Keto Diet: Short- and Long-Term Side Effects

The keto diet is a much-discussed eating plan, but it’s no walk in the park. Essentially, when following a keto diet food list, you’re eating mostly keto-friendly fat (60 to 80 percent of your calories) and very low carbohydrates, starting with 20 to 30 grams (g) per day. This allows your body to use fat for energy rather than carbohydrates, sending you into a state called ketosis.

While fans talk about the amazing weight loss results you may see from cutting so many carbs, there’s one vital truth to keep in mind: “It’s very important to understand that the ketogenic diet doesn’t work the same for everyone,” says the New York City–based dietitian Kristen Mancinelli, RD, author of The Ketogenic Diet: A Scientifically Proven Approach to Fast, Healthy Weight Loss.

Your unique body chemistry dictates how you’ll respond to eating this way, but there are some universal truths. Here’s what you can and can’t expect to happen on the diet, including the possible benefits and risks of keto!

You’ll slim down if you try the keto diet — and it’ll probably be fast. “When a person goes on a keto diet, they lose a lot of weight. That’s just what happens,” says Mancinelli. Indeed, a review found that the keto diet was effective in spurring weight loss. (1)

Your metabolism may increase slightly from keto, says Mancinelli, but the weight loss benefits derive more from the fact that you likely won’t be as hungry. “Are you really going to sit down and eat three avocados and an entire roast chicken? Nope. You won’t be eating the same number of calories because you won’t want to eat that much,” she says. The ketogenic diet may help decrease your appetite.

But the keto diet won’t fix all your health woes. Another study points out that the health benefits are controversial. (2) Some animal research points to potential problems like nonalcoholic fatty liver disease (NAFLD) from long-term adherence to the diet. Even though these are animal studies, they do point toward the need for more research into the implications of the diet. And in a report from 2020, researchers doing a review of related literature involving studies with humans cited potential beneficial effects of a keto diet on NAFLD. (3)

In the beginning, you’ll have to go very low in carbs, around 20 to 30 g a day, to ensure your body goes into ketosis, advises Mancinelli. (Some people call a keto diet anything that limits carbs to less than 50 g a day, though there are various types of keto.) And that can be difficult. When you’re eating a high-fat and very-low-carbohydrate diet, you’re eliminating many foods, including fruit, most dairy, starchy veggies, whole grains, and legumes (and certainly all sugar). It’s difficult and at times uncomfortable.

In the short term, before you begin to lose weight, you may also experience what people call the keto flu, a state that lasts about a week as your body adjusts to the diet. That can include extreme fatigue and foggy-headedness. (Not everyone feels this, Mancinelli allows.) Knowing that keto flu is a possibility can help you plan the best time to start the diet. You can also mentally prepare for this transition period — reminding yourself that it’s only temporary can help immensely.

For these reasons, “some people won’t be able to stay on it,” says Mancinelli. You should aim to give it at least six weeks. After that, if you feel exhausted or hate eating the food, then the diet is probably not for you. However, if you feel energetic after keto flu symptoms subside within a couple of weeks, which some people report, you may see success with the plan.

“I don’t recommend staying on the keto diet permanently. It can be really restrictive,” says Mancinelli.

Long-term research on the keto diet is limited, so there’s no telling for sure what cutting out certain major food groups and cutting carbs will eventually do to your body. Registered dietitians warn that nutrient deficiencies may be possible if you’re on it for too long.

A 2019 statement from the National Lipid Association Nutrition and Lifestyle Task Force, which focused broadly on low and very low carbohydrate diets, did not rate this class of diets as superior to other weight loss diets. Their report did cite some potential benefits — for example, effects on managing appetite and cholesterol levels — but it found no clear evidence for claims like cardiovascular benefits, and reiterated that a decision to try these diets should be carefully reviewed with medical experts. (5)

Because it’s not a forever diet, there will be life after keto. But even when you go off it, you can expect that your taste preferences will have changed. “You likely won’t want as many carbs, and things will taste extremely sweet,” she says.

While everyone’s nutrition requirements differ, generally, you may eat 40 to 45 percent fat and stick to 120 to 150 g of carbs per day to help you keep the weight off, she says.

The ketogenic diet isn’t a miracle diet for weight loss or a fix for all health problems. For certain people (like those with kidney disease), it can be dangerous. And because it’s not a long-term plan, you have to manage your diet carefully after stopping keto.

Think about how your lifestyle fits with such a restrictive plan, as well as what you’re willing to give up temporarily (like eating the same foods as your family). Let that be your guide to help you determine if the keto diet is right for you.