Low-fat diet confers less energy intake vs. low carb, opposite of prior predictive models
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Contrary to prior models, a plant-based, low-fat diet was associated with less ad libitum energy intake compared with an animal-based, low-carbohydrate diet, for reasons still unclear, a speaker reported.
According to a presentation at the Cardiometabolic Health Congress, Kevin D. Hall, PhD, clinical researcher at the National Institute of Diabetes and Digestive and Kidney Diseases, cited a special communication written by David Ludwig, MD, PhD, professor of nutrition at Harvard School of Public Health, and Cara Ebbeling, PhD, associate professor of pediatrics at Harvard Medical School, published in JAMA Internal Medicine, in which they described the carbohydrate-insulin model of obesity.
“The carbohydrate-insulin model of obesity (CIM) proposes that a high-carbohydrate diet — including large amounts of refined starchy foods and sugar, as commonly consumed in the low-fat diet era — produces postprandial hyperinsulinemia, promotes deposition of calories in fat cells instead of oxidation in lean tissues, and thereby predisposes to weight gain through increased hunger, slowing metabolic rate, or both,” the authors wrote.
In January 2021, Hall and colleagues published an analysis in Nature Medicine that assessed the effects of a plant-based, low-fat diet compared with an animal-based, ketogenic diet on as libitum energy intake.
For this analysis, researchers enrolled 20 inpatients at the NIH Clinical Center in Bethesda, Maryland(meanage, 30 years; mean BMI, 27.8kg/m2), and randomly assigned them to consume ad libitum either a minimally processed plant-based diet with high glycemic load (10.3% fat; 75.2% carbohydrate; 85g1,000kcal) or a minimally processed ketogenic diet with low glycemic load (75.8% fat; 10% carbohydrate; 6g1,000kcal). Participants were on their respective diets for 2 weeks after which time they were swapped to the opposite diet for an additional 2 weeks. The primary outcomes were mean daily ad libitum energy intake between each 2-week diet period and between the final week of each diet.
Following assessment using a visual analog scale, researchers observed similar outcomes for hunger, satisfaction, fullness and eating capacity between the two diets.
Hall reported that participants on the plant-based diet had less energy intake compared with compared with the ketogenic diet, with a difference in energy intake of 689 kcal per day (P < .0001).
“Contrary to the carbohydrate insulin model, it was during the period of the low-fat diet that they had the lower energy intake, by almost 700 calories per day, as compared to when the same people consumed the lower carbohydrate diet,” Hall said during the presentation. “That's exactly opposite to the carbohydrate insulin model predictions, and so one of our conclusions from this paper was that the primacy of carbohydrates, glycemic index and insulin on regulating people's appetite does not seem to be supported by our data, in this particular case.”
According to the presentation, people on the low-carbohydrate diet lost weight faster and had greater loss of fat-free mass, but people on the low-fat diet had more body fat loss.
"Think about what that means for patients who go on low-carb diets, in terms of the feedback that they're getting as they step on the scale and seem to lose a lot of weight. Unfortunately, that weight loss is a little bit misleading because it's all coming from loss of fat-free mass, primarily body water, we believe," Hall said during the presentation. "The low-fat diet caused people to eat 700 calories less per day and yet you didn't see as great of a weight loss, but that was because they're preserving their fat-free mass and they experienced a greater rate of body fat changes as measured by [dual-energy X-ray absorptiometry] ... So, what was the mechanism for the diet differences and ad libitum energy intake?"
Thinking participants might have liked one diet over the other, the researchers evaluated the patients with regard to food pleasantness and familiarity but observed a similar level of each for both the plant-based and ketogenic diets.
According to the presentation, from a variety of studies, researchers know that different gut hormones, such as GLP-1, PYY and ghrelin can influence appetite in a way that might explain the differences observed in the analysis. However, Hall reported that postprandial measurements of GLP-1, PYY and ghrelin each predicted the opposite effect on energy intake.
“These hormones, while they're probably very important, don't tell the whole story,” Hall said. “There's something else going on that we still don't fully understand about the low-fat high-carbohydrate plant-based diet that led people to consume many fewer calories, despite the fact that these hormone changes suggest that the opposite should have occurred. We've got a lot more research to do to figure this out.”
References:
- Hall KD, et al. Nat Med. 2021;doi:s41591-020-01209-1.
- Ludwig DS, et al. JAMA Intern Med. 2018;doi:10.1001/jamainternmed.2018.2933.
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Hall KD. Session I: Obesity & Lifestyle. Presented at: Cardiometabolic Health Congress; Oct. 14-17, 2021; National Harbor, Md. (hybrid meeting).
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