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Variability in adult BMRs

Inter-individual variability

It is generally recognised that in a group of apparently healthy and comparable individuals there is a considerable between-individual or inter-individual variation in habitual, total daily energy expenditure. This, however, is not as large as the inter-individual variation in energy intakes. Edholm (1961) reviewed a number of studies where repeated measurements of total energy expenditure had been made and reported that the coefficient of inter-individual variability was of the order of ± 12.5% on a body weight basis. In recent studies, in which energy expenditure was measured in a respiratory chamber and both the intake and the physical activity levels were controlled, the inter-individual coefficient of variation (CV) ranged between 7.5 and 17.9% (Garby et al, 1984; De Boer, 1985). lt appeared that the CV depended upon the variations in body size; the larger the variation in body weight among subjects, the larger the CV of total energy expenditure.

Comparisons of subjects of similar body weight and body composition showed an inter-individual CV of BMR of 13% (Jéquier & Schutz, 1981). Other reports suggest that the inter-individual CV of BMR varies between 7.9 and 12.0% in both male and female subjects when measurements are made under conditions of controlled intake and physical activity (Schulz, 1984; De Boer, 1985; Daly et al, 1985). The inter-individual CV of BMR was 9.2% when intake was controlled at two levels of physical activity in males (Dallosso & James, 1984) and of the order of 11.7% in free-living males who had a CV of body weights of 15.2% (Shetty et al, 1986). In the few instances where the CVs of inter-individual variation in BMR and TEE have been simultaneously computed (in male subjects who maintained body weight) they were of the order of 10.2% and 10.3%, respectively (Dallosso et al, 1982). This last report emphasises that the inter-individual CV of TEE is reflected in the CV of BMR, since the latter makes a substantial contribution to the total energy output of an individual.

Intra-individual variability in BMR

Sukhatme & Margen (1982) argued that within individual variations in intakes are more important than between-individual variations, and that the observed inter-individual variations can largely be explained in terms of the intra-individual variations. These investigators contend that the well-documented variation in intakes observed among apparently healthy individuals indulging in similar levels of activity is evidence that different individuals operate at different levels within what they consider to be the intra-individual range of 'costless' adaptation. This has resulted in an unsubstantiated claim that intra-individual variations in energy expenditure are also large, with a high coefficient of variation even in subjects accustomed to similar levels of physical activity every day, and that this wide variation needs to be considered when assessing the energy requirements of an adult (Sukhatme & Narain, 1983). Table 2 summarises some recent data on intra-individual variations in BMR obtained from repeated measurements in the same individual when: (1) energy intake and physical activity were controlled while in a respiration chamber Jéquier & Schutz, 1981); (2) energy intake alone was controlled and BMR measurements were made on two levels of physical activity over a 24-h period (Dallosso & James, 1984); (3) physical activity was kept constant over 24 h but the energy intake was varied at two different levels (Dallosso et al, 1982) and (4) when BMR measurements were made in free-living subjects in whom neither intake nor activity were regulated (Soares & Shetty, 1986). In these studies, the CV of the measured BMR has never exceeded 5% and is frequently below 3%.

Estimation of the CV of 24-h energy expenditure measurements by whole body calorimetry also leads to similar conclusions (Table 2). Several studies (Dallosso et al, 1982; Webb & Abrams, 1983; Webb & Annis, 1983; Garby et al, 1984; De Boer, 1985) have confirmed the low CV of intra-individual measurements of 24-h energy output when both energy intake and physical activity are tightly regulated, as is usual in a calorimetry protocol. Even when energy intakes are varied at two different levels, but the activity patterns when inside the calorimeter are maintained constant, the within individual CVs do not vary by more than 2.4 or 2.6% (De Boer, 1985). When energy intakes are unaltered, but 24-h energy expenditure is varied at two different levels of activity in the same subject, a large CV (of the order of 9.8%) is seen. This is to be expected, since the 24-h energy output has been deliberately altered in these subjects. However, even in these experimental situations the CV of the measured BMR in the same subjects at the two different levels of activity while in the calorimeter is no more than 2.2% (Dallosso & James, 1984).

Table 2 Intra-individual variations in basal metabolic rate and total energy expenditure

	Sex	CV (%)
Basal metabolic rate
1. Energy intake and physical activity controlled Jéquier & Schutz (1981)	F	2.0
2. Energy intake controlled; physical activity varied Dallosso & James (1984)	M	2.2
3. Energy intake varied: physical activity controlled Dallosso et al (1982)	M	2.8
4. Energy intake and physical activity uncontrolled Soares & Shetty (1986)	M	2.9
Total energy expenditure (24 hour)
1. Energy intake and physical activity controlled
Dallosso et al (1982)	M	1.5
Webb & Abrams (1983)	F	3.3
Webb & Annis (1983)	F	6.0
Garby et al (1984)	M	2.2
De Boer (1985)	F	1.9
2. Energy intake varied; physical activity controlled
De Boer (1985)	F	2.4
De Boer (1985)	F	2.6

The intra-individual variation of total energy expenditure obtained from repeated measurements based on doubly-labelled water studies, where body weight, activity and physiological status remained unaltered, have also been recently compiled. Data from nine such studies are summarised in Table 3. They confirm that the CV is reasonably small, despite measurements being made with doubly-labelled water in the free-living state (Black et al, 1995). The mean within-subject CV of 79 individuals, in whom more than one doubly-labelled water measurement was made, was 8.9%. This includes both the methodological error and the variation in activity levels.

