What Really Happens to Your Body When You Bounce In and Out of Ketosis

If you're intrigued by the keto diet — or are currently following the high-fat, low-carb eating regimen — you're likely very aware that this diet is super restrictive, making it one of the most challenging diets to follow. But what exactly happens when you break the keto diet's guidelines?

This is one diet that doesn't lend itself to falling off the bandwagon for an evening or weekend. We spoke to experts to uncover how your body reacts if you stray from the diet's rules, bouncing in and out of ketosis as a result.

First, a quick refresher on how the keto diet works, and why it can be effective: The goal of the ketogenic diet is to send your body into a state of ketosis, which is optimal for fat-burning, explains Jeannine McGown, a registered dietitian and health mentor for EHE Health.

But what exactly happens during ketosis? Since you're severely limiting carbohydrates, the cells in your body can't use blood sugar drawn from these carbs as the body's primary energy source, per Harvard Health Publishing. Instead, after burning the carbs your body stores as glycogen, it burns stored fat for energy, which produces a byproduct called ketones. This process is known as ketosis.

"During ketosis, the majority of the body's cells will use ketones to fuel the body with energy until carbohydrates are consumed," McGown says.

It's not easy to achieve ketosis.

Most people up their fat and protein intake while decreasing carbs and almost entirely cutting out sugars, McGown says. On a typical keto diet, just 5 to 10 percent of what you eat will be carbohydrates, according to the Harvard T.H. Chan School of Public Health. That means if you're eating 2,000 calories a day, you'll only have 40 grams of carbohydrates a day — that is, less than a regular-sized bagel, which contains 55 grams of carbs, per the USDA, for the entire day.

It can take upwards of a week of carefully monitoring your meals to hit ketosis.

And, when you lapse — maybe having a sugar-filled dessert one night, sneaking a bun to accompany your burger or otherwise shifting your macros from what's required in the keto diet — you'll exit ketosis. When this happens, you may experience some unpleasant side effects.

Here's what to expect throughout your body when you bounce in and out of ketosis.

Typically, the brain gets its energy from glucose, which comes from carbs. Once you trim carbs from your body, the brain eventually turns to ketones instead. It can take a bit of time for your body to get accustomed to the new energy source. That's one of the contributing factors to "keto flu" or the cluster of flu-like symptoms, including brain fog, that can hit when you first get on the diet.

Once that transition is over, many people report a sense of mental clarity while in a state of ketosis, McGown says. You might feel like a total rockstar, zipping through deadlines and crushing your to-do list.

But when you're out of ketosis, those feel-good, productive vibes may become sluggish once again. This is because your body starts to source its energy from glucose again, which creates undeniable highs and lows.

One of the reasons so many people swear by keto is that it solves one of the most significant issues dieters have: hunger. People who are in ketosis feel less hunger, and have less of a desire to eat, per a January 2015 meta-analysis in ‌Obesity Reviews‌.

But while you may feel less hungry, transitioning into the keto state isn't very pleasant. Insulin and blood glucose levels fall, which causes people to feel tired and cranky, McGown says.

And, once you stop following the diet, your hunger will return. After all that deprivation, it can be tempting to reach for the restricted foods, like sweets and other forms of simple carbs. Not only will this undo any potential weight loss you had while following the keto diet, but these types of foods aren't very good for your overall health.

The key once your hunger returns is to opt for good-for-you carbs — like oats, beans and whole fruit, not donuts — and avoid sugar bombs, per the Cleveland Clinic. Basically, even though you're no longer following the keto diet, you'll still want to eat healthy foods.

A byproduct of drastically cutting down on carbs is limiting your fiber intake. When it comes to your GI system, that's bad news: Fiber helps ease constipation, and fiber-rich foods also often contain prebiotics, which in turn help fuel probiotics, the good-for-you bacteria that reside in your gut, per the Academy of Nutrition and Dietetics.

Eating low-carb, high-fiber options like nuts and avocado can help while you're in ketosis, but when you go off the keto guidelines, you may find yourself eating all sorts of fiber-rich foods: whole grains, fruits, vegetables and so on. While these are healthy, good-for-you foods, the sudden influx of fiber-rich food could cause tummy issues (think: gas, bloating and cramps) since your system grew used to not having this nutrient, McGown says.

To ease symptoms, take it slowly when it comes to adding fiber back in your diet if you're taking a break from keto guidelines. A gradual approach with fiber will reduce unpleasant GI symptoms such as gas and diarrhea, per the U.S. National Library of Medicine (NLM).

Often the weight loss from this diet simply isn't sustainable, says registered dietitian nutritionist Erica Christ. In fact, many people see a rapid increase in weight ‌gain‌ when they return to regular eating habits again, she says.

When you first begin the keto diet, you may experience some weight loss that's due to water weight, per the MD Anderson Cancer Center. This is due to the body using up those glycogen stores. Glycogen holds a fair amount of water, so once carbohydrates are reintroduced, that water weight will return.

And it's not just water weight that people gain after they break from the keto diet guidelines. In her experience, Christ says, "those who have lost weight on a ketogenic diet actually gain back more than what they originally lost."

Bouncing in and out of a keto diet can also be called "keto-cycling," but only if you follow a specific regimen.

This routine is when someone follows keto for a set number of days, then introduces carbs for a shorter time, says Vincent Pedre, MD, board-certified internist and the founder of Happy Gut Life.

Typically, people who are keto-cycling have five to six days on the keto diet, followed by one to two days off of it, he says. "The latter is called 're-feeding' days because they are meant to replenish your body's glycogen stores — the source of glucose for high-intensity exercise," Dr. Pedre says.

Athletes often use this practice as a means to enhance muscle growth and exercise performance. But for everyday people who aren't competing at a national or international level, keto-cycling is an intentional way to work toward metabolic flexibility.

According to Dr. Pedre, this means the ability to cycle complex carbs to replenish your glycogen deficiency without affecting the keto state. "This can only happen once your body is used to burning fat for fuel. Usually, it takes at least two weeks to get to that keto state," he says.

During those days off from the keto diet, the amount of carbs you're eating may vary depending on your fitness level and goals, but they should be complex carbs. The break allows you to have whole grains and fiber- and carb-rich foods like beans. This isn't a moment to sneak in sweets, simple carbs or other unhealthy foods.

And, while keto-cycling may seem tempting, since you'll great a break from a truly restrictive diet, be aware more research is needed to back up the potential benefits of this diet modification.