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Chapter: Introduction to Human Nutrition: Energy Metabolism

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Energy balance(Human Nutrition) in various conditions

Changes in energy intake during infancy have been well characterized.

Energy balance in various conditions

Infancy and childhood

Changes in energy intake during infancy have been well characterized. During the first 12 months of life, energy intake falls from almost 525 kJ/kg per day in the first month of life to a nadir of 399 kJ/kg per day by the eighth month, then rises to 441 kJ/kg per day by the 12th month. However, total energy expendi-ture in the first year of life is relatively constant at around 252–294 kJ/kg per day. In infants, the large difference between total energy expenditure and energy intake is explained by a positive energy balance to account for growth. In the first 3 months of life it is estimated that the energy accretion due to growth is 701.4 kJ/day, or approximately 32% of energy intake, falling to 151.2 kJ/day, or 4% of energy intake, by 1 year of age. Individual growth rates and early infancy feeding behavior are at least two known factors that would cause variation in these figures.


There is now substantial evidence to suggest that existing recommendations may overestimate true energy needs, based on measurement of total energy expenditure in infants. In the first year of life, tradi-tional values of energy requirements overestimate those derived from measurement of total energy expenditure and adjusted for growth by 11%. Between 1 and 3 years of age the discrepancy is more striking, where the traditional values for requirements are 20% higher than those derived from total energy expendi-ture and adjusted for growth. For example, in 3 year old children total energy expenditure by DLW aver- ages 5.1 MJ/day, while the currently recommended intake for these children is 6.2 MJ/day. Thus, newer estimates of the energy requirements of infants are needed based on assessment of total energy expendi-ture data.


Several laboratories have reported measurements of total energy expenditure in young, healthy, free-living children around the world. Despite marked dif-ferences in geographical locations, the data are similar, although environmental factors such as season and sociocultural influences on physical activity can influ-ence total energy expenditure and thus energy require-ments. In the average 5 year old child weighing 20 kg, total energy expenditure is approximately 5.5–5.9 MJ/ day, which is significantly lower than the existing rec-ommended daily allowance for energy in children of this age, by approximately 1.7–2.1 MJ/day. Thus, as with infants, newer estimates of energy needs in chil-dren are needed based on assessment of total energy expenditure data.


In the elderly, two different problems related to energy balance can be recognized. In one segment of the elderly population there is a decline in food intake that is associated with dynamic changes in body com-position where there is a tendency to lose FFM, which leads to loss in functionality. In others there is a tendency to gain fat mass, which increases the risk for obesity, cardiovascular disease, and noninsulin-dependent diabetes. These two opposing patterns suggest that the ability to self-regulate whole body energy balance may diminish with aging. Thus, pre-scription of individual energy requirements may serve as a useful tool to prevent the age-related deteriora-tion of body composition. Other special consider-ations in the elderly relate to meeting energy needs in special populations, such as those with Alzheimer’s and Parkinson’s disease, which frequently can lead to malnourished states and a diminishing of body weight. It was thought that these neurological condi-tions may lead to body weight loss because of an associated hypermetabolic condition in which meta-bolic rate may increase above normal, thus increasing energy needs. However, more recent studies have clearly shown that the wasting or loss of body weight often associated with these conditions is explained by a reduction in food intake, probably owing to a loss in functionality.

Energy requirements in physically active groups

The DLW technique has been used to assess energy requirements in highly physically active groups of people. The most extreme case is a study that assessed the energy requirements of cyclists performing in the 3 week long Tour de France bicycle race. The level of total energy expenditure recorded (PAL factor of 5.3, or approximately 35.7 MJ/day) was the highest recorded sustained level in humans. In another study involving young male soldiers training for jungle warfare, energy requirements were 19.9 MJ/day (PAL factor of 2.6). The total energy expenditure of four mountaineers climbing Mount Everest was 13.6 MJ/day (PAL 2.0–2.7), which was similar to energy expenditure during on-site preparation prior to climbing (14.7 MJ/day). Total energy expenditure in free-living collegiate swimmers was almost 16.8 MJ/ day in men and 10.9 MJ/day in women. In elite female runners previously performed studies of energy intake suggested unusually low energy requirements. However, in a study in nine highly trained young women, free-living energy expenditure was 11.9 ± 1.3 MJ/day, compared with the reported energy intake of 9.2 ± 1.9 MJ/day. This study suggests that elite female runners underreport true levels of energy intake and confirms the absence of energy-saving metabolic adaptations in this population.


