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Editor,—The recent articles on sleeping metabolic rate (SMR) in infants are commendable in addressing the relationship between heat balance and the thermal environment.1 2There is increasing awareness that this relationship may be relevant to sudden infant death syndrome.3
However, although agreeing with the authors that SMR can vary considerably between individuals due to genetic and thermal environmental factors, we suspect that the variation reported in the studies would prove less extreme if variation in body size and sleep state were taken into account.
In the first study,1 SMR per kg body weight was assumed to be an index of SMR adjusted for body size, and differences in SMR/kg were assumed to be equivalent to differences in thermal balance. However, we have shown previously that, for a given age group, expressing metabolic rate on a per kilogram basis does not give an index of SMR that is independent of weight.4 Rather, SMR/kg underestimates the metabolic rate of large infants and overstimates that of small infants, whereas a mass independent index of SMR would not be related to weight. Thus the plotting of SMR/kg against age1 is likely to have resulted in the artificial exaggeration of variation in SMR, since the effects of body size are not fully removed. This is firstly due to individual differences in body fatness, as fat free mass is the true body size determinant of SMR. However, even after taking fatness into account, a relationship between SMR/kg and body size persists,4 which is due to size related variation in the ratio of organs, with higher metabolic rate, to muscle tissue, with lower metabolic rate.5 These factors are equally applicable to the issue of surface area, where SMR expressed on a per m2 basis is not constant between large and small infants.
In the second study,2 repeat measurements of SMR were made on infants. In both studies measurements lasted 15 minutes, but it is generally accepted that infants sleep in cycles lasting approximately one hour, during which sleep state influences metabolic rate.6 During the cycles, metabolic rate has been shown to fall to a minimum of approximately 89% of the average hourly SMR value.4 7 Thus comparison of measurement periods that fail to take into account sleep cycle variation will almost certainly exaggerate interindividual variation, whereas comparison of mean hourly SMR or minimum SMR4 7 would be a more appropriate methodology.
Finally, the authors conclude that metabolic rate per unit surface area is not higher in infants compared to older age groups. It is now generally accepted that fat free mass is the most appropriate aspect of body size in terms of which to express metabolic data.8Resting energy expenditure per mass independent unit of fat free mass declines during infancy and subsequently, because the ratio of organ tissue to muscle tissue declines as the infant grows.5However, mass-independent total energy expenditure is influenced additionally by physical activity level and has a more complex development with age.
We applaud the investigation of factors associated with the exchange of heat between infants and their environment, but we suggest that the extent of true variation, both between and within individuals, may have been substantially overestimated in these studies.
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