People living in areas of high climatic seasonality are well aware of the nutritional impact of seasonality, as indicated by the language they use to define such seasonal stress periods. The Massa of Cameroon call the month of July in the middle of the wet season the month of 'Did you call me for food?' and they have a word to define 'hunger with threat,' when food shortage has been too long and life is in danger. However, it was not until the 1950s that the scientific community started to appreciate the presence of a nutritional impact of seasonality, and its functional significance is still a matter of discussion.
As described earlier, seasonal climates may affect nutritional status via a combination of reduced dietary intake, increased physical activity, and increased disease incidence, and may occur to a variable extent in different populations and socioeconomic groups. As a result, both energy balance and micronutrient status may be affected, and this is confirmed by the observation of changes in body weight and body composition in adults, growth performance in children, pregnancy weight gain, and birth weight as well as by changes in micronutrient status.
Body weight changes between 1 and 4 kg, corresponding to 2-5% of body weight have been observed in adults in areas of medium and high seasonality. Larger body weight changes occur in draught years or among pastoralists. Remarkable interindividual differences also feature in parallel with such mean values. Smaller changes are usually suffered by women and by people with a lower body mass index (BMI).
Socioeconomic differences may go in both directions: wealthier people may either lose more weight, because they own land and have the opportunity and the need to perform more intense agricultural work, or lose less, because they rely on hired labor.
The composition of the tissue lost varies according to the size of the loss and to the initial energy stores. In a rural population in Mali the mean BMI was low (19.8 kg m-2 in men and 19.3 kg m-2 in women), the weight loss in young active men (16-35 years) was 2.6kg, corresponding to a 3.8% change in body weight, and changes in body fat were in the order of 1.5%. Arm muscle area was also reduced. In Northeast Thailand, where BMI in men was similar (19.8 kg m-2) but the weight change smaller (1 kg), a 1.5% reduction of the fat mass was also observed, but no reduction in the fat-free mass could be detected by anthropometry.
The vulnerable groups of the population are not spared by the seasonal stress. During the wet season, energy imbalance leads to the utilization of fat reserves in pregnant women and the women's own energy requirements compete with that of the fetus, leading to increased reproductive risk. In Sierra Leone, at the time of planting and harvesting, pregnant women are expected to continue working and are also more affected by malaria, anemia, and pregnancy-induced hypertension; as a result, in this season birth weights are the lowest in the year. In Taiwan predelivery skin-fold thickness of a cohort of women measured in the cold season was greater than that of a comparable cohort measured in the warm/wet season and a 150 g difference in mean birth weight was observed. In The Gambia a 0.4 kg month-1 weight gain was observed among pregnant women during the rainy season, as opposed to 1.4 kg month-1 during the dry season. Dry season mean birth weight was 160 g higher than in the wet season, and the prevalence of low birth weight was 13% in the dry season and 35% in the wet season. Perinatal and infant mortality were also higher in wet season cohorts.
Seasonal stress continues after birth for both mothers and children. In The Gambia lactating women lost on average 0.74 kg month-1, at the same rate of nonlactating women. As shown earlier, during the wet season very young children get less attention and less breast milk from their mothers and their growth is affected. In Taiwan, children born in the hot wet summer season were smaller, but could catch-up in the following 3 months, while those born in the dry season had a larger birth weight but had a slower postnatal growth.
Seasonal impact is more evident at critical times when a more intense growth effort is required, in order to catch-up from previous delays or at the mid-infancy growth spurt. In a rural area of The Gambia the lower weight and height gains observed in the wet season were not followed by corresponding increases at other times of the year. In Malawi the weight-for-age Z scores and height-for-age Z scores declined more rapidly during the rainy season among 1 to 6-month-old babies and among 13 to 36-month-old children, but not among the 7 to 12-month-old babies.
Weight and height increments are more sensitive indicators than achieved weight and height. In Bangladesh monthly height gain in children under 5 years ranged from a minimum of 12-20% of the reference value to 200-240% (respectively, in boys and girls), while height gain fluctuated between 5260% and 165-180%. In Ethiopian children, height growth velocity showed a marked seasonal pattern, with values close to normal (—0.2 SD units) in July to December, a period characterized by better food availability, and lower values (—3.0SD units) in January to June, a period characterized by intensive farm labor and heavy rains. Therefore, there was never an opportunity to recover from growth faltering and stunting was a continuous process in the first 5 years of life.
Unlike the younger children, the seasonal variables did not have a permanent effect in older children and adolescents. In the Ethiopian study, girls above the age of 10 years showed accelerated growth in the first semester of the year and delayed growth in the second semester, characterized by the wet season, so that the mean yearly growth rates were normal overall. This was also observed in Senegalese adolescents, in whom arm circumference and triceps skinfold were significantly lower during the rainy season, followed by a recovery in the postharvest season, but no change in the growth rate.
In young children, labor burden is not a critical variable, but the reduction of food availability, combined with the greater incidence of infectious diseases, especially diarrhea, leads to impaired growth. This is supported by the observation that seasonality may also affect urban children. In an urban area of The Gambia height-for-age showed little seasonal variation, but weight gain was poor during the rains and was not compensated by catch-up growth during the dry season. In older children and adolescents, the seasonal effects may instead be related to increased physical activity.
Figure 4 illustrates the magnitude of seasonal effects on mean weight changes observed in adult men and women in different regions of the world. In both sexes the observed values range between 1 and 5 kg, although the values greater than 4 kg have been documented during extreme environmental stress, as in drought.
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