The Carbohydrate-Insulin Model Wars

Writing on diet and health, I have been bemused to see the scientific heat that has raged over whether a lowcarb diet leads people to burn more calories, other things equal. It is an interesting question, because it speaks to whether in the energy balance equation

weight gain (in calorie equivalents) = calories in - calories out

the calories out are endogenous to what is eaten rather than simply being determined directly by conscious choices about how much to exercise.

My own view is that, in practice, the endogeneity of calories in to what is eaten is likely to be a much more powerful effect than the endogeneity of calories out to what is eaten. Metabolic ward studies are good at analyzing the endogeneity of calories out, but by their construction, abstract from any endogeneity of calories in that would occur in the wild by tightly controlling exactly what the subjects of the metabolic ward study eat.

The paper flagged at the top by David Ludwig, Paul Lakin, William Wong and Cara Ebeling is the latest salvo in an ongoing debate about a metabolic ward study done by folks associated with David Ludwig (including David himself). Much of the discussion is highly technical and difficult for an outsider to fully understand. But here is what I did manage to glean:

  1. Much of the debate is arising because the sample sizes in this and similar experiments are too small. I feel the studies that have been done so far amply justify funding for larger experiments. I would be glad to give input on my opinions about how such experiments could be tweaked to deliver more powerful and more illuminating results.

  2. One of the biggest technical issues beyond lower-than-optimal power involves how to control statistically for weight changes. Again, it is not so easy to fully understand all the issues with the time it is appropriate for me to devote to a single blog post, but I think weight changes need to be treated as an indicator of amount of fat burned with a large measurement error due to what I have called “mass-in/mass-out” effects. (See “Mass In/Mass Out: A Satire of Calories In/Calories Out.”) Whenever a right-hand-side variable is measured with error relative to the the exactly appropriate theoretical concept, a measurement error model is needed in order to get a consistent statistical estimate of the parameters of interest. I’ll write more (see “Adding a Variable Measured with Error to a Regression Only Partially Controls for that Variable”) about what happens when you try to control for something by using a variable afflicted with measurement error. (In brief, you will only be partially controlling for what you want to control for.)

  3. David Ludwig, Paul Lakin, William Wong and Cara Ebeling are totally correct in specifying what one should focus on as the hypothesis of interest:

Hall et al. set a high bar for the Carbohydrate-Insulin Model by stating that “[p]roponents of low-carbohydrate diets have claimed that such diets result in a substantial increase in … [TEE] amounting to 400–600 kcal/day”. However, the original source for this assertion, Fein and Feinman [18], characterized this estimate as a “hypothesis that would need to be tested” based on extreme assumptions about gluconeogenesis, with the additional qualification that “we [do not] know the magnitude of the effect.” An estimate derived from experimental data—and one that would still hold major implications for obesity treatment if true—is in the range of 200 kcal/day [3]. At the same time, they set a low bar for themselves, citing a 6-day trial [16] (confounded by transient adaptive responses to macronutrient change [3]) and a nonrandomized pilot study [5] (confounded by weight loss [8]) as a basis for questioning DLW methodology. Elsewhere, Hall interpreted these studies as sufficient to “falsify” the Carbohydrate-Insulin Model [19]—but they do nothing of the kind. Indeed, a recent reanalysis of that pilot study suggests an effect similar to ours (≈250 kcal/day) [20].

Translated, this says that a 200-calorie-a-day difference is enough to be interesting. (Technically, the authors say “kilocalories,” but dieters always call kilocalories somewhat inaccurately by the nickname “calories.”) That should be obvious. For many people, 200 calories would be around 10% of the total calories they would consume and expend in a day. If a 200-calorie-a-day difference isn’t obvious beyond statistical noise, a metabolic ward study is definitely underpowered and needs a bigger sample!

Conclusion. In conclusion, let me emphasize again that the big issue with the worst carbs is that they make people hungry again relatively quickly, so that they eat more. (See “Forget Calorie Counting; It's the Insulin Index, Stupid” for which carbs are the worst.) Endogeneity of calories in might be a bigger deal than endogeneity of calories out. Moreover, because it is difficult for the body to switch back and forth between burning carbs and burning fat, a highcarb diet makes it painful to fast, while a lowcarb highfat diet when eating makes it relatively easy to fast. And fasting (substantial periods of time with no food, and only water or unsweetened coffee and tea as drinks) is powerful both for weight loss and many other good health-enhancing effects.

Update: David Ludwig comments on Twitter:

Perhaps: “endogeneity of calories in to what is eaten is likely to be a much more powerful effect than the endogeneity of calories out to what is eaten.” But the latter is a unique effect predicted by CIM. And if CIM is true, both arise from excess calorie storage in fat cells.

For annotated links to other posts on diet and health, see:

Here are some diet and health posts on authors involved in the Carbohydrate-Insulin Model Wars: