Dr. Michael Eades linked to an interesting study yesterday on his Health and Nutrition blog. It's entitled "Vegetable-Rich Food Pattern is Related to Obesity in China."
It's one of these epidemiological studies where they try to divide subjects into different categories of eating patterns and see how health problems associate with each one. They identified four patterns: the 'macho' diet high in meat and alcohol; the 'traditional' diet high in rice and vegetables; the 'sweet tooth' pattern high in cake, dairy and various drinks; and the 'vegetable rich' diet high in wheat, vegetables, fruit and tofu. The only pattern that associated with obesity was the vegetable-rich diet. The 25% of people eating closest to the vegetable-rich pattern were more than twice as likely to be obese as the 25% adhering the least.
The authors of the paper try to blame the increased obesity on a higher intake of vegetable oil from stir-frying the vegetables, but that explanation is misleading. A cursory glance at table 3 reveals that the vegetable-eaters weren't eating any more fat than their thinner neighbors. Dr. Eades suggests that their higher carbohydrate intake (+10%) was partially responsible for the weight gain, but I wasn't satisfied with that explanation so I took a closer look. Dr. Eades also pointed to their higher calorie intake (+120 kcal/day), which makes sense to me.
One of the most striking elements of the 'vegetable-rich' food pattern is its replacement of rice with wheat flour. The 25% of the study population that adhered the least to the vegetable-rich food pattern ate 7.3 times more rice than wheat, whereas the 25% sticking most closely to the vegetable-rich pattern ate 1.2 times more wheat than rice! In other words, wheat flour rather than rice was their single largest source of calories. This association was much stronger than the increase in vegetable consumption itself!
All of a sudden, the data make more sense. Wheat seems to associate with health problems in many contexts. Perhaps the reason we don't see the same type of association in American epidemiological studies is that everyone eats wheat. Only in a culture that has a true diversity of diet can you find a robust association like this. The replacement of rice with wheat may have caused the increase in calorie intake as well. Clinical trials of low-carbohydrate diets as well as 'paleolithic diets' have shown good metabolic outcomes from wheat avoidance, although one can't be sure what role wheat plays from those data.
I don't think the vegetables had anything to do with the weight gain, they were just incidentally associated with wheat consumption. But I do think these data are difficult to reconcile with the idea that vegetables protect against overweight.