A study from the University of Bath reveals that, while helping with weight loss, ketogenic low-carbohydrate diets can increase cholesterol levels and reduce beneficial gut bacteria.

Published in Cell Reports Medicine, the research from the Centre for Nutrition, Exercise, and Metabolism involved 53 healthy adults for up to 12 weeks. Participants followed either a moderate sugar diet (control), a low-sugar diet (less than 5% of calories from sugar), or a ketogenic (keto) low-carbohydrate diet (less than 8% of calories from carbohydrates).

Key findings from the study include:

• Weight loss: Keto diet resulted in an average of 2.9 kg fat mass loss per person, whilst the sugar restricted diet followed with an average 2.1 kg fat mass loss per person at 12 weeks.. 

• Increased cholesterol: The keto diet raised LDL (bad) cholesterol levels by 16%, particularly in small and medium sized LDL particles. It also increased apolipoprotein B (apoB), which causes plaque buildup in arteries. In contrast, the low-sugar diet significantly reduced cholesterol in LDL particles.

• Reduced favourable gut bacteria: The keto diet altered gut microbiome composition, notably decreasing bifidobacteria, which helps produce B vitamins and has bee linked to a stronger immune system. Sugar restriction did not significantly impact the gut microbiome composition.

• Glucose Tolerance: The keto diet reduced glucose tolerance, reducing the body’s efficiency in handling carbohydrates.

Lead researcher Dr. Aaron Hengist highlighted the concerning cholesterol findings: “Despite reducing fat mass, the ketogenic diet increased the levels of unfavourable fats in the blood of our participants, which, if sustained over years, could have long-term health implications such as increased risk of heart disease and stroke.”

Commenting on the potential impact on gut health, Dr. Russell Davies, who led the microbiome research, said: “Dietary fibre is essential for the survival of beneficial gut bacteria like bifidobacteria. The ketogenic diet reduced fibre intake to around 15 grams per day, half the NHS recommended intake. This reduction in bifidobacteria might contribute to significant long-term health consequences such as an increased risk of digestive disorders like irritable bowel disease, increased risk of intestinal infection and a weakened immune function.”

The full paper – Ketogenic diet (but not free-sugar restriction) alters glucose tolerance, lipid metabolism, peripheral tissue phenotype, and the gut microbiome – is published in Cell Reports Medicine.

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