The ‘income related reversal of fatness in the female’ is the paradoxical changes that occur to female body fat levels in individuals from different socioeconomic backgrounds. In this regard, females of low socioeconomic status are thinner than their wealthier counterparts, but as these women age, the low socioeconomic females becomes fatter, relative to the more affluent females. Put another way we can say that relative leanness predominated in female girls of low socioeconomic status, whereas relative leanness predominates in women of higher socioeconomic status. Such findings have been confirmed in large scale population surveys such as the Health and Nutrition Examination Survey and the Ten State Nutrition Survey. For the paradox to occur, the rate of body fat accumulation in low socioeconomic females must outstrip the accumulation seen in high socioeconomic status females following adolescence. The same patterns of paradoxical changes however are not seen in males.
Researchers have investigated the changes in body fat that occur in populations by observing groups of individuals over time in prospective studies. For example in one study1, researchers followed over 1000 boys and girls over an 18 year period to assess their long term body fat changes. The authors used measurements of skinfold thickness to measure body fat in the boys and girls as this has been shown previously to more accurately represent total body fat compared to many other clinical measures. The results showed that the rates of body fat accumulation were faster for children belonging to parents of lower socioeconomic status compared to those belonging to parents of higher socioeconomic status. The authors suggested that the rate of body fat gain in the children explained the income related reversal of fatness in females. The gain in subscapularis and triceps skinfold thickness was as much as 7mm greater in the lower socioeconomic group, and this would equate to around a 20 to 30 % greater increase in body fat.
The greater obesity rates reported in the poor are interesting because the reason for this trend is not known. It should be pointed out however, that this trend is only present in Western developed nations. Developing nations actually show an opposite trend, in that those of lower socioeconomic status tend to be leaner than those of higher socioeconomic status. In fact the differences in the obesity rates between the poor and the rich in developing and developed nations gives clues as to the real cause. One of the most likely explanations for the differing rates of obesity relates to the availability of Western foods. In developing nations Western style foods tend to only be available to the rich, whereas the poor tend to eat more traditional diets. In contrast, in developed nations the poor tend to have limited access to higher quality foods (that tend to be more expensive) whereas the rich are able to consume these foods. However, it is unclear why girls from poorer families are leaner than those from richer families.
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