 1989 to 100 fluid ounces in 2002 among US adults, all from caloric beverages. 19
Effects of aging on fluid intake regulation
The thirst and fluid ingestion responses of older persons to a number of stimuli have been compared to those seen in younger persons. 20 Following water deprivation older persons are less thirsty and drink less fluid compared to younger persons. 21 , 22 The decrease in fluid consumption is predominantly due to a decrease in thirst as the relationship between thirst and fluid intake is the same in young and old persons. Older persons drink insufficient water following fluid deprivation to replenish their body water deficit. 23 When dehydrated older persons are offered a highly palatable selection of drinks, this also failed to result in an increased fluid intake. 23 The effects of increased thirst in response to an osmotic load have yielded variable responses with one group reporting reduced osmotic thirst in older individuals 24 and one failing to find a difference. In a third study, young individuals ingested almost twice as much fluid as old persons, despite the older subjects having a much higher serum osmolality. 25
Overall these studies support small changes in the regulation of thirst and fluid intake with aging. Defects in both osmoreceptors and baroreceptors appear to exist as well as changes in the central regulatory mechanisms mediated by opioid receptors. 26 Because of their low water reserves, it may be prudent for the elderly to learn to drink regularly when not thirsty and to moderately increase their salt intake when they sweat. Better education on these principles may help prevent sudden hypotension and stroke or abnormal fatigue can lead to a vicious circle and eventually hospitalization.
Hydration status is critical to the body’s process of temperature control. Body water loss through sweat is an important cooling mechanism in hot climates and in physical activity. Sweat production is dependent upon environmental temperature and humidity, activity levels, and type of clothing worn. Water losses via skin (both insensible perspiration and sweating) can range from 0.3 L/h in sedentary conditions to 2.0 L/h in high activity in the heat and intake requirements range from 2.5 to just over 3 L/d in adults under normal conditions, and can reach 6 L/d with high extremes of heat and activity. 27 , 28 Evaporation of sweat from the body results in cooling of the skin. However, if sweat loss is not compensated for with fluid intake, especially during vigorous physical activity, a hypohydrated state can occur with concomitant increases in core body temperature. Hypohydration from sweating results in a loss in electrolytes, as well as a reduction in plasma volume, and can lead to increased plasma osmolality. During this state of reduced plasma volume and increased plasma osmolality, sweat output becomes insufficient to offset increases in core temperature. When fluids are given to maintain euhydration, sweating remains an effective compensation for increased core temperatures. With repeated exposure to hot environments, the body adapts to heat stress, and cardiac output and stroke volume return to normal, sodium loss is conserved, and the risk for heat-stress related illness is reduced. 29 Increasing water intake during this process of heat acclimatization will not shorten the time needed to adapt to the heat, but mild dehydration during this time may be of concern and is associated with elevations in cortisol, increased sweating, and electrolyte imbalances. 29
Children and the elderly have differing responses to ambient temperature and different thermoregulatory concerns than healthy adults. Children in warm climates may be more susceptible to heat illness than adults due to greater surface area to body mass ratio, lower rate of sweating, and slower rate of acclimatization to the heat. 30 , 31 Children may respond to hypohydration during activity with a higher relative increase in core temperature than adults do, 32 and sweat less, thus losing some of the benefits of evaporative cooling. However, it has been argued that children can dissipate a greater proportion of body heat via dry heat loss, and the concomitant lack of sweating provides a beneficial means of conserving water under heat stress. 30 Elders, in response to cold stress, show impairments in thermoregulatory vasoconstriction and body water is shunted from plasma into the interstitial and intracellular compartments. 33 , 34 With respect to heat stress, water lost through sweating decreases water content of plasma, and the elderly are less able to compensate for increased blood viscosity. 33 Not only do they have a physiological hypodipsia, but this can be exaggerated by central nervous system disease 35 and by dementia 36 . In addition, illness and limitations in activities of daily living can further limit fluid intake. Coupled with reduced fluid intake, with advancing age there is a decrease in total body water. Older individuals have impaired renal fluid conservation mechanisms and, as noted above, have impaired responses to heat and cold stress 33 , 34 . All of these factors contribute to an increased risk of hypohydration and dehydration in the elderly.
