Something has changed, but what? Well, the most parsimonious explanation is that we're simply eating more. Here is a graph I created of our calorie intake (green) overlaid on a graph of obesity prevalence (blue) between 1970 and 2008:
As you can see, the fit is extraordinary. For those who may note that calorie intake appears to increase after obesity, I have to point out that the calorie intake graph has a point for each year, while the obesity graph has no points between 1978 and 1991. Therefore, all we know is that obesity began to climb at some point between the 1976-1980 and 1988-1994 survey periods. This is consistent with obesity being a result of increased calorie intake.
The careful work of Dr. Eric Ravussin and colleagues, based on what we know about the relationship between calorie intake and body fat, has shown that the increase in calorie intake over the last 40 years can single-handedly explain the increase in obesity in this country (2, 3)*. This is something I discussed in my Boing Boing feature piece "Seduced by Food: Obesity and the Human Brain".
The question remains, why are we eating more calories today than we used to? The answer is related to increased availability and consumption of energy-dense, seductive and palatable foods, including sweetened beverages, and advertising that makes us crave them (4). However, nothing about human physiology or behavior is simple, and as always there's more to the story.
In future posts, I'll explore in more detail why we eat. From this position of knowledge, based on the work of many outstanding researchers, I think it will be fairly clear why we eat more today than we used to.
* I know someone out there is thinking about the "alternative hypothesis" that we're eating more because our fat mass is expanding. Anyone can come up with a hypothesis, but not all hypotheses are supported by evidence from the scientific literature. There is a mountain of evidence demonstrating that experimentally increasing calorie intake increases fat mass, regardless of whether the calories are coming from carbohydrate or fat, and regardless of changes in circulating insulin (5, 6, 7, 8). When someone publishes evidence that the reverse occurs-- that fat mass expansion causes increased food intake in humans, and that this is not only biologically possible but actually occurs in common obesity-- I'll begin taking the idea seriously. Until that time, it will remain unfounded speculation that is unnecessary due to the existence of explanations that are more parsimonious and better supported.
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