Effects of High-Carb Diets on BMI & Insulin Resistance

A September 2018 16-week randomised controlled clinical trial by the Physician’s Committee for Responsible Medicine1 looked at whether a plant-based high-carbohydrate, low-fat diet in overweight individuals would have a more beneficial effect on body weight, body composition and insulin resistance when compared with a standard Western diet. Controversy in the media is rife about whether or not a carbohydrate-rich diet is healthy, so will this research study help settle the argument once and for all?

Study method

75 participants were randomised to follow either a plant-based high-carbohydrate, low-fat (vegan) diet or to maintain their current diet.

The intervention group was asked to follow a low-fat vegan diet consisting of vegetables, grains, legumes, and fruits, and avoiding animal products and added oils. There were no limits on energy or carbohydrate intake. The control group participants were asked to maintain their current diets, which included meat and dairy products, for the 16-week intervention period.

All study participants were asked not to alter their physical activity and to continue their preexisting medication regimens for the duration of the study, except as modified by their personal physicians.

Measurements were taken before and at the end of the trial.

Study results

The results can be seen in the charts below.

So, in the vegan group:

  • fat mass reduced
  • insulin resistance significantly reduced
    • and these results remained significant after adjustment for energy intake

The more total and insoluble fibre consumed, the greater the drop in the following:

  • BMI
  • fat mass
  • volume of visceral fat
    • again, these results remained significant even after adjustment for energy intake

Increased consumption of carbohydrate and fibre, as part of a plant-based high-carbohydrate, low-fat diet, were associated with beneficial effects on the following:

  • weight
  • body composition
  • insulin resistance


Obesity – a pandemic?

Percent of Adults with BMI ≥30. Source: WHO, Global Database on Body Mass Index.

The World Health Organisation estimates that more than 1.9 billion adults worldwide have excess body weight 2 3 .  Hence, obesity rates are rising rapidly. Since increased body weight is associated with a higher all-cause mortality 4 , it’s vital that sustainable and practical solutions are found.

Causes of obesity

It’s pretty obvious to anyone who does a little research: poor dietary habits (such as high intakes of processed meat products and sodium with low intakes of fruits and vegetables) are one of the leading contributors.  This single avoidable behavioural factor contributes to huge numbers of deaths through a range of chronic diseases – including around half of cardio-metabolic deaths in the United States alone 3 5 6 .

Solutions to obesity

In contrast to the above, plant-based diets have been shown repeatedly to represent an effective strategy for improving nutrient intake 7 , being clearly associated with decreased all-cause mortality and decreased risk of obesity, type 2 diabetes, and coronary heart disease 8 .

The following dietary elements of a plant-based diet have been shown to have independent advantages for weight loss and weight management:

  • whole grains 9 10
  • legumes 11
  • fruits and vegetables 12 13 14
  • high carbohydrate (low glycaemic index) intake 15 16
  • high fibre content 17

Final thoughts

Above, I asked whether this study would settle once and for all the question of whether or not a carbohydrate-rich diet is healthy. As long as the carbohydrates are what’s found in wholefood plants and not in processed junk food, it appears to me that this, as well as many other studies mentioned above, do indeed favour a positive answer to this question.

Naturally, however, the industries benefiting from producing, advertising and selling processed junk foods and animal products will continue in their attempts to deny/ignore/marginalise the facts with the aim of confusing the public. That, unfortunately, is the way business is done. But each of us can take responsibility for what foods we consume, and thus exert complete dietary control to help ourselves avoid so many chronic diet-related diseases.


