The ketogenic diet—also known as the “keto diet” or just “keto”—has become the latest big thing in weight-loss plans, touted recently by celebs like Jenna Jameson, Mama June, and Halle Berry. 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. Animal studies have also suggested that the diet may have anti-aging, anti-inflammatory, and cancer-fighting benefits, as well.
“Some people report that when they start ketosis, they just feel sick,” says Kristen Kizer, RD, a nutritionist at Houston Methodist Medical Center. “There can sometimes be vomit, gastrointestinal distress, a lot of fatigue, and lethargy.” This so-called keto flu usually passes after a few days, she adds.
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,” he says. “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
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,” says Axe.
Diarrhea can also be due to a lack of fiber in the keto diet, says 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.
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?’” he says.
In a recent 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,” Weiss continues. “In reality, the benefits of weight loss could be at least partially canceled out by reductions in performance.”