What is the human energy consumption by organ? - Biology
Human energy requirements are estimated from measures of energy expenditure plus the additional energy needs for growth, pregnancy and lactation. Recommendations for dietary energy intake from food must satisfy these requirements for the attainment and maintenance of optimal health, physiological function and well-being. The latter (i.e. well-being) depends not only on health, but also on the ability to satisfy the demands imposed by society and the environment, as well as all the other energy-demanding activities that fulfil individual needs.
Energy balance is achieved when input (i.e. dietary energy intake) is equal to output (i.e. total energy expenditure), plus the energy cost of growth in childhood and pregnancy, or the energy cost to produce milk during lactation. When energy balance is maintained over a prolonged period, an individual is considered to be in a steady state. This can include short periods during which the day-to-day balance between intake and expenditure does not occur. An optimal steady state is achieved when energy intake compensates for total energy expenditure and allows for adequate growth in children, and pregnancy and lactation in women, without imposing metabolic, physiological or behavioural restrictions that limit the full expression of a persons biological, social and economic potential.
Within certain limits, humans can adapt to transient or enduring changes in energy intake through possible physiological and behavioural responses related to energy expenditure and/or changes in growth. Energy balance is maintained, and a new steady state is then achieved. However, adjustments to low or high energy intakes may sometimes entail biological and behavioural penalties, such as reduced growth velocity, loss of lean body mass, excessive accumulation of body fat, increased risk of disease, forced rest periods, and physical or social limitations in performing certain activities and tasks. Some of these adjustments are important and may even increase the chances of survival in times of food scarcity.
An adequate, healthy diet must satisfy human needs for energy and all essential nutrients. Furthermore, dietary energy needs and recommendations cannot be considered in isolation of other nutrients in the diet, as the lack of one will influence the others. Thus, the following definitions are based on the assumption that requirements for energy will be fulfilled through the consumption of a diet that satisfies all nutrient needs.
Energy requirement is the amount of food energy needed to balance energy expenditure in order to maintain body size, body composition and a level of necessary and desirable physical activity consistent with long-term good health. This includes the energy needed for the optimal growth and development of children, for the deposition of tissues during pregnancy, and for the secretion of milk during lactation consistent with the good health of mother and child.
The recommended level of dietary energy intake for a population group is the mean energy requirement of the healthy, well-nourished individuals who constitute that group.
Based on these definitions, a main objective for the assessment of energy requirements is the prescription of dietary energy intakes that are compatible with long-term good health. Therefore, the levels of energy intake recommended by this expert consultation are based on estimates of the requirements of healthy, well-nourished individuals . It is recognized that some populations have particular public health characteristics that are part of their usual, "normal" life. Foremost among these are population groups in many developing countries where there are numerous infants and children who suffer from mild to moderate degrees of malnutrition and who experience frequent episodes of infectious diseases, mostly diarrhoeal and respiratory infections. Special considerations are made in this report for such sub-populations.
2.1.1 Daily energy requirements and daily energy intakes
Energy requirements and recommended levels of intake are often referred to as daily requirements or recommended daily intakes . These terms are used as a matter of convention and convenience, indicating that the requirement represents an average of energy needs over a certain number of days, and that the recommended energy intake is the amount of energy that should be ingested as a daily average over a certain period of time. There is no implication that exactly this amount of energy must be consumed every day, nor that the requirement and recommended intake are constant, day after day. Neither is there any biological basis for defining the number of days over which the requirement or intake must be averaged. As a matter of convenience, taking into account that physical activity and eating habits may vary on some days of the week, periods of seven days are often used when estimating the average daily energy expenditure and recommended daily intake.
2.1.2 Average requirement and inter-individual variation
Estimates of energy requirements are derived from measurements of individuals. Measurements of a collection of individuals of the same gender and similar age, body size and physical activity are grouped together to give the average energy requirement - or recommended level of dietary intake - for a class of people or a population group . These requirements are then used to predict the requirements and recommended levels of energy intake for other individuals with similar characteristics, but on whom measurements have not been made. Although individuals in a given class have been matched for characteristics that may affect requirements, such as gender, age, body size, body composition and lifestyle, there remain unknown factors that produce variations among individuals. Consequently, there is a distribution of requirements within the class or population group (WHO, 1985) (Figure 2.1).
