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Energy requirements of adults: an update on basal metabolic rates (BMRs) and physical activity levels (PALs)

Introduction
Variability in adult BMRs
Predictive equations to estimate bmrs of adults
Ethnic differences in BMR
Effects of migration from tropical to temperate climate on BMR
Adaptation and energy requirements
Total energy expenditure (TEE) and physical activity levels (PAL) in adults: doubly-labelled water data
References

PS Shetty¹, CJK Henry², AE Black³ and AM Prentice³

¹ Human Nutrition Unit, Department of Public Health & Policy, London School of Hygiene and Tropical Medicine, 2 Taviton Street, London WC1H OBT; ² School of Biological & Molecular Sciences, Oxford Brookes University, Headington, Oxford OX3 OBP; ³ Dunn Clinical Nutrition Centre, Hills Road, Cambridge CB2 2DH.

Descriptors: Human energy requirements, basal metabolic rate, adaptation, total energy expenditure, physical activity level

Introduction

The FAO/WHO/UNU Expert Consultation (1985) adopted the principle of relying on estimates of energy expenditure, rather than energy intake from dietary surveys, to estimate the energy requirements of adults. Obtaining data on the energy expenditure of adult males and females has thus gained importance. Since the available reliable data on habitual energy expenditure in free-living adults has been limited hitherto, measures and/or predictions of basal metabolic rate (BMR) have also attracted more attention. The BMR of an individual can simply be defined as the minimal rate of energy expenditure compatible with life. It is measured under standard conditions of immobility, in the fasted state (12-14 h after a meal), and in an ambient environmental temperature between 26 and 30º C, to prevent activation of heat generating processes such as shivering. It can be quantified by direct or indirect calorimetric techniques, the former measuring the heat output directly, while the latter measure oxygen consumption (and carbon dioxide production), which are then appropriately converted to their energy equivalents. The BMR of adults can also be predicted with reasonable accuracy (i.e. with a coefficient of variation of 8%) from predictive equations. Since BMR constitutes between 60 and 70% of the total energy expenditure, it now forms the basis of the factorial approach for the assessment of energy requirements of adults. In this paper, several issues related to BMRs of adults, their relationship to total energy expenditure and physical activity levels and how they influence the estimation of adult energy requirements, are discussed. These include the methodology of BMR measurement, variability of BMRs and total energy expenditure (TEE), and the constancy of BMR over time in adults. A discussion on the usefulness and limitations of equations for the prediction of BMRs from anthropometric parameters, such as body weight. and the likelihood of ethnic variations in BMR and the effects of migration from one climatic zone to another are also included. The chapter also deals with metabolic adaptation and discusses the relevance of this to estimating adult energy requirements. It concludes with a discussion on the factorial approach to assessing total energy expenditure by the use of physical activity levels (PALs) and provides a review of the data to date on doubly labelled water measurements of total energy expenditure and PALs of free-living healthy adults.

Methodological implications of basal metabolic rate measurement in adults

Whether the methods or techniques used to measure the BMR of adults contribute to variability in BMRs is a question that needs to be addressed if BMRs are the basis for estimating total energy expenditure by the factorial method. BMR measurements involve, in the first instance, an estimation of the oxygen consumption of the individual which is then converted into units of heat or energy output. Comparisons of techniques using different equipment, such as Douglas bags, oxylogs, metabolators, and ventilated hoods, show no significant differences between estimates of oxygen consumption of adults obtained by two or more of these techniques in the same individual at the same time (Segal, 1987; Soares et al, 1989).

During the subsequent conversion of oxygen consumption (in ml or litres of oxygen) to energy output or expenditure (in kcals, kJ or MJ) many assumptions are made that can introduce errors into estimates of the BMR. The most important of these potential sources of error are:

(1) the value attributed to the non-protein respiratory quotient (NPRQ or RQ) when the method used does not actually measure the NPRQ from the CO₂ production;
(2) the equations used in the calculations to convert O₂ consumption and CO₂ production when measured (whether or not nitrogen excretion in the urine is estimated) into units of energy output and
(3) the corrections that are made for differences in the volumes of inspired and expired air when both CO₂ output and O₂ consumption are measured.

It is often believed that these factors do not influence the final results over the physiological range of observed RQs. This is not correct. The difference between the true NPRQ of the subject and the assumed RQ (be it 0.82 or 1.0) in the calculation can introduce an error of over 5% for the same measure of O2 consumption in a subject, if the true RQ is as low as 0.77 (Shetty et al, 1986) (Table 1). For instance, when an oxylog which assumes an RQ = 1 is used and the true RQ of the subject is less than 0.8, the difference in the estimate of energy expenditure is reported to be of the order of 4.6% (Garlick et al, 1987).

Brockway (1987) conclusively demonstrated that differences in the final estimate of BMR due to the various formulae used in the conversion of O2 consumption values, i.e. those of Weir (1949), Consolazio et al (1963), Brouwer (1965) and Passmore & Eastwood (1986), may extend over a range of about 3%.

McLean (1984) has argued that the not uncommon assumption that O₂ consumption = outlet ventilation × O₂ concentration difference, can introduce an error in the estimate of O2 consumption of the order of ± 6%. This large error emphasises the importance of correcting for differences in volume flow of inspired and expired air when measuring BMR. McLean (1984) states that this large error is fortunately cancelled out by an error in the calorific value of O₂ consumed as the RQ of the subject varies. However, this is not necessarily true. For example, the calculation of energy output from values for an O₂ deficit of, say, 5%, for a fixed volume of 5 1/min at an assumed RQ of 0.82 using Weir's formula introduces a net error of between +4.5 and -4.0% for a range of physiological RQs of between 0.71 and 1.0, respectively (Shetty et al, 1986).

The errors that arise as a result of the assumptions made in the calculation of BMR from measures of O₂ consumption, even with the tacit assumption that the measures of O₂ consumption are fairly accurate in themselves, imply that differences in BMR between individuals or groups of individuals of the order of 5% do not have biological significance unless the methodology, the assumptions made and the calculations used to arrive at the BMR values are comparable. It is, of course, assumed that certain stipulated minimal experimental prerequisites, such as absence of gross muscular activity, a post-absorptive state, thermoneutral environment, etc. are strictly met in order to ensure basal levels of metabolism, so that measurements made in different individuals, or in the same individual over time, are comparable and that biological significance can be imparted to differences that are observed under these conditions.

Table 1 Sources of error in conversion of oxygen consumption to energy output between assumed and true respiratory quotients (RQs)^a

^aFor expired air volume = 51/min; O₂ deficit = 5%; and Weir's equation [calorific value of 11 of O₂ = 3.9+1.1(RQ)].
^b [(Assumed-true)/true] × 100.
^c Corrected volume indicates the value of O₂ consumed that has been corrected for difference in inspired and expired volume at the given RQ.

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