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ENERGY. Sufficient dietary energy is essential to the survival and health of all animals. For understanding the biology and health of humans, energy is particularly important for a number of reasons. First, food and energy represent critical points of interaction between humans and their environment. The environments in which humans live determine the range of food resources that are available and how much energy and effort are necessary to procure those resources. Indeed, the dynamic between energy intake and energy expenditure is quite different for a subsistence farmer of Latin America than it is for an urban executive living in the United States. Beyond differences in the physical environment, social, cultural, and economic variation also shape aspects of energy balance. Social and cultural norms are important for shaping food preferences, whereas differences in subsistence behavior and socioeconomic status strongly influence food availability and the effort required to obtain food.

Additionally, the balance between energy expenditure and energy acquired has important adaptive consequences for both survival and reproduction. Obtaining sufficient food energy has been an important stressor throughout human evolutionary history, and it continues to strongly shape the biology of traditional human populations today.

This article examines aspects of energy expenditure and energy intake in humans. How energy is measured is first considered, with a look at how both the energy content of foods and the energy requirements for humans are determined. Next, aspects of energy consumption and the chemical sources of energy in different food items are examined. Third, the physiological basis of variation in human energy requirements is explored, specifically a consideration of the different factors that determine how much energy a person must consume to sustain him- or herself. Finally, patterns of variation in energy intake and expenditure among modern human populations are examined, with the different strategies that humans use to fulfill their dietary energy needs highlighted.

Calorimetry: Measuring Energy

The study of energy relies on the principle of calorimetry, the measurement of heat transfer. In food and nutrition, energy is most often measured in kilocalories (kcal). One kilocalorie is the amount of heat required to raise the temperature of 1 kilogram (or 1 liter) of water 1°C. Thus, a food item containing 150 kilocalories (two pieces of bread, for example) contains enough stored chemical energy to increase the temperature of 150 liters of water by 1°C. Another common unit for measuring energy is the joule or the kilojoule (1 kilojoule [kJ] = 1,000 joules). The conversion between calories and joules is as follows: 1 kilocalorie equals 4.184 kilojoules.

To directly measure the energy content of foods, scientists use an instrument known as a bomb calorimeter. This instrument burns a sample of food in the presence of oxygen and measures the amount of heat released (that is, kilocalories or kilojoules). This heat of combustion represents the total energetic value of the food.

Basic principles of calorimetry are also used to measure energy expenditure (or requirements) in humans and other animals. Techniques for measuring energy expenditure involve either measuring heat loss directly (direct calorimetry) or measuring a proxy of heat loss such as oxygen consumption (O₂) or carbon dioxide (CO₂) production (indirect calorimetry). Direct calorimetry is done under controlled laboratory conditions in insulated chambers that measure changes in air temperature associated with the heat being released by a subject. Although quite accurate, direct calorimetry is not widely used because of its expense and technical difficulty.

Thus, methods of indirect calorimetry are more commonly used to quantify human energy expenditure. The most widely used of these techniques involve measuring oxygen consumption. Because the body's energy production is dependent on oxygen (aerobic respiration), O₂ consumption provides a very accurate indirect way of measuring a person's energy expenditure. Every liter of O₂ consumed by the body is equivalent to an energy cost of approximately 5 kilocalories. Consequently, by measuring O₂ use while a person is performing a particular task (for example, standing, walking, or running on a treadmill), the energy cost of the task can be determined.

With the Douglas bag method for measuring O₂ uptake, subjects breathe through a valve that allows them to inhale room air and exhale into a large collection bag. The volume and the O₂ and CO₂ contents of the collected air sample are then measured to determine the total amount of oxygen consumed by the subject. Recent advances in computer technology allow for the determination of O₂ consumption more quickly without having to collect expired air samples. One computerized system for measuring oxygen consumption, like the Douglas bag method, determines energy costs by measuring the volume and the O₂ and CO₂ concentrations of expired air samples.

Sources of Food Energy

The main chemical sources of energy in our foods are carbohydrates, protein, and fats. Collectively, these three energy sources are known as macronutrients. Vitamins and minerals (micronutrients) are required in much smaller amounts and are important for regulating many aspects of biological function.