Distribution of energy requirements of a population group or class of individuals*
* It is assumed that individual requirements are randomly distributed about the mean requirement for the class of individuals, and that the distribution is Gaussian.
Source. WHO, 1985.
For most specific nutrients, a certain excess of intake will not be harmful. Thus, when dietary recommendations are calculated for these nutrients, the variation among individuals in a class or population group is taken into account, and the recommended level of intake is an amount that will meet or exceed the requirements of practically all individuals in the group. For example, the recommended safe level of intake for proteins is the average requirement of the population group, plus 2 standard deviations. This approach cannot be applied to dietary energy recommendations, because intakes that exceed requirements will produce a positive balance, which may lead to overweight and obesity in the long term. A high level of energy intake that assures a low probability of energy deficiency for most people (e.g. the average requirement plus 2 standard deviations) also implies a high probability of obesity for most people owing to a dietary energy excess (Figure 2.2). Therefore, in agreement with earlier reports, this expert consultation concluded that the descriptor of the dietary energy intake that could be safely recommended for a population group is the estimated average energy requirement of that group.
Probability that a particular energy intake is inadequate or excessive for an individual*
* Individuals are randomly selected among a class of people or a population group. The two probability curves overlap, so the level of energy intake that assures a low probability of dietary energy deficiency is the same level that implies a high probability of obesity owing to dietary energy excess.
Source: WHO, 1985.
2.2 Sources of dietary energy
Energy for the metabolic and physiological functions of humans is derived from the chemical energy bound in food and its macronutrient constituents, i.e. carbohydrates, fats, proteins and ethanol, which act as substrates or fuels. After food is ingested, its chemical energy is released and converted into thermic, mechanical and other forms of energy.
This report refers to energy requirements that must be satisfied with an adequately balanced diet, and does not make specific recommendations for carbohydrates, fats or proteins. Reports from other FAO and WHO expert groups address those topics. Nevertheless, it should be noted that fats and carbohydrates are the main sources of dietary energy, although proteins also provide important amounts of energy, especially when total dietary energy intake is limited. Ethanol is not considered part of a food system, but its contribution to total energy intake cannot be overlooked, particularly among populations that regularly consume alcoholic beverages. Allowing for the mean intestinal absorption, and for the nitrogenous portion of proteins that cannot be completely oxidized, the average values of metabolizable energy provided by substrates in a mixed diet are 16.7 kJ (4 kcal) per gram of carbohydrate or protein, and 37.7 kJ (9 kcal) per gram of fat. Ethanol provides 29.3 kJ (7 kcal) per gram. The energy value of a food or diet is calculated by applying these factors to the amount of substrates determined by chemical analysis, or estimated from appropriate food composition tables. A recent related report from a FAO technical workshop provides more information on this topic (FAO, 2003).
2.3 Components of energy requirements
Human beings need energy for the following:
Basal metabolism . This comprises a series of functions that are essential for life, such as cell function and replacement the synthesis, secretion and metabolism of enzymes and hormones to transport proteins and other substances and molecules the maintenance of body temperature uninterrupted work of cardiac and respiratory muscles and brain function. The amount of energy used for basal metabolism in a period of time is called the basal metabolic rate ( BMR ), and is measured under standard conditions that include being awake in the supine position after ten to 12 hours of fasting and eight hours of physical rest, and being in a state of mental relaxation in an ambient environmental temperature that does not elicit heat-generating or heat-dissipating processes. Depending on age and lifestyle, BMR represents 45 to 70 percent of daily total energy expenditure, and it is determined mainly by the individuals age, gender, body size and body composition.
Metabolic response to food . Eating requires energy for the ingestion and digestion of food, and for the absorption, transport, interconversion, oxidation and deposition of nutrients. These metabolic processes increase heat production and oxygen consumption, and are known by terms such as dietary-induced thermogenesis , specific dynamic action of food and thermic effect of feeding . The metabolic response to food increases total energy expenditure by about 10 percent of the BMR over a 24-hour period in individuals eating a mixed diet.
