When you set out to lose weight, you expect to give up a few of your favorite foods. Donuts? So long. Ice cream? Goodbye. Forgoing grains, fruits, and any sugars, real and artificial, is a whole new level of self-restraint, though.
But it’s par for the course for the ketogenic diet, a high-fat, low-carb eating plan attracting a lot of attention these days. “The keto diet is a calorie-reduced diet plan that focuses on eating most of your calories from fat, a moderate amount of protein, and only 20 or so grams of carbs per day—that’s about the amount in one apple,” says Julie Upton, R.D., a registered dietitian and co-founder of Appetite for Health.
Translation: You’ll basically avoid any food that’s high in carbs. Besides the obvious ones like pasta and dessert, this means you should also skip all fruits (except berries), low-fat dairy, snack/protein bars, alcohol, and sugary condiments and sauces.
As a result, the keto diet can cause some serious changes in your body, both positive and negative. From fatigue to muscle cramps, we talked to R.D.s to figure out what to expect.
RAPID WEIGHT LOSS
Basically, the keto diet works by changing your body’s primary fuel source. “When you eat foods containing carbs, your body stores the excess carbs in your muscles as glycogen to use as energy, along with some extra fluid,” explains Victoria Lindsay, R.D., a registered dietitian in Washington, DC.
Then, when you greatly restrict your carb intake, your body dips into your muscles’ glycogen stores for energy. When those are used up, you lose the fluid that was stored along with the glycogen as well. That leads to pretty rapid fat loss at first—even though it’s mostly “water weight,” Lindsay says.
AND MUSCLE LOSS
As you continue to eat this way, your body will enter into ketosis, when you start burning stored fats as fuel, leading to further weight loss. But while you’re losing fat tissue, you will usually lose some muscle tissue as well. “This is because carbohydrate plays a major role in muscle synthesis,” Lindsay says. “While protein gets all the credit for building and repairing muscles, numerous studies have shown that adding in carbohydrate along with protein after a workout results in more muscle growth and better recovery.”
What’s more: If you aren’t eating enough calories, your body will respond by breaking down muscle tissue—not good! “Muscle tissue helps keep our metabolisms revved and our bodies strong and healthy,” Lindsay explains.
LOW ENERGY LEVELS
Feeling lethargic? It’s not all in your head. “As your body adjusts to this switch in fuel sources, it won’t be as efficient at tapping into its energy sources, causing fatigue,” explains Mike Roussell, Ph.D., professor of nutrition at Penn State University and author of The MetaShred Diet.
Another source of initial fatigue is calorie restriction, so when you’re starting a very low-carb diet, make sure you’re eating enough calories at first, he recommends. “It just takes some time for your body to adapt to this new physiological ‘normal.’”
Another unfortunate cost of rapid weight loss: The dreaded “keto flu,” which often crops up in the first few weeks. You may experience headaches, have trouble focusing, feel nauseous, have trouble sleeping, and more.
Pro tip: Eat a relatively low-carb diet for a couple weeks before fully committing to keto, Upton suggests, which can help your body prepare for ketosis. Either way, the “keto flu” should only last a few days.
Another side effect of a keto diet is digestive distress—think bloating, gas, and constipation. The culprit: You may not get enough fiber in your diet when you’re avoiding foods like fruits, (starchy) veggies, whole grains, and legumes, Lindsay says.
To combat these issues, try to stay active (do low-intensity workouts if your energy is flagging) and make sure you’re drinking plenty of fluids. Roussell also recommends taking a fiber supplement and eating more high-fiber veggies.
“Some people might experience some initial carbohydrate cravings due to a blood sugar response from a lack of carbs,” Lindsay says. “Others may find themselves wanting to eat something they aren’t allowed to have simply because it’s so-called ‘forbidden.’”
The good news: These cravings tend to go away or improve after a few weeks, although it does depend on the person, Lindsay adds. Even better news: Some people may not experience hunger pangs or cravings at all, since large amounts of proteins and fats tend to be very satiating.
When your body creates ketones, it excretes them in a variety of ways, Roussell says. One way is through the lungs. The most readily excreted ketone is acetone, which has a “fruity” taste and is the main culprit of bad breath on a very low-carb diet.
Sugar-free breath mints and flavored water (and of course, your toothbrush!) are your BFFs in this case.
The hormone insulin stimulates your kidneys to retain sodium, so when you go on a very low-carb diet your insulin levels will be very low, Roussell explains. Insulin is thus no longer stimulating your kidneys to retain sodium, which can lead to leg cramps. You’re also not eating much fruit (if any), which contains potassium and other nutrients which can help to mitigate cramps.
Your body should eventually adjust to the lower sodium levels, but you can also ask your doctor about taking supplements to help mitigate these issues, he says.