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Nutrition - Challenges In Assessing Energy Requirements

age physiological differences daily expenditure water labeled

There are numerous problems that have hindered the establishment of nutritional recommendations in older adults. These problems are due, in part, to the reliance on measuring caloric intake to derive daily energy needs. Research has shown that when validated against newly developed techniques, the older methods used to assess caloric intake, such as self-recorded diaries, dietary interviews, and twenty-four-hour dietary recalls, consistently underestimate actual daily energy needs. In addition, the heterogeneity of the elderly population presents additional challenges. For instance, the current recommended daily allowances (RDAs) are suggested for individuals up to age fifty and to those over fifty-one years of age. However, this broad categorization of persons over fifty-one years does not take into consideration the numerous physiological differences and differing nutritional needs of individuals within this broad age group.

An alternative to assessing energy requirements by using energy intake methodologies is the utilization of daily energy expenditure, or the sum of calories burned throughout the course of a day. When an individual is weight stable, daily energy expenditure can be used as a proxy measure of daily energy needs.

Daily energy expenditure is comprised of three components: resting metabolic rate, the thermic effect of feeding, and the thermic effect of physical activity. Resting metabolic rate represents the greatest proportion of daily energy expenditure (60–75 percent), and is a measure of the energy required to sustain homeostasis and basic physiological functions. The thermic effect of feeding constitutes approximately 10 percent of daily energy expenditure and represents the energy cost of digesting, absorbing, storing, and metabolizing a meal. The most variable component of daily energy expenditure is the thermic effect of physical activity (15–50 percent), which includes the energy expended through both voluntary exercise and involuntary activities, such as shivering, fidgeting, and postural control. Research has suggested that age is associated with a decrease in all three of these components of daily energy expenditure.

A new and more accurate method to assess energy expenditure is the doubly labeled water technique, which was first validated for use in humans in the 1980s. This technique involves the oral administration of two isotopes of water (deuterium and oxygen-18), and is based on the fact that deuterium labeled water is lost from the body through the usual routes of water loss (urine, sweat, evaporation), whereas oxygen-18 labeled water is eliminated from the body not only as water, but also as carbon dioxide. Approximately one to three weeks following administration of the isotopes, urine samples are collected and analyzed to calculate carbon dioxide production, which is determined by the difference in the turnover rates of the two isotopes. Carbon dioxide production, in turn, is used to determine oxygen consumption, and hence energy expenditure.

The advantages of the doubly labeled water technique are: 1) it provides an accurate, noninvasive assessment of daily energy expenditure in the free-living environment; 2) measurements are performed over extended periods of time (two to three weeks), which helps account for the daily perturbations in energy expenditure; and 3) when used in combination with indirect calorimetry, it is possible to assess the thermic effect of physical activity. The disadvantages of the method lie mainly in its cost, which makes it impractical for widespread use in large-scale studies. Despite this limitation, however, doubly labeled water provides a more accurate measure of energy requirements than previous methods.

To support this claim, a 1996 summary of six studies utilizing doubly labeled water concluded that energy requirements in older individuals may be higher than the given recommendations. Other studies have also concluded that current recommendations underestimate energy needs of older, Caucasian men and women and African American men, but not African American women. These studies suggest that doubly labeled water may help us understand true energy needs and alleviate the various problems that have hindered the development of adequate nutritional recommendations for elderly persons. Furthermore, studies highlight the heterogeneity of the elderly population and the necessity to focus on the development of individual, rather than group, recommendations.

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