New research shows that people who follow a keto diet to lose weight or treat diabetes should avoid sudden spikes in blood sugar levels.

Diets require discipline, and it is not always easy for people to follow them without indulging in a "cheat day." One day may not make a big difference in the long-term, but a recent study from the University of British Columbia in Okanagan, Canada (UBCO), found that when it comes to the keto diet, a single dose of carbohydrates may have dangerous side effects.

The keto diet aims to provide the body with more calories from protein and fat and less from carbohydrates. Ratios depend on the individual's body mass and activity level, but they usually stay in the following ranges: 60%–75% of calories from fat, 15%–30% of calories from protein, and 5%–10% of calories from carbohydrates.

When the body receives less than 50 grams of carbohydrates a day, it will run out of its preferred fuel source—glucose—and this will happen 3 to 4 days after the beginning of the diet. At this point, the body will start to break down protein and fat to get energy. This metabolic process is called ketosis.

Keto diet for weight loss and diabetes

People often use the keto diet to lose weight, but this method can also help to manage certain health conditions, including diabetes. Studies showed that the keto diet might help to control hunger and reduce body weight, and it may lower blood sugar levels in those with diabetes. "The ketogenic—or keto—diet has become very common for weight loss or to manage diseases such as type 2 diabetes," says Jonathan Little, senior author of the recent study and associate professor in the School of Health and Exercise Sciences at UBCO.

Little explains that during ketosis, the body's chemistry changes. The process in which the body starts to aggressively burn its fat stores to get energy may lead to significant weight loss and improvement in symptoms of diseases, for example, type 2 diabetes.

The impact of one dose of glucose

It is common for people to indulge in "cheat days," and so researchers at UCBO wanted to know what happens when people reintroduce a dose of glucose to the body. The study was looking for inflammatory responses or reduced tolerance to blood glucose.

"Since impaired glucose tolerance and spikes in blood sugar levels are known to be associated with an increased risk in cardiovascular disease, it made sense to look at what was happening in the blood vessels after a sugar hit," says Cody Durrer, study first author and doctoral student at UBCO.

Nine healthy young males participated in this study, which appears in the journal Nutrients. The researchers asked them to follow a 7-day, high-fat, low-carbohydrate diet that was similar to the keto diet, consisting of 70% fat, 10% carbohydrates, and 20% protein. They also had to consume a 75-gram glucose drink before and after the diet.

The inflammatory and blood glucose concerns were not what the researchers discovered, however. "What we found instead were biomarkers in the blood, suggesting that vessel walls were being damaged by the sudden spike in glucose," Durrer continues. The results were alarming because even though the participants were all young males, the condition of their blood vessels after consuming the glucose drink was more similar to that of people with poor cardiovascular health. Little explains that the damage to blood vessels is due to the body's own metabolic response to spikes in blood sugar levels, which may lead to the death of blood vessel cells.

Researchers cautioned that the study only included nine people, and more work is needed to verify the results. However, it still points to worrying issues, the senior author suggests. "My concern is that many of the people going on a keto diet—whether it's to lose weight, to treat type 2 diabetes, or some other health reason—may be undoing some of the positive impacts on their blood vessels if they suddenly blast them with glucose," Little concludes. These findings should give those on a keto diet pause when considering a "cheat day." The data suggest that the keto diet is not something that people should do for 6 days a week and take a day off. This consideration is especially important for people who are at a higher risk for cardiovascular disease.