

Experts break down the ketogenic diet and explain common keto diet side effects.

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The keto diet has become the most buzzed about and controversial eating plans out there, with celebrities, dietitians, and even doctors warring over different sides of the fence. While Halle Berry, Vanessa Hudgens, and Al Roker are fans and have said that the diet has helped them improve their health, trainer Jillian Michaels says the high-fat, low-carb diet is "terrible for your health, immunity, longevity, and anti-aging." So what gives? To help you decide whether the diet is right for you, we break down everything you need to know about the keto diet, including some gnarly keto diet side effects and the infamous keto flu.

These celebrities swear by the keto diet. Here's why:

What is the keto diet?

Originally started in the 1920s to help treat people with epilepsy, the keto diet is a high-fat, low-carb eating plan that emphasizes weight loss through burning fat. By eating foods high in fat and limiting carbs, your body can safely hit ketosis, a state in which your body uses fat—in the form of ketones—as fuel instead of carbohydrates. Because your body's preferred fuel source is carbs, it will always turn to those first. But if you eat fewer carbs, your body will burn through them faster and start breaking down fat for energy.

A strict keto diet requires as little as 20 grams of net carbs per day in order to sustain weight loss. If that sounds restrictive, that's because it is. 20 grams is less carbs than what you'd find in one medium-sized apple, for perspective. On the keto diet, 80 percent of the diet is comprised of fat, 15 percent is protein, and a mere 5 percent of calories come from carbohydrates. Other low-carb diets calls for 20 to 60 grams of carbs a day, while the average person's diet is comprised of 55 percent carbohydrates, 30 percent fat, and 15 percent protein.

Does the keto diet work?

The keto diet is known for helping people lose a few pounds very quickly, says Becky Kerkenbush, RD, a clinical dietitian at Watertown Regional Medical Center. Carbs hold on to more water than protein or fat, so when you stop eating them, your body releases all that extra water weight by making you pee more. As a result, the scale might read a few pounds lower, and you may look a bit leaner. And because you're eating high-fat foods, you'll feel fuller longer and curb cravings for sweets.


But all those studies were very small, and not all research on the keto diet is as promising. One American Society for Clinical Nutrition May 2006 study of 20 participants found that those on the diet didn’t lose more weight than those on a non-keto diet. But they did have fouler moods and higher levels of inflammation, which has been linked to a variety of conditions, including heart disease and cancer. Another thing to consider is that studies that show the benefits of following low-carb diets don't spell out whether they're as low carb as the keto diet.

What are the downsides of following the keto diet?

As we mentioned, there's not a lot of research and science supporting the weight-loss benefits of following the keto diet. In fact, following a restrictive low-carb diet, like keto, can be detrimental to your health. A February 2019 study from Nutrients suggests that following a low-carb, high-fat diet, similar to keto, can damage blood vessels and increase risk for type 2 diabetes and heart disease.

Moreover, an August 2018 study from The Lancet, which followed more than 15,000 people in the U.S., found that those who limited their carb intake (less than 30 percent of their daily calories from carbs) would live an average of four years less than someone who eats a moderate amount of carbs (5o to 55 percent of their daily calories from carbs). What's more, the keto diet might not be as effective for improving sports performance as many athletes think it can. A February 2017 study in Nutrition & Metabolism, which followed 42 athletes, found that a restrictive ketogenic diet can actually negatively impact physical performance, causing faster exhaustion.

Keto diet results

Some people really do lose weight.



But for others, the keto diet doesn't work.


How to start the keto diet

Keto dieters eat mostly meat, healthy fats, and and non-starchy vegetables like leafy greens. And...that's pretty much it.


Keto diet foods list

Best keto foods

  • Fats: Olive oil, coconut oil, butter, avocado oil, MCT oil, avocados
  • Proteins: Beef, poultry, eggs, fish

    Keto foods to eat sparingly

    • Full-fat dairy: Milk, cheese, yogurt
    • Medium-starchy veggies: Carrots, beets, parsnips, peas, artichokes, potatoes
    • Legumes: Beans, chickpeas, lentils, peanuts
    • Nuts and seeds: Almonds, cashews, walnuts, pumpkin seeds, sunflower seeds
    • Fruits: Berries, bananas, melons

      Keto foods to avoid

      • All types of sugar: Including honey, agave, maple syrup
      • Grains: Wheat, oats, all types of rice, corn
      • All foods made with flour: Breads, pastas
      • Processed foods: Anything that comes in a bag or a box

        Despite how limiting the keto diet is, diet experts say that with careful planning, you can still fit in all of your vital vitamins and minerals. However, to play it safe, you should work with a dietitian to come up with a well-rounded keto meal plan to avoid nutrition deficiencies.

        Keto diet side effects you should know about

        Cutting down on carbs (way down) can really mess with your body—at least at first. Here are all the potential keto side effects that could happen:

        Increased thirst

        Carbs hold fluids in your body (think of how bread soaks up water compared to a chicken breast), so when you cut back on the nutrient, extra water is excreted in your urine. This makes it vital for those on the keto diet to stay hydrated.

        Increased risk of kidney stones



        “You might find that you fatigue more quickly or that exercise seems harder than normal,” says Seattle-based nutritionist and Academy of Nutrition and Dietetics spokesperson Ginger Hultin, MS, RDN.

        Cholesterol issues


        Keto breath

        As we mentioned earlier, when your body enters ketosis, it starts to store fat in the form of ketones. This includes acetone, the same chemical in nail polish remover, which can cause "keto breath," a nasty, metallic taste in your mouth. Don't worry, though, it's a side effect that will pass as you adjust to the diet.


        Because you're limiting your carb intake significantly, that means you're eating less vegetables, fruits, whole grains, and beans. Pair that with the fact that you lose more water weight, and you've got the recipe for constipation. But you can improve your digestion by eating more fiber-rich, keto-approved foods like avocados, nuts, and non-starchy veggies.

        Keto diarrhea

        In the same vein, whenever you drastically change your diet, it can cause some digestive distress. Filling your plate with mostly fat can cause diarrhea. Moreover, some keto-friendly snacks use artificial sweeteners and sugar alcohols that can upset your stomach, so be sure to avoid those if you're sensitive to them.

        The "keto flu"

        A cluster of symptoms called the keto flu typically kicks in a day or two after starting the keto diet, says Mancinelli, and come on as your body goes through carb withdrawal.

        Is the keto diet safe?




        Bottom line: Talk to a registered dietitian to make sure the keto diet is right for you

        You’ve heard this line before, but really: Check with a doctor before trying the keto diet and ask for a recommendation for a registered dietitian. A registered dietitian will be able to better advise on the pros and cons of following the diet, as well as offer tips on how to properly follow it. The keto diet is an extreme eating regimen that could be harmful to certain people. It may not be right for those with heart disease, or metabolic diseases like type 1 diabetes or type 2 diabetes not controlled by insulin.

        Although experts like Kizer have concerns about the keto diet, if you’re set on trying it, a doctor can help you determine if it is right for you and provide you with tools and information for accurately tracking your ketosis status.


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