This article was medically reviewed by Rachel Lustgarten, R.D., C.D.N., a clinical dietitian and member of the Prevention Medical Review Board.
The keto diet has become one of the most popular nutrition fads in recent years, with celebrities from Katie Couric to the Kardashians extolling its benefits. But, of course, nothing is perfect and there are keto side effects to be aware of if you’re considering the diet.
While keto fans swear the diet leads to weight loss, better energy levels, and less cravings, plenty also acknowledge that it comes with some unusual downsides like the keto flu and possible digestive issues. Still, the keto diet requires some careful thought and planning before you can even get to the point where you might experience side effects.
In a typical keto diet plan, fat provides as much as 80% of your daily calories, while a mere 5% comes from carbohydrates, explains Jessica Cording, R.D., author of The Little Book of Game-Changers. (Worth noting: That’s 40-60% fewer carbs than the usual recommended dietary amount.) So, if you eat 2,000 calories a day, that means that only 100 of them are coming from carbs—including healthy carbs like fruits and vegetables.
The main goal of the keto diet is to put you into a state called ketosis, where your body burns fat for energy instead of the usual carbs, Cording explains. But, again, that can lead to some side effects.
Keto side effects can vary for everyone—if you have them at all, says Scott Keatley, R.D., co-owner of Keatley Medical Nutrition Therapy. Typically, though, you’ll have side effects in the beginning of the diet as your body gets used to the new eating plan. “The general timeline is about five to seven days of feeling low energy, i.e., the keto flu,” Keatley says. “In some people, this will manifest itself in general tiredness or brain fog and in others it may be nausea.” That could be followed by unusual side effects like “foul-smelling poop” for a few days and a fruity-smelling breath, Keatley says. (More on those in a moment.) Eventually, though, your symptoms should level off.
Keto isn’t for everyone and some people feel that the potential side effects just aren’t worth it. So, what are they and why do they happen? Here’s what you need to know.
Keto flu is a real thing. Cutting your carbs to the bone and going into a state of ketosis (where your body burns fat for energy) can bring on a cluster of uncomfortable symptoms, such as headaches, fatigue, muscle aches, nausea, and diarrhea. The side effects are the result of your body transitioning to using fat as its primary source of energy instead of carbs, explains Kristen Mancinelli, M.S., R.D.N., author of The Ketogenic Diet. Once it adapts to the new fuel source (usually within a week or two), you’ll start to feel better.
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When you’re on a low carb diet you may not be getting the carbohydrates needed to produce serotonin, a brain chemical that helps regulate mood, as well as sleep and appetite—two other factors that can mess with your disposition, says Laura Iu, R.D., registered dietitian and nutrition therapist certified intuitive eating counselor based in New York City.
The keto diet is notorious for delivering a quick initial slim down. That’s because carbs hold on to more water than protein or fat, says Becky Kerkenbush, R.D., a clinical dietitian at Watertown Regional Medical Center. So when you stop eating them, all that extra H2O gets released through urination. As a result, the scale might read a few pounds lower, and you may look a bit leaner.
That first drop might be mostly water weight. But research suggests that the keto diet is good for fat loss, too. An Italian study of nearly 20,000 obese adults found that participants who ate keto lost around 12 pounds in 25 days. However, there aren’t many studies looking at whether the pounds will stay off long-term, researchers note. Most people find it tough to stick with such a strict eating plan, and if you veer off your diet, the pounds can easily pile back on.
Constipation is a common side effect of low-carb eating plans, including the ketogenic diet. Severely curbing your carb intake means saying goodbye to high-fiber foods like whole grains, beans, and a large proportion of fruits and vegetables, says Ginger Hultin, M.S., R.D.N., Seattle-based nutritionist and spokesperson for the Academy of Nutrition and Dietetics.
Combine that with the fact that your body is excreting more water, and you have a potential recipe for clogged pipes. You can keep things moving by getting some fiber from keto-friendly foods like avocado, nuts, and limited portions of non-starchy vegetables and berries, says David Nico, Ph.D., author of Diet Diagnosis. Upping your water intake helps, too.
“When we eat foods with fat, our liver releases bile into the digestive system to help break it down. Following a high fat diet like keto means that the liver needs to release extra bile—and bile is natural laxative, so too much can loosen stool and speed up how fast it moves through your system, leading to diarrhea,” says Iu.
When your body goes into ketosis, it will start to produce by-products called ketones. This includes acetone—yes, the same chemical found in nail polish remover, which your body actually naturally makes on its own, according to a 2015 review of research. “One of the ways ketones are released from the body is through exhaling, and breath usually has a distinct odor thats different than the common bad breath experienced when there’s a build up of bacteria in the mouth,” says Iu.
Cutting out carbs can cause the brain to release a chemical calledneuropeptide-Y (NPY), which tells the body that we need carbs; when we don’t get those carbohydrates our body needs, this chemical builds up and can intensify cravings, which can increase the risk of developing disordered eating patterns like binge eating, says Iu. “It has nothing to do with not having enough ‘will power,’ it’s more to do with the body’s biological response to deprivation,” she says.
Don’t be surprised if you find yourself parched while you’re on the keto diet. Excreting all that extra water will likely cause a spike in thirst—so make it a point to drink up, Mancinelli advises. There’s no hard and fast recommendation for how much water you should be having on a keto diet. But in general, aim to drink enough so your urine is clear or pale yellow. If it’s any darker, bump your intake.
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Plagued by pimples? You may start to notice a difference in your skin on the keto diet, especially if you were a former sugar addict. Consuming lots of empty carbs is linked to worse acne—in part because these foods trigger inflammation and signal the release of hormones that up the production of pore-clogging oils, according to a review published in the Journal of the Academy of Nutrition and Dietetics. Some findings suggest that curbing your carb intake could help solve these problems, improving your skin as a result.
The kidneys play an important role in metabolizing protein, and it’s possible that eating too much of the nutrient can have a negative impact on kidney function. While ketogenic diets are supposed to be much higher in fat than they are in protein, many keto eaters make the mistake of loading up on lots of meat, Mancinelli says. The result? You could end up eating way more protein than you actually need.
Here’s the tricky part: There’s no definitive answer for how much protein you’d have to eat before you run into trouble. “It really depends on how much protein a person is consuming versus how much they need, as well as the health of their kidneys at baseline,” Hultin says. That’s why it can be helpful to speak with a nutritionist or doctor who can help you tailor your diet before going keto.
Eating an ultra-low carb diet is linked to a lower rate of obesity, along with improved HDL cholesterol, all of which can translate to a lower risk for heart disease.
But your heart health might depend on what you actually eat. Research published in the New England Journal of Medicine suggests that low-carb diets based mostly on plant sources of fat and protein (like avocados or nuts) can lower heart disease risk by 30%. But those benefits didn’t hold for people who ate mostly animal-based proteins and fats. (Think: bacon, butter, and steak.)
Plus, the American Heart Association says that going overboard on saturated fat—which can be easy to do on a keto diet if you eat a lot of meat, butter, and cheese—can up your risk for heart problems. While you’re on the keto diet, you should have your cholesterol levels and heart health assessed by a doctor on a regular basis, Hultin says.
Some side effects are worse than others, and your body simply may not be OK with going on a keto diet. If you have consistent constipation, diarrhea, or vomiting, “you may want to take on another diet as your body, in its current state, cannot handle what is happening to it,” Keatley says.
Eating a keto diet can have some short-term health perks. But in the long run, it also has the potential to create some serious health problems. That’s why many experts say you shouldn’t attempt it on your own. “In general, if a person follows a ketogenic diet, they should only do so for a brief time and under close medical supervision,” says Hultin.