|November 1, 2023

7 Keto Side Effects and What to Do About Them

By Molly Apel
Reviewed by Emily Gonzalez, ND for Scientific Accuracy

The keto diet is safe for almost everyone, but some side effects are red flags that you should revisit your eating habits.
The shift in your body’s metabolism can cause brain fog, trouble sleeping and flu-like symptoms like muscle cramps.
Read on to learn the small but important shifts in your diet that can help alleviate these keto side effects.

You may have heard that the ketogenic diet is easy. Just eat steak, avocado and dark chocolate, and easy-peasy—you’re burning fat. Some people start the keto diet for its weight loss benefits, but eating a low-carb, high-fat diet doesn’t work for everyone. Your body might give you some negative feedback, and when it does, there are ways to help your body adjust during the first week of keto and beyond. If you start to notice keto side effects, try these tips.

As with any diet plan, talk to your doctor to determine what’s best for you.

What are keto side effects?

The keto diet causes your body to shift where it sources energy—from carbohydrates to fat. Once you enter a state of ketosis, you’ll experience fat-burning health benefits like fewer cravings, lasting energy and more brain power.

But until you get there, it’s common to experience certain side effects that can make you feel pretty lousy (aka the keto flu). Other side effects can sneak up on you after following keto for a while, like trouble sleeping and digestive issues.

Here are some common keto diet side effects and tips to feel better. As with all lifestyle changes, watch out for the small signs that things aren’t working, and be ready to make adjustments. Keeping a food journal is a good way to track your progress. Write down what you eat, whether you notice any side effects you might experience and when. This way, you can go back and connect food changes with reactions.

1. Dehydration and muscle cramps

Storing carbs takes a lot of water, and storing fat doesn’t. On the keto diet, you store less water. Once you’re in ketosis, your kidneys actively expel sodium instead of holding onto it. That means it’s easy to get dehydrated eating keto, especially during the first few weeks. If you get dehydrated and deplete your electrolytes, your muscles might start cramping, too.

Magnesium, sodium and potassium, your body’s three main electrolytes, are essential keto supplements. Make sure to drink plenty of water. This is particularly important if you work out while on keto. Staying hydrated will also help you avoid symptoms of the keto flu.

2. Low energy

Your body’s preferred source of energy is glucose from carbohydrates. During the first week or so of keto, fatigue is a common side effect because you’re cutting your carb intake. Your body’s go-to energy source is suddenly dwindling. Once you switch to fat-burning, your energy should level out—but even after you’ve entered a state of ketosis, your body’s energy needs can change. This is especially true if you become highly active or stressed (or both!).

If you’re dealing with energy slumps that you just can’t shake, experiment with cyclical keto. On this style of eating, you periodically bump up your carb intake and eat approximately 150 grams of net carbs one to two days per week. Feeling better? A lower-carb diet might be a better option for you than the standard keto diet.

3. Trouble sleeping

There isn’t a lot of research on keto and sleep problems, but some people report the keto side effect of waking up in the middle of the night. If you find you have trouble sleeping on keto, you may be better off eating some high-quality carbs at night.

Eat a very small amount of carbs before bed, such as a teaspoon of raw honey. This can help give your body the carbs it needs to get through the night and let you to sleep. If the sweet stuff isn’t cutting it, try taking magnesium before bed to help your body relax. You can find magnesium citrate (plus a science-backed blend of ingredients to reduce stress) in Bulletproof Zen Mode.

4. Constipation

If you’re eating fewer than 20 grams of net carbs a day, it can be hard to get enough fiber. Fiber intake that’s lower than the recommended daily intake can cause some serious backup. Yes, that’s code for constipation.

On a low-carb diet, most of your carb intake should come from leafy, colorful veggies that are high in fiber, such as spinach, collard greens and brussels sprouts. And to support gut health, add a scoop of keto-friendly Bulletproof InnerFuel Prebiotic to your favorite drinks. Prebiotics feed good gut bacteria and support healthy digestion.

5. Diarrhea

If you’re consuming too much fat, too soon, your body won’t be able to break it down. Digestive distress is a common side effect when you start a high-fat, low-carb diet, and you may experience bloating, cramping or diarrhea.

Everyone has different levels of fat tolerance, so you’ll have to experiment to find what works for you (especially if you’re used to eating low-fat foods). That’s why it’s a good idea to start slowly with a high-fat diet.

For example, if you want to blend up a cup of keto-friendly Bulletproof Coffee but you’re new to MCT oil, start with just a teaspoon of Bulletproof Brain Octane C8 MCT Oil and grass-fed butter. Build up to a full serving as your body adjusts to having more quality fats in your diet.

Since you may not be properly digesting fats, ask your doctor about supplementing with a digestive enzyme. Two good ones to try are lipase, because it’s an enzyme that digests fat in the body, or hydrochloric acid (HCL), because it helps increase stomach acid and support digestion.

And of course, make sure to drink plenty of water. Frequent urination means the keto diet is working.

6. Brain fog

Keto might leave you feeling foggy or dazed. Many people experience a bit of a mental slump at the start of their keto diet. This is usually a short-term side-effect that’s the result of low blood sugar during the span of time it takes your body to adjust to burning fat instead of carbs.

Give your brain some energy it can use immediately. Gradually start supplementing with Brain Octane C8 MCT Oil. It contains pure C8 MCTs (medium-chain-triglycerides), the most ketogenic MCT oil. These saturated fatty acids deliver fast energy to your brain in the form of ketones. That means Brain Octane C8 MCT Oil helps keep your mind sharp, especially as it adapts to keto.

Since it can take a little bit of time for your digestive system to get used to fats like MCT oils, start with 1 teaspoon at a time and work your way up from there. Not sure how to add MCT oil to your routine? Try these 33 MCT Oil Recipes That Can Fit Any Diet.

7. Keto rash

A very small number of unlucky people who try keto can develop an itchy, red rash on the back, chest, neck or armpit area. Keto rash, also called prurigo pigmentosa (PP), is not a common side effect. The exact causes still aren’t understood, but one study suggests it’s due to a shift in hormones, gut bacteria or exposure to allergens as potential triggers.

If it happens, talk to your doctor. Some people suggest upping your carb intake, avoiding tight clothes and scented products and supporting your skin with anti-inflammatory foods.

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