Physical activity . This is the most variable and, after BMR, the second largest component of daily energy expenditure. Humans perform obligatory and discretionary physical activities. Obligatory activities can seldom be avoided within a given setting, and they are imposed on the individual by economic, cultural or societal demands. The term "obligatory" is more comprehensive than the term "occupational" that was used in the 1985 report (WHO, 1985) because, in addition to occupational work, obligatory activities include daily activities such as going to school, tending to the home and family and other demands made on children and adults by their economic, social and cultural environment.
Discretionary activities, although not socially or economically essential, are important for health, well-being and a good quality of life in general. They include the regular practice of physical activity for fitness and health the performance of optional household tasks that may contribute to family comfort and well-being and the engagement in individually and socially desirable activities for personal enjoyment, social interaction and community development.
Growth . The energy cost of growth has two components: 1) the energy needed to synthesize growing tissues and 2) the energy deposited in those tissues. The energy cost of growth is about 35 percent of total energy requirement during the first three months of age, falls rapidly to about 5 percent at 12 months and about 3 percent in the second year, remains at 1 to 2 percent until mid-adolescence, and is negligible in the late teens.
Pregnancy . During pregnancy, extra energy is needed for the growth of the foetus, placenta and various maternal tissues, such as in the uterus, breasts and fat stores, as well as for changes in maternal metabolism and the increase in maternal effort at rest and during physical activity.
Lactation . The energy cost of lactation has two components: 1) the energy content of the milk secreted and 2) the energy required to produce that milk. Well-nourished lactating women can derive part of this additional requirement from body fat stores accumulated during pregnancy.
2.4 Calculation of energy requirements
The total energy expenditure of free-living persons can be measured using the doubly labelled water technique (DLW) or other methods that give comparable results. Among these, individually calibrated heart rate monitoring has been successfully validated. Using these methods, measurements of total energy expenditure over a 24-hour period include the metabolic response to food and the energy cost of tissue synthesis. For adults, this is equivalent to daily energy requirements. Additional energy for deposition in growing tissues is needed to determine energy requirements in infancy, childhood, adolescence and during pregnancy, and for the production and secretion of milk during lactation. It can be estimated from calculations of growth (or weight gain) velocity and the composition of weight gain, and from the average volume and composition of breastmilk.
2.4.1 Factorial estimates of total energy expenditure
When experimental data on total energy expenditure are not available, it can be estimated by factorial calculations based on the time allocated to activities that are performed habitually and the energy cost of those activities. Factorial calculations combine two or more components or "factors", such as the sum of the energy spent while sleeping, resting, working, doing social or discretionary household activities, and in leisure. Energy spent in each of these components may in turn be calculated by knowing the time allocated to each activity, and its corresponding energy cost.
As discussed in the following sections of this report, the experimental measurement of total energy expenditure and the assessment of growth and tissue composition allow sound predictions to be made regarding energy requirements and dietary recommendations for infants and older children around the world. Special considerations and additional calculations assist the formulation of recommendations for children and adolescents with diverse lifestyles.
Total energy expenditure has also been measured in groups of adults, but this has been primarily in industrialized countries. Variations in body size, body composition and habitual physical activity among populations of different geographical, cultural and economic backgrounds make it difficult to apply the published results on a worldwide basis. Thus, in order to account for differences in body size and composition, energy requirements were initially calculated as multiples of BMR. They were then converted into energy units using a known BMR value for the population, or the mean BMR calculated from the populations mean body weight. To account for differences in the characteristic physical activity of the associated lifestyles, energy requirements of adults were estimated by factorial calculations that took into account the times allocated to activities demanding different levels of physical effort.
The extra needs for pregnancy and lactation were also calculated using factorial estimates for the growth of maternal and foetal tissues, the metabolic changes associated with pregnancy and the synthesis and secretion of milk during lactation.
2.4.2 Expression of requirements and recommendations
Measurements of energy expenditure and energy requirement recommendations are expressed in units of energy (joules, J), in accordance with the international system of units. Because many people are still used to the customary usage of thermochemical energy units (kilocalories, kcal), both are used in this report, with kilojoules given first and kilocalories second, within parenthesis and in a different font (Arial 9). In tables, values for kilocalories are given in italic type. 
