Wellness Nutrition Eat Well 7 Side Effects of Going Keto The low-carb, high-fat plan promises quick weight loss, but health experts worry about these side effects and complications. By Amanda MacMillan Amanda MacMillan Amanda MacMillan is a health and science writer and editor. Her work appears across brands like Health, Prevention, SELF, O Magazine, Travel + Leisure, Time Out New York, and National Geographic's The Green Guide. health's editorial guidelines Updated on August 23, 2022 Medically reviewed by Chika Anekwe, MD Medically reviewed by Chika Anekwe, MD Chika Anekwe, MD, MPH, is an obesity medicine physician at Massachusetts General Hospital. She is board-certified in general preventive medicine, public health, obesity medicine and by the National Board of Physician Nutrition Specialists. learn more Share Tweet Pin Email The ketogenic diet—also known as the "keto diet" or just "keto"—is a big thing in weight-loss plans, touted by celebs like Halle Berry, Kourtney Kardashian, and Jenna Jameson. The diet involves cutting way back on carbohydrates, to 50 grams a day or less, to help the body achieve a state of ketosis, in which it has to burn fat (rather than sugar) for energy. Doctors say that the keto diet can be helpful in treating epilepsy; it's unclear exactly why, but something about a ketogenic state seems to reduce the frequency of seizures. A July 2021 review published in Frontiers in Nutrition confirmed the benefits ketogenic diets can have in reducing seizure frequency in individuals with drug-resistant epilepsy, in addition to reducing weight. However, the review also notes that without more data on long-term safety, it appears that the risks associated with this diet may outweigh the benefits. Jillian Michaels Isn't the Only One Who Hates Keto—These 5 Other Experts Say Ditch the Diet As a general weight-loss plan, keto is controversial. Some health experts warn against it entirely, citing unpleasant side effects, health risks, and the diet's unsustainable nature. Even many keto diet proponents admit that if the diet's not done "the right way," it can be the opposite of healthy. Here are a few things you should know about the ketogenic diet before you try it as a way to lose weight. Yes, you might drop pounds, but you should also watch out for the following side effects or complications. What Is Keto 2.0—And Is It Any Healthier Than the Standard Keto Diet? 1. The "Keto Flu" "Some people report that when they start ketosis, they just feel sick," said Kristen Kizer, RD, a dietician at SportsTec Clinic in Kingston, Australia. "There can sometimes be vomit, gastrointestinal distress, a lot of fatigue, and lethargy." This so-called keto flu usually passes after a few days, said Kizer. Josh Axe, a doctor of natural medicine and clinical nutritionist, estimates that about 25% of people who try a keto diet experience these symptoms, with fatigue being the most common. "That happens because your body runs out of sugar to burn for energy, and it has to start using fat," said Axe. "That transition alone is enough to make your body feel tired for a few days." You may be able to minimize the effects of keto flu by drinking plenty of water and getting plenty of sleep. Axe, who sells keto-related supplements on his website, also recommends incorporating natural energy sources to battle fatigue, like matcha green tea, organic coffee, or adaptogenic herbs (herbs that may help your body deal with stress and fatigue). The 6 Biggest Keto Diet Mistakes 2. Diarrhea If you find yourself running to the bathroom more often while on a ketogenic diet, a quick internet search will show you that you're not alone. (Yes, people are tweeting about keto diarrhea.) This may be due to the gallbladder—the organ that produces bile to help break down fat in the diet—feeling "overwhelmed," said Axe. Diarrhea can also be due to a lack of fiber in the keto diet, said Kizer, which can happen when someone cuts way back on carbs (like whole-grain bread and pasta) and doesn't supplement with other fiber-rich foods, like vegetables. It can also be caused by an intolerance to dairy or artificial sweeteners—things you might be eating more of since switching to a high-fat, low-carb lifestyle. What Causes Diarrhea? 10 Things That Can Trigger Loose Bowel Movements 3. Reduced Athletic Performance Some athletes swear by the ketogenic diet, not just for weight loss but for improved performance in their sport, as well. But Edward Weiss, PhD, associate professor of nutrition and dietetics at Saint Louis University, doesn't buy it. "I hear cyclists say all the time that they're faster and better now that they're on the keto diet, and my first question is, 'Well, how much weight did you lose?'" said Weiss. In a 2018 study in the Journal of Sports Medicine and Physical Fitness, Weiss and his colleagues 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. Weiss says that the body is in a more acidic state when it's in ketosis, which may limit its ability to perform at peak levels. "Just losing a few pounds is enough to give you a huge advantage on the bike, but I'm very concerned that people are attributing the benefits of weight loss to something specific in the ketogenic diet," continued Weiss. "In reality, the benefits of weight loss could be at least partially canceled out by reductions in performance." 