Is insulin really the root cause of fat gain? Or has the carbohydrate-insulin model of obesity been oversold?
In this episode, Dr. Layne Norton breaks down what insulin actually does in the body, why short-term biochemistry snapshots don’t tell the whole story, and what the highest-quality human evidence really shows about low-carb vs. low-fat diets when calories and protein are controlled.
We cover metabolic ward trials, meta-analyses of controlled feeding studies, overfeeding research, and why GLP-1 drugs (which increase meal insulin) produce massive fat loss. Spoiler: insulin is not a wizard that can violate thermodynamics.
Citations
Metabolic ward / tightly controlled feeding studies (Kevin Hall et al.)
- Calorie for Calorie, Dietary Fat Restriction Results in More Body Fat Loss than Carbohydrate Restriction in People with Obesity
Hall KD et al. Cell Metabolism. 2015.
(2015 NIH metabolic ward study: isocaloric, protein-matched carb-restricted vs fat-restricted diets; greater fat loss on the low-fat diet despite higher insulin.)
Link: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4603544/ - Energy expenditure and body composition changes after an isocaloric ketogenic diet in overweight and obese men
Hall KD et al. American Journal of Clinical Nutrition. 2016.
(Follow-up metabolic ward trial: isocaloric ketogenic vs high-carb baseline; lower insulin and higher fat oxidation on keto, but no greater fat loss.)
Link:: https://pmc.ncbi.nlm.nih.gov/articles/PMC4962163/ - Effect of a plant-based, low-fat diet versus an animal-based, ketogenic diet on ad libitum energy intake
Hall KD et al. Nature Medicine. 2021.
(Inpatient ad libitum study: plant-based high-carb/low-fat vs animal-based low-carb/keto; participants ate ~550–700 fewer kcal/day on the high-carb diet despite higher post-meal insulin.)
Link: https://pubmed.ncbi.nlm.nih.gov/33479499/
Meta-analysis of controlled feeding studies
- Obesity Energetics: Body Weight Regulation and the Effects of Diet Composition
Hall KD, Guo J. Gastroenterology. 2017.
(Includes the meta-analysis of 32 controlled feeding studies with calories and protein matched, varying carb/fat: low-fat diets showed ~26 kcal/day higher energy expenditure and ~16 g/day greater fat loss.)
Link: https://pubmed.ncbi.nlm.nih.gov/28193517/
Longer-term free-living randomized trial
- 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
Gardner CD et al. JAMA. 2018.
(DIETFITS: >600 adults, healthy low-fat vs healthy low-carb for 12 months; no significant difference in weight loss; no interaction with insulin secretion.)
Link: https://jamanetwork.com/journals/jama/fullarticle/2673150
Systematic review (Cochrane)
- Low-carbohydrate versus balanced-carbohydrate diets for reducing weight and cardiovascular risk
Naude CE et al. Cochrane Database of Systematic Reviews. 2022.
(61 RCTs, ~7,000 participants: little to no difference in weight loss between low-carb and balanced/high-carb weight-reducing diets up to 2 years.)
Link: https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD013334.pub2/full
Overfeeding studies
- Fat and carbohydrate overfeeding in humans: different effects on energy storage
Horton TJ et al. American Journal of Clinical Nutrition. 1995.
(Isoenergetic 50% overfeeding with carb vs fat: progressive increase in carb oxidation and energy expenditure with carb overfeeding; fat stored more efficiently.)
Link: https://pubmed.ncbi.nlm.nih.gov/7598063/
Ludwig / carbohydrate-insulin model supporting study (critiqued in the episode)
- Effects of a low carbohydrate diet on energy expenditure during weight loss maintenance: randomized trial
Ebbeling CB et al. (Ludwig group). BMJ. 2018.
(Reported higher energy expenditure on low-carb during weight-loss maintenance via doubly labeled water; Norton notes methodological issues including missing body-weight reporting and DLW overestimation on low-carb diets.)
Link: https://www.bmj.com/content/363/bmj.k4583
Historical high-carb diet example
- Treatment of Massive Obesity With Rice/Reduction Diet Program: An Analysis of 106 Patients With at Least a 45-kg Weight Loss
Kempner W et al. Archives of Internal Medicine. 1975.
(White rice / high-refined-carb, very-low-fat diet in massively obese patients; large average weight losses ~64 kg, with improvements in glucose/insulin-related markers.)
Link: https://pubmed.ncbi.nlm.nih.gov/1200726/
Insulin and appetite meta-analysis
- Associations between postprandial insulin and blood glucose responses, appetite sensations and energy intake in normal weight and overweight individuals: a meta-analysis of test meal studies
Flint A et al. British Journal of Nutrition. 2007.
(Meta-analysis of 7 test-meal studies: higher post-meal insulin associated with decreased hunger and greater satiety, opposite to the carbohydrate-insulin model prediction.)
