1. Serious Muscle Loss Is a Possible Side Effect of Keto

“Muscle loss on the ketogenic diet is an ongoing area of research,” says Edwina Clark, RD, a dietitian in private practice in San Francisco. “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.” Meanwhile, according to a small study published in March 2018 in the journal Sports, people following the keto diet for three months lost about the same amount of body fat and had about the same muscle mass changes as people following normal diets. Yet the folks on keto did lose more leg muscle.

“Loss of muscle mass as we age has a number of serious consequences,” notes Clark. “Muscle is metabolically active and helps boost daily energy expenditure and mitigate age-related weight gain.” Losing muscle mass can also decrease functional strength and heighten the risk of falls, notes Clark. Falls are the top cause of death from injury in older populations, according to the Institute of Medicine Division of Health Promotion and Disease Prevention.

2. Keto Can Put Stress on the Kidneys and Possibly Give You Kidney Stones

Kidney stones are a well-noted potential side effect of the ketogenic diet. Researchpublished in the Journal of Child Neurology observed that among children following the keto diet as a treatment for epilepsy, 13 out of 195 subjects developed kidney stones. Children supplementing with potassium citrate in the study noticed a decreased likelihood of kidney stones. Speak with your healthcare practitioner about supplementing if kidney stones are a concern.

“If you’re going to do keto, there’s a better and a worse way to do it,” says Yawitz. “Loading your plate with meats, and especially processed meats, may increase your risk for kidney stones and gout,” which is a painful type of arthritis. “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,” adds Yawitz.

And the ketogenic diet can be dangerous for people with kidney disease, as people with kidney disease need to follow an individualized diet as prescribed by their doctor. According to Davita Kidney Care, people with kidney disease often need to consume a low-protein diet, which may not align with the type of keto you’re following.

3. The Possibility of Low Blood Sugar Can Make Keto Risky for People With Diabetes

Carbohydrates help control blood sugar levels, which are of particular importance for people with diabetes. A study published in May 2018 in the journal Diabetic Medicine shows that while a keto diet may help control HbA1c levels (a two- to three-month average of blood sugar levels), the diet may also cause episodes of hypoglycemia, which is a dangerous drop in blood sugar. Echoing many registered dietitians, the Lincoln, Nebraska–based sports dietitian Angie Asche, RD, says she is “hesitant to recommend a ketogenic diet for individuals with type 1 diabetes.”

The same goes for people with type 2 diabetes. While some preliminary research suggests the keto diet may be safe and effective for certain people with type 2 diabetes, there’s still the risk for low blood sugar, especially for those on insulin, and the keto diet omits certain food groups known to benefit those with this disease. For example, a study published in September 2016 in the journal Nutrients highlights the importance of whole grains for helping to control weight as well as episodes of high blood sugar. Whole grains are off-limits on the ketogenic diet.

The best course of action is to proceed with caution and consult a professional before diving in. “If you have a medical condition or are on medications, you should always consult with your doctor before beginning a ketogenic diet,” Asche says.

4. Because It’s Hard to Follow, Keto Can Lead to Yo-yo Dieting

“Rapid, significant weight loss is a common side effect of the keto diet because of the water losses that occur as carbohydrate stores are depleted,” says Clark. In a study in the American Journal of Clinical Nutrition, obese men following a modified version of the ketogenic diet, with high protein and low carbs, lost about 14 pounds in one month, compared with the control group, which lost about 10 pounds on a high-protein, medium-carb diet.

Jalali says people following the diet have the best chance of keeping the weight off if they stay on it long term. And that’s not always easy to accomplish. The weight may come back if you go back to your regular eating habits. And regaining weight may lead to other negative effects. “Chronic yo-yo dieting appears to increase abdominal fat accumulation and diabetes risk,” notes Clark.

“Studies have shown that the ability to stick to a diet is more important for long-term success than the type of diet that’s followed,” says Yawitz. “Keto is incredibly restrictive and is particularly tough for those who have frequent social engagements or are prone to carb cravings.” The Mediterranean diet allows you to eat carbs, like as many fruits and veggies as you want, along with whole grains. Not to mention, the Mediterranean diet has been linked to a number of other health benefits, including a lower risk of Alzheimer’s and Parkinson’s disease, certain types of cancer, and heart disease, according to the Mayo Clinic.

5. The Ketogenic Diet Can Lead to Dehydration and a Loss of Electrolytes

“Suddenly and drastically reducing carbohydrates sets your body up for a double whammy of sorts,” says Yawitz. “The brain’s favorite fuel is glucose, which is most easily created from carbohydrates. In very-low-carb diets, the brain has to adjust to using ketones from digested fats for energy. To add to this discomfort, your kidneys release more electrolytes as insulin levels fall.” Additionally, your total body water decreases as carbohydrates become depleted on a keto diet, notes Clark. The result? What’s known as the keto flu, which could cause constipation, nausea, headache, fatigue, irritability, cramps, and other symptoms. Don’t fret, though: Many of these symptoms are short term and should last only a few days to weeks. Make sure to drink plenty of water to help your body cope with these symptoms. And call your doctor if symptoms — especially nausea — are prolonged, advises Yawitz.

