New research suggests the keto diet could put us at risk of heart disease, stroke, type 2 diabetes, and irritable bowel syndrome, to name a few concerns.
The study was small, but the results might make people think twice about whether a keto diet is worth the trouble.
Led by researchers from the University of Bath (UoB) in the UK, the team had 53 healthy adults follow one of three diets for 4 to 12 weeks: a low-carb ketogenic (keto) diet, a low-sugar diet, or a diet with moderate sugar and carbohydrates.
They found the keto diet increased cholesterol, reduced good gut bacteria, and reduced the body's ability to tolerate sugars, shifting the body's energy source from glucose to fats.
Both the low-sugar diet and the keto diet led to fat loss without affecting people's physical activity levels. It seems people hoping to achieve a leaner body are better off cutting added sugars, not carbs, from their diet.
"The ketogenic diet is effective for fat loss, but it comes with varied metabolic and microbiome effects that may not suit everyone," explains physiologist Dylan Thompson from UoB.
"In contrast, sugar restriction supports government guidelines for reducing free sugar intake, promoting fat loss without apparent negative health impacts."
Those on the keto diet consumed less than 8 percent of their calories from carbohydrates. The low-sugar diet included less than 5 percent of energy intake from free sugars – those added to food and drinks or present in fruit juice, syrups, and honey.
These were compared to a control diet with moderate sugar and carbohydrates, where free sugar contributed around 18 percent of energy intake.
By week four of the keto diet, participants' gut microbe diversity had changed, with massive reductions in the relative abundance of Bifidobacterium, which you may know as one of the heroes of probiotics.
These bacteria rely on dietary fibers that were cut by 40 percent in the keto diet compared to the moderate sugar and carb diet, and the fact that this shift in the gut ecosystem persisted at week 12 of the keto diet is evidence that the diet can cause this internal chaos.
The keto diet's effect on cholesterol was particularly concerning to the researchers, while participants on the low-sugar diet had actually reduced their total cholesterol concentrations by week 12 of the study.
"Despite reducing fat mass, the ketogenic diet increased the levels of unfavorable fats in the blood of our participants," says UoB nutrition scientist Aaron Hengeist.
"If sustained over years, [this] could have long-term health implications such as increased risk of heart disease and stroke."
While the keto diet reduced fasting glucose levels, it also reduced the body's ability to handle carbs from a meal, the researchers explain.
"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," says UoB human physiologist Javier Gonzalez.
People on the keto diet had increased levels of the enzyme PDK4 in their skeletal muscle, which is also seen in type 2 diabetes. The diet also reduced levels of the enzyme AMPK and the protein GLUT4, which play important roles in glucose metabolization.
"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," Gonzalez says.
This research was published in Cell Reports Medicine.