When You Are in Ketosis Are You Burning Fat Rather Than Muscle?

The trouble with restrictive diets is that they might make bold claims but it's always a little iffy exactly how they're going to affect your body. Take a ketogenic diet for example — keto is hailed for fast and easy fat loss, but could keto cause muscle loss as well?

The keto diet does have its benefits, but because it lacks certain nutrients (not only vitamins and minerals but also carbohydrates and protein) it isn't the best long-term diet choice, especially for people who are living an active lifestyle.

To get a better idea of the link between keto and muscle loss or whether there's any way of building muscle on the keto diet, it's important to look at what a strict keto diet is, what nutritional requirements you need to build muscle, and whether there are any alterations you can make to a keto diet to make it more conducive to athletic performance.

You've heard of low-carb diets before, so you might be wondering what it is about keto that sets it apart from Atkins or South Beach. As the American Academy of Family Physicians explains, the idea behind the keto diet is that it sends the body into ketosis, a metabolic state where the body burns fat for energy.

A more in-depth explanation from the Academy of Nutrition and Dietetics stresses that a ketogenic diet must be ultra-restrictive with carbohydrates. Dieters are supposed to consume fewer than 50 grams of carbohydrates a day. The keto diet encourages getting most of your calories from fat instead.

As with most low-carb diets, this means restricting foods like bread, cereal, rice, oatmeal and other grains. But keto is even more restrictive, meaning that dieters may also have to cut out foods like fruits and vegetables. When the body goes into this metabolic state of ketosis, it's as if the body is starving, and it starts breaking fat down into molecules called ketones to use as energy. Northwestern Medicine notes that a body usually has to go through carbohydrate elimination for a period of about three weeks before the body goes into ketosis.

During these first few weeks, most people go through what is known as the "keto flu," during which time they suffer mood swings and have difficulty focusing. This is because the brain isn't getting the sugar from carbohydrates that it is used to getting.

Despite this, many people who follow keto like it because they lose weight quickly and easily. Experts, such as those with Academy of Nutrition and Dietetics and Northwestern Medicine, explains that these dieters are not only in the metabolic state of ketosis but are also likely consuming fewer calories by eliminating so many foods.

However, the high fat content still keeps them feeling satiated. For some people, the idea of eating high-fat foods like red meats, cheese, butter, fatty fish, nuts and so forth and still managing to lose weight is an appealing one — they don't miss pasta and bread if they can eat plenty of bacon and gouda.

Kristin Kirkpatrick, MS, RD, LD, of the Cleveland Clinic reflects on her experience on the keto diet by explaining that she felt tired and had wild sugar cravings at first, but she later felt great. For her, a sample day on the keto diet looked like this:

• Breakfast: Protein smoothie or eggs with a side of avocado
• Lunch: Salad with lots of dressing or avocado, or zucchini noodles with pesto and grilled salmon
• Dinner: Wild salmon with broccoli, kale or spinach; or hearty mushroom soup with veggies, cream and butter or bone broth; or grass-fed beef on a low-carb wrap with a ton of roasted veggies
• Dessert: Occasionally a no-added-sugar coconut bar
• Snacks: Nuts, dried cheese, jerky or no-added-sugar coconut chips

In addition to being an easy method of weight loss, keto has also been hailed for helping people with diabetes or pre-diabetes control their blood sugar. And before it became popular as a weight-loss or weight-control method, a ketogenic diet was touted for helping patients with epilepsy control their seizures.

But the keto diet is not without its drawbacks. As the Cleveland Clinic's Kirkpatrick notes, there's not a lot of evidence on how a high-fat diet like keto would affect a person's health in the long run, so people could potentially be setting themselves up for heart disease or high cholesterol.

As a means of weight loss, keto isn't foolproof. Northwestern Medicine notes that it's very difficult to sustain because it requires a full commitment. It's not something you do halfheartedly — in order to keep the body in ketosis, a dieter must continue restricting carbs to fewer than 50 grams a day.

The restrictive nature can also have damaging psychological effects, as people feel they are not "allowed" to have certain foods or they refuse to listen to what their body needs. In the long term, this can lead to binging and other problems related to restrictive eating.

Finally, as the Academy of Nutrition and Dietetics notes, keto could be especially bad for people with eating disorders or a history of eating disorders, pancreatic disease, liver conditions, thyroid problems, gallbladder disease or those who have had their gallbladder removed. Furthermore, people on keto can run the risk of being deficient in vitamins and minerals typically found from fruit and vegetable sources, such as vitamin A, C, K and folate.