Gender, age and body weight are the main determinants of total energy expenditure. Thus, energy requirements are presented separately for each gender and various age groups, and are expressed both as energy units per day and energy per kilogram of body weight. As body size and composition also influence energy expenditure, and are closely related to basal metabolism, requirements are also expressed as multiples of BMR.
2.5 Recommendations for physical activity
A certain amount of activity must be performed regularly in order to maintain overall health and fitness  , to achieve energy balance and to reduce the risk of developing obesity and associated diseases, most of which are associated with a sedentary lifestyle. This expert consultation therefore endorsed the proposition that recommendations for dietary energy intake must be accompanied by recommendations for an appropriate level of habitual physical activity. This report provides guidelines for desirable physical activity levels, and for the duration, frequency and intensity of physical exercise as recommended by various organizations with expertise in physical activity and health. It also emphasizes that appropriate types and amounts of physical activity can be carried out during the performance of either obligatory or discretionary activities and that recommendations must take into account the cultural, social and environmental characteristics of the target population.
2.6 Glossary and abbreviations
In addition to those defined in the preceding sections, the following terms and abbreviations are used in this report. They are consistent with the definitions used in other related WHO and FAO documents (FAO, 2003 James and Schofield 1990 WHO, 1995).
Basal metabolic rate (BMR) : The minimal rate of energy expenditure compatible with life. It is measured in the supine position under standard conditions of rest, fasting, immobility, thermoneutrality and mental relaxation. Depending on its use, the rate is usually expressed per minute, per hour or per 24 hours.
Body mass index (BMI) : The indicator of weight adequacy in relation to height of older children, adolescents and adults. It is calculated as weight (in kilograms) divided by height (in meters), squared. The acceptable range for adults is 18.5 to 24.9, and for children it varies with age.
Doubly labelled water (DLW) technique : A method used to measure the average total energy expenditure of free-living individuals over several days (usually 10 to 14), based on the disappearance of a dose of water enriched with the stable isotopes 2 H and 18 O.
Energy requirement (ER) : The amount of food energy needed to balance energy expenditure in order to maintain body size, body composition and a level of necessary and desirable physical activity, and to allow optimal growth and development of children, deposition of tissues during pregnancy, and secretion of milk during lactation, consistent with long-term good health. For healthy, well-nourished adults, it is equivalent to total energy expenditure. There are additional energy needs to support growth in children and in women during pregnancy, and for milk production during lactation.
Heart rate monitoring (HRM) : A method to measure the daily energy expenditure of free-living individuals, based on the relationship of heart rate and oxygen consumption and on minute-by-minute monitoring of heart rate.
Total energy expenditure (TEE) : The energy spent, on average, in a 24-hour period by an individual or a group of individuals. By definition, it reflects the average amount of energy spent in a typical day, but it is not the exact amount of energy spent each and every day.
Physical activity level (PAL) : TEE for 24 hours expressed as a multiple of BMR, and calculated as TEE/BMR for 24 hours. In adult men and non-pregnant, non-lactating women, BMR times PAL is equal to TEE or the daily energy requirement.
Physical activity ratio (PAR) : The energy cost of an activity per unit of time (usually a minute or an hour) expressed as a multiple of BMR. It is calculated as energy spent in an activity/BMR, for the selected time unit.
FAO. 2003. Food energy - methods of analysis and conversion factors. Report of a technical workshop. FAO Food and Nutrition Paper No. 77. Rome.
James, W.P.T. & Schofield, E.C. 1990. Human energy requirements. A manual for planners and nutritionists . Oxford, UK, Oxford Medical Publications under arrangement with FAO.
WHO. 1985. Energy and protein requirements: Report of a joint FAO/WHO/UNU expert consultation. WHO Technical Report Series No. 724. Geneva.
WHO. 1995. Physical status: The use and interpretation of anthropometry. Report of a WHO expert committee. WHO Technical Report Series No. 854. Geneva.
Conversion from ATP to ADP
Adenosine triphosphate (ATP) is the energy currency of life and it provides that energy for most biological processes by being converted to ADP (adenosine diphosphate). Since the basic reaction involves a water molecule,
this reaction is commonly referred to as the hydrolysis of ATP.
