There is no question that Americans are getting fatter. The CDC animated graphic tells the tale – state by state statistics of the percentage of population that are obese. The big question is, what’s causing it? There are three main hypotheses, which are not mutually exclusive. The first is that activity levels are down. People, especially children, are spending more time indoors in front of computer screens and TVs and less time outside running around. The second is that people are eating more calories. And the third is that the type of calories we are eating is playing a significant role. There are two main camps in this third group: those who blame fat consumption and those who blame carbohydrates.

I do not feel that the evidence supports the third group – blaming calorie type. This hypothesis is great for selling books advocating one fad diet or another, but there is just no convincing evidence that altering the type of calories consumed has a significant effect on weight. Certainly the low-carb craze has not caused a blip in the steady rise of obesity in the country, just like the low-fat craze failed to have an effect. You can argue that this is because not enough people actually adopted an effective diet. However, if book sales are any indication, millions of people tried low-carb diets, and they do not appear to have struck upon the secret of easy weight loss. The clinical data also shows that weight loss is generally a factor of total calories, not calorie type.

It is true that Americans are becoming more sedentary, and it’s hard to imagine that this is not contributing to some degree to the obesity problem. But the data is not clear. The evidence shows an overall association between obesity and greater time spent in sedentary activities. However, recent data suggests that obesity causes lower activity levels (at least in children), not the other way around.

So we are left with the primary factor in actually causing the obesity epidemic being an increased intake in total calories. Multiple independent lines of evidence point in this direction. First of all – if we simply calculate the total amount of food being consumed by Americans it looks as if we are consuming 500 calories per day per person more than we did in 1970. Further – that increased caloric intake completely accounts for the rising trends in obesity.

If we accept that increased caloric intake is the primary culprit, then the deeper question becomes – why are we consuming more calories? It could just be culture, it could be the “supersized” fast food industry, or it could be changing trends in eating habits. One factor often raised is the fact that families with both parents working have less time to prepare traditional meals, and therefore there is more reliance on fast food, eating out, and store-prepared foods – all of which tend to have more calories.

A new study supports the notion that eating out is a significant contributor to caloric intake. The study finds:

The study determined that increased energy intake (+179 kcal/day) by children from 1977-2006 was associated with a major increase in calories eaten away from home (+255 kcal/day). The percentage of calories eaten away from home increased from 23.4% to 33.9% from 1977-2006.

This study shows that kids are eating more calories outside the home, and this contributes to greater overall caloric intake. This does not necessarily mean that such food sources are inherently more caloric – it could just mean that kids have greater access to food outside the home and this is adding incrementally to their overall calorie intake.

However, restaurant portion sizes and overall calories have been creeping up over the years. Prepared foods also compete for tastiness, and that often results in higher calories (even when labeled low fat or low carb). To address this issue there is an increasing push for labeling calories on menus. I find this extremely useful. It’s difficult to be in denial about how many calories a restaurant meal may have when the calories are printed right there on the menu. This does tend to affect food selection and aids those who are trying to estimate their caloric intake.

A more aggressive public health strategy would be to regulate the food industry to reduce the number of calories in prepared food. Such draconian measures are not popular, but as our health care system strains under the weight of the obesity epidemic such measure may become more palatable.

Conclusion

There is still a need for more and better research into the many questions and sub-questions surrounding the obesity epidemic, but at this point, the evidence is pointing to increased overall caloric intake as the primary culprit. There are no easy answers to this problem, but it is becoming increasingly important that we find practical solutions.