The growing foetus is extremely sensitive to external stimuli, particularly the health and nutrition of the mother. Recent evidence suggests that the future health outcomes of an individual may be determined during this sensitive foetal period. While the traditional viewpoint suggests that obesity is caused by a positive energy balance as a result of increase energy intake (greed) and decrease physical activity (laziness), a growing alternative theory suggests that this simplistic model is seriously flawed. Increasingly, evidence suggests that sub-optimal foetal growth might sensitise the individual to particular environmental stimuli in later life. In particular, evidence suggests that this sensitisation may make some individuals respond to certain food stimuli with deleterious physiological reactions. A number of associations between birth weight and anthropometric characteristics have been identified which suggests that the development of obesity in part may be related to the period of growth in the womb.
For example, researchers1 have investigated the associations between birth weight and various anthropometric measurements in 78 obese women with a mean body mass index of 34 km/m2 and a mean age of 36.7 years. Body fat, lean mass and bone mass were assessed using duel energy x-ray absorptiometry, resting metabolic rate (RMR) was measured using indirect calorimerty and insulin resistance was measured using the homeostasis assessment model insulin resistance (HOMA-IR). The results showed that the birth weight Z-score (a measure of the birth weight in relationship to the mean birth weight) was associated with lean mass and RMR. In addition, when the subjects were placed on a low energy diet, birth weight Z-score was associated with changes in fat mass and changes in the fat mass to lean mass ratio. However, there were no associations between birth weight and insulin resistance or blood glucose levels.
A lower birth weight relative to the mean value therefore is associated with a lower muscle and bone mass later in adult life, and this in turn is associated with a lower RMR. In this study, after 12 weeks of the low energy diet, the study participants lost an average of 7.7 kg. However, there were differences in the way the study participants responded to this low energy diet, depending on their birth weight. Individuals were less likely to lose body fat and more likely to lose lean mass if they had a lower than average (mean) birth weight. This inability of the low birth weight individuals to respond to a low energy diet with fat loss may have been a result of their lower RMR and lower lean mass, which would normally account for between 50 to 80 % of their total energy expenditure. However, care must be taken when interpreting these data, as many confounding variables could exist.
These results support the contention that obesity is not caused by a simple positive energy balance, as stated by the traditional theory of weight gain. Importantly, these results show that obese individuals may have different physiological responses to energy restrictive diets if they had a low birth weight, and this may need consideration in the implementation of weight loss strategies. That some of the women in this study were obese but had normal birth weights argues that low birth weight is not the cause of obesity. However, it might be a contributory factor, and the relationship between a low birth weight and RMR suggests that such individuals might struggle lose excess body fat. Because low birth weight is already known to sensitise individuals to the effects of sodium on blood pressure (here), it is not surprising that other health outcomes are also affected.
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