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Energy requirements of older individuals

Introduction
Methodology for determination of total energy expenditure
Total energy expenditure of older individuals
Relationship between total energy expenditure and physical activity
Effects of energy balance on nitrogen balance
Summary of energy requirements in older individuals and recommendations for future research
References
Discussion

SB Roberts

The Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University, 711 Washington Street, Boston, MA 02111, USA

Descriptors: energy metabolism, body composition, isotopes, metabolizable energy

Introduction

Current recommendations on dietary energy intake (FAO/WHO/UNU, 1985; National Research Council, 1989) define expected average amounts of metabolizable energy required for sustaining normal metabolic processes, together with desirable or expected levels of physical activity in healthy individuals. In weight-stable adults, energy requirements are thus equal to total energy expenditure (TEE). In re-evaluating the energy requirements of older individuals, several key issues need to be addressed:

1. Traditionally, TEE has been estimated by a factorial approach involving summation of all the expected components of energy expenditure, taking into account the energy costs of different activities and their durations. Does this approach remain the most suitable method, or alternatively should requirements now be based on more direct measurements of TEE determined by the doubly labeled water technique? An additional methodological issue is whether the current procedure of expressing requirements as multiples of predicted resting energy expenditure (pREE) should be recommended for continuing use, i.e. the PAL (physical activity level) system.
2. Do current recommendations on the energy requirements of older individuals accurately predict usual energy needs for persons in each decade over 50 years of age?
3. What is the relationship between physical activity and energy requirements in older individuals?

Methodology for determination of total energy expenditure

In the past, the lack of an accurate method for the direct determination of TEE has necessitated estimation of TEE either from measured energy intake in healthy populations, as used in the early editions of recommended dietary allowances, or by summing estimates of the principal components of energy expenditure (Roberts, 1995). In this second, 'factorial' approach (which has been in use since the early 1950s; see Roberts, 1995), information on the energy cost of major activities as measured by direct or indirect calorimetry is combined with expected durations of activities to provide an indirect assessment of the TEE over 24 h The primary limitation of the factorial approach to determining TEE is the well-recognized problem that it is hard to categorize and time the many varied activities performed during the course of a typical 24-h period. Thus, estimates of TEE are subject to substantial error and in addition may tend to underestimate TEE because of the tendency to ignore activity not associated with specific tasks.

Correspondence to: SB Roberts.

The doubly labeled water method, which was refined for use in humans during the 1980s (Schoeller, 1988; Roberts, 1989), now provides a more direct assessment of the TEE of free living individuals. Calorimetric validation studies in adults and infants with a wide range of body composition have demonstrated that the method can be accurate as well as reasonably precise. However, there remain several unresolved issues that need to be addressed in relation to using the method for reevaluating energy requirements.

The first concern is that there is currently substantial between-laboratory variability in the analytical capabilities in the increasing number of institutions that now have the capacity to make doubly labeled water measurements of TEE. A recent investigation of this variability in the technique in 19 laboratories from around the world has indicated that the analytical component of the method has the potential to introduce substantial error, resulting in estimates for TEE differing from calorimetrically-measured values by as much as 600% in some laboratories (Roberts et al, 1995). This finding implies that it may be misleading to compare values for TEE between subjects measured in different laboratories unless each of the laboratories has conducted its own validation of the method Either indirect calorimetry or the intake-balance method are a suitable reference method for determining the overall accuracy of the doubly labeled water technique (the key issue for studies of energy requirements in population groups) although indirect calorimetry has the added advantage that the precision of the method can also be assessed.

The second issue concerning the use of the doubly labeled water technique in studies of energy requirements is that it does not measure TEE directly and there is significant potential for influencing the predicted value for TEE through the choice of calculation procedures and correction factors. Of relevance to this issue is the fact that there remains no general agreement on a single calculation method for data processing and several different approaches are used (Prentice, 1990). Although it has been presumed that currently used calculation options should differ only slightly in their effect on predicted TEE, this does not apply to all data sets that have been published. In particular, the question of whether the measured ratio of the dilution spaces (²H₂O: H₂ ¹⁸O) or a fixed ratio of 1.03 (²H₂O: H₂ ¹⁸O) should be used remains unanswered. In a recent study by Goran & Poehlman (1992), values for TEE calculated using the mean of measured dilution spaces differed by 10.5% from values calculated using a fixed ratio of 1.()3. In addition, it has been suggested on the basis of preliminary data (Roberts et al, 1992) that the correction for isotopic fractionation should be lower in older individuals than in young adults, a factor estimated to influence calculated values for TEE by approximately 2%.

Table 1 Pearson correlation coefficients (R) for the relationship between total energy expenditure and selected variables in 35 young and older men^a

^a SB Roberts, P Fuss & VR Young, includes unpublished data.
* P<0.05; ** P<0.01; *** P<0.001.

Taking into account these considerations on the analytical and calculation aspects of the doubly labeled water method, it appears clear that the technique provides an important new approach to determining energy requirements, but that the following recommendations should be taken into account when individual data sets are examined:

1. It is highly desirable that laboratories producing doubly labeled water data for the purpose of reevaluating requirements should have validated their technique against a reference method.
2. It is also desirable, when studying the effects of aging on energy requirements, that groups of individuals of different ages be included within the same study. In this way, differences due to age should be accurate, even if absolute mean values are subject to error.

Table 2 Number of male (M) and female (F) subjects in doubly labeled water studies including older individuals

Another issue related to the methodology for determining energy requirements is whether values for TEE should continue to be expressed as multiples of the pREE (i.e. the PAL index). To evaluate this issue, we examined Pearson correlation coefficients for several potential predictors of TEE in a population of 35 young and older men (Table 1). It can be seen that measured resting energy expenditure (mREE) was the best predictor of TEE in young and older men, and was also a significant predictor in young and older men when they were analyzed as separate groups. The pREE determined from body weight (FAO/WHO/UNU, 1985) was the second best predictor in the two age groups combined as well as a significant predictor in each age group separately. Body weight was an equally good predictor of TEE in young and older men separately (because the predicted REE was determined from body weight) but was not a significant predictor in young and older men combined because of the change in the relationship between REE and body weight that occurs with aging. These observations justify the continuing use of pREE as a means of normalizing TEE data.

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