For years, we’ve heard that to lose weight, you have to cut carbs. And there’s a good reason why many people have seen success with this approach. But what if it isn’t the whole story? A new study from the U.K. suggests that a ketogenic diet, which severely restricts carbohydrates, may lead to unwanted side effects that a simple low-sugar diet doesn’t.
Published in the journal Cell Reports Medicine, the research reveals that while both diets can help you shed fat, the ketogenic diet can disrupt your body’s metabolism and even change the bacteria in your gut. These findings raise a critical question: when it comes to losing weight, does the way you get there matter just as much as the result?
Not All Diets Are Created Equal
The study followed 53 healthy adults over a period of 12 weeks. They were split into three groups: one group on a typical Western diet (the control group), one on a low-sugar diet, and a third on a ketogenic, low-carbohydrate diet. The researchers made sure the participants stuck to their diets by providing them with specific foods, but otherwise, the participants were allowed to live their lives as they normally would.
At the end of the study, both the low-sugar and the keto groups had lost a significant amount of fat. This happened because cutting out a major food group—whether it was sugar or carbs—led to them eating fewer calories overall.
Unexpected Side Effects of the Keto Diet
While both diets achieved fat loss, the low-sugar diet had a cleaner, more beneficial effect on the body. It lowered levels of LDL-C, a type of cholesterol often called “bad” cholesterol because it can lead to plaque buildup in your arteries. A low-sugar approach did this without causing other major metabolic shifts.
The keto diet, however, had a different story. In addition to a rise in LDL-C, it also led to an increase in something called apolipoprotein B (apoB). This protein is considered a key risk factor for heart disease.
Beyond cholesterol, the keto diet also made participants’ bodies less efficient at processing carbohydrates. This is known as reduced glucose tolerance. While this isn’t the same as having diabetes, it’s a concerning sign that the body’s ability to handle sugar is impaired.
Perhaps most surprising was the effect on the gut. The ketogenic diet significantly changed the gut microbiome by decreasing the amount of Bifidobacteria, a type of beneficial bacteria often found in probiotics. These bacteria are important for producing B vitamins and can even help lower cholesterol. The low-sugar diet did not cause these same changes.
What This Means for Your Diet
These findings challenge the idea that fat loss is the only measure of a diet’s success. The study points to a difference between a low-sugar diet and a ketogenic one—the former helps with fat loss without the unwanted side effects on cholesterol, glucose tolerance, or the gut microbiome.
When you’re considering a diet, it’s worth thinking about more than just the number on the scale. For most people, this research suggests that a simpler approach—like reducing sugar—may be a safer and more effective path to better health.
Paper Summary
Methodology
This was a 12-week randomized controlled trial with 53 healthy adults. The participants were divided into three groups: a control group on a moderate-sugar diet, a low-sugar group (less than 5% of calories from free sugars), and a ketogenic group (less than 8% of calories from carbs). The study was conducted in a “free-living” environment where participants were given specific foods to help them stick to their assigned diets. The study was registered at clinicaltrials.gov (NCT03574987).
Results
Both the low-sugar and ketogenic diets resulted in fat loss. The ketogenic diet led to an average loss of 2.9 kg of fat, and the low-sugar diet resulted in an average loss of 2.1 kg. However, the ketogenic diet also increased apolipoprotein B (apoB) levels, which are associated with heart disease, and reduced glucose tolerance. It also altered the gut microbiome by decreasing beneficial Bifidobacteria. The low-sugar diet did not cause these same negative effects and also lowered LDL-C (bad cholesterol).
Limitations
The study had a smaller final sample size than originally planned due to dropouts and the COVID-19 pandemic. Although the sample size was sufficient for most analyses, it did reduce the statistical power for the primary outcome.
Funding and Disclosures
The research was supported by a grant from The Rank Prize Funds, with additional funding from Cosun Nutrition Center, The University of Bath, and Ian Tarr. Some of the investigators received fellowship support from the British Heart Foundation and the Medical Research Council.
Publication Information
The paper is titled “Ketogenic diet but not free-sugar restriction alters glucose tolerance, lipid metabolism, peripheral tissue phenotype, and gut microbiome: RCT.” It was published on August 20, 2024, in the journal Cell Reports Medicine. The lead contact for the study is Professor Javier Gonzalez ([email protected]).