Link: https://pubmed.ncbi.nlm.nih.gov/17524176/
Traditional Okinawans
High-carbohydrate (~85% of energy from carbs, mostly sweet potatoes and vegetables), low-fat (~6%), relatively low-calorie diet associated with low obesity, low rates of chronic disease, and exceptional longevity.
- Willcox BJ, Willcox DC, Todoriki H, et al. Caloric Restriction, the Traditional Okinawan Diet, and Healthy Aging. Ann N Y Acad Sci. 2007.
https://doi.org/10.1196/annals.1396.037 - Willcox DC, Willcox BJ, et al. The Okinawan Diet: Health Implications of a Low-Calorie, Nutrient-Dense, Antioxidant-Rich Dietary Pattern Low in Glycemic Load. J Am Coll Nutr. 2009.
https://doi.org/10.1080/07315724.2009.10718117 - Okinawa Centenarian Study overview: https://okinawacentenarian.org/the-study
Kitavans (Trobriand Islands, Papua New Guinea)
High-carbohydrate (~69% from tubers such as yams, sweet potatoes, taro, plus fruit), ~21% fat (much from coconut), low protein; subsistence horticulturalists who were lean (low BMI), had low fasting insulin that declined with age, low blood pressure, and virtually absent cardiovascular disease/stroke.
- Lindeberg S, Eliasson M, Lindahl B, Ahrén B. Low serum insulin in traditional pacific islanders—The Kitava study. Metabolism. 1999.
https://doi.org/10.1016/S0026-0495(99)90258-5
Pima Indians (Mexico vs. Arizona)
Traditional-living Mexican Pimas (higher physical activity, higher-fiber/lower-fat diet) had dramatically lower rates of obesity and type 2 diabetes than genetically similar Arizona Pimas living in a Westernized environment with higher calorie intake.
- Schulz LO, Bennett PH, Ravussin E, et al. Effects of traditional and western environments on prevalence of type 2 diabetes in Pima Indians in Mexico and the U.S. Diabetes Care. 2006.
https://doi.org/10.2337/dc06-0138
Kuna of Panama
Traditional island Kuna consumed substantial refined sugar (often estimated in the range of ~80–160 g/day, frequently in cocoa drinks) yet were typically lean (BMI ~22–23 in classic reports) with low blood pressure; obesity was not spontaneous. Modernization has since increased diabetes/obesity risk.
Supporting discussions appear in work on Kuna cacao/flavonoid intake and blood pressure (e.g., Hollenberg et al.) and later prevalence surveys noting rising metabolic disease with dietary transition. Classic low-BP/cacao observations are summarized in reviews of indigenous Kuna health; more recent data show higher rates with modernization (e.g., 2019 survey of San Blas Kuna).These are ecological/observational comparisons, not controlled trials. Norton uses them to show that high carbohydrate (and thus elevated post-meal insulin) is not sufficient by itself to produce obesity when total energy intake is not excessive.
GLP-1 claims
Norton’s points: GLP-1 receptor agonists (semaglutide, tirzepatide, etc.) are among the most effective obesity treatments (typically 15–25% body-weight loss), increase meal-stimulated insulin, act as strong appetite suppressants, and thereby contradict the idea that higher meal insulin necessarily traps fat and drives overeating. Fasting insulin falls later secondary to weight loss.Weight-loss efficacy (15–25% range):
- Wilding JPH et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). N Engl J Med. 2021. ~15% mean weight loss.
https://www.nejm.org/doi/full/10.1056/NEJMoa2032183 - Jastreboff AM et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1). N Engl J Med. 2022. Up to ~21–22.5% with higher doses.
https://www.nejm.org/doi/full/10.1056/NEJMoa2206038 - Aronne LJ et al. Tirzepatide as Compared with Semaglutide for the Treatment of Obesity (SURMOUNT-5). N Engl J Med. 2025. Tirzepatide ~20.2% vs semaglutide ~13.7%.
https://www.nejm.org/doi/full/10.1056/NEJMoa2416394
Increased insulin secretion / improved β-cell function (glucose-dependent):
- Mather KJ et al. Effects of Tirzepatide vs Semaglutide on β-Cell Function, Insulin Sensitivity, and Glucose Control During a Meal Test. J Clin Endocrinol Metab. 2024.
https://pubmed.ncbi.nlm.nih.gov/38795393/ - Heise T et al. Effects of subcutaneous tirzepatide versus placebo or semaglutide on pancreatic islet function and insulin sensitivity… Lancet Diabetes Endocrinol. 2022.
https://doi.org/10.1016/S2213-8587(22)00085-7
Mechanism review (appetite suppression + insulinotropic effects + large weight loss):
- Holst JJ. GLP-1 physiology in obesity and development of incretin-based drugs for chronic weight management. Nat Metab. 2024.
https://www.nature.com/articles/s42255-024-01113-9