6. Because Keto Severely Limits Carbs, You May Develop Nutrient Deficiencies 

When carbohydrate intake is low, fiber consumption tends to be low, too. “This doesn’t come as a surprise when you consider fruits, whole grains, and starchy vegetables are decreased in your diet,” says Asche. This can lead to one especially uncomfortable side effect (more on that below).

Another possible nutrient deficiency: potassium, a mineral important for both electrolyte balance and blood pressure control, notes MedlinePlus. “Inadequate intake of potassium is likely when consumption of fruits and starchy vegetables are decreased,” says Asche. She recommends adding lower-carb sources of potassium to the diet, including avocado and spinach — as well as lower-carb sources of fiber, such as chia seeds and flaxseed (be sure to enjoy ground for the best health benefits).

7. Bowel Problems, Such as Constipation, Are Also Common on Keto

Let’s talk about a keto side effect that may not be so sexy: constipation. “Many of the richest sources of fiber, like beans, fruit, and whole grains are restricted on the ketogenic diet,” says Clark. “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.” Indeed, in a long-term study in the Journal of Pediatrics in April 2015, constipation was noted as a very common side effect in children receiving ketogenic diets for epilepsy treatment.

In addition to constipation, diarrhea can crop up as a side effect of the keto diet — especially in the first few weeks of following it. “Some people have difficulty digesting large amounts of dietary fat, which can lead to greasy diarrhea,” notes Yawitz.

Other causes of diarrhea on the keto diet include consuming a diet low in fiber (fiber helps ward off diarrhea by bulking up stool) and eating processed low-carb foods like shakes and bars that may contain sugar alcohols. These sugar alcohols can ferment in the gut and cause gastrointestinal discomfort. Yawitz suggests limiting foods labeled “sugar free” if you’re prone to gas or diarrhea when you eat them. And you may want to gradually adjust your carbs downward and your fats upward. “Also build your diet around [naturally] high-fiber, low-carb foods like avocado and nonstarchy vegetables such as broccoli, cauliflower, and asparagus,” she says. Other keto-friendly ways to get more fiber include chia seeds, almonds, and coconut.

8. As Your Body Adjusts to Ketosis, You’ll Probably Have Bad Breath

Considered a symptom of the keto flu, your breath on this diet often smells fruity at first. This is because acetone is a by-product of ketosis and is eliminated mostly through the lungs and the breath, according to a study in the journal International Journal of Environmental Research and Public Health in February 2014. Acetone is a type of ketone known for having a fruity aroma in smaller concentrations. “It’s hard to say exactly how long it will last as it depends on the person, but it’s common for someone to experience this side effect for a few weeks,” says Asche.

9. Your Period on Keto Might Undergo Some Changes

“Periods may become irregular or completely stop on the keto diet,” says Yawitz. “This is more due to rapid weight loss than the diet itself and happens because of drops in gonadotropin-releasing hormone, follicle-stimulating hormone, luteinizing hormone, estrogen, and progesterone.”

Long-term disruption of menstruation can bring on serious side effects, including low bone density. “This is because estrogen is very important to bone health,” says Yawitz. “Studies have also found prolonged menstrual irregularity to increase risk for cardiovascular disease, depression, anxiety, and sexual dysfunction. It’s important to contact your ob-gyn if your cycles become irregular or if you stop having periods.”

But wait, there’s one loophole. Have polycystic ovary syndrome (PCOS)? Then the keto diet may help regulate your periods. “Women with PCOS have high insulin levels, which cause sex hormone imbalances,” notes Yawitz. In a small study published in the journal Nutrition & Metabolism, subjects with PCOS following a ketogenic diet for six months noted improvements in their menstrual cycles — and a small number of women became pregnant, overcoming previous infertility obstacles. “This study was very small, so we can’t make recommendations for all women with PCOS based on its findings,” says Yawitz. “And really, any diet that leads to weight loss should help in PCOS.”

10. Keto Could Cause Your Blood Sodium to Dip

“When you start the keto diet, you lose sodium and other electrolytes in the urine due to reductions in insulin,” says Yawitz. “This is a major contributor to symptoms of keto flu.” So it’s important to replenish sodium through the diet, especially if you exercise or sweat a lot. “This can help ward off more serious side effects that are seen with long-term sodium deficiencies,” says Yawitz. These include lethargy and confusion — and in extreme cases, seizures, coma, and death, according to the Mayo Clinic.

11. Keto May Lead to High Cholesterol and an Increased Risk for Heart Disease

The ketogenic diet doesn’t put a cap on saturated fat or even trans fats. The latter are fats you should always avoid. Read ingredient labels and avoid any food with partially hydrogenated oils, aka trans fats. These fats heighten your LDL (“bad”) cholesterol levels and lower your HDL (“good”) cholesterol levels. They also raise your risk of heart disease and stroke, according to the American Heart Association.

“Certainly, the quality of fat counts,” says Yawitz. “There’s a big difference nutritionally between bacon and almonds. As much as possible, people set on the keto diet should emphasize plant-based, unsaturated fats like nuts, seeds, olive oil, and avocado, which have even been shown to protect the heart.” If you have high cholesterol or other risk factors for heart disease, you should speak with your doctor before beginning the keto diet. This is because the diet may — but doesn’t have to — include large amounts of saturated fat. Some studies have shown increases in cholesterol and triglycerides in people following the diet, while other research reveals that the keto diet may actually decrease heart disease risk as well as saturated fat intake.