Ketogenic diet may reduce gut bacteria and raise cholesterol levels

Key findings include:

• Increased cholesterol: The keto diet increased cholesterol levels, especially in small and medium-sized LDL particles. The diet increased apolipoprotein B (apoB), which promotes plaque buildup in arteries. In contrast, a low-carb diet significantly reduced cholesterol in LDL particles.

• Reduced gut bacteria: The keto diet changes the composition of the gut microbiome, especially BifidobacteriaA beneficial bacterium often found in probiotics. This bacterium has a wide range of benefits, including the production of vitamin B, inhibition of pathogenic and harmful bacteria, and lowering cholesterol. Carbohydrate restriction did not significantly affect the composition of the gut microbiota.

• Glucose tolerance: The keto diet reduces glucose tolerance, making the adult body less efficient at processing carbohydrates.

• Both diets resulted in fat loss: The keto diet resulted in an average fat loss of 2.9 kg per person, while the low carb diet resulted in an average fat loss of 2.1 kg per person over 12 weeks.

• Metabolism: Researchers also found that the keto diet caused significant changes in lipid metabolism and muscle energy utilization, shifting the body's fuel preference from glucose to fat.

• Physical activity levels: Both low carb and keto diets achieved fat loss without changing physical activity levels. Previous research from the Center for Nutrition, Exercise, and Metabolism has shown that skipping breakfast and intermittent fasting lead to decreased physical activity.

Lead researcher Dr Aaron Hengist highlighted the worrying findings regarding cholesterol.

“Despite reducing fat mass, the ketogenic diet increased levels of harmful fats in participants' blood. If continued for years, this could have long-term health effects, including an increased risk of heart disease and stroke.”

Dr Russell Davies, who led the microbiome research, explained the impact on gut health:

“Fibre is essential for the survival of gut bacteria such as Bifidobacteria. On a ketogenic diet, fibre intake is reduced to around 15g per day, half the NHS recommended intake. Reduced Bifidobacteria may lead to significant long-term health problems, including an increased risk of digestive diseases such as irritable bowel disease, an increased risk of intestinal infections and reduced immune function.”

Professor Javier Gonzalez, who oversaw the study, commented on the glucose findings:

“The ketogenic diet lowered fasting blood sugar levels, but it also reduced the body's ability to process dietary carbohydrates. Measuring protein in muscle samples from the participants' legs, we found that this is likely an adaptive response to reducing daily carbohydrate intake and reflects insulin resistance to storing carbohydrates in muscle. This insulin resistance is not necessarily a bad thing for someone on a ketogenic diet, but if these changes continue when they return to a high-carb diet, it may increase the risk of developing type 2 diabetes in the long term.”

In light of this new study, the researchers conclude that if you are considering dieting, a low-carb diet is better for most people. Further research is needed to understand how individuals benefit from each diet. The government recommends limiting free sugars (those added to foods and drinks, or found naturally in honey, syrups, fruit juice and smoothies) to less than 5% of total energy intake. Professor Dylan Thompson, who oversaw the study, said:

“Ketogenic diets are effective for fat loss, but have different effects on metabolism and microbiota and may not be suitable for everyone. In contrast, carbohydrate restriction supports government guidelines to reduce discretionary sugar intake and promotes fat loss without any obvious adverse health effects.”

The research was supported by an initial grant from The Rank Prize Funds, with subsequent funding from the Cosun Nutrition Centre, University of Bath and Ian Tarr. The exploratory analysis included fellowship support for the researchers from the British Heart Foundation and the Medical Research Council.