Does keto produce more weight loss when calorie targets are the same? A new meta-analysis examined this question in adults with overweight or obesity.

Overview

• What did they test? The researchers reviewed randomized controlled trials comparing ketogenic diets with higher-carbohydrate diets in adults with overweight or obesity. Both groups had to receive the same prescribed calorie intake or targeted calorie deficit. 
• What did they find? Across 259 participants in six trials, ketogenic diets produced ~1.49 kg more weight loss. However, when protein was matched there was no difference. Overall, the evidence was rated low quality, most studies lasted 6–12 weeks, and changes in fat mass, total body water, or lean mass were not analyzed.
• What does it mean for you? Keto can cause weight loss and this meta-analysis suggests a small advantage in body weight under comparable calorie prescriptions, but the difference in body weight can likely be attributed to changes in total body water or glycogen.

What’s the Problem?

Low-carbohydrate diets are often promoted as a better way to lose weight, partly because reducing carbohydrate intake lowers insulin. The carbohydrate–insulin model proposes that higher insulin levels promote fat storage, increase hunger, and subsequently increase body weight. However, controlled feeding studies have failed to support key predictions of this model 1. This raises a question about why some studies still report greater weight loss with carbohydrate restriction.

Indeed, several meta-analyses have found an advantage for low-carbohydrate diets. Sackner-Bernstein et al. (2015) analyzed 17 trials involving 1,797 participants and reported ~2 kg more weight loss than with low-fat diets 2. However, their definition allowed up to 120 g of carbohydrate per day, so the diets were not necessarily ketogenic. Mansoor et al. (2016) found a similar advantage of 2.2 kg across 11 trials lasting at least six months 3. However, in that review, most low-carbohydrate groups could eat without a prescribed calorie limit.

The duration of the diet is also a key consideration. Bueno et al. (2013) found a smaller advantage of ~0.9 kg for ketogenic diets in trials with at least 12 months of follow-up 4. Silverii et al. (2022) analyzed 25 trials in adults with obesity and found greater weight loss with low-carbohydrate diets at 3–4 and 6–8 months, but no significant differences at 10–14 or 18–30 months 5. Their definition also included diets with considerably more carbohydrate than a typical ketogenic diet.

A somewhat recent Cochrane review reached a more cautious conclusion. Across 61 trials, they found that low-carbohydrate and balanced-carbohydrate diets probably produce little to no difference in weight loss through two years 6. Most trials used similar approaches to calorie restriction, but the review included a broad range of carbohydrate intakes.

These reviews leave uncertainty about ketogenic diets because differences in carbohydrate intake, calorie prescriptions, protein intake, and follow-up duration make it difficult to determine what explains the reported weight-loss advantages. Therefore, the present review narrowed the comparison to randomized trials using diets that provided <50 g of carbohydrate per day or <10% of total calories. Both groups also had to receive the same prescribed calorie intake or targeted calorie deficit. This addresses two limitations of earlier reviews: broad definitions of low-carbohydrate diets and unequal calorie prescriptions. Whether an advantage remains under these conditions is the question the authors set out to answer.

Purpose