The claimEvidence A · guideline-tier
Insulin is the sole driver of fat gain
The evidence
In metabolic-ward trials where every meal was controlled (Hall 2015 and 2021), a low-fat diet lost more body fat than a low-carb one at equal calories; eating freely, the low-fat plant-based group ate 689 fewer kcal a day. Under controlled testing, the carbohydrate-insulin model fails.
The mechanism, in brief
You and a colleague eat about the same, yet they stay lean and you gain weight. What is to blame? Nutrition science has spent a decade arguing over the serious version of that question: do people get fat because they eat too much, or because they eat the wrong things? So far, tightly controlled feeding trials favor…
Sources (4)
- Hall, K. D., Bemis, T., Brychta, R., Chen, K. Y., Courville, A., Crayner, E. J., Goodwin, S., Guo, J., Howard, L., Knuth, N. D., Miller, B. V., Prado, C. M., Siervo, M., Skarulis, M. C., Walter, M., Walter, P. J., & Yannai, L. (2015). Calorie for calorie, dietary fat restriction results in more body fat loss than carbohydrate restriction in people with obesity. Cell Metabolism, 22(3), 427–436. 19 adults with obesity in a metabolic ward, each receiving both isocaloric diets for 6 days in random order after a 5-day baseline: cutting carbohydrate raised fat oxidation and lost 53 ± 6 g/day of body fat; cutting fat left fat oxidation unchanged but lost 89 ± 6 g/day (P = 0.002). Model simulations predicted the body minimises such differences over longer periods (abstract, PMID 26278052).
- Hall, K. D., Guo, J., Courville, A. B., Boring, J., Brychta, R., Chen, K. Y., Darcey, V., Forde, C. G., Gharib, A. M., Gallagher, I., Howard, R., Joseph, P. V., Milley, L., Ouwerkerk, R., Raisinger, K., Rozga, I., Schick, A., Stagliano, M., Torres, S., … Chung, S. T. (2021). Effect of a plant-based, low-fat diet versus an animal-based, ketogenic diet on ad libitum energy intake. Nature Medicine, 27(2), 344–353. 20 adults randomised to ad libitum minimally processed plant-based low-fat (10.3% fat, 75.2% carbohydrate) or animal-based ketogenic (75.8% fat, 10.0% carbohydrate) diets for 2 weeks each, inpatient crossover: the low-fat diet led to 689 ± 73 kcal/day less intake over 2 weeks and 544 ± 68 less in the final week, contrary to the carbohydrate-insulin model's prediction; one participant withdrew with hypoglycaemia on the low-carbohydrate diet (abstract, PMID 33479499).
- Gardner, C. D., Trepanowski, J. F., Del Gobbo, L. C., Hauser, M. E., Rigdon, J., Ioannidis, J. P. A., et al. (2018). Effect of low-fat vs low-carbohydrate diet on 12-month weight loss in overweight adults and the association with genotype pattern or insulin secretion: the DIETFITS randomized clinical trial. JAMA, 319(7), 667-679. 609 adults, 12 months; no significant weight-loss difference between healthy low-fat and healthy low-carbohydrate diets, and neither genotype pattern nor baseline insulin secretion predicted which diet worked better.
- Ludwig, D. S., & Ebbeling, C. B. (2018). The carbohydrate-insulin model of obesity: beyond 'calories in, calories out'. JAMA Internal Medicine, 178(8), 1098-1103. Narrative review setting out the model: processed, high-glycemic-load carbohydrates produce hormonal changes that promote calorie deposition in fat tissue, increase hunger and lower energy expenditure. The authors cite mechanistic, genetic and animal evidence and behavioural-trial meta-analyses (greater weight loss with reduced-glycemic-load than low-fat diets, with poor long-term compliance), and state that feeding studies have lacked the rigor and duration to test the model; pending definitive studies they offer low-glycemic-load principles as a practical alternative (abstract, PMID 29971406).