You’ve undoubtedly heard about the ketogenic diet — a high-fat, low-carb approach that forces the body to burn fat for fuel — and decided to consider it for weight loss, more energy, or any of the other touted benefits of the plan. But you’ve heard the stories: Several days into the keto diet, people start to feel sluggish with headaches, difficulty concentrating, and muscle cramps. Some get diarrhea (or alternatively, constipation).

Such symptoms are commonly referred to as the keto flu, says Amy Gorin RDN, in Jersey City, New Jersey, owner of Amy Gorin Nutrition. But they aren’t the result of an infection of any sort like influenza. Rather, they’re a byproduct of the body adjusting to a very-low-carb diet as it depletes stored glucose. The body turns to stored fat for energy instead, triggering a state of ketosis, when the liver starts breaking down fat into acids called ketones. For most people, the adjustment period may be mildly unpleasant but not dangerous to their health. Yet people with diabetes who require insulin, those with kidney disease or a history of kidney stones, as well as individuals with a history of eating disorders are among those who should avoid the diet, says Gorin.

Like Gorin, Lynn Grieger, RDN, a health coach in private practice in Prescott, Arizona, and a medical reviewer for Everyday Health, has observed an adjustment period of one to two weeks in people who are on the ketogenic diet. She says her clients report low energy levels, which can disrupt their usual exercise routines, along with focus issues and irritability.

What the Keto Flu Is and What Causes These Unpleasant Symptoms

During the initial few days of the keto diet, the body is essentially adjusting its metabolic machinery, says Stephen Phinney, MD, chief medical officer and co-owner of Virta Health, in San Francisco, a company experimenting with nutritional ketosis in its treatment of type 2 diabetes.

Symptoms that Grieger and Gorin have seen in ketogenic dieters include:

Fatigue or lethargy
Nausea
Headaches
Bad breath
Bloating
Constipation
Mood swings
Difficulty concentrating or “brain fog”

It often takes up to two weeks for symptoms to pass. But Gorin notes that bloating and constipation, which can result from not getting enough of the fiber that many carb-rich foods contain, may persist as long as you’re on the keto diet.

In other cases, symptoms can actually be serious, as was the case for Michelle Alley, a blogger in Utah who writes about health and nutrition for iFit. Alley says she lost feeling in her limbs after following a version of keto where she ate three parts of fat for every one part of protein and carbs. “They call it the pins and needles,” says Alley, explaining she stopped the diet after seven days.

Indeed, there are numerous types of the keto diet out there. Grieger says some of her clients who have tried it simply eliminate grains and fruit, and increase their fat intake with foods such as nuts, oils, bacon, and avocado. But the popular form of the plan calls for getting 70 to 80 percent of your calories from fat, 20 to 25 percent from protein, and 5 to 10 percent from carbs.

Groups Who May Be Harmed by the Effects of the Keto Diet

Before even trying the keto diet, you’ll want to check with your healthcare team to ensure the approach is right for you. It is for some: For instance, previous research has shown the diet to provide anti-seizure benefits to children with epilepsy, although it’s not given to them for weight loss. There's also a growing body of research that suggests the keto diet may be effective in adults with epilepsy, too.

But people with the following conditions should avoid it, say Gorin and Kristen Mancinelli, RD, who is in private practice in New York City:

Eating Disorders The strict limitations of the diet could increase the risk of compulsive overeating.

Kidney Disease or a History of Kidney Stones The ketogenic diet may increase the risk of kidney stones. In fact, one study suggests 3 to 10 percent of children with epilepsy who are on the diet have kidney stones, compared with one in several thousand in the general population. The diet may also put people with kidney disease at risk, according to the National Institute of Diabetes and Digestive and Kidney Diseases. People with kidney disease must follow a specific diet plan and should work with their healthcare provider to determine what they should be eating.

Diabetes People with type 1 diabetes or those with type 2 diabetes who are on insulin or medication to lower their blood sugar can develop diabetic ketoacidosis, a dangerous condition in which ketones build up in the bloodstream and urine too quickly, making the blood too acidic. And if you have type 2 diabetes and are on medication, a dramatic change in your carbohydrate intake can put your blood sugar levels at risk of dipping to dangerously low levels. Work with your diabetes care team to determine a safe timetable for lowering your carb intake and any medication adjustments that may be required to help keep your blood sugar stable. Those with type 1 diabetes should not follow a keto diet.

High Cholesterol The risk of ingesting too much saturated fat, which can easily happen on a keto diet, can increase LDL (bad) cholesterol and further raise your risk for heart disease. People with the genetic condition familial hypercholesterolemia, which prevents the body from removing LDL cholesterol from the blood, should also avoid the keto diet, according to the National Library of Medicine.

The dietitians caution anyone with an active health condition to consult a doctor before making any drastic dietary change, including those required by a ketogenic diet.

You’ll also want to keep in mind that the keto diet is difficult to follow. In fact, in its 2018 rankings, U.S. News & World Report rated keto second-to-last in Easiest Diets to Follow and Best Diets Overall, and last for Best Diets for Healthy Eating. “People tend to lose weight quickly the first few weeks following a keto diet, and then the rate of weight loss slows and they usually find it difficult to continue to follow the diet’s principles,” Grieger says.

