Factors influencing variation in basal metabolic rate
Nutrition Research Newsletter, Dec, 2005
The major factor determining basal metabolic rate (BMR) is fat-free mass (FFM), with some studies finding an additional contribution of fat mass (FM), but others failing to find such an effect. Even when these factors are known, however, substantial residual variation remains. Understanding the physiologic nature of this variability is important because it has been implicated in the obesity epidemic in two separate ways. First, low BMR (after mass effects are removed) may be a predisposing risk factor for the development of obesity, and cross-sectional studies often report that formerly obese subjects have BMRs that are 3% to 5% lower than expected. However, although some long-term studies have indicated that variation in BMR is associated with subsequent weight gain, other longitudinal studies have failed to replicate this effect. Second, BMR may show an adaptive response to caloric restriction, which may predispose individuals to subsequent weight regain. Many studies have called into question whether such changes reflect metabolic adaptation or simply disproportional losses of metabolically active FFM. In the present study the researchers aimed to define the roles of FFM, FM, age, and sex and to examine whether residual variation in BMR once these factors had been accounted for was linked to concentrations of circulating leptin and thyroid hormones [T3 and thyroxine (T4)].
One hundred fifty adults (n = 107 women, 43 men) aged between 21 years and 64 years and having a BMI (in kg/[m.sup.2]) in the range of 16.7 to 49.3 were recruited to participate in a study investigating genetic and environmental influences on body weight. The subject group was a representative sample of the Scottish population, with 1 in 5 obese (BMI > 30) and 54% of the population collectively overweight and obese (BMI > 25). Subjects were included only if they were not consuming any special medical diet; had a stable weight (weight change > 2 kg in the previous 3 months); were otherwise healthy on the basis of an extensive medical examination, screening blood tests (full blood count and renal, liver, and thyroid function), and electrocardiogram; and took no regularly prescribed medication or vitamin or mineral supplements.
The subjects attended the Rowett's Human Nutrition Unit for measurements of body composition and metabolic rate under standardized conditions. For estimates of basal energy requirements, BMR (MJ/d) was measured by indirect calorimetry with a ventilated hood system. Body composition was assessed by dual-energy X-ray absorptiometry. Measurements were conducted in the morning immediately after the BMR and standard anthropometry measurements while the subjects were still in a fasted condition. Circulating leptin in plasma was measured by using a radioimmunoassay kit.
Only 2% of the observed variability in BMR was attributable to within-subject effects, of which 0.5% was analytic error. Of the remaining variance, which reflected between-subject effects, 63% was explained by FFM, 6% by FM, and 2% by age. The effects of sex and bone mineral content were not significant. 26% of the variance remained unexplained. This variation was not associated with concentrations of circulating leptin or T3. T4 was not significant in women but explained 25% of the residual variance in men.
Many previous studies have quantified the repeatability of BMR measurements when using hood-based measurement systems. Previous estimates of within-subject repeat-ability fell in the range of 2% to 4% of the mean estimate, with the extent of repeatability depending on the interval between measurements. The present estimate of repeatability of 2.8% falls within the expected range given the short time scale between repeated measurements in this study. Because BMR is a highly repeatable trait, the contribution of within subject variation to the total variance was trivially small (2%), which indicates that most of the variance in BMR resided between the individuals under study.
Consistent with previous studies the dominant factor influencing this variation between individuals was the extent to which the individuals varied in their FFM. The additional role played by FM as an independent factor influencing BMR has been a matter of previous debate, with some studies finding a significant independent effect and others failing to replicate this result. This study confirmed that, in this sample of adults, FM was an independent factor--although the contribution to the total variance that was explained by differences in FM was relatively small compared with that explained by FFM.
Like many other studies, this work has indicated a large between-individual variation in BMR that cannot be explained by morphologic characteristics, including sex and age. The researchers show here that two important physiologic candidates that might explain this variability, namely, variations in circulating concentrations of leptin and T3, do not account for this unexplained variation in a population of 150 white adults. In men, however, 25% of the residual variance was associated with variation in circulating concentrations of T4. No equivalent effect was found in women.
A. Johnstone, S. Murison, J. Duncan, K. Rance, and J. Speakman. Factors influencing variation in basal metabolic rate include fat-free mass, fat mass, age, and circulating thyroxine but not sex, circulating leptin, or triiodothyronine. AJCN; 82(5):941-948 (November 2005). [Correspondence: A. Johnstone, Rowett Research Institute, Aberdeen, Scotland, UK, AB21 9SB. E-mail:
a.johnstone@rowett.ac.uk].
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