5 of the worst keto diet side effects

The keto diet can help with weight loss, but at what cost?

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  • In addition to weight loss, there are a few well-known side effects of the keto diet, some of which can be unpleasant.
  • Some side effects of the keto diet are bound to occur, though others only happen when the diet is implemented poorly.
  • The keto diet doesn't have to lead to a host of negative side effects, but anyone considering undertaking the diet over the long term should be especially careful.

The keto diet is often called a fad diet. Make no mistake: it is. But unlike other trendy diets, the keto diet is unique because it actually pushes the body into an alternate, natural metabolic state called ketosis. When this happens, you can reliably expect a few negative side effects, notably those that come with the "keto flu." But other side effects emerge only when people implement the keto diet poorly, typically by failing to eat balanced, nutrient-rich foods as a part of a high-fat, low-carb diet.

Here are a few of the worst side effects of the keto diet, most of which can be avoided with some careful planning.

​Constipation

What's likely to happen when you cut out common sources of fiber from your diet? Constipation. A 2015 study involving children on the keto diet showed that regular constipation was extremely common among participants, affecting about 65 percent of them.

"Many of the richest sources of fiber, like beans, fruit, and whole grains are restricted on the ketogenic diet," registered dietician Edwina Clark told Everyday Health. "As a result, ketogenic eaters miss out on the benefits of fiber-rich diet such as regular laxation and microbiome support. The microbiome has been implicated in everything from immune function to mental health."

Still, the keto diet doesn't need to lead to fiber deficiency: avocados, flaxseed, almonds, pecans and chia seeds can all provide fiber while still keeping you in ketosis — when consumed in the right amounts.

Vitamin deficiency

Any diet that prohibits you from eating many types of fruits, vegetables and other foods is bound to leave you vulnerable to vitamin and mineral deficiencies, and it's for this reason many doctors only advise going on the keto diet over the short term.

"Keto is not a great long-term diet, as it is not a balanced diet," Nancy Rahnama, M.D., M.S., an internal medicine and bariatric specialist, told Reader's Digest. "A diet that is devoid of fruit and vegetables will result in long-term micronutrient deficiencies that can have other consequences. The keto diet can be used for short-term fat loss, as long as it is under medical supervision."

On the keto diet, your body begins to shed fat, water and glycogen, and as this happens you lose key electrolytes, such as sodium, potassium and magnesium. When you're running low on these electrolytes, you might experience headaches or extreme fatigue. These losses are most pronounced during the first few weeks after you enter ketosis, so if you're going to start the keto diet it's best to plan ahead to make sure you get healthy amounts of these electrolytes — and other vitamins and minerals — either through supplements or a thoughtfully-designed meal plan.

​Muscle loss

Some research suggests that the keto diet can lead to the loss of lean body mass, which includes muscle protein.

"Muscle loss on the ketogenic diet is an ongoing area of research," Clark told Everyday Health. "Small studies suggest that people on the ketogenic diet lose muscle even when they continue resistance training. This may be related to the fact that protein alone is less effective for muscle building than protein and carbohydrates together after exercise."

The website sci-fit, which compiled a survey of the research on the keto diet, found:

"We generally see greater lean body mass (LBM) loss in ketogenic diet groups. Note that lean body mass contains water, glycogen, and muscle protein, by definition. It is hard to say with certainty that LBM loss implies greater "dry" muscle protein loss. "Wet" LBM can come and go quickly because it consists of water and glycogen."

In terms of gaining muscle, it seems protein alone doesn't do as well as it does when paired with complex carbs. These carbs don't become part of the muscle fiber, but they do help speed up the process, in part by helping cells regain glycogen — a key source of fuel during exercise.

The 'keto flu'

One of the most immediate side effects of the keto diet is the "keto flu," a suite of symptoms that many experience in the first couple weeks after entering ketosis. Similar to the flu, these symptoms can include fatigue, brain fog, dizziness, vomiting, nausea and stomach pain.

"The keto flu is definitely real," registered dietician Scott Keatley told Everyday Health. "Your body functions really well on carbohydrates — that's what it was designed for. When it switches to fat burning, it becomes less efficient at making energy."

The keto flu — and the accompanying sugar cravings – often leads people to give up the diet and begin scarfing down carbs, but those who stick it out usually report that the symptoms clear up after a few days or a couple weeks.

Kidney damage

Some people inflict damage on their kidneys when they switch to the kidney diet because they eat too much meat and don't drink enough water. This can lead to an increase in uric acid, which is known to cause kidney stones.

"If you're going to do keto, there's a better and a worse way to do it," registered dietician Kim Yawitz told Everyday Health. "Loading your plate with meats, and especially processed meats, may increase your risk for kidney stones and gout... High intake of animal proteins makes your urine more acidic and increases calcium and uric acid levels. This combination makes you more susceptible to kidney stones, while high uric acid can increase your risk for gout."

Of course, a responsible keto diet plan need not result in damage to the kidneys. In addition to monitoring meat consumption, a 2007 study on kidney stone development within young participants on the keto diet found that taking oral potassium citrate tablets seemed to be effective at preventing kidney stones.

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