The false dichotomy of carbs and obesity

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.

10 thoughts on “The false dichotomy of carbs and obesity

  1. “one of the biggest things we don’t understand is how diet composition affects food intake”


    Anecdotally and unscientifically, my observation has always been that HFCS-sugary/processed-carby diets cause higher intake – and that these culturally often are served together with energy-dense fats, tempting abuse. Of course, this combines with the people eating junk food tending to be the most vulnerable to temptation of all types (“diet composition affecting food intake” is something that will disparately affect people of different discipline levels).


  2. But doesn’t Taubes argue precisely that even in low-calorie diets, people can gain weight, if the diet is very high in carbs and, presumably, through the suppression of BMR? He specifically discusses historical instances of mothers with starving children who are nonetheless very overweight.


  3. Taubes absolutely argues this point but if you take it to the limit it will eventually fail. There are 4 Calories in a gram of sugar. No human can survive on 4 Calories a day forever. It takes at least a thousand or more calories to stay alive even if you are in a coma. Hence, what he really means is that carbs are more efficient at the margin. Then it becomes a quantitative question. In terms of the obese mothers and malnourished kids, we have no idea what they are eating. Kids also have very different protein requirements then adults. It could be true that kids are not getting enough protein and micronutrients and hence are malnourished even though their mothers are obese.


  4. I don’t think Taubes disagrees with your last statement. In fact, a large portion of his book is devoted to arguing that yes, eating more fat and less carbohydrates, shifts the way in which your ingested calories are partitioned into less being stored as fat and more being expended as energy (or heat). In such a situation yes, eating fewer carbohydrates would make you have less of an appetite since you might have less of a physiological need for more calories. I think Taubes would disagree with you that this is all a function of the brain however, but rather a homeostatic mechanism at the level of tissues that doesn’t need to involve the brain at all.


  5. I’m not sure you’ve correctly portrayed what Taubes was saying in that book and the articles he’s written. I didn’t understand Taubes’ point regarding “a calorie is not just a calorie” to mean that you could ingest an infinite amount of one kind of calorie and not gain weight, or could ingest a tiny amount of another kind (one sugar cube a day) and survive just fine, as you seem to have understood him saying.

    I took his thesis to be that calories from fat and protein affect us differently from calories from carbohydrates, and not that we could eat infinite amounts of them, but rather eat as much as we feel like, until we’re satisfied, much like animals in the wild often do, and not become obese, just as they don’t. In addition, he points out how carbs induce insulin responses which have a complex effect on whether calories are stored or burned.

    Another way to put it is that he’s saying that if you don’t overload with carbs, you *do* tend to “burn everything you eat” as you put it, and you do so for a variety of reasons.

    I’ve read your post twice now and I think to some degree you’re making “calories don’t matter” into a straw man of your own interpretation and then arguing against that, but I don’t think it’s a fair representation of his argument.


  6. That is precisely my point. Taubes doesn’t imply that calories don’t matter, just that they are less important than diet composition. I’m just pointing out this out. From the email I receive, not everyone gets this.


  7. my 2 cents (or 50–hip hop)—if you dont eat any protein, no matter how much fat or carbs you eat you will eventually lose weight and even starve to death.


  8. “one of the biggest things we don’t understand is how diet composition affects food intake”
    Agreed 100%. The association between carbs and obesity has to be at least partially attributed to how easy it is to overeat carb-rich foods like junk food and soda.


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