Keto diets involve getting most of your calories from fatShutterstock/George Dolgikh
Eating a keto diet, which entails consuming most of your calories from fat and very few from carbohydrates, does lead to fat loss, but may also clog arteries and negatively affect the gut microbiome.
Also known as a ketogenic diet, this way of eating forces the body to use a different type of fuel. Instead of relying on glucose from carbohydrates, it starts to rely on ketone bodies, a type of fuel the liver produces from stored fat, which leads to weight loss.
Previous studies have tracked health outcomes in people who follow a ketogenic diet compared with those who don’t, which suggest they have both harms and benefits.

Now, Javier Gonzalez at the University of Bath in the UK and his colleagues have carried out a randomised controlled trial, the best kind of medical evidence. The researchers recruited 53 people with an average age of 34, none of whom were obese. They randomly assigned roughly a third to eat a ketogenic diet, in which all types of carbohydrates made up less than 8 per cent of their energy intake and fat constituted more than 70 per cent.
Another third of participants were told to eat a low-sugar diet, where “free” sugars – a type of carbohydrate found in foods such as syrups, cakes and biscuits – made up 5 per cent of their energy intake, while their non-free-sugar carbohydrate and fat intakes were 45 per cent and 35 per cent, respectively.
The remaining third ate a diet with moderate levels of free sugars, making up just under 20 per cent of their energy intake, while their non-free-sugar carbohydrate and fat intakes were around 30 per cent and 35 per cent, respectively. These participants acted as the control group. Protein consumption was between 15 and 18 per cent across all of the diets.
A month later, X-rays revealed that those on the ketogenic diet had lost 1.6 kilograms of fat, on average. The team confirmed that the participants had followed the ketogenic diet by measuring levels of ketone bodies in their blood, urine and breath. Food diaries also gauged diet adherence.
Meanwhile, those on the low-sugar diet lost 1 kg of fat, on average, while those on the moderate diet lost no fat. By having the participants intermittently wear movement monitors and by estimating their energy intake, the team found that the fat loss was caused by consuming fewer calories, rather than exercising more, something that was previously unclear, says Gonzalez.
But while there was greater fat loss in the ketogenic group, these individuals had 16 per cent higher levels of low-density lipoprotein (LDL) “bad” cholesterol than those in the control group. They also had 26 per cent higher levels of a protein called apolipoprotein B, which clogs arteries, raising the risk of heart-related events. In contrast, those on a low-sugar diet had 10 per cent lower levels of LDL cholesterol and no change to their apolipoprotein B levels, compared with those in the control group.
The researchers also found that, compared with the moderate-sugar diet, those on the ketogenic diet had reduced levels of a type of gut bacteria called Bifidobacterium, which helps produce B vitamins and has been linked to a stronger immune system. The same was not true for those on the low-sugar diet. This is probably because those on the ketogenic diet ate less fibre, which increases Bifidobacterium levels, says Gonzalez.
But whether a ketogenic diet really negatively affects the gut microbiome is unclear, because no one is entirely sure what constitutes an optimal one or what the full effects of Bifidobacterium are, says Natasha Schoeler at Great Ormond Street Hospital in London.

Moreover, while raised apolipoprotein B levels are concerning, the increased cholesterol seen in the study aren’t necessarily harmful if they don’t reach dangerous thresholds, says Schoeler.
Long-term research is also required to determine if a ketogenic diet’s weight-loss benefits outweigh potential concerns about cholesterol and gut health among people who are obese, says Schoeler.
Ultimately, the main challenge of losing weight among those who are obese or overweight is adhering to a diet, says Schoeler. “Whether it is a normal-carbohydrate diet but low in calories, or the same amount of calories with less carbohydrate – if you stick to it, generally, you will lose weight.”

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