Seasonal variations in energy balance among agriculturalists in central Mali: compromise or adaptation?

OBJECTIVE

This paper considers whether seasonal variations in the energy balance of adult agriculturalists in Central Mali exceed acceptable risk, or whether they represent a short-term adaptive response without functional consequence.

DESIGN

Prospective/longitudinal study design.

SETTING

Agricultural village in rural Mali.

SUBJECTS

From a total population of 166 adults aged > or = 16 years, two cohorts of 63 men and 73 non-pregnant women with complete records were retained for analysis.

INTERVENTIONS

Over a 12-month period, monthly anthropometry and biweekly retrospective assessments of morbidity were made on every adult in the sample. Direct measures of household food consumption were collected twice in harvest, dry and rainy seasons respectively. Continuous 15-h observations of time-use and work intensity were conducted on a sub-sample of active adults and used to calculate mean seasonal energy expenditure.

RESULTS

Energy balance as measured by body mass index (BMI; kg/m2) declines significantly in the rainy season (P < 0.01), corresponding to a seasonal weight loss of 2-3 kg. A similar seasonal trend in % body fat (P < 0.0001) suggests that most of this loss represents adipose tissue. A rainy season increase in arm muscle area (AMA) implies that there is no seasonal compromise in lean tissue. Among men and women with BMI > 18.5, seasonal fat loss is significantly greater than their leaner counterparts. Considering the likely determinants of variations in energy balance, an increase in the duration and incidence of adult morbidity is observed; however, no significant intercorrelations exist between it and seasonal nutritional indices. Comparing trends in energy expenditure and intake, a disturbance in energy homeostasis is apparent as energy expenditure increases in the rainy season relative to constant household food consumption across seasons. While seasonal variations in energy expenditure are not as dramatic in the female sample, they appear to have less opportunity to recover from so-called 'heavy' levels of expenditure experienced in successive dry and rainy seasons. Periods of 'light' activity in both harvest and dry seasons provide the male sample with a reasonable length of time in which to reconstitute energy stores after the physical demands of the rainy season.

CONCLUSIONS

In the year of study, the modest loss of body fat recorded in the rainy season (< 5% body mass) represents a successful physiological response to energy imbalance, and is unlikely to compromise adult productive and reproductive function.