The law of the excluded middle is one of the foundations of logic. It says that if a proposition is false then the opposite must be true. There is no room for a middle ground in classical logic. However, one must be extremely careful when applying the law to biology where hypotheses are generally situational and rest on many assumptions. In order to apply the law of the excluded middle, one must have only two alternatives and this is seldom true in biology and in particular human metabolism. Gary Taubes argued quite successfully in his book Good Calories, Bad Calories that fat probably doesn’t cause heart disease and in some cases may even be beneficial. A major theme of that book was that scientists can become irrationally attached to hypotheses and willfully ignore any evidence to the contrary. He recently penned a New York Times opinion piece arguing that the medical establishment is equally misguided in asserting that salt is unhealthy. One of the hypotheses that Taubes dislikes the most is that “a calorie is a calorie”, which proposes what you eat is not as important as how much you eat when it comes to weight gain and obesity. Taubes thinks that carbs and especially sugar is what makes you fat (and causes heart disease). This is summarized in his Times opinion piece today, which covers the recent JAMA result that I posted about recently (see here).
It may very well be true that a calorie is not a calorie but that still may not mean carbs are the cause of the US obesity epidemic. I’ve posted on this a few times before (e.g. see here and here) but I thought it was important enough to reiterate and simplify the points here. In short, the carbs are bad argument is that 1) carbs induce insulin and insulin sequesters fat, and 2) carbs are metabolically more efficient so you burn fewer calories when you eat them compared to fat and protein. Even if this is true (and it may not all be) that still doesn’t mean that calories are unimportant. I don’t care how metabolically efficient carbs may be, you would starve to death if you only ate one sugar cube each day. Conversely, no matter how many excess calories you may burn eating fat, you will become obese if you eat two pounds of butter each day. Hence, even if a calorie is not a calorie, calories still matter. It is then a matter of degree. If you manage to burn everything you eat then your body won’t change. This is true if you eat a high carb or a low carb diet. Now it could be true that you could have a different amount of body fat and weight for the same calorie diet depending on diet composition. So a plausible hypothesis for the cause of the obesity epidemic is that we switched from a high fat diet to a low fat diet and everyone became fatter as a result. This is something that I’m planning to test using the same data that we used to show how the increase in food production is sufficient to explain the obesity epidemic. Ultimately though, the brain is what decides how much we eat and one of the biggest things we don’t understand is how diet composition affects food intake. It could be that low carb diets do make you thinner but the reason is that we tend to eat less when we’re on them.
2012-7-2: changed fat to carb in last sentence.