Recent evidence thus supports the conclusion that within-subject variations in BMR are small and insignificant, even when energy intake and physical activity are uncontrolled, (Shetty & Soares, 1988). This effectively refutes the Sukhatme-Margen hypothesis.

The constancy of BMR of adults over time

A critical analysis of the historical data on variations in BMR over long periods of time indicates that the BMR of an individual is constant over time. (Shetty & Soares, 1988; Henry, Hayter & Rees, 1989). More recent data, shown in Table 4, confirm this. BMR in 14 subjects (controls and obese), each tested on five consecutive days, had a CV of about 2% (Jéquier & Schutz, 1981); the BMRs of 166 male subjects studied on two separate occasions had a CV of less than 3% (Dallosso et al 1982). Other studies (Garby et al, 1984; Lammert et al, 1987; Soares & Shetty, 1986; 1987; Henry, Hayter & Rees,1989) support the view that the intra-individual, variations in BMR, measured over a period of days, weeks or even months or years, are small and probably not significant.

Table 3 Within-subject coefficient of variation in doubly labelled water measurement of total energy expenditure where activity, weight and physiological status are unchanged

Subjects	No. of subjects	No. of measurements	CV (%)
Adolescents confined to a metabolic facility during two periods of experimental diet. No control on activity	9	2	6.8
Twice in the calorimeter with the same imposed exercise	4	2	9.1
Mothers measured pre-pregnant and at 16 weeks of pregnancy	9	2	7.4
Mothers in weeks 4, 8 and 12 of lactation	10	3	7.9
Males living in a metabolic facility but following normal occupation. First and last measurements at same weight and activity	8	2	8.1
Males living in metabolic facility but pursuing usual sedentary occupation	7	3	7.1
Physiotherapy students. No apparent change in activity	5	2	10.5
Free-living men	17	2 or 3	11.0
Free-living men during two experimental diets	10	2	10.9
Mean of 9 studies			8.9

CV = within-subject coefficient of variation.

Table 4 Intra-individual variations in BMR (MJ/d) with time

			Coefficient of variation (%)
	Sex	n	Days	Weeks	Months	Years
Jéquier & Schutz (1981)	F	(14?)	2
Garby & Lammert (1984)	M	(22)	2.4
	M	(23)		2.2
Lammert et al (1987)	M	(7)	3.5	4.3
	M	(7)		4.8
Soares & Shetty (1986)	M	(5)		3.1
Soares & Shetty (1987)	M	(5)		2.9
	M	(10)			2.5
Henry et al (1989)	M	(9)				4.0

Table 5 Intra-individual variations in energy expenditure and body weight over time

				CV (%)
	Group	n	Time intervala (months)	EE	Body weight
BMRb Males	Entire	(10)	18.2 ± 2.3	2.5	2.5
			(7.0 33.0)
	Weight stabled	(5)	14.4 ± 2.9	3.2	0.6
			(7.0-21.0)
	Weight change	(5)	22.0 ± 3.0	1.8	4.3
			(15.0-33.0)
24-h EEc Females	Entire	(10)	9.5 ± 2.0	2.4	2.4
			(2.0-24.0)
	Weight stabled	(5)	7.2 ± 2.0	2.0	1.1
			(2.0-13.0)
	Weight change	(5)	11.8 ± 3.3	2.7	4.1
			(5 0-24.0)

^a Mean ± s.e.m.; figures in parentheses = range.
^b Soares & Shetty (1987).
^c De Boer (1985) (recalculated).
^d Considered stable if change < 2.0% of initial body weight.

A critical analysis of the variations seen in BMRs or in 24-h energy output over a period of up to 2 years, when intakes and activity patterns were not controlled over this length of time, is presented in Table 5. The BMRs of 10 male subjects measured on at least three occasions over a period of 6-36 months showed a mean CV of intra-individual differences (separated from measurement error) of the order of 2.5% (Soares & Shetty, 1987). Five of the 10 individuals who had body weight changes of > 2% had even smaller CVs (1.8%) than those who had smaller changes in body weight over the period of time. Measurements of 24-h expenditure by calorimetry in 10 females over a period of 24 months also showed small CVs of 2.4%; however, smaller CVs were seen in those five women who had <2% body weight change over this period (De Boer 1985). In these females, neither intakes nor activity patterns were controlled, except during calorimetry. These recent data confirm the conclusion that BMRs of individuals are relatively constant over a period of several years, despite reasonable fluctuations in body weight, when no attempt is made to regulate either energy intake or physical activity patterns.

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