London: There’s a lot of competition for the most contentious issue in weight loss, but I’d have to give the nod to ketogenic diets. Now a study about them sheds some interesting new light — although I’m not holding out hope for kumbaya.
First though, a brief overview of the theory of ketogenic diets. When you eat carbohydrates, your body processes them with insulin, which shuttles blood glucose into fat stores, leaving you hungry. It’s important to keep in mind that nothing, including keto diets, can defeat the calorie balance equation:
To lose weight, you have to burn more calories than you absorb. But there are two mechanisms by which a keto diet might help you do that: It could leave you satiated on fewer calories, so you take in less, and it could increase the rate at which your body burns energy, so you expend more.
Does keto actually do those things?
Let’s take appetite first, which brings us back to that interesting new study. It’s by the National Institutes of Health’s Kevin Hall, the same scientist who found that people eating ultra-processed food ate 500 calories more per day than people eating a diet of whole-ish foods. This time, he compared a keto diet to a low-fat, high-carb plant-based diet.
If you’re of a certain age, you may recall that low-fat diets have their own theory, about which much was made back in the ’90s. Because fat is calorie-dense — nine calories per gram vs. four for carbs and protein — high-fat diets lead to overconsumption. Hall’s study wanted to test the battling theories by comparing the diets head-to-head.
Twenty subjects, in-patients at an NIH facility so all their consumption could be monitored, were fed either the keto diet or the low-fat diet for two weeks, and then switched over to the other. (These studies are very expensive, which is why they tend to be small and short.)
The low-fat. By a lot: nearly 700 calories per day. This, despite the fact that insulin levels on the low-fat diet were, Hall told me, “through the roof.” The low-fat group also lost a little more fat (only about a pound, not enough for statistical significance). The keto group lost more fat-free mass, but Hall points out that a big component of that is water, which you always lose when you cut carbs.
But wait! This is not the last nail in the coffin of keto satiety. Hall made a point of highlighting the fact that, on the keto diet, consumption dropped by 300 daily calories in the second week, possibly because of a satiety effect kicking in. Would it have dropped more had the study gone longer? There’s no way to know, but it’s certainly possible.
Is Keto Really Better For Weight Loss?
Another piece of evidence comes from Hall’s previous study, the one about processed foods.
The energy density of the keto diet in the new study was comparable to that of the highly processed diet in his previous study, Hall said, but subjects on the keto diet didn’t overeat, while subjects on the processed diet did. I think it’s safe to say that the preponderance of the evidence indicates that keto diets may indeed be more satiating than some other diets.
So let’s move on to the second question: Does eating keto up your energy expenditure?
Hall has researched that, too. In a study he conducted, he found that energy expenditure increased 57 calories per day on the keto diet, although there was no accompanying fat loss. In a review he did of other studies, though, he found no advantage.
There are studies that find that keto diets increase energy expenditure, and the issue is very contentious. Scientists argue about methodology and accuracy and the various ways to measure energy expenditure, and I don’t think there’s a clear consensus. Me, I’m absolutely ready to believe that energy expenditure can vary on different diets, because our bodies handle different foods in different ways, but I think it’s unlikely that the difference will be big enough to matter much. If keto diets make you burn, say, several hundred more calories every day, it shouldn’t be that hard to detect it in the lab.
So where does that leave us? Ketogenic diets may suppress appetite, at least a little, and they may even help you burn more calories. In practice, though, they don’t do better than other diets in the long term. In the short term, several studies have found that keto dieters lose more weight. A 2013 review found that, among studies that followed people for at least a year, keto dieters lost about two pounds more than low-fat dieters, a finding the authors called “of little clinical significance.” In 2019, the National Lipid Association concluded: “Low-CHO [carbohydrate] and very-low-CHO diets are not superior to other dietary approaches for weight loss