Carbohydrates and proteins have similar energy contents; each provides 4 kilocalories of metabolic energy per gram. In contrast, fat is more calorically dense; each gram provides about 9 to 10 kilocalories. Alcohol, although not a required nutrient, also can be used as an energy source, contributing 7 kcal/g. Regardless of the source, excess dietary energy can be stored by the body as glycogen (a carbohydrate) or as fat. Humans have relatively limited glycogen stores (about 375–475 grams) in the liver and muscles. Fat, however, represents a much larger source of stored energy, accounting for approximately 13 to 20 percent of body weight in men and 25 to 28 percent in women.

The largest source of dietary energy for most humans is carbohydrates (45–50 percent of calories in the typical American diet). The three types of carbohydrates are monosaccharides, disaccharides, and polysaccharides. Monosaccharides, or simple sugars, include glucose, the body's primary metabolic fuel; fructose (fruit sugar); and galactose. Disaccharides, as the name implies, are sugars formed by a combination of two monosaccharides. Sucrose (glucose and fructose), the most common disaccharide, is found in sugar, honey, and maple syrup. Lactose, the sugar found in milk, is composed of glucose and galactose. Maltose (glucose and glucose), the least common of the disaccharides, is found in malt products and germinating cereals. Polysaccharides, or complex carbohydrates, are composed of three or more simple sugar molecules. Glycogen is the polysaccharide used for storing carbohydrates in animal tissues. In plants, the two most common polysaccharides are starch and cellulose. Starch is found in a wide variety of foods, such as grains, cereals, and breads, and provides an important source of dietary energy. In contrast, cellulose—the fibrous, structural parts of plant material—is not digestible by humans and passes through the gastrointestinal tract as fiber.

Fats provide the largest store of potential energy for biological work in the body. They are divided into three main groups: simple, compound, and derived. The simple or "neutral fats" consist primarily of triglycerides. A triglyceride consists of two component molecules: glycerol and fatty acid. Fatty acid molecules, in turn, are divided into two broad groups: saturated and unsaturated. These categories reflect the chemical bonding pattern between the carbon atoms of the fatty acid molecule. Saturated fatty acids have no double bonds between carbons, thus allowing for the maximum number of hydrogen atoms to be bound to the carbon (that is, the carbons are "saturated" with hydrogen atoms). In contrast, unsaturated fatty acids have one (monounsaturated) or more (polyunsaturated) double bonds. Saturated fats are abundant in animal products, whereas unsaturated fats predominate in vegetable oils.

Compound fats consist of a neutral fat in combination with some other chemical substance (for example, a sugar or a protein). Examples of compound fats include phospholipids and lipoproteins. Phospholipids are important in blood clotting and insulating nerve fibers, whereas lipoproteins are the main form of transport for fat in the bloodstream.

Derived fats are substances synthesized from simple and compound fats. The best known derived fat is cholesterol. Cholesterol is present in all human cells and may be derived from foods (exogenous) or synthesized by the body (endogenous). Cholesterol is necessary for normal development and function because it is critical for the synthesis of such hormones as estradiol, progesterone, and testosterone.

Proteins, in addition to providing an energy source, are also critical for the growth and replacement of living tissues. They are composed of nitrogen-containing compounds known as amino acids. Of the twenty different amino acids required by the body, nine (leucine, isoleucine, valine, lysine, threonine, methionine, phenylalanine, tryptophan, and histidine) are known as "essential" because they cannot be synthesized by the body and thus must be derived from food. Two others, cystine and tyrosine, are synthesized in the body from methionine and phenylalanine, respectively. The remaining amino acids are called "nonessential" because they can be produced by the body and need not be derived from the diet.

Determinants of Daily Energy Needs

A person's daily energy requirements are determined by several different factors. The major components of an individual's energy budget are associated with resting or basal metabolism, activity, growth, and reproduction. Basal metabolic rate (BMR) represents the minimum amount of energy necessary to keep a person alive. Basal metabolism is measured under controlled conditions while a subject is lying in a relaxed and fasted state.

