By now, you've probably heard all about the ketogenic diet, the high-fat, low-carb eating style everyone's talking about. And while the benefits of keto are pretty well-known—weight loss and a suppressed appetite—the downsides are usually minimized by keto advocates. "The positive aspects of a ketogenic diet are spoken about much more than the drawbacks," says Alix Turoff, R.D., C.D.N., C.P.T., a dietitian and personal trainer.
"The diet was introduced in the 1920s as a way to treat epilepsy and then sort of fell out of popularity with the introduction of anti-seizure drugs," Turoff says. What's more, ketosis (the goal of keto, a state where the body uses fat for energy instead of carbs) is something seen in people during periods of starvation—including in people with anorexia nervosa. "The body is deprived of carbohydrates and thus has to turn to ketone bodies as a fuel source," Turoff explains. "People really need to understand that it's not just a low-carbohydrate, high-fat diet—it actually changes the way your body uses fuel."
Turoff doesn't think the keto diet should be a go-to solution for weight loss, and she's not alone in this opinion. Many of the dietitians Shape spoke to for this story had similar thoughts on the keto diet, which is why many of them strongly encourage anyone who is thinking about trying a ketogenic diet to chat with a registered dietitian first. (Related: Why This Dietitian Is Completely Against the Keto Diet)
In the meantime, here are some things to keep in mind before you go full-on keto:
No one knows what the long-term effects are.
The side effects of extreme low-carb diets are still a mystery. "The first major drawback is the fact that we really don't have any long-term research about how keto followers fare 10, 20, 30 years down the line," Turoff says. "What will the results look like? Will they regain the weight? Will they suffer health consequences? We also thought smoking and diet pills were a good idea, until long-term studies showed us that we were wrong."
"Keto flu" is not a fun experience.
Keto dieters often commiserate about experiencing symptoms as their bodies adjust to the diet, but experts say this period of transition is no joke. "When your body first enters ketosis, you may experience a series of side effects termed the 'keto flu,'" says Jennifer M. Brown, R.D., a faculty associate at Arizona State University's School of Nutrition and Health Promotion. "These include fatigue, dizziness, lightheadedness, poor sleep, difficulty with exercise, and constipation, all resulting from extreme restriction of carbohydrates." While these symptoms usually subside after the body adjusts to relying on fat for fuel, it can take anywhere from a few days to a few weeks. Is losing weight really worth potentially feeling ill for three weeks?! "Given that low-carb diets have not been shown to have an advantage when it comes to weight loss, I would say these side effects aren't worth it and are unnecessary," Brown says. (Related: Is It Possible to Follow a Vegetarian Keto Diet?)
It can throw your body's hydration levels out of whack.
"Electrolyte imbalances can also be common if keto followers aren't aware of hydration needs," Turoff says. It's important to stay hydrated on keto, she says, especially in the beginning. "As you limit carbohydrates, your body produces less insulin, and glycogen stores (how carbs are stored) in the muscles and liver are depleted. For every 1 gram of glycogen that's depleted, you lose about 3 grams of water." This causes the kidneys to flush out more water, and along with it, electrolytes your body needs like magnesium, calcium, sodium, and potassium. "Imbalanced electrolytes can lead to muscle cramps, irregular heartbeat, fatigue, cognitive distortions, and lack of body temperature control," Turoff says. Some people may also be more likely to get kidney stones while on keto due to inadequate hydration, according to Turoff. (PS: These Are the Hydration Tips Every Fit Girl Needs.)
The initial weight loss is mostly water.
Your kidneys flushing out all that water is also part of the reason people see dramatic scale drops in the beginning of their keto journeys. "I wish people knew that the weight loss they will initially experience on this diet is largely due to water loss," Brown says. That means you're most likely not actually losing fat in the first couple of weeks, but instead losing water that will come back in the form of glycogen stores if and when you start eating carbs again. (BTW, Here's How to Safely and Effectively Come Off the Keto Diet.)
You could have some seriously funky breath.
This one might not seem like a big deal, but it's worth knowing about. "When the body goes into ketosis, it's common for the breath to start smelling like acetone because of the breakdown of acetoacetic acid," Turoff says. For reference: "Acetone is the chemical that gives nail polish its smell." Uh, noted.
The brain fog you've heard about is a real thing.