One reason could be that many people find the high-fat regimen onerous over the long term, says Mancinelli, who followed the keto diet for about five months while writing her book The Ketogenic Diet: A Scientifically Proven Approach to Fast, Healthy Weight Loss. “If you can’t eat a lot of fat, you can’t successfully do a ketogenic diet. Many people believe it’s just a low-carb diet where you avoid grains and starch and sugar, but it’s really much more restrictive than that.” For example, depending on your carb limit, you may need to severely limit your veggie intake as well.

That said, keto has been shown to lead to quick weight loss anecdotally and in research, including a review published in February 2014 in the International Journal of Environmental Research and Public Health, which suggested the approach could be useful for treating obesity.

Additionally, a review published in September 2016 in the Journal of Obesity & Eating Disorders suggested that a keto diet can confer benefits to people with type 2 diabetes, such as lowering A1C levels (a measure of the two- to three-month average of glucose in the blood), weight loss, lowering triglycerides (a type of fat in the blood) and LDL cholesterol. Again, if you have diabetes, check with a doctor before starting a keto diet.

Tips for Managing Symptoms of the Keto Flu and Following the Ketogenic Diet Safely

If you’re thinking of trying keto and have cleared it with your healthcare team, be prepared for the aforementioned side effects. As Alley’s experience demonstrates, in the event of dehydration and an imbalance of electrolytes, keto side effects may not only be unpleasant — they could be serious.

1. Talk to Your Doctor About Supplementing With Sodium if Your Blood Pressure Is Normal

Because the production of ketones spurs the kidneys to remove sodium from your body more quickly, you’ll need to make sure you’re getting enough sodium through your food while on keto. If you don’t, your blood pressure may fall to dangerous levels, Dr. Phinney says. The “pins and needles” feeling and heavy limbs that Alley described may be due to a drop in blood pressure, which reduces blood flow to the limbs, he explains.

Phinney advises anyone trying the keto diet who doesn’t have high blood pressure to talk to their doctor about adding more sodium to their diet. The amount will vary depending on your health needs. Just know that the American Heart Association recommends no more than 2.3 gram (g) of salt daily for adults, including what is ingested in food, with an ideal limit of no more than 1.5 g.

By making sure you have enough salt in your diet, “the light-headedness, the dizziness, the fatigue, the heat intolerance, the headache, and constipation, almost always completely resolve,” Phinney says.

2. Drink Lots of Water, Too

Staying hydrated can help with headaches, says Grieger. Furthermore, old-fashioned H2O can help to minimize (though not eliminate) another unpleasant side effect of the keto diet: bad, fruity breath, often called “keto breath.”

3. Don’t Ditch Electrolyte-Rich Vegetables

Getting more electrolytes from vegetables, such as magnesium and potassium, can also help to minimize headaches, says Grieger. Yet many of our favorite vegetables are “starchy” or high in carbohydrates, such as acorn squash, potatoes, sweet potatoes, green peas, and corn, and therefore are avoided by ketogenic dieters. In doing so, these individuals are missing out on key sources of electrolytes. Opt for low-carb, electrolyte-rich fare, such as leafy green vegetables, to replenish these nutrients. Gorin recommends avocado and broccoli, in particular, as potassium sources.

4. Take It Easy

“Usually, I recommend people reduce exercise and avoid high-intensity exercise until their energy levels increase. Getting more sleep or even taking a nap during the day can also help manage energy levels,” says Grieger.

5. Add Fiber-Rich, Low-Carb Foods to Your Diet to Reduce Constipation

Staying hydrated, along with incorporating fiber-rich foods in your diet, such as nonstarchy leafy green vegetables, like broccoli and kale, can also help reduce constipation. “Ketogenic diets are often low in fiber intake due to restrictions on grains, beans, legumes, fruits, and many vegetables,” says Courtney Schuchmann, RD, an outpatient dietitian at The University of Chicago Medical Center in the department of gastroenterology, hepatology, and nutrition services in Illinois. She suggests reaching for foods like flaxseed and chia seeds, which are high in fiber and low in net carbs, and incorporating them into recipes such as chia pudding, pizza crust, salads, or keto-friendly baked goods.

A Final Word on What to Expect From the Keto Flu on the Ketogenic Diet

After your body has transitioned to burning ketones for energy you need, the side effects will likely dissipate. “I felt very energetic. I didn’t have energy dips throughout the day,” says Mancinelli, who says she lost 9 pounds while on the diet.

Still, not everyone is convinced that keto is worth the trouble for people who want to lose weight and keep it off.

“There isn’t good scientific evidence that a keto diet leads to long-term weight loss, and there is a wealth of evidence on the importance of eating whole grains, fruit, vegetables and legumes,” says Grieger, who does not recommend the diet. Indeed, the two “best” diets on U.S. News & World Report’s 2018 list, the Mediterranean diet and the DASH diet, both maximize whole grains, fruit, vegetables and legumes, while minimizing saturated fat, which is in keto-friendly foods such as butter and coconut oil. If you’re looking for a regimen you can stay on for the long-haul, they may be better options for you.

Why the Keto Flu Happens and How to Manage the Symptoms