Keto Side Effects to Know About Before Trying This Year's Buzziest Diet

Yes, keto flu could happen to you.

Steak with fresh salad
Photo: Claudia Totir/Getty Images

You’ve probably heard a thing or two about the keto diet and its purported benefits — weight loss, decreased blood sugar levels, and lower blood pressure among them. From celebrities like Kourtney Kardashian to influencers like Amber Fillerup Clarkwho reportedly shed 50 lbs. by following the diet after pregnancy — the high-fat, low-carb diet has been one of the buzziest diets this year.

The gist of the ketogenic diet (or keto for short)? Followers are required to cut back on added sugars, grains, starchy veggies, and processed foods in order to go into ketosis, which is when the body begins to use fat for fuel. Yeah, so the keto lifestyle is not for the faint of heart, but the reported health benefits do make it hard to ignore.

But if you've been at the diet for a bit, then there's a chance you've also begun to notice a few not-so-pleasant keto side effects, ranging from headaches to diarrhea.

Here's everything you need to know about why the keto diet can lead to certain unpleasant side effects — and how to, hopefully, find relief.

VIDEO: What is the Keto Diet?

It might actually hurt your health in the long term.

This one’s kind of important to know right off the bat: The long-term effects of the keto diet haven’t been studied. While it has proven benefits for certain medical conditions, it is also heavy in red meat which is notoriously unhealthy when consumed in excess. According to a 2019 report by the National Institutes of Health, eating red meat daily triples the levels of a chemical linked to heart disease.

In a 2018 study published in Lancet, a low-carb, high-protein diet was linked to a higher risk of death compared to diets that included whole grain carbs in moderation (as well as plant-derived protein and fat, such as vegetables and nuts). And a 2018 study showed people on a low-carb diet were at the highest risk of dying from cancer, stroke and cardiovascular conditions. Plus, the high-fat component of the diet has been linked to an increased risk of diabetes.

In the long term, keto staples like these could hurt your health, which is probably why this diet is not meant to be done for an extended period of time (experts typically recommend a maximum of 90 days). You can also try a method known as keto (or carb) cycling, which allows you to come in and out of ketosis. This can include upping your carb intake every other week (or in some cases, every few days), depending on your individual needs.

You could end up with “keto flu".

No matter the diet you choose, a drastic change in the way you eat can cause symptoms ranging from headaches to hangryness. But the keto diet comes with its own special set of side effects that have been dubbed the “keto flu.”

Yep, some people report feeling flu-like symptoms — including muscle aches, dizziness, fatigue, nausea, decreased concentration, brain fog, and diarrhea — during the first couple of weeks on keto, says Emmie Satrazemis, R.D., nutrition director at Trifecta.

While it's unclear why exactly it happens, experts say it has to do with the metabolic shift happening when you go into ketosis. “These [symptoms] occur because your body has to go through a transition period as you’re switching from carbs to fat as your primary source of fuel,” Satrazemis says.

The symptoms typically last anywhere from 12 hours to a week, although this timeframe can vary from person to person, according to Jillian Kubala, R.D., owner of Hamptons Clinical Nutrition in Southampton, New York.

You might have headaches, a lack of energy, or feel dizzy.

If you notice you’ve been feeling a little parched while trying the keto diet or notice headaches and dizziness, you’re not alone, Kubala says. Like any low-carb diet, the keto diet can cause the body to shed excess water, depleting you not only of water, but some seriously key minerals, like potassium, magnesium, and sodium. In short, “drink plenty of water,” when on the keto diet, Kubala says.

Satrazemis agrees, adding that helpful supplements include electrolyte drops and sugar-free sports drinks can keep you hydrated and might even help control sugar cravings.

You can also try bumping up your salt and water intake by mixing a bit of salt into a glass of water and sipping on it when you feel headache-y, tired, nauseous, or dizzy. Bone broth or chicken stock can have a similar effect and are, arguably, a tastier option.

Constipation is on the menu.

If the less-than-fiber-rich keto main ingredients (like steak, avocados, and hard cheeses) have you feeling, let’s say, unproductive in the bathroom department, then you might want to change up the types of veggies you are eating, Kubala says. Try dark, leafy greens, broccoli and cauliflower to help reduce constipation. These foods also provide another source of minerals, including potassium and magnesium, that are often lacking in a keto diet.

It could hurt your workouts.

Since you're cutting out carbs, don’t be surprised if going keto initially zaps your energy levels and negatively impacts your workouts, Satrazemis says.

“It may require some adjustments if you typically participate in high-intensity training and heavy weight lifting,” she says, adding that your plummeting energy levels might never improve while on a keto diet.

You can help your less-than-enviable mojo by getting more sleep, Satrazemis says, a measure that studies show can also help curb cravings and overall appetite (and keto-related cravings can be particularly challenging).

If you're really strugging in the energy department, Satrazemis suggests increasing your carbs just a bit to see if it helps. “If you “This works particularly well around the times you are working out or more active in general.”

You can gain weight.

If you do want to give keto a shot, then Satrazemis says calorie control and choosing the right foods still matters. Yes, that means the diet is not “a license to eat as much bacon and cheese as you’d like,” she says.

“In order to promote fat loss and be successful, it is key to learn how many calories you need to eat per day for weight loss and stick to this amount consistently,” she says. “And tracking your daily food intake is one of the best ways to ensure you are hitting your carb goals and calorie needs.” (You can keep your calories in check by using a keto calculator like the ones offered by Kiss My Keto and KetoVale.)

Weight gain could have something to do with the high-fat component of the keto diet, an aspect that has been shown to increase estrogen production — a known culprit behind weight gain — when studied in mice.

You might be stressed out, or experience mood swings.

There is also research that points at low-carb diets causing a jump in cortisol (aka the stress hormone), which can lead you to feel more irritable and even lead to low libido. Plus, simply being on any restrictive diet has the potential to mess with your mood, Kubala says.

Overall, if you’re discovering that the keto diet means more misery than results for you, then it just might not be the diet for you, Kubala says.

The good news? “Simply following a diet rich in whole, unprocessed foods is the best way to improve health and boost weight loss (if desired) in most people,” Kubala says.

Which means you can have your (healthy, whole grain) carbs and skip keto flu, too.

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