"Your brain runs on glucose (a simple form of carbohydrates)," notes Adrien Paczosa, R.D., L.D., a registered dietitian and eating disorder specialist. "When you cut out carbs, your body has to 'make' carbs from breaking down other parts of the body. This results in what I like to call a hungry brain. You experience slower cognition, memory loss, headaches, and confusion." That doesn't sound so great, right? "Also, those who suffer from depression and anxiety may have higher levels of those periods when not eating carbs on a regular basis." Because of this, keto might not be the best choice for those with mental health issues.
You'll eat fewer fruits and vegetables.
"On a ketogenic diet, your intake of fruits and vegetables is extremely limited, and we all know how important the fiber, vitamins, minerals, and other compounds in fruits and veggies are," says Libby Parker, R.D., a dietitian who specializes in treatment and prevention of eating disorders. It's true: There are a lot of healthy but high-carb foods you can't eat on the ketogenic diet. That's one of the reasons keto is usually pretty low in fiber, which isn't so great long-term. "Fiber is not only protective against many gastrointestinal cancers, it is also a big factor in fullness and weight loss," Parker explains. "Furthermore, constipation is very common on low-fiber diets like the keto diet." (That said, These Healthy, High-Fat Foods Can Help You Feel Full Longer.)
It may negatively affect your athletic performance.
There are people who say that keto helps your athletic performance, and those who say it hurts it. "The literature is mixed on how an ultra low-carbohydrate diet like the keto diet affects athletic performance," says Grant Cox, C.S.C.S., head coach at Iron Tribe Fitness. But one thing is pretty clear: "A lot of it points to consistent decreases when athletes are looking for maximal power output (in weightlifting, vertical jumps, sprinting, etc.). Along the same lines, you'd be hard-pressed to put on weight and strength on such a low carbohydrate diet," he says.
That might be part of the reason the keto diet is thought to be a better fit for endurance athletes. But even for them, evidence is mixed. "The keto diet has been popular with athletes that engage in long endurance events, because it requires them to rely on less frequent carbohydrate feedings to fuel their exercise (think gummies, drinks, and goos) and more on their body fat stores," Brown explains. "This can decrease uncomfortable gastrointestinal symptoms (fewer stops at the porta-potties!), but a keto diet can be as challenging to maintain for an athlete as it is for a layperson."
Plus, "some research has found that athletes on a keto diet become less efficient in their exercise," Brown says. Because of this, the researchers involved in the study recommended that instead of going for a keto diet, athletes who want to take advantage of their body's ability to use fat as fuel alternate between training in a carb-depleted state (for example, doing a fasted workout in the morning after not eating carbs the night before) and eating carbs as normal. (Related: 8 Things You Need to Know About Exercising on The Keto Diet)
It may trigger disordered eating.
Keto is pretty much a no-go for those who are prone to disordered eating. "Being on such a restrictive diet can quickly turn obsessive and if you're prone to disordered eating, this can be even more dangerous," Turoff says. To be fair, this isn't exclusive to the keto diet, but it applies to any eating style that restricts entire food groups.
Even if you don't have a history with eating disorders, keto can still leave some people (although certainly not everyone!) with negative feelings around food. "For the vast majority of people, keto is not sustainable, meaning they don't stick with it for more than a few months," Brown says. "Not 'succeeding' with this diet and returning to one's usual eating habits can trigger feelings of guilt and failure." Those two feelings can actually trigger disordered eating in some people in the worst-case scenario, says Brown. (Also, We Seriously Need to Stop Thinking of Foods As "Good" and "Bad")
It can be emotionally exhausting.
"Most people who wind up trying a ketogenic diet and then deciding not to continue do so because of the emotional and lifestyle consequences," Turoff says. To put it simply, people miss eating carbohydrates. "That doesn't mean that you should be eating pizza or cupcakes every day, but what about having a sweet potato with a meal, or beans in a chili? Or fresh watermelon in the summer?" Turoff asks. "Whether or not we want to admit it, food plays much more than just a physical role in our lives and having such restrictions on the types of foods you can and can't eat can really take a toll. It might be easy in the short-term to go for carb-free foods but at a certain point, the thought of not being able to eat your favorite foods again can become daunting."
Eating in a restrictive way can be socially isolating, Brown says. "You may avoid parties and other gatherings because you may worry about what food will be available for you, or even what foods may tempt you. Having to plan and control what you are eating so closely may lead to increased anxiety and stress. And who doesn't want to enjoy a beer with a friend or a piece of cake on your birthday?" As you probably know, emotional well-being and physical health are closely linked, which is all the more reason to choose foods that make you feel good both mentally and physically. If keto does that for you, great! But if it doesn't, know you're not alone.