  1. Nutrients. 2018 Sep 14;10(9). pii: E1302. doi: 10.3390/nu10091302. A Plant-Based High-Carbohydrate, Low-Fat Diet in Overweight Individuals in a 16-Week Randomized Clinical Trial: The Role of Carbohydrates. Kahleova H, Dort S, Holubkov R, Barnard ND. []
  2. NCD Risk Factor Collaboration (NCD-RisC). Trends in adult body-mass index in 200 countries from 1975 to 2014: A pooled analysis of 1698 population-based measurement studies with 19.2 million participants. Lancet 2016, 387, 1377–1396. []
  3. GBD 2015 Risk Factors Collaborators. Global, regional, and national comparative risk assessment of 79 behavioural, environmental and occupational, and metabolic risks or clusters of risks, 1990–2015: A systematic analysis for the Global Burden of Disease Study 2015. Lancet 2016, 388, 1659–1724. [] []
  4. Di Angelantonio, E.; Bhupathiraju, S.N.; Wormser, D.; Gao, P.; Kaptoge, S.; de Gonzalez, A.B.; Cairns, B.J.; Huxley, R.; Jackson, C.L.; Joshy, G.; et al. Body-mass index and all-cause mortality: Individual-participant-data meta-analysis of 239 prospective studies in four continents. Lancet 2016, 388, 776–786. []
  5. Murray, C.J.L.; Atkinson, C.; Bhalla, K.; Birbeck, G.; Burstein, R.; Chou, D.; Dellavalle, R.; Danaei, G.; Ezzati, M.; Fahimi, A.; et al. Burden of Disease Collaborators The state of US health, 1990–2010: Burden of diseases, injuries, and risk factors. JAMA 2013, 310, 591–608. []
  6. Micha, R.; Peñalvo, J.L.; Cudhea, F.; Imamura, F.; Rehm, C.D.; Mozaffarian, D. Association between dietary factors and mortality from heart disease, stroke, and type 2 diabetes in the United States. JAMA 2017, 317, 912–924. []
  7. Rizzo, N.S.; Jaceldo-Siegl, K.; Sabate, J.; Fraser, G.E. Nutrient profiles of vegetarian and nonvegetarian dietary patterns. J. Acad. Nutr. Diet. 2013, 113, 1610–1619. []
  8. Fraser, G.E. Vegetarian diets: What do we know of their effects on common chronic diseases? Am. J. Clin. Nutr. 2009, 89, 1607S–1612S. []
  9. Pol, K.; Christensen, R.; Bartels, E.M.; Raben, A.; Tetens, I.; Kristensen, M. Whole grain and body weight changes in apparently healthy adults: A systematic review and meta-analysis of randomized controlled studies. Am. J. Clin. Nutr. 2013, 98, 872–884. []
  10. Harland, J.I.; Garton, L.E. Whole-grain intake as a marker of healthy body weight and adiposity. Public Health Nutr. 2008, 11, 554–563. []
  11. Kim, S.J.; de Souza, R.J.; Choo, V.L.; Ha, V.; Cozma, A.I.; Chiavaroli, L.; Mirrahimi, A.; Blanco Mejia, S.;
    Di Buono, M.; Bernstein, A.M.; et al. Effects of dietary pulse consumption on body weight: A systematic review and meta-analysis of randomized controlled trials. Am. J. Clin. Nutr. 2016, 103, 1213–1223. []
  12. Bertoia, M.L.; Mukamal, K.J.; Cahill, L.E.; Hou, T.; Ludwig, D.S.; Mozaffarian, D.; Willett, W.C.; Hu, F.B.; Rimm, E.B. Changes in intake of fruits and vegetables and weight change in United States men and women followed for up to 24 years: Analysis from three prospective cohort studies. PLoS Med. 2015, 12, e1001878. []
  13. Tapsell, L.C.; Batterham, M.J.; Thorne, R.L.; O’Shea, J.E.; Grafenauer, S.J.; Probst, Y.C. Weight loss effects from vegetable intake: A 12-month randomised controlled trial. Eur. J. Clin. Nutr. 2014, 68, 778–785. []
  14. Boeing, H.; Bechthold, A.; Bub, A.; Ellinger, S.; Haller, D.; Kroke, A.; Leschik-Bonnet, E.; Müller, M.J.; Oberritter, H.; Schulze, M.; et al. Critical review: Vegetables and fruit in the prevention of chronic diseases. Eur. J. Nutr. 2012, 51, 637–663. []
  15. Thomas, D.E.; Elliott, E.J.; Baur, L. Low glycaemic index or low glycaemic load diets for overweight and obesity. Cochrane Database Syst. Rev. 2007, CD005105 []
  16. Schwingshackl, L.; Hoffmann, G. Long-term effects of low glycemic index/load vs. high glycemic index/load diets on parameters of obesity and obesity-associated risks: A systematic review and meta-analysis. Nutr. Metab. Cardiovasc. Dis. 2013, 23, 699–706 []
  17. Slavin, J.L. Dietary fiber and body weight. Nutrition 2005, 21, 411–418. []