9 Fruits You Can Actually Eat on the Keto Diet 4. Ketoacidosis If you have type 1 or type 2 diabetes, you shouldn't follow the keto diet unless you have your doctor's permission and close supervision, said Kizer. "Ketosis can actually be helpful for people who have hyperglycemia issues, but you have to be very mindful of your blood sugar and check your glucose levels several times a day," explained Kizer. That's because, for people with diabetes, ketosis can trigger a dangerous condition called ketoacidosis. This occurs when the body stores up too many ketones—acids produced as a byproduct of burning fat—and the blood becomes too acidic, which can damage the liver, kidneys, and brain. Left untreated, it can be fatal. Although this complication is quite rare, ketoacidosis was also reported in a breastfeeding woman who did not have diabetes and was following a low-carb, high-fat diet, according to a 2015 case report from the Journal of Medical Case Reports. Symptoms of ketoacidosis include a dry mouth, frequent urination, nausea, bad breath, and breathing difficulties. If you experience any of these while following the keto diet, check in with a healthcare provider right away. Keto Diet Constipation Is a Legit Issue—Here's How to Deal 5. Weight Regain Because the keto diet is so restrictive, health experts say it's not an appropriate plan to follow long-term. (Even Axe said it's best done for 30 to 90 days, followed by a more sustainable diet plan.) But the problem with that, said Kizer, is that most people will regain a lot of the weight they lost as soon as they go back on carbs. "It's an issue with any fad diet, but it seems to be extra common with ketosis," said Kizer. "When people tell me they want to try it because their friends lost weight, I always tell them, 'Just watch, I almost guarantee that they'll gain it all back.'" These types of weight fluctuations can contribute to disordered eating, said Kizer, or can worsen an already unhealthy relationship with food. "I think the keto diet appeals to people who have issues with portion control and with binge eating," said Kizer. "And in many cases, what they really need is a lifestyle coach or a professional counselor to help them get to the bottom of those issues." Why 'Keto-Approved' Food Labels Are Misleading 6. Less Muscle Mass, Decreased Metabolism Another consequence of keto-related weight changes can be a loss of muscle mass, said Kizer—especially if you're eating much more fat than protein. "You'll lose weight, but it might actually be a lot of muscle," said Kizer, "and because muscle burns more calories than fat, that will affect your metabolism." When a person goes off the ketogenic diet and regains much of their original weight, it's often not in the same proportions, said Kizer: Instead of regaining lean muscle, you're likely to regain fat. "Now you're back to your starting weight, but you no longer have the muscle mass to burn the calories that you did before," explained Kizer. "That can have lasting effects on your resting metabolic rate, and on your weight long-term." 5 Long-Term Health Risks of Going Keto 7. Increased Risk of Heart Disease and Diabetes Axe said that, when done right, the keto diet includes lots of vegetables and lean sources of animal protein. In other words, it's not an excuse to eat butter and bacon—although some people may try to do just that. That's why many health experts are concerned about people on the ketogenic diet, especially those who try it without the guidance of a doctor or nutritionist. Doctors say that high-fat diets like this one may raise cholesterol levels, and the July 2021 review mentioned above concluded by saying these diets can be associated with dramatic increases in LDL-C, the "bad" form of cholesterol. Some have even called the ketogenic diet a "cardiologist's nightmare." 10 Keto-Friendly Vegetables You Should Eat More Of Heart problems are not the only concern when it comes to the keto diet. A study published in The Journal of Physiology in August 2018 suggests that these diets increase the risk of diabetes. Another study published in the European Heart Journal in April 2019 found that while a short-term low-carb diet helps improve body weight, blood pressure, and other health markers, this diet is associated in the long term with the risk of death from cardiovascular disease, cancer, stroke, and all other causes. A study published in The Lancet in August 2018, also found that people who followed diets that were low in carbs and high in animal proteins (typical of the keto diet) had a higher risk of early death compared to those who consumed carbs in moderation. (The opposite was true, however, for low-carb dieters who opted for plant-based proteins over meat and dairy.) "Whether you're in the paleo camp or the keto camp or the vegan camp, everyone agrees that we want to have a nutrient-rich diet," said Axe: "Lots of vegetables, herbs, spices, and plant-based sources of fat and protein, too." "If you're not doing that, you're promoting disease in the body—it's that simple," said Axe. (And yes, that's true even if you still lose weight in the beginning.) "If you're just going to eat butter and bacon," added Axe, "I'd rather you not do the keto diet at all." Was this page helpful? Thanks for your feedback! Tell us why! Other Submit Sources Health.com uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy. Crosby L, Davis B, Joshi S, et al. Ketogenic diets and chronic disease: weighing the benefits against the risks. Front Nutr. 2021;8:702802. doi:10.3389/fnut.2021.702802 Crosby L, Davis B, Joshi S, et al. Ketogenic diets and chronic disease: weighing the benefits against the risks. Front Nutr. 2021;8:702802. doi:10.23736/S0022-4707.18.08318-4 von Geijer L, Ekelund M. Ketoacidosis associated with low-carbohydrate diet in a non-diabetic lactating woman: a case report. J Med Case Rep. 2015;9:224. Published 2015 Oct 1. doi:10.1186/s13256-015-0709-2 Grandl G, Straub L, Rudigier C, et al. Short-term feeding of a ketogenic diet induces more severe hepatic insulin resistance than an obesogenic high-fat diet: A short-term ketogenic diet induces hepatic insulin resistance. J Physiol. 2018;596(19):4597-4609. Mazidi M, Katsiki N, Mikhailidis DP, Sattar N, Banach M. Lower carbohydrate diets and all-cause and cause-specific mortality: a population-based cohort study and pooling of prospective studies. European Heart Journal. 2019;40(34):2870-2879. Seidelmann SB, Claggett B, Cheng S, et al. Dietary carbohydrate intake and mortality: a prospective cohort study and meta-analysis. The Lancet Public Health. 2018;3(9):e419-e428.