Here's How the Keto Diet Affects Your Ability to Build Muscle

Many people like to argue that the ketogenic diet is an efficient way to build muscle. Your strength will skyrocket, they posit, and you’ll feel less sore and recover faster. Critics of the diet, however, often say the exact opposite: Ketogenic diets limit your ability to train hard, the theory goes. Trying to build muscle without carbs is like Batman patrolling the streets of Gotham without his utility belt. There’s no way, they say, to add muscle while you’re in ketosis.

So who’s right? First, let’s take a look at the science: Back in 2002, researchers from the University of Connecticut looked at how six weeks of low-carb dieting affected body composition in two groups of healthy, normal-weight men. One group switched to a ketogenic diet for six weeks, while the rest continued with their regular diets. The men who went keto gained just over two pounds of muscle. The control group, on the other hand, gained just under one pound. On the face of it, that sounds like a win for the low-carbers; they gained twice as much muscle in the same amount of time.

When you look under the hood at how the study was done, however, there were more than a few problems that limit the conclusions we can draw. For one, there was a big difference in protein intake between the two groups. Subjects on the ketogenic diet ate twice as much protein as those in the control group, which by itself could account for the extra muscle growth.

In an ideal world, both groups would have followed the same training program. But they didn’t. Basically, everyone just carried on doing their own thing, so any differences in muscle growth between the two groups could have come down to a better training program rather than diet alone. More recently, a team of Florida researchers ran a similar study. This time, protein intake was matched between the two groups, and everyone in the study followed the same training program. What happened?

From weeks 1 to 11, the keto group gained roughly twice as much lean mass as subjects on the regular higher-carb diet. Gains in muscle thickness, measured using ultrasound, were also significantly greater in the keto group. On the surface, this research appears to provide strong evidence that keto diets are the way to go if you want to build muscle. But only until you take a closer look at the way the study was done.

The keto group “carbed up” in the final week of the study, which led to a seven-pound gain in lean body mass. In other words, much of the increase in lean tissue came from glycogen (the name given to carbohydrate stored in the body) and water. If you look at the results in the first ten weeks, before the keto group bumped up their carb intake, there was no significant difference in the rate of muscle growth between the two groups.

Even the researchers write that it’s “likely that both groups gained similar amounts of muscle mass throughout the entire study.” When it comes to building muscle, most research shows that ketogenic diets offer no advantage over their higher carb counterparts.

For example, a team of Brazilian researchers took a group of overweight men and women, and got them to train with weights three times a week for eight weeks. Half the subjects were told to restrict their carb intake, while the other half followed a diet that was higher in carbs and lower in fat. Both groups ate a similar amount of protein—roughly 0.7 grams per pound of bodyweight.

There was very little difference in results between the low carb and conventional diet groups. They both got stronger, lost fat, and reduced their waist size. There was also no significant difference in muscle growth—measured with ultrasound at the biceps, triceps and quadriceps— between the two groups.

Similar results were found in a three-month study of men with metabolic syndrome, and a ten-week study of overweight women. Combining resistance training with a ketogenic diet had no beneficial or adverse effects on the preservation of muscle mass during weight loss compared to the same training program paired with a conventional diet.

Ketogenic diets can be useful under certain circumstances for people who know what they’re doing and why they’re doing it. After a period of intelligent experimentation, they seem to do better with fewer carbs in their diet. You may be one of those people. If the diet is working and you’re feeling good, then stick with it.

Most low-carb diets get a lot of things right—the focus is usually on eating simple, wholesome, nutrient-dense foods that tend to fill you up on fewer calories. By almost completely cutting out a major macronutrient from your diet, you’re going a long way towards simplifying your dietary choices.

And your muscles don’t actually need carbs to grow. Lifting weights triggers an increase in muscle protein synthesis, which is the key driving force behind muscle growth. But you don’t need carbs for it to happen. Carbohydrate comes in handy because it helps you put in the work that stimulates muscle growth, not because it makes a direct contribution to growth per se.

Ketogenic diets, however, do have a number of potential downsides: They’re very restrictive, and you have to monitor your carb intake very carefully. When you know you can’t have something, it’s human nature to want it all the more. So if you’re “not allowed” to eat carbs, carbs are exactly what you’re going to want.

What’s more, the low carb approach does tend to leave some people struggling in the gym with low energy levels. They feel tired and mentally fuzzy. If you do a lot of intense exercise, the quality of your workouts may decline.

You don’t have to go full keto to get the benefits of restricting your carb intake. Many people do just fine with a moderate intake of carbs, cutting out the sugary snacks and replacing some of the starchy carbs with fruit and vegetables. But cutting carbs even further leaves them feeling worse rather than better, and they don’t stick with it for very long.

To sum it all up, it’s possible to gain muscle on a ketogenic diet. What’s more, there are several studies out there to show that ketogenic diets do just as well as their higher carb counterparts when it comes to preserving muscle while you lose fat. There’s no compelling evidence, however, to show that ketogenic diets offer any muscle-building benefits that you don’t get with a higher-carb diet that provides adequate amounts of protein.

If you want to get rid of your gut while building some muscle at the same time, a ketogenic diet is a viable option. But if you’re relatively lean, training hard 3 or 4 times a week, and your main goal is to add mass to your frame, there’s little point in being so restrictive. Indeed, a 2018 study shows that a group of resistance-trained men failed to gain any muscle at all after two months of lifting weights on a ketogenic diet.

Christian Finn is a UK-based personal trainer with a masters in exercise science.

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