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."

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

"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."

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

"The ketogenic diet reduced fasting glucose levels but also reduced the body's ability to handle carbs from a meal. By measuring proteins in muscle samples taken from participants' legs, we think this is probably an adaptive response to eating less carbohydrates day-to-day and reflects insulin resistance to storing carbs in muscle. This insulin resistance is not necessarily a bad thing if people are following a ketogenic diet, but if these changes persist when people switch back to a higher carbohydrate diet it could increase the risk of developing type 2 diabetes in the long-term"

In light of this new research, the academics conclude that if you're considering a diet, a low sugar one will be better for most people. More work is needed to understand how individuals may benefit from each type of diet. The government recommends that free sugars (those added to food or drink or found naturally in honey, syrups, fruit juices and smoothies) should be restricted to less than 5% of total energy intake. Professor Dylan Thompson, who also oversaw the work, said:

"The ketogenic diet is effective for fat loss, but it comes with varied metabolic and microbiome effects that may not suit everyone. In contrast, sugar restriction supports government guidelines for reducing free sugar intake, promoting fat loss without apparent negative health impacts."

This work was supported by an initial grant from The Rank Prize Funds, with subsequent funding from Cosun Nutrition Center, The University of Bath, and Ian Tarr. Exploratory analyses included fellowship support for some investigators from the British Heart Foundation and Medical Research Council.