The structure of ATP has an ordered carbon compound as a backbone, but the part that is really critical is the phosphorous part - the triphosphate. Three phosphorous groups are connected by oxygens to each other, and there are also side oxygens connected to the phosphorous atoms. Under the normal conditions in the body, each of these oxygens has a negative charge, and as you know, electrons want to be with protons - the negative charges repel each other. These bunched up negative charges want to escape - to get away from each other, so there is a lot of potential energy here.
If you remove just one of these phosphate groups from the end, so that there are just two phosphate groups, the molecule is much happier. If you cut this bond, the energy is sufficient to liberate about 7000 calories per mole, about the same as the energy in a single peanut.
Living things can use ATP like a battery. The ATP can power needed reactions by losing one of its phosphorous groups to form ADP, but you can use food energy in the mitochondria to convert the ADP back to ATP so that the energy is again available to do needed work. In plants, sunlight energy can be used to convert the less active compound back to the highly energetic form. For animals, you use the energy from your high energy storage molecules to do what you need to do to keep yourself alive, and then you "recharge" them to put them back in the high energy state.
The basal metabolic rate (BMR) is the quantity of calories burned by the whole body per unit time at rest. It increases by some 30% over the course of pregnancy from about 1300 kcal/day to 1700 k/day ( Butte et al., 2004 ). However, the relative and absolute change in BMR depends strongly on the pre-pregnancy body mass index (BMI = body weight/height 2 ) and weight gain over gestation. This is shown in Fig. 2 ( Butte et al., 2004 ). BMR was found to correlate with body weight, fat free mass, fat mass, cardiac output, maternal plasma insulin-like growth factor-1 concentration and thyroid hormone (T3) concentration as well as fetal body mass. As individual organs adapt to their new role in supporting the pregnancy, their oxygen consumption rises. Thus, basal oxygen consumption increases most in the uterus to support the fetus but oxygen utilization rates are also increased in breast, kidney, heart, and respiratory muscles. As expected, total oxygen consumption in pregnancy increases in parallel to BMR and by some 50 mL O2/min by term. Much of the energy cost is expended for augmenting ATP production. Extra energy is also required for generating heat, cellular processes, and to support added organ activity in kidney, heart, and breast.
Fig. 2 . Basal metabolic rate in pregnant women by trimester and Body Mass Index (BMI, weight/height 2 ). Blue bars = low BMI green bars = medium BMI red bars = high BMI.
Data from Butte, N. F., Wong, W. W., Treuth, M. S., Ellis, K. J. and O’Brian Smith, E. (2004). Energy requirements during pregnancy based on total energy expenditure and energy deposition. American Journal of Clinical Nutrition 79, 1078–1087.
How to Detect Energy Fields
It takes someone with clairvoyant ability to see the second, third, fourth, and fifth layers, which can, but not always, look completely different from one individual to the other. The layers may also be perceived in ways that do not involve third eye visualization. For example, some energy practitioners can sense a person's aura via touch, scent, or sound. To people with these special abilities, these layers are living energies with a pulse that can be measured.
The environmental impact of transport is significant because it is a major user of energy, and burns most of the world’s petroleum. This creates air pollution, including nitrous oxides and particulates, and is a significant contributor to global warming through emission of carbon dioxide, for which transport is the fastest-growing emission sector. By subsector, road transport is the largest contributor to global warming.
Figure 3. Interstate 10 and Interstate 45 near downtown Houston, Texas in the United States.
Environmental regulations in developed countries have reduced the individual vehicles emission however, this has been offset by an increase in the number of vehicles, and more use of each vehicle. Some pathways to reduce the carbon emissions of road vehicles considerably have been studied. Energy use and emissions vary largely between modes, causing environmentalists to call for a transition from air and road to rail and human-powered transport, and increase transport electrification and energy efficiency.
Other environmental impacts of transport systems include traffic congestion and automobile-oriented urban sprawl, which can consume natural habitat and agricultural lands. By reducing transportation emissions globally, it is predicted that there will be significant positive effects on Earth’s air quality, acid rain, smog and climate change.