The ketogenic or “keto” diet hype shows no signs of slowing: The low-carb regimen is still massively popular, with celebrities like Al Roker and Jenna Jameson crediting the diet for serious weight loss.

It’s easy to see why a diet that promises quick results—and that, technically, allows you to still enjoy foods like burgers and cheese—would be so tempting. But before you try it, it’s important to realize that keto can also have its downsides and that there are a lot of health experts who still don’t know about its long-term effects on the body.

Following the keto diet for an extended period of time can be difficult, and even some of its top proponents warn against sticking to its strict guidelines. This includes cutting back carbohydrates to 50 grams a day or less, for at least two to three weeks up to six to 12 months, per the National Library of Medicine. Other researchers warn that sticking to the diet long-term could even be dangerous. Here are a few reasons why.

Limiting carbs to 50 grams a day or less likely means you’re cutting out unhealthy foods like white bread and refined sugar. But it also means you may have to cut back on fruits and certain vegetables, which are also sources of carbohydrates, according to MedlinePlus.

That’s a concern, Annette Frain, RD, program director with the Weight Management Center at Wake Forest Baptist Health, told Health, especially if someone is spending more than a few weeks on this type of diet. “Fruits and vegetables are good for us; they’re high in antioxidants and full of vitamins and minerals,” said Frain. “If you eliminate those, you aren’t getting those nutrients over time.”

It may also be hard to get enough fiber while you’re cutting back so severely on carbohydrates since whole grains are one of the biggest sources of this important nutrient. Therefore, you might be missing out on the many benefits of fiber. According to UpToDate, a high-fiber diet can reduce your risk of heart disease, stroke, and type 2 diabetes. It can also help with digestive problems such as constipation or chronic diarrhea.

There’s no shortage of athletes who have jumped on the keto bandwagon, but some researchers worry that they could actually be sabotaging their strength and fitness. In a 2019 study published in the Journal of Sports Medicine and Physical Fitness, researchers found that participants performed worse on high-intensity cycling and running tasks after four days on a ketogenic diet compared to those who’d spent four days on a high-carb diet.

The body is in a more acidic state when it’s in ketosis, lead researcher Edward Weiss, PhD, associate professor of nutrition and dietetics at Saint Louis University, told Health, which may limit its ability to perform at peak levels.

Sure, keto can help athletes lose weight, which can be helpful for speed and endurance. “But I’m very concerned that people are attributing the benefits of weight loss to something specific in the ketogenic diet,” Weiss said. “In reality, the benefits of weight loss could be at least partially canceled out by reductions in performance.”

Because the keto diet is so strict, many variations of the diet recommend incorporating several stages. The first stage, usually the first one to three months, is extremely low-carb and allows for very few “cheat days,” if any at all. It also requires keeping close track of your carbohydrate and fat consumption to ensure your body is entering ketosis.

But then, people may transition to a more relaxed form of keto that allows for more carbohydrates or less monitoring—sometimes known as lazy keto, keto cycling, or “maintenance mode.” The problem here, said Frain, is that weight regain is almost inevitable.

“Keto can be a great jump-start to weight loss, but the reality is that most people can’t adhere to it for very long,” said Frain. “Often, people are going into ketosis and losing weight, then coming out and gaining it back and falling into this yo-yo pattern, and that’s not what we want.” In addition to being extremely frustrating, said Frain, these types of weight fluctuations are also linked to a higher risk of early death.

The type of weight you gain back is important as well. If you lost weight when you first started on keto, you likely lost some muscle mass along with fat tissue, Kristen Kizer, RD, a nutritionist at Pura Vida Behavioural Nutrition, told Health. Now, since you’re following a high-fat diet, you will probably gain back more fat and less lean muscle—which not only looks and feels different on the body but also burns calories at a slower rate. This can affect your metabolism and make it more difficult to lose weight again in the future.

Enjoying a “cheat day” in the short term on the ketogenic diet can also have long-term consequences, say researchers from the University of British Columbia. In a 2019 study published in Nutrients, they found that indulging in a high-sugar treat (like a large bottle of soda) while on a high-fat, low-carb diet can actually damage blood vessels.

“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,” senior author Jonathan Little, associate professor in the School of Health and Exercise Sciences, said in a press release. “Our data suggests a ketogenic diet is not something you do for six days a week and take Saturday off.”

Health experts worry about how a long-term keto-style diet can affect the heart and arteries. A study published in 2019 in the Journal of the American College of Cardiology, found that people on low-carb diets are more likely to develop atrial fibrillation (AFib), compared to those who eat moderate amounts of carbohydrates. According to the American Heart Association (AHA), this type of arrhythmia raises the risk of blood clots, stroke, and heart failure.

It’s not just the heart they’re worried about either. A 2018 study published in Lancet found that low-carb dieters who consumed large amounts of meat and dairy had a higher risk of early death compared to those who consumed carbs in moderation or who consumed mostly plant-based protein. Research published in European Heart Journal in 2019 also found that people who followed low-carb, high-fat diets had an increased risk of dying from cancer and all other causes during the study period.

However, most of this research is still observational—meaning that it’s only been able to find associations with certain health outcomes and not cause-and-effect relationships. Frain said that, overall, there’s not enough long-term research to know exactly what the ketogenic diet does to the body over an extended period of time—or why it seems to affect some people differently than others.

The main hallmark of a keto diet is eating more fat and less carbohydrates. While some people claim that it is helpful for weight loss, many experts caution against the ketogenic way of eating due to varying health concerns.

Frain advised anyone who’s thinking about trying keto to strive for balance, not for extremes. “It’s important to look at what you’re missing in a diet and what is really sustainable for you,” Frain said. “You want to make sure you have satisfaction and satiety from the foods you’re eating and that you feel good and are getting great nutrition from a variety of foods. That’s what will help you keep it up and keep the weight off.”