The low carb war continues

Last month, a paper in the British Journal of Medicine on the effect of low carb diets on energy expenditure, with senior author David Ludwig, made a big splash in the popular press and also instigated a mini-Twitter war. The study, which cost somewhere in the neighborhood of 12 million dollars, addressed the general question of whether a person will burn more energy on a low carbohydrate diet compared to an average or high carb diet. In particular, the study looked at the time period after weight loss where people are susceptible to regaining weight. The argument is that it will be easier to maintain weight loss on a low carb diet since you will be burning more energy. Recent intensive studies by my colleague Kevin Hall and others have found that low carb diets had little effect if any on energy expenditure, so this paper was somewhat of a surprise and gave hope to low carb aficionados. However, Kevin found some possible flaws, which he points out in an official response to BMJ and a BioRxiv paper, which then prompted a none-too-pleased response from Ludwig, which you can follow on Twitter. The bottom line is that the low carb effect size depends on the baseline point you compare too. In the original study plan, the baseline point was chosen to be energy expenditure prior to the weight loss phase of the study. In the publication, the baseline point was changed to after the weight loss but before the weight loss maintenance phase. If the original baseline was chosen, the low carb effect is no longer significant. The authors claim that they were blinded to the data and changed the baseline for technical reasons so this did not represent a case of p-hacking where one tries multiple combinations until something significant turns up. It seems pretty clear to me that low carbs do not have much of a metabolic effect but that is not to say that low carb diets are not effective. The elephant in the room is still appetite. It is possible that you are simply less hungry on a low carb diet and thus you eat less. Also, when you eliminate a whole category of food, there is just less food to eat. That could be the biggest effect of all.

2 thoughts on “The low carb war continues

  1. I saw some of that debate. D Ludwig posted a summary of his research on the free online mag medium. (An engineering professor commented on that and gave a summary of a model he had for that; while much of his research is very applied engineering, his degree is from Duke and his PhD was on chaotic dynamics of the Duffing equation which is theory.)

    I tend to think the low carb theory has some truth to it—a calorie is not a calorie, just as economists say ‘a dollar is not a dollar’—i forget the exact term but it may be price discrmination . Your dollar on one side of a street or town may not be worth the same as on the other side due to ‘market imperfections’, path dependence, transaction costs, etc. (Or maybe its like, if i recall, the ‘winding number’ in math and arahanov- bohm effect one may have to go around a circle 2 times to get to the same you started. ) (And in financial econ, there are several kinds of money—something c1, 2, 3..which are not perfectly convertible . E =/ mc^2 in practice. i have heard a high granite diet is excellent for weight loss–rock soup, and good for the teeth too.)

    I also think drinking a half gallon of unsweetened orange juice with 880 calories in it is not the same as drinking coca cola with 880 sugar calories (i wonder if that is also 1/2 gallon—probably close). . Context is important

    (Thats one thing alot of behavioral scientists sometimes tend to deny–eg people like R Plomin (genetics, UK) and B Caplan (he says he does what he calls ‘economics’ at GMU) who say the school you go to doesn’t matter for your education—because your genes choose your environment, and (also somewhat paradoxically or inconsistantly) your educational outcome doesn’t depend on what school or environment you are in or from, because the genes are sort of like air bubbles in a well stirred liquid—they always know exactly where they are going so they rise to the top, except genes, unlike air bubbles are heterogenous particles, so each kind goes as far as it can get. I think there are lots of papers on mixing of fluids and heterogereous particles. ).

    Rate also matters. If you drink a gallon of juice in one day, if you drink it all at once it wont be same as spread out. You will probably fall asleep in first case and hence excercize less. (One could also compare effects of equal calorie amounts of juice and vodka, or vodka and coke, or gin and juice. )


  2. Totally agree the ‘elephant in the room’ is appetite. Infuriating how it remains so neglected in the research and in the debates. Maybe it is harder to study than the physics of energy metabolism because in the end it is a feeling. But I would think that it would be possible to infer appetite effects from simply measuring how much people eat in different circumstances.


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