• A new study compared the keto diet with a diet low in added sugars and found that both diets reduced fat mass compared to a control diet.
• However, the keto diet was associated with negative metabolic changes, such as raised “bad” cholesterol and changes in the gut microbiome.
• The diet low in added sugars was not linked to negative health outcomes.

A diet low in added sugars is recommended for overall health. The World Health Organization (WHO) recommends that free sugars make up less than 10% of total energy intake because of their links to chronic diseases.

Studies have shown that reducing intake of free sugars helps reduce fat mass. Similarly, carbohydrate restriction, as with the ketogenic diet, is a proven weight loss method that has recently exploded in popularity.

A new study published in the journal Cell Reports Medicine investigates how the low carb, high fat keto diet compared to a diet low in free sugars (sugar added to foods or drinks) and a control diet. The researchers wanted to find out how these eating patterns influenced weight loss and metabolism. The study involved 53 participants and ran for 12 weeks.

According to a news release, the scientists also found that compared with the control diet, the low-free sugar diet and keto diet reduced fat mass by 2.1 and 2.9 kilograms, respectively. They determined this weight loss was unrelated to changes in physical activity — all groups maintained similar activity levels.

However, the scientists found the keto diet increased levels of “bad” cholesterol and other markers associated with cardiovascular disease risk compared to the low-free sugar diet and the control diet. The keto diet was also linked to reduced “good” gut bacteria.

Meanwhile, the diet low in free sugars did not have these negative side effects.

Can a keto diet cause high cholesterol?

Participants following the keto diet had elevated levels of low-density lipoprotein (LDL) cholesterol, sometimes called “bad” cholesterol.

In particular, they had higher levels of small and medium-sized LDL particles. When it comes to cholesterol, the smaller the particle, the worse it is for health.

In contrast, the low-sugar diet significantly reduced LDL cholesterol.

Participants following the keto diet also had increased levels of a molecule called apolipoprotein B (apoB), a compound found in LDL cholesterol. This protein is linked to atherosclerosis, where the lining of blood vessels develops plaques, increasing the risk of cardiovascular disease.

“This is a meaningful reduction,” study author Javier Gonzalez, PhD, a professor at the University of Bath in the United Kingdom, told Healthline.

“We observed a 0.5 millimoles per liter (mmol/L) reduction. Genetic studies indicate that if the levels of LDL cholesterol were reduced by just 0.35 mmol/L, then over a lifetime, this would equate to a 21% lower relative risk of atherosclerotic cardiovascular disease, including heart disease and stokes,” Gonzales said.

How does the keto diet affect gut health?

Unlike the reduced-sugar and control diets, the keto diet significantly affected the gut microbiome — the trillions of bacteria that live in the lower intestine.

In particular, they found reduced Bifidobacteria, a “good” gut bacteria. Reduced numbers of these species are linked to increased risks of metabolic, immune, and gut issues.

Fiber — a carbohydrate that the digestive system cannot break down — feeds our gut bacteria. Fiber intake is severely reduced on the keto diet, which may explain why this dietary pattern would impact the microbiome.

Keto diet impairs glucose tolerance

Carbohydrates are broken down into glucose in the gut. Glucose then enters the bloodstream and travels around the body. This is healthy, but if levels stay too high for too long, blood vessels may become damaged.

In response to rising blood glucose levels, the body releases insulin, which helps the liver and muscles take up the glucose and reduces the levels in the blood.

In this study, those following the keto diet had lower glucose levels in their blood before eating.

However, participants also had reduced glucose tolerance. In other words, glucose was removed from their blood less efficiently after eating, which would cause a longer blood sugar response.

What are the long-term effects of the keto diet?

When asked what these keto-related metabolic changes might mean in the long run, Gonzales explained this is a “challenging question to answer.”

Some metabolic changes determined by this study were beneficial (i.e., lower fasting glucose levels), and some were negative (i.e., the increase in apoB).

“This may mean that the long-term effects on health depend on the status of the individual,” Gonzales said. “For example, does someone have a particular problem with controlling their fasting glucose or their blood lipids?”

Gonzalez said he plans to continue this research. “We are keen to explore if there are individual responses to ketogenic diets that may warrant personalized use and whether we can mitigate against some of the unfavorable effects with targeted supplementation strategies,” he said.

“We have just received a large Medical Research Council grant for some of this work and are actively seeking more funding for the rest.”

Sarah Herrington, a nutritionist for Brio-Medical, explained the potential long-term implications of following the keto diet: “Long-term effects of a ketogenic diet may include more significant changes to the gut microbiome, potentially leading to less overall diversity.”

“It may also worsen glucose tolerance — the body will be more sensitive to sugars and carbohydrates when they are reintroduced into the diet,” Herrington continued.

“A ketogenic diet is more likely to alter the way the body uses energy, increasing fat metabolism and making positive short-term improvements in blood sugar regulation, but may have implications in the health of the microbiome long- term.”

Takeaway

Restricting free sugars or following the keto diet can lead to weight loss. But a new study shows the keto diet may have negative health consequences, such as raising bad cholesterol and harming gut health.

However, a diet low in added sugars did not appear to harm health.

“The free sugar restriction achieves this with almost entirely favorable health effects, whereas the ketogenic diet achieves the weight loss with some trade-offs we may need to be cautious about,” Gonzales said.