So keto has its pros and cons — but what's the science behind keto and muscle loss? And is it impossible to be building muscle on a keto diet?

Read more: How to Find the Best Weight-Loss Diet for You

Building muscle is as much about proper nutrition and eating well as it is about working out. Make no mistake, exercise is necessary — as the Academy of Nutrition and Dietetics explains, regular resistance training is what will help you build and maintain muscle. You should do muscle strengthening resistance training like lifting weights, using resistance bands, or doing pushups, pullups and sit-ups at least twice a week.

But on top of this exercise, you need to make sure you are feeding yourself properly to fuel your workouts and to recover from them. And because a balanced diet with protein, carbs and fat is necessary, you should be wary of any diet that heavily restricts or discourages consumption of one of these nutrients.

Don't think that building muscle is all about eating excessive amounts of protein. The Academy of Nutrition and Dietetics notes that you need to get about 10 to 35 percent of your calories from protein, and that more isn't necessarily better.

The Mayo Clinic emphasizes that poor nutrition makes you tired, so you are less likely to achieve peak performance during a workout, more likely to injure yourself and less likely to recover properly from the workout. In fact, your body requires plenty of carbs for high-intensity workouts, so a high-fat, low-carb diet isn't going to benefit your athletic performance.

So how exactly should a person be eating to get the most out of their workout and ultimately get stronger? The Mayo Clinic and the Academy of Nutrition and Dietetics agree that carbs are necessary for fueling your muscles. That's because carbs are the body's preferred source of energy.

The Mayo Clinic points out that people who are already in shape and are doing a light-intensity workout will require about 3 to 5 grams of carbohydrates for every kilogram of body weight — for someone who is 150 pounds, or 68 kilograms, that equates to 200 to 340 grams of carbohydrates, far more than the 50 grams a person is restricted to on keto.

Longer workouts or multiple sessions in one day require even more carbohydrates. If an athlete is working out for more than an hour, they need about 6 to 10 grams of carbs per kilogram of bodyweight. That in-shape 150-pound individual would then need 408 to 680 grams of carbohydrates a day if they're going to fuel a 90-minute or two-hour workout. However, someone who is not a high-performing athlete (i.e. the average person) can adequately fuel themselves on fewer carbs.

Finally, the International Sports Sciences Association notes that low-carb diets aren't the best option for athletes because of their negative effect on muscle anabolism. Restricting glycogen for energy while still engaging in intense physical activity means loss of muscle, and athletes should consume more carbohydrates for energy on days when they do heavy lifting.

Read more: 16 Diet-Friendly Healthful Carbs

While most experts agree that carbohydrates are a necessary nutrient for active individuals and for people trying to build or maintain muscle, keto isn't as bad as starvation. When explaining the idea behind the keto diet, the Academy of Nutrition and Dietetics notes that a total fast or starvation would typically cause the body to start breaking down lean muscle mass for energy. But because the keto diet still provides the body with calories and is not a full fast, it doesn't cause this same muscle breakdown.

People might still struggle building muscle on keto. According to a small study published in July 2018 in the Journal of the International Society of Sports Nutrition that looked at 24 men on an eight-week program, eating keto can decrease fat mass without decreasing lean body mass, but it was not ideal for increasing muscle mass, even in the men who were doing resistance training and eating a caloric surplus.

Although people don't focus on keto's restriction of protein quite as much as they focus on its restriction on carbs, some sources fear keeping protein so low might be harmful for people with increased protein needs.

One of the reasons the AARP discourages the keto diet for older adults is that it doesn't have the protein necessary to build and maintain muscle mass. Because seniors and athletes have increased protein needs, consuming limited calories from protein can be harmful for them. Although keto diets, the AARP says, might mean you burn fat more efficiently, the potential loss of muscle mass is not worth it.

There may still be one way of building muscle on a keto diet. According to Keto Resource, an organization connecting those interested in the keto diet to exchange expert advice, recipes and tips, the best way to gain muscle on a keto diet is to have a high-carb day once or twice a week while aiming to enter ketosis for the other five to six days. This pattern is called a cyclical ketogenic diet, and it emphasizes that having a high-carb day to reload your glycogen stores could benefit athletic performance.

Read more: Which Is the Best Low-Carb Diet: High-Fat or High-Protein?

Ultimately, no two individuals are the same. If restricting your carbs and eating plenty of fats makes you feel better, even during your workouts, then a keto lifestyle might be for you. Talk to your doctor before doing any sort of diet so you can ensure you're getting everything your body needs and not doing yourself any long-term harm.