The Not-So-Sexy Side Effects of Keto

By: Abbey Sharp, R.D.

Constipation, headaches, bad breath—these are only some of the surprising and potentially dangerous side effects of the ketogenic diet.

You've seen your Facebook friends' before-and-after photos. You've watched the YouTube videos of people's favorite low-carb snacks. You're ready to go keto. A diet that lets you eat bacon, cheese and avocados all day and drop those stubborn pounds sounds pretty good.

But before you hop on the oh-so-popular keto train, be aware that it's not all weight loss and roses out there. We've compiled some of the most annoying potential side effects of the keto diet to help prepare you for the potential discomfort to come.

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1. Keto Flu

As if starting a diet is bad enough, now you've got to get sick as well? Don't worry, this isn't a real flu. It's just your body's natural response to switching over to using ketones as energy, and is generally characterized by brain fog, headaches, nausea and fatigue. This under-the-weather feeling is related to plummeting electrolyte levels (like sodium, magnesium and potassium) as your kidneys flush out excess water. The good news is that this is likely temporary and can also be mitigated by increasing the fluids and electrolytes in your diet. Drink lots of homemade broths for sodium, eat nut butter for magnesium, and try cooked spinach for potassium to get your electrolyte fix.

2. Bad Breath

It might be best not to plan a first date when you're easing into ketosis. One of the biggest complaints keto dieters (and their partners) have about the diet is related to their embarrassingly bad breath. Some even say their mouths taste metallic. Yummy. One of the ketone bodies (compounds that are created from fatty acid metabolism) is the stinky volatile substance called acetone. Yep, that's the same chemical that gives nail polish its infamous odor. Ketones, like acetone, are released from the body through exhalation, sweating and urination, so smelling like a nail salon is actually a good indication that you're actively in a state of ketosis. The good news is that as your body adapts to running on ketones, this side effect generally will disappear. Until then, you might want to keep your toothbrush handy.

3. Hair Loss

While hair loss isn't inevitable on the keto diet, it's also not uncommon. One small study on teens doing keto found that two out of the 45 participants experienced hair thinning, likely for a few potential reasons. First, most keto dieters are not only restricting carbs, but they are also restricting calories in general, and are losing weight, both of which are related to hair thinning and loss. Other potential reasons for hair loss are vitamin and mineral deficiencies (specifically zinc and biotin), and unfavorable shifts in the microbiome due to the restrictive and high-fat nature of the diet. Ensure you're not cutting calories or protein too low, up your probiotics and consult with your doctor about supplementing with zinc and biotin if necessary.

4. Constipation

One of the biggest health concerns with the ketogenic diet is the lack of carb-rich whole grains, pulses, fruits and starchy vegetables like sweet potatoes, all of which contribute fiber to your diet. Unless you're being really diligent about supersizing your greens, there's a very good chance you're not getting your recommended 25-38 grams of fiber each day. Not surprisingly, many keto dieters experience constipation—and no one likes to feel backed up. We recommend filling your plate with the highest-fiber low-carb foods, like avocado, broccoli, artichokes, Brussels sprouts, chia seeds, flaxseed and almonds, and making sure you're getting enough water and salt.

5. Diarrhea

While constipation is more common, keto dieters are not immune to disruptions at the other end of the digestive spectrum as well. A lot of keto dieters are big fans of adding coconut oil or straight-up MCT oil (a supplement of concentrated medium-chain triglycerides from coconut or palm kernel) to their smoothies and morning Joe. The problem is that while MCTs are a type of fat that our bodies process more quickly—providing fast fuel while also helping stave off weight gain—too much can overload your kidneys and not get processed efficiently. The result? Well, the keto crew calls it "disaster pants," and it's not as funny as it sounds. To avoid diarrhea, go easy on the MCT and other added fats, and make sure you're pairing fats with food and drinking plenty of water.

6. Weight Regain

Dieters praise the keto approach as a way to drop a dress size or two ASAP, but they're often discouraged when they see the weight pile back on after a "cheat" day or when they relax the rules even a bit. The reason why low-carb dieters seem to see such immediate results is that a lot of the initial weight lost is water, since our bodies store every gram of carbs with about 4 grams of H2O. You can see how quickly that weight adds up. For that reason, any quick jump in carb consumption might mean a bump in the reading on the scale. Don't panic. If you find your body rebounding as you attempt a more moderate carb intake, pay attention to portion sizes, focus on quality high-fiber carbs, and always pair them up with sources of protein and healthy fat.

Bottom Line

Nobody said the keto diet was going to be easy or fun, but at least you'll be a bit more prepared for what's to come.

Related:
Ketogenic Diet 101
Should You Try the Keto Diet?
Healthy Recipes to Eat When You're on the Keto Diet