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Chapter: Introduction to Human Nutrition: Energy Metabolism

Energy requirements - Human Nutrition: Energy Metabolism

How much energy do we need to sustain life and maintain our body energy stores?

Energy requirements

How much energy do we need to sustain life and maintain our body energy stores? Why do some people require more energy and others less? In other words, what are the energy requirements of different types of people? Based on our earlier definition of energy balance, the energy needs or energy require-ments of the body to maintain energy balance must be equal to total daily energy expenditure. Total daily energy expenditure is the sum of the individual com-ponents of energy expenditure as discussed previ-ously, and represents the total energy requirements of an individual that are required to maintain energy balance. Until recently, there was no accurate way to measure total energy expenditure or energy needs of humans. The DLW technique has provided a truly noninvasive means to measure accurately total daily energy expenditure, and thus energy needs, in free-living humans. Before DLW, energy requirements were usually assessed by measurement or prediction of RMR, the largest component of energy require-ments. However, since the relationship between RMR and total energy expenditure is highly variable because of differences in physical activity, the estimation of energy needs from knowledge of RMR is not that accurate and requires a crude estimate of physical activity level. Nevertheless, reasonable estimates can be made to estimate daily energy budgets for indi-viduals (Table 3.4).


Following the validation of DLW in humans, this technique has been applied to many different popula-tions. Total energy expenditure is often compared across groups or individuals using the ratio of one’s total energy expenditure to RMR, or physical activity level (PAL). Thus, for example, if the total energy expenditure was 12.6 MJ/day and the RMR was 6.3 MJ/day, the PAL factor would be 2.0. This value indicates that total energy expenditure is twice the RMR. The PAL factor has been assessed in a variety of types of individual. 

A low PAL indicates a sedentary lifestyle, whereas a high PAL represents a highly active lifestyle. The highest recorded sustained PAL in humans was recorded in cyclists participating in the Tour de France road race. These elite athletes could sustain a daily energy expenditure that was up to five times their RMR over extended periods. Smaller animals, such as migrating birds, have a much higher ceiling for achieving higher rates of total energy expen-diture, which can reach up to 20 times their RMR.

Factors such as body weight, FFM, and RMR account for 40–60% of the variation in total energy expenditure. Total energy expenditure is similar between lean and obese individuals after taking into account differences in FFM. Thus, fatness has small, but important, additional effects on total energy expenditure, partly through RMR, as discussed previ-ously, but also by increasing the energetic cost of any physical activity.


WWith regard to age, some studies suggest that only a limited change in total energy expenditure (relative to RMR) occurs from childhood to adulthood, but that a decline occurs in the elderly. Recent data also suggest a gender-related difference in total energy expenditure, in addition to that previously described for RMR. In a meta-analysis that examined data from a variety of published studies, absolute total energy expenditure was significantly higher in males than in females by 3.1 MJ/day (10.2 ± 2.1 MJ/day in females, 13.3 ± 3.1 MJ/day in males), and nonresting energy expenditure remained higher in men by 1.1 MJ/day.

Individuals who have sedentary occupations and do not participate frequently in leisure pursuits that require physical activity probably have a PAL factor in the region of 1.4. Those who have occupations requiring light activity and participate in light physi-cal activities in leisure time probably have a PAL around 1.6 (this is a typical value for sedentary people living in an urban environment). Individuals who have physically active occupations and lifestyles prob-ably have a PAL greater than 1.75. It has been sug-gested that the optimal PAL that protects against the development of obesity is around 1.8 or higher. Increasing one’s physical activity index from 1.6 to 1.8 requires 30 min of daily vigorous activity, or 60 min of light activity (Table 3.4).


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