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.