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7 Dangerous Side Effects of the Keto Diet, According to Experts

Is it even safe to eat this way? We asked the experts.
FACT CHECKED BY Kiersten Hickman

For all its popularity as a weight-loss strategy, the keto diet isn't all sunshine, rainbows, and descending digits on the scale. In fact, this high-fat, low-carb eating plan has a bit of a dark side. We're not just talking about a lack of long-term research on the effects of keto on health conditions like diabetes and high cholesterol. In the short-term, you may experience some seriously unpleasant, and even dangerous, side effects of the keto diet.

Watch out for these seven potentially harmful side effects of the keto diet. And for more healthy eating tips, be sure to check out our list of 15 Underrated Weight Loss Tips That Actually Work.

Your thinking might get fuzzy.

keto foods

The ketogenic diet tweaks the body's metabolism into a fat-burning state called ketosis. It does this by (nearly) eliminating carbs and focusing on fats. This can reap major dividends for rapid weight loss, but it can also do a number on your clarity of mind.

"A rapid reduction in carbohydrate intake results in less available glucose for the brain—leading to that feeling of 'brain fog' where you may feel like your thinking and concentration are impaired," says diabetes educator Erin Palinski-Wade, RD, CDE.

If you're about to interview for a job (or, uh, operate heavy machinery), you may want to hold off on starting keto. That said, brain fog will usually wear off after a relatively brief interval.

"The brain is able to use ketones [a byproduct of fat] for energy in the absence of carbohydrate. As the body becomes more efficient at producing ketones, brain fog typically resolves within a week's time," Palinski-Wade says.

Here's What Happens To Your Body When You Stop Eating Carbs.

You may lose muscle mass.

keto dish

A true keto diet isn't just low-carb—it's quite low-protein, too. This means that some of the losses on the scale won't just be body fat. You could lose muscle, too.

"When protein intake dips so low, it's only natural for there to be loss of muscle mass, even with people who exercise on a regular basis," says Molly Kimball, RD, CSSD. "A lot of times someone isn't aware that they've lost muscle mass because they're so excited that the scale is going down."

To keep whittling your waistline—without whittling down your muscles—Kimball encourages a modified keto diet.

"This could look like not going so low-protein by adding foods like fish or Greek yogurt," says Kimball. "You might not achieve ketosis, but that's not necessarily a bad thing. You'll still likely see results."

For a more in-depth look at this popular eating plan, check out our guide to doing keto safely.

You could get all stopped up.

Woman holding plate of keto foods at a table

Unfortunately, dialing down carbs for weight loss means you'll reduce good carbs, like fiber, too. (Yep, fiber is a carb!) Without fiber-rich roughage from fruits, vegetables, beans, and grains, your digestion will likely slow down—way down.

Occasional constipation doesn't tend to present major health problems, but over time it can create harmful issues like abdominal pain, distention, hemorrhoids, and bowel obstruction. Struggling to "go" while on keto? Be sure to drink plenty of water and make your carbs count by choosing high-fiber, lower-carb foods like berries, avocados, and nuts.

If you're not sure, read up on our guide on 9 Warning Signs You're Not Eating Enough Fiber.

You may be at increased risk of kidney stones.

Table of keto diet foods on a table

It's easy to understand how changing your macros on keto could affect your GI tract, but could it interfere with your kidneys, too? In some cases, yes. It all comes down to the uptick in animal products that often accompanies a high-fat diet.

"A high intake of animal proteins can increase both calcium and uric acid levels in urine," says Palinski-Wade. "As these levels rise, so does your risk of developing kidney stones."

If you're already prone to kidney stones, Palinski-Wade recommends consulting with your doctor or dietitian before experimenting with keto. And for those who've never dealt with the particular misery of passing a stone, she has advice, too: "Since this risk can be higher with large amounts of processed meats, it is best to limit your intake of processed meats and high-fat meats and select leaner, unprocessed options such as fish and white meat poultry along with eggs."

It could throw off your blood sugar.

healthy fats keto

Some people with diabetes tout keto as a miracle for stabilizing their blood sugar. There's even some clinical evidence that a low-carb ketogenic eating plan could improve glycemic control after about four months. But Palinski-Wade cautions that keto and diabetes aren't always a good mix, especially for people taking glucose-lowering meds. "If you are on medication to lower blood sugar and you are eating few carbohydrates, there is a risk that blood sugar will drop dangerously low to hypoglycemia."

To avoid the woozy, nauseous feelings of hypoglycemia, it's critical for people with blood sugar issues to develop a diet plan with a healthcare professional. "I would not recommend a keto diet or a very low carbohydrate diet for a person on glucose-lowering medication or insulin unless they work closely with a registered dietitian and/or diabetes educator," says Palinski-Wade.

Meal planning with diabetes? Don't miss these 50 best foods for diabetics!

You might experience mood disturbances.

High protein keto breakfast of bacon and eggs

Food and mood go hand in hand—for better or for worse. (After all, we all know what it is to be "hangry.") As you deprive your body of carbohydrates, mood swings may result.

"When the brain gets less sugar from the blood for energy, this can impact mood and result in feeling irritated, fatigued, and difficulty concentrating," says Palinski-Wade.

Feelings of restriction or the inability to socialize through food on a keto diet might make you feel extra blue. Before diving in, give some thought to how much you're willing to sacrifice to attain keto success.

It could increase your cholesterol.

Many saying no to carbs so he is cutting carbs and choosing a keto diet instead

A typical keto diet involves getting up to 85% of your daily calories from fat. With all that fat, it's not surprising that the diet isn't exactly considered heart-healthy.

"When we look at the long-term effects of a diet chronically lower in fruits and veggies and higher in animal saturated fats, odds are it's not helping heart health," says Kimball. "I'm not saying that the keto diet is inherently bad for cholesterol or heart and vascular health, but elevated cholesterol is a side effect that is not uncommon."

On the other hand, because of the scarcity of long-term research on keto and heart health, it's tough to say how things might play out if you stick to the diet for years. According to Kimball, keto can often improve blood pressure, stabilize blood sugar, and help people maintain a healthy weight—benefits that might outweigh the downsides of higher cholesterol.

"There are just so many factors involved," she says. "It's something that should be managed on an individual basis with your physician or dietitian."

For healthier ways to lose weight this year, check out our list of The Best Healthy Eating Tips For 2021, According to Dietitians.

Sarah Garone, NDTR, CNC
Sarah Garone, NDTR, is a registered nutrition and dietetic technician, and a health, nutrition, and food writer. Read more about Sarah