In addition to basal requirements, energy is expended to perform various types of work, such as daily activities and exercise, digestion and transport of food, and regulating body temperature. The energy costs associated with food handling (i.e., the thermic effect of food) make up a relatively small proportion of daily energy expenditure and are influenced by amount consumed and the composition of the diet (e.g., high-protein meals elevate dietary thermogenesis). In addition, at extreme temperatures, energy must be spent to heat or cool the body. Humans (unclothed) have a thermoneutral range of 25 to 27°C (77–81°F). Within this temperature range, the minimum amount of metabolic energy is spent to maintain body temperature. Finally, during one's lifetime, additional energy is required for physical growth and for reproduction (e.g., pregnancy, lactation).

In 1985 the World Health Organization (WHO) presented its most recent recommendations for assessing human energy requirements. The procedure used for determining energy needs involves first estimating BMR from body weight on the basis of predictive equations developed by the WHO. These equations are presented in Table 1. After estimating BMR, the total daily energy expenditure (TDEE) for adults (18 years old and above) is determined as a multiple of BMR, based on the individual's activity level. This multiplier, known as the physical activity level (PAL) index, reflects the proportion of energy above basal requirements that an individual spends over the course of a normal day. The PALs associated with different occupational work levels for adult men and women are presented in Table 2. The WHO recommends that minimal daily activities such as dressing, washing, and eating are commensurate with a PAL of 1.4 for both men and women. Sedentary lifestyles (e.g., office work) require PALs of 1.55 for men and 1.56 for women. At higher work levels, however, the sex differences are greater. Moderate work is associated with a PAL of 1.78 in men and 1.64 in women, whereas heavy work levels (for example, manual labor, traditional agriculture) necessitate PALs of 2.10 and 1.82 for men and women, respectively.

In addition to the costs of daily activity and work, energy costs for reproduction also must be considered. The WHO recommends an additional 285 kcal/day for women who are pregnant and an additional 500 kcal/day for those who are lactating.

Energy requirements for children and adolescents are estimated differently because extra energy is necessary for growth and because relatively less is known about variation in their activity patterns. For children and adolescents between 10 and 18 years old, the WHO recommends the use of age-and sex-specific PALs. In contrast, energy requirements for children under 10 years old are determined by multiplying the child's weight by an ageand sex-specific constant. The reference values for boys and girls under 18 years old are presented in Table 3.

Human Variation in Sources of Food Energy

Compared to most other mammals, humans are able to survive and flourish eating a remarkably wide range of foods. Human diets range from completely vegetarian (as observed in many populations of South Asia) to those based almost entirely on meat and animal foods (for example, traditional Eskimo/Inuit populations of the Arctic). Thus, over the course of evolutionary history, humans have developed a high degree of dietary plasticity.

This ability to utilize a diverse array of plant and animal resources for food is one of the features that allowed humans to spread and colonize ecosystems all over the world.

Table 4 presents information on per capita energy intakes and the percentage of energy derived from plant and animal foods for subsistence-level (i.e., food-producing) and industrial human societies. The relative contribution of animal foods varies considerably, ranging from less than 10 percent of dietary energy in traditional farming communities of tropical South America, to more than 95 percent among traditionally living Inuit hunters of the Canadian Arctic.

Subsistence-level agricultural populations, as a group, have the lowest consumption of animal foods. Among hunting and gathering populations, the contribution of animal foods to the diet is variable, partly reflecting the environments in which these populations reside. For example, the !Kung San, who live in arid desert environments of southern Africa, have among the lowest levels of animal food consumption among hunter-gatherers. In contrast, hunters of the Arctic rely almost entirely on animal foods for their daily energy. Foragers living in forest and grassland regions of the tropics (for example, the Ache and the Hiwi) have intermediate levels of animal consumption.

Regardless of whether they are from plant or animals, the staple foods in most human societies are calorically dense. Indeed, one of the hallmarks of human

evolutionary history has been humankind's success at developing subsistence strategies that maximize the energy returns from available food resources. The initial evolution of human "hunting and gathering" economies some 2 million years ago is an example of this. By incorporating more meat into their diet, man's hominid ancestors were able to increase the energy contents of their diets.

