For appetite control, a low-fat, plant-based diet has advantages over a low-carbohydrate, animal-based ketogenic diet, although the keto diet wins when it comes to keeping post-meal glucose and insulin levels in check, new research suggests.
In a highly controlled crossover study conducted at the National Institutes of Health (NIH), people consumed fewer daily calories when on a low-fat, plant-based diet, but their insulin and blood glucose levels were higher than when they followed a low-carbohydrate, animal-based diet.
“There is this somewhat outdated idea now that higher-fat diets, because they have more calories per gram, tend to make people overeat ― something called the passive overconsumption model,” senior investigator Kevin Hall, PhD, National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), told Medscape Medical News.
The other more popular model these days, he explained, is the carbohydrate-insulin model, which holds that following a diet high in carbohydrates and sugar that causes insulin levels to spike will increase hunger and cause a person to overeat.
In this study, Hall and colleagues tested these two hypotheses head to head.
“The short answer is that we got exactly the opposite predictions from the carbohydrate-insulin model of obesity. In other words, instead of making people eat more and gaining weight and body fat, they actually ended up eating less on that diet and losing body fat compared to the higher-fat diet,” Hall said.
“Yet, the passive overconsumption model also failed, because despite them eating a very energy-dense diet and high fat, they didn’t gain weight and gain body fat. And so both of these models of why people overeat and gain weight seem to be inadequate in our study,” he told Medscape Medical News. “This suggests that things are a little bit more complicated.”
The study was published online January 21 in Nature Medicine.
Pros and Cons to Both Diets
For the study, the researchers housed 20 healthy adults who did not have diabetes for 4 continuous weeks at the NIH Clinical Center. The mean age of the participants was 29.9 years, and the mean BMI was 27.8 kg/m2.
The participants were randomly allocated to consume ad libitum either a plant-based, low-fat diet (10.3% fat, 75.2% carbohydrate) with low energy density (~1 kcal g−1), or an animal-based, ketogenic, low-carbohydrate diet (75.8% fat, 10.0% carbohydrate) with high energy density (~2 kcal g−1) for 2 weeks. They then crossed over to the alternate diet for 2 weeks.
Both diets contained about 14% protein and were matched for total calories, although the low-carb diet had twice as many calories per gram of food than the low-fat diet. Participants could eat what and however much they chose of the meals they were given.
One participant withdrew, owing to hypoglycemia during the low-carbohydrate diet phase. For the primary outcome, the researchers compared mean daily ad libitum energy intake between each 2-week diet period.
They found that energy intake from the low-fat diet was reduced by approximately 550 to 700 kcal d−1 compared to the low-carbohydrate keto diet. Yet, despite the large differences in calorie intake, participants reported no differences in hunger, enjoyment of meals, or fullness between the two diets.
Participants lost weight on both diets (about 1 to 2 kg on average), but only the low-fat diet led to a significant loss of body fat.
“Interestingly, our findings suggest benefits to both diets, at least in the short term,” Hall said in a news release.
“While the low-fat, plant-based diet helps curb appetite, the animal-based, low-carb diet resulted in lower and more steady insulin and glucose levels. We don’t yet know if these differences would be sustained over the long term,” he said.
Hall added that it’s important to note that the study was not designed to make diet recommendations for weight loss, and the results might have been different had the participants been actively trying to lose weight.
“In fact, they didn’t even know what the study was about; we just said we want you to eat the two diets, and we’re going to see what happens in your body either as you eat as much or as little as you want,” he told Medscape Medical News.
“It’s a bit of a mixed bag in terms of which diet might be better for an individual. I think you can interpret this study as that there are positives and negatives for both diets,” Hall said.
Commenting on the study for Medscape Medical News, Taylor Wallace, PhD, adjunct professor, Department of Nutrition and Food Studies, George Mason University, Fairfax, Virginia, said it’s important to note that “a ‘low-carb diet’ has yet to be defined, and many definitions exist.
“We really need a standard definition of what constitutes ‘low-carb’ so that studies can be designed and evaluated in a consistent manner. It’s problematic because, without a standard definition, the ‘diet tribe’ researchers (keto vs plant-based) always seem to find the answer that is in their own favor,” Wallace said. “This study does seem to use less than 20 grams of carbs per day, which in my mind is pretty low-carb.”
Perhaps the most important caveat, he added, is that in the real world, “most people don’t adhere to these very strict diets ― not even for 2 weeks.”
The study was supported by the NIDDK Intramural Research Program, with additional NIH support from a National Institute of Nursing Research grant. One author has received reimbursement for speaking at conferences sponsored by companies selling nutritional products, serves on the scientific advisory council for Kerry Taste and Nutrition, and is part of an academic consortium that has received research funding from Abbott Nutrition, Nestec and Danone. Hall and the other authors have disclosed no relevant financial relationships. Wallace is principal and chief executive officer of the Think Healthy Group; editor of the Journal of Dietary Supplements; and deputy editor of the Journal of the American College of Nutrition.
Nat Med. Published online January 21, 2021. Abstract