7 Dangers of Going Keto

The low-carb, high-fat plan promises quick weight loss, but health experts worry about these side effects and complications.


August 29, 2018

The ketogenic diet—also known as “keto”—has become the latest big thing in weight-loss plans, touted recently by celebs like Jenna JamesonMama 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 keto 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-aginganti-inflammatory, and cancer-fighting benefits, as well.

But as a general weight-loss plan, keto is more controversial. Some health experts warn against it entirely, citing unpleasant side effects, health risks, and the diet’s unsustainable nature. Even many keto 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.

The “keto flu”

“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 ketogenic 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 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.

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 keto, 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.

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.”


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, says 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,” she says.

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.

Ketoacidosis has also been reported in people without diabetes who were following low-carb diets, although this complication is quite rare. Symptoms of ketoacidosis include a dry mouth, frequent urination, nausea, bad breath, and breathing difficulties; if you experience these while following the keto diet, check in with a doctor right away.

RELATED: Keto Diet Constipation Is a Legit Issue—Here’s How to Deal

Weight regain

Because the keto diet is so restrictive, health experts say it’s not an appropriate diet to follow long-term. (Even Axe says it’s best done for 30 to 90 days, followed by a more sustainable diet plan.) But the problem with that, says 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,” says 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 back-and-forth weight fluctuations can contribute to disordered eating, Kizer says, or can worsen an already unhealthy relationship with food. “I think this diet appeals to people who have issues with portion control and with binge eating,” she says. “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.”

Less muscle mass, decreased metabolism

Another consequence of keto-related weight changes can be a loss of muscle mass, says Kizer—especially if you’re eating much more fat than protein. “You’ll lose weight, but it might actually be a lot of muscle,” she says, “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, says 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,” she says. “That can have lasting effects on your resting metabolic rate, and on your weight long-term.”

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Increased risk of heart disease and diabetes

Axe says that, when done right, the ketogenic 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 keto 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 some studies suggest that they increase the risk of diabetes. Some have even called it a “cardiologist’s nightmare.”

Just this week, a 25,000-person study presented at the European Society of Cardiology Congress in Munich suggested that people on the lowest-carb diets had the highest risk of dying from cancer, cardiovascular conditions, and all other causes. Another study, published this month in the Lancet, also found that people who followed diets that were low in carbs and high in animal proteins 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,” Axe says: “Lots of vegetables, herbs, spices, and plant-based sources of fat and protein, too.”

The Keto Diet for Athletes: How Low Carb Should You Go?

“A lot of controversy surrounds the traditional ketogenic dieting practice – especially when it comes to performance athletes. Can athletes eat a keto diet?”

The truth is that athletes, especially strength and combat athletes, require enough protein and some carbs if they are going to be able to perform. The goal here isn’t just to get into ketosis. It’s to leverage fat adaptation for increased strength, endurance, and recovery. This is why we see so many athletes crashing and burning when they follow popular low carb recommendations.

But how many grams of protein and carbs can athletes sustain while still being able to benefit from fat adaptation?

To answer this question, we’ll explore one of the most important studies of the benefits of keto-adaptation for athletes over the past few years.

The Study

In 2016, Dr. Jeff Volek and his team published “Metabolic Characteristics of Keto-Adapted Ultra-Endurance Runners1.” This study is important for a few reasons:

  • It’s one of the few randomized control trials using live human subjects that compares low carb athletes with high carb athletes in athletic trials.
  • It’s the only study that compares carbohydrate and fat oxidation between low carb and high carb athletes during performance. We know what fuel sources these athletes were actually using while they were doing the trial.
  • It’s the only study that compares glycogen concentrations pre and post exercise between high carb and low carb athletes.

The population studied was 20 ultra endurance athletes who were active competitors in ultra marathons or triathlons.

For 20 months prior to the trial, 10 of these athletes ate a high carb low fat (HCLF) diet consisting of approximately 60% carbs, 25% fats, and 15% protein, and 10 of these athletes ate a low carb high fat (LCHF) diet consisting of approximately 10% carbs, 70% fat, and 20% protein.

After following their habitual diets they were brought into the lab for a series of trials including a VO2 Max test and a submaximal 3 hour run done at 64% of their VO2 Max. Fat and carbohydrate oxidation rates were measured in both trials using indirect calorimetry.

Muscle glycogen retention was measured before, during, and after the run from muscle biopsies taken from the outer thigh. Blood tests were also taken before, during, and after the 3 hour run.

Here’s what they found:

  • During the VO2 Max test and the submaximal run, the LCHF athletes oxidized fat at twice the rate of the HCLF athletes.
  • During the VO2 Max test, the LCHF athletes had a peak fat oxidation rate at about 70% of their VO2 Max, while the HCLF athletes had a peak fat oxidation rate at about 55% of their VO2 Max. Previously, it was thought that a disadvantage of low carb diets was that athletes would not be able to perform high intensity exercise without switching to glucose. This study demonstrated that fat adapted athletes could burn fat even during intense exercise.
  • The most surprising finding was that the LCHF and the HCLF athletes had similar amounts of glycogen in their muscles before and after the three hour run despite the fact that the LCHF were limiting their dietary carbs habitually for an average of 20 months. This means that the LCHF athletes were actually more efficient at using and retaining glycogen than the carb fueled HCLF athletes.

So why should you, a strength or physique athlete, care about a study involving a bunch of endurance athletes?

You should care because better ability to oxidize fat while retaining glycogen means that fat adaptation is a potential way for you to be able to burn fat efficiently while building muscle, gaining strength, and improving athletic performance.

But How Can I Build Muscle on a Low Carb and Low Protein Diet?

Yeah, yeah, I can hear you now, “my cousin/friend tried a low carb diet for his show and he got weak, tired, flat and sick.”

Normally, when this happens, it’s because athletes are following extremely strict recommendations that aren’t meant for athletes.

If you’re using the keto diet as a therapy for cancer or epilepsy, then you probably need to follow the very high fat with very low protein and very low carb recommendations.

If you’re overweight, sedentary, and don’t have the ability to train with any intensity, then a more strict form of the keto diet may also be necessary.

But this kind of strictness may not be necessary for most athletes.

For example, in Volek’s study, the LCHF athletes weighed an average of 69 kilograms (152 pounds) and were eating an average of 226 grams of fat, 82 grams of carbs, and 139 grams of protein.

Carbohydrate wise, that’s way above the 30-50 gram recommendations that most keto “gurus” are telling people to eat.

Bear in mind, the guy who put this study together was Dr. Jeff Volek, who along with Stephen Phinney, literally wrote the book on low carb dieting for athletes.

In it they address carb flexibility in a single phrase “…some people may need to stay under 30 grams while others can consume as much as 100 grams per day of total carbs and still remain in nutritional ketosis2.”  I would add that, if you’re a healthy individual who’s lifting weights and doing intense exercise, that this number is probably going to be closer to 100 grams.

Protein wise, these athletes were actually eating an average of 2.1 grams of protein per kilogram of bodyweight or close to 1 gram of protein per pound of bodyweight. That’s well above the scientifically recommended protein thresholds for building muscle3.

The point here is that, in practice, a properly formulated low carbohydrate diet for athletes is most certainly not a low protein diet. Of course, if your protein is ridiculously high at 2-3 grams of protein per pound of bodyweight, you definitely won’t be able to adapt. But if it is too low, your performance and ability to build muscle will plummet.