With the evolution of agriculture, human populations began to manipulate relatively marginal plant species so as to increase their productivity, digestibility, and energy content. Today, staple agricultural crops such as rice, wheat, and other cereal grains are calorically dense (more than 300 kilocalories per 100 grams), and are much richer sources of energy than the wild plants from which they evolved.

Novel methods of food processing also allow humans to increase the energy content and digestibility of their foods. The most fundamental of these techniques is the use of fire for cooking, a strategy adopted by man's hominid ancestors at least 400,000 years ago. Cooking makes plant foods more digestible by helping to break down complex carbohydrates. Recent work has shown that cooking can increase the energy content of starchy tubers (potatoes, cassava) by more than 70 percent.

Another interesting example of processing food to raise its energy content is seen among populations living in the high Andes of South America. Here, small potatoes are left outside for several days to be repeatedly frozen during the cold nights and then dried under the intense daytime sun. The resulting product, called chuño, can be stored for many months and has an energy content more than three times that of a fresh potato (330 kilocalories per 100 grams versus 90 kilocalories per 100 grams).

Human Variation in Energy Expenditure

Humans also show considerable variation in levels of energy expenditure. Recent work by Allison E. Black and colleagues indicates that daily energy expenditure in human groups typically ranges from 1.2 to 5.0 times BMR (i.e., PAL = 1.2–5.0). The lowest levels of physical activity, PALs of 1.20 to 1.25, are observed among hospitalized and nonambulatory populations. In contrast, the highest levels of physical activity (PALs of 2.5–5.0) have been observed among elite athletes and soldiers in combat training. Within this group, Tour de France cyclists have the highest recorded daily energy demands of 8,050 kcal/day (a PAL = 4.68)!

Table 5 presents data on body weight, total daily energy expenditure, and PALs of adult men and women from selected human groups. Men of the subsistence-level populations (that is, foragers, pastoralists, and agriculturalists) are, on average, 20 kilograms (45 pounds) lighter than their counterparts from the industrialized world, and yet have similar levels of daily energy expenditure (2,897 versus 2,859 kcal/day). The same pattern is true for women; those from subsistence-level populations are 12.5 kilograms (28 pounds) lighter than women of industrialized societies, but have higher levels of daily energy expenditure (2,227 versus 2,146 kcal/day).

Thus, daily energy needs are expressed relative to BMR; it is found that adults living a "modern" lifestyle in the industrialized world have significantly lower physical activity levels than those living more "traditional" lives. Among men, PALs in the industrialized societies average 1.67 (range = 1.53 to 1.84), as compared to 1.90 (range = 1.58 to 2.38) among the subsistence-level groups. Physical activity levels among women average 1.63 in the industrialized world (range = 1.48 to 1.69) and 1.78 (range = 1.56 to 2.03) among the subsistence-level societies.

The differences in daily energy demands between subsistence-level and industrialized populations are further highlighted in Figure 1, which shows daily energy expenditure (kilocalories/day) plotted relative to body weight (in kilograms). The two lines denote the best-fit regressions for both groups. These regressions show that at the same body weight, adults of the industrialized world have daily energy needs that are 600 to 1,000 kilocalories lower than those of people living in subsistence-level societies.

It is these declines in physical activity and daily energy expenditure associated with "modern" lifestyles that are largely responsible for the growing problem of obesity throughout the world. In the United States, rates of obesity have increased dramatically over the last twenty years, such that now over half of the adult American population is either overweight or obese. Equally disturbing has been the emergence of obesity as a problem in part of the developing world where it was virtually unknown less than a generation ago. In some sense, obesity and other chronic diseases of the modern world (diabetes and

cardiovascular disease, for example) represent a continuation of trends that started early in man's evolutionary history. Humans have developed a diet that is extremely rich in calories while at the same time minimizing the amount of energy necessary for physical work and activity. Ongoing work in nutritional science is attempting to better understand the biological and environmental factors that influence patterns of energy consumption and expenditure to promote human health and well-being.

See also Assessment of Nutritional Status; Body Composition; Hunting and Gathering; Inuit; Nutrition Transition: Worldwide Diet Change; Physical Activity and Nutrition.

William R. Leonard