4.1. Basal metabolic rate (BMR)

A decrease in BMR, persisting even after adjusting for body weight and fat-free mass (FFM) losses, represents one of the earliest and best known adaptive responses to energy deficit, and has been extensively described under experimental and controlled laboratory conditions. However, the seasonal impact on BMR has not been systematically investigated, and little can be found on this topic in the literature.

A study conducted in rural Gambia showed that 21 unsupplemented women, who lost 5 kg of body weight during the rainy period (Figure 8), lowered their BMR by a maximum of 50 kcal/d (LAWRENCE et al., in press). This drop is statistically significant (p<0.01), but obviously can have only a very doubtful energy-saving value. Furthermore, the drop appears to have slightly preceded the loss of body weight. Since there was a concomitant reduction of energy intake and thus an energy deficit, these findings suggest that BMR was as early a response to energy imbalance as the loss of body weight.

Women in Benin lost 1.5 kg between April and June, at the end of the rainy season, while their BMR did not change (SCHULTINK et al., in preparation). Ethiopian women lost about 1 kg and experienced a simultaneous decrease of energy intake. Their BMR appeared to follow energy intake changes more closely than body weight changes (FERRO-LUZZI, SCACCINI and DENTE, in preparation).

In conclusion, the changes in BMR are neither consistent throughout the various reports, nor biologically significant. Our understanding of their meaning is limited, and it is hardly possible to draw firm conclusions from these scarce and conflicting data. Studies conducted by SUZUKY (1959) in Japan revealed a significant seasonal fluctuation in BMR, closely related to average temperature. There were also fluctuations in energy intake, although it would be rather unlikely, given the social status of the study subjects, that there were external limitations to food availability. Therefore, as for growth performance in children, it could well be that at least part of the observed seasonal fluctuations of BMR in LDC may be explained by factors other than energy imbalance.