Can Energy Intake Be Accurately Measured In Humans

Self-recorded food intake has been the traditional method of estimating energy requirements. However, available methods for estimating food intake are fraught with limitations and methodological problems. While there is a clear need to provide well-founded recommendations for dietary energy, there have been major technical, physiologic and conceptual problems in doing so. The establishment of individual energy requirements has been problematic because of reliance on (a) measurement of energy intake from self-recorded diaries and/or dietary interviews, (b) the use of a multiple of BMR (or RMR) to predict energy needs, and (c) the failure of current recommended daily energy requirements to take into account the diversity of the population with respect to body composition and physical activity. The shortcomings of each of these approaches are briefly discussed below.

Self-recording of energy intake depends on the cooperation of the volunteer, and the very act of recording energy intake may actually alter ingestive behavior, even in compliant volunteers who wish to "please" the investigator. Thus, recording food intake becomes an unreliable tool on which to base guidelines for determining energy needs. Several recent studies suggest consistent underreporting of actual energy intake when validated against measures of total daily energy expenditure from doubly labeled water (16, 17 and 18). Data from our laboratory suggest a significant underreporting of energy intake by as much as 30% in older individuals, compared with measurement of daily energy expenditure (16). Underreporting was more pronounced in women (30%) than in men (15%). Thus, it is apparent that using measures of energy intake to estimate energy requirements lacks scientific credibility because of the uncertainty and unreliability of subject reporting.

An alternative method of estimating energy needs uses multiples of RMR (19). In this approach, estimates of daily energy expenditure are not derived directly but by a factorial approach in which RMR and the estimated energy expenditure from various physical activities are summed (20). This method suffers from a number of methodological problems. First, it does not consider the components of daily energy expenditure that contribute to individual variation in daily energy expenditure. These "neglected" components include (a) the TEF, which contributes approximately 10 to 15% of daily energy expenditure (21), and (b) the thermic effect of physical activity. Data from our laboratory showed that under free-living conditions, physical activity is highly variable in normal persons and can range from as low as 187 kcal/day to 1235 kcal/day (11, 16). Furthermore, knowledge of RMR alone provides insufficient information for explaining variation in daily energy expenditure, as variation in RMR explains less than half of individual variation in daily energy expenditure ( 16).

Another "general method" of assessing energy needs is based on recommended daily allowances (22) (see Appendix I.a.b.I.e...Jl-.A-2-.a.:.1..). The current RDAs divide the adult population into two age groups those who are 19 to 50 years old and those 51 years old and older. The frequent use of the category of "51 and older" is recognized as inappropriate, because normal and diseased aging produces increased heterogeneity in almost all physiologic measurements. The physiologic status and energy requirements of individuals who are 50 to 60 years old are very different from those of persons who are 80 to 90 years old. Furthermore, the RDAs do not take into account energy recommendations for individuals who vary in physical activity or disease state. It is evident, however, that the use of a single energy value is far too crude an approach and should be abandoned for medical, nutritional, and planning purposes. These methods were necessitated, until recently, by the lack of a direct method to measure daily energy expenditure under free-living conditions.

The World Health Organization Consultative Panel has stated that future guidelines should be based on measurements of energy expenditure "if and when these became available" (19). As noted above, the doubly labeled water technique (2H218O) provides a measure of free-living energy expenditure. In the adult individual, daily energy expenditure defines the level of energy intake to maintain energy balance ( 2.3). Measurement of total daily energy expenditure with the doubly labeled water technique therefore acts as a proxy indicator of the amount of energy intake that is required to maintain energy balance and body energy stores.

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