Regular participation in exercise is traditionally thought to elevate energy requirements through the additional direct cost of the activity, as well as through an increase in RMR. However, in some situations energy requirements are not necessarily altered by participation in regular physical activity. For example, in a study of an elderly group of healthy volunteers, there was no significant change in total energy ex-penditure in the last 2 weeks of an 8 week vigorous endurance training program. The failure to detect an increase in total energy expenditure occurred despite a 10% increase in RMR (6703.2 ± 898.8 to 7404.6 ± 714 kJ/day), as well as an additional 630 kJ/ day associated with the exercise program. These increases in energy expenditure were counteracted by a significant reduction in the energy expenditure of physical activity during nonexercising time (2.4 ± 1.6 versus 1.4 ± 1.9 MJ/day). The lack of increase in total energy expenditure in this study is probably explained by a compensatory energy-conserving adaptation to this vigorous training program leading to a reduction in spontaneous physical activity and/or a reduction in voluntary physical activities, similar to that observed in several animal studies. Thus, it should not automatically be assumed that energy require-ments are elevated by participation in activity programs, and the ultimate change in energy requirements may be dictated by the intensity of the training program and the net sum of change in the individual components of energy expenditure. An important area of research is to identify the optimal program of exercise intervention in terms of exercise mode, type, duration, and intensity that can have optimal effects on all components of energy balance.

Energy requirements in pregnancy and lactation

Pregnancy and lactation are two other examples in which energy metabolism is altered in order to achieve positive energy balance. The specific changes in energy requirements during pregnancy are unclear and the various factors affecting this change are complex. Traditional government guidelines suggest that energy requirements are raised by 1.3 MJ/day during pregnancy. This figure is based on theoretical calculations based on the energy accumulation asso-ciated with pregnancy. However, these figures do not include potential adaptations in either metabolic effi-ciency or PAL during pregnancy. In a study that per-formed measures in 12 women every 6 weeks during pregnancy the average increase in total energy expenditure was 1.1 MJ/day. The average energy cost of pregnancy (change in total energy expenditure plus change in energy storage) was 1.6 MJ/day. However, there was considerable variation among the 12 subjects for the increase in average total energy expenditure (264.6 kJ/day to 3.8 MJ/day) and the average energy cost of pregnancy (147 kJ/day to 5.2 MJ/day).

Metabolic adaptations during lactation have been examined in well-nourished women using the DLW technique. The energy cost of lactation was calculated to be 3.7 MJ/day. Just over half of this energy cost was achieved by an increase in energy intake, while the remainder was met by a decrease in physical activity energy expenditure (3.2 MJ + 873.6 kJ/day at 8 weeks of lactation compared with 3.9 + 1.1 MJ/day in the same women prior to pregnancy).

Energy requirements in disease and trauma


The DLW technique has been used in various studies to assess the energy requirements of hospitalized patients. Information on energy requirements during hospitalization for disease or trauma is important because:


      energy expenditure can be altered by the disease or injury


      physical activity is often impaired or reduced


      both underfeeding and overfeeding of critically ill patients can lead to metabolic complications; there-fore, correct assessment of energy requirements during recovery is an important part of therapy.


The metabolic response during recovery from a burn injury includes an increase in RMR, although this is not necessarily a function of the extent of the burn. The widely used formulae to predict energy needs in burn patients are not based on measurement of energy expenditure and estimate that most patients require 2–2.5 times their estimated RMR. However, using the DLW technique, total energy expenditure was 6.7 + 2.9 MJ/day in 8 year old children recovering from burn injury, which was equivalent to only 1.2 times the onfasting RMR. The lower than expected values for total energy expenditure in children recov-ering from burns suggest that RMR is not as elevated in burn patients as previously speculated, and that RMR is not a function of burn size or time after the injury, probably owing to improvements in wound care which reduce heat loss. In addition, energy requirements in patients recovering from burn injury are reduced because of the sedentary nature of their hospitalization.

In a study of patients with anorexia nervosa, total energy expenditure was not significantly different than controls (matched for age, gender, and height). However, physical activity-related energy expendi-ture was 1.3 MJ/day higher in anorexia nervosa patients, which was compromised by a 1.3 MJ/day lower RMR. Thus, energy requirements in anorexia nervosa patients are normal, despite alterations in the individual components of total energy expenditure. In infants with cystic fibrosis, total energy expendi-ture was elevated by 25% relative to weight-matched controls, although the underlying mechanism for this effect is unknown.


Developmental disabilities appear to be associated with alterations in energy balance and nutritional status at opposite ends of the spectrum. For example, cerebral palsy is associated with reduced fat mass and FFM, whereas half of patients with myelodysplasia are obese. It is unclear whether the abnormal body com-position associated with these conditions is the end-result of inherent alterations in energy expenditure and/or food intake, or whether alterations in body composition are an inherent part of the etiology of the specific disability. In addition, it is unclear how early in life total energy expenditure may be altered and whether reduced energy expenditure is involved with the associated obese state. Nevertheless, pre-scription of appropriate energy requirements may be a useful tool in the improvement of nutritional status in developmental disabilities.

Total energy expenditure has been shown to be lower in adolescents with both cerebral palsy and myelodysplasia, partly owing to reduced RMR but primarily to reduced physical activity. Based on measurements of total energy expenditure, energy requirements of adolescents with cerebral palsy and myelodysplasia are not as high as previously specu-lated. In nonambulatory patients with cerebral palsy, energy requirements are estimated to be 1.2 times RMR, and in the normal range of 1.6–2.1 times RMR in ambulatory patients with cerebral palsy.

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