Inequality, Evolution & Obesity


A recent study on child obesity by researchers at Harvard has received a good deal of media attention lately. In the NY Times, a synopsis of the study was one of the most emailed articles in the country (“Baby Fat May Not Be So Cute After All,” March 22). The key sentence from that article:

“More and more evidence points to pivotal events very early in life — during the toddler years, infancy and even before birth, in the womb — that can set young children on an obesity trajectory that is hard to alter by the time they’re in kindergarten.”


There have been previous studies suggesting similar patterns – that early life events can predispose individuals to higher body fat later in life. Why should we expect anything different? After all, life doesn’t begin in kindergarten. If anything, the embryonic and fetal periods of life, when we are most plastic and our organs are first forming, should have the greatest impact on our developmental trajectories.

The odd thing is that some studies have found that both low and high birth weight infants are at an increased risk for obesity later in life compared to infants in the middle range of the birth weight spectrum (Curhan et al., 1996). High birth weight infants may carry more body mass for whatever reason: genes, maternal diet, exposure to maternal diabetes. On the other hand, in developed countries rates of overweight and obesity are higher among the impoverished. Consistent with this fact is that low birth weight infants and prenatal malnutrition may predispose individuals to obesity as well, which seems paradoxical on the surface. Interestingly, when one looks more closely at the bodies of low birth weight infants beyond just weight alone, it is apparent that they are not lighter in all tissues equally. One study in India, for example, found that infants were much lighter than infants in England, but had very similar amounts of body fat (Yajnik et al., 2003). In effect, Indian babies were proportionately fatter than English babies. Why should this be?

For mammals, body fat stores are crucial in infancy since they have one food source – breast milk. If that food source is somehow interrupted, then the helplessness of an infant precludes it from obtaining nutrients in some other way. Human infants in particular are quite fat when compared with other mammals, with body fat percentage peaking around age 6 months (Kuzawa, 1998). This is probably related to a few different factors. First, human infants are incredibly helpless and reliant on others for nutrients. Second, they have really large, metabolically expensive brains that require a steady stream of nutrients. Third, the stresses of weaning (which, not coincidentally, often begins around age 6 months), such as infection from diarrhea from non-sterile foods, may exacerbate nutritional stress. Given these conditions, there are good reasons that we are predisposed to think that a fat baby is a healthy baby. Body fat is an infant’s backup plan in times of scarcity. It seems likely, then, that low birth weight babies make an effort to retain more body fat at the expense of other tissues. It appears to be an adaptive ‘strategy’ favored by natural selection, or what Gluckman and Hanson have termed a ‘predictive adaptive response’ to get ready for lean times. It is possible that this trajectory remains embedded in their physiology long-term.

The problem is that with the obesity epidemic in the United States, and much of the rest of the world as well, people may associate body fat with pathology a bit too hastily (recall the headline of the NY Times article: “Baby Fat May Not Be So Cute After All”). While it is true that excess body fat is a risk factor for many chronic diseases and may be too much of a good thing, it’s also important to remember why body fat is important to begin with.

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References

Curhan GC et al. (1996) Birth weight and adult hypertension and obesity in women. Circulation 94: 1310-5.

Gluckman P, Hanson M (2005) The fetal matrix: Evolution, development and disease.  Cambridge: Cambridge University Press.

Kuzawa CW (1998) Adipose tissue in human infancy and childhood: An evolutionary perspective. Yearbook of Physical Anthropology 41: 177-209.

Yajnik CS et al. (2003) Neonatal anthropometry: The thin-fat Indian baby. The Pune Maternal Nutrition Study. International Journal of Obesity and Related Metabolic Disorders 27: 173-80.

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