As muscle and fat tissue are completely different substances on a molecular level, it is not possible to turn one into the other. However, it is possible to lose muscle while gaining fat and vice versa which gives the appearance that one is turning into the other.

Though I know what’s meant by the question, it’s not a good question. It’s sort of like asking if you can turn a piece of wood into a piece of metal. They’re completely different materials at the molecular level and the process for milling lumber is entirely different from the process of steelmaking.

Interestingly you could burn wood and generate heat that’s then used in the process of making metal. Similarly, your body can get energy from your fat stores that can then be used in weight training that stimulates muscle growth. In fact that’s the entire premise of the Keto (ketogenic) Diet. But, “no”, you can’t turn fat into muscle or muscle into fat – not directly.

Muscles are built with proteins and fat cells enlarge as excess fatty acids and sugars are stored. Different building blocks; different processes.

What Happens to Muscle when We Get Fat?

If you’ve seen the movie Avengers: Endgame, you saw “Fat Thor”. It was a hilarious transformation in one sense but in another his “big” change is too close to home for many guys to be too funny. Here you have the normally ripped “God of Thunder” ripping out of his pants because of an ever expanding waistline. Where did those muscles go? For the most part they were just under the fat. Thor was still strong as ever and, if it were real life, he may be even stronger. Bottom line, the muscle didn’t turn into fat; the fat just got stacked on top of the muscle.

When our bodies “gain” or “lose” weight that generally means when we gain fat we may also be gaining muscle. Likewise, when we lose fat we may be losing muscle too.

A friend of mine (in his 50’s) decided to clean up his diet a few years ago and he lost about 80 pounds. He did it mostly through cutting out sugar and junk food and adding some cardio workouts to his day. Though he was excited about the fat loss he was shocked to discover that he’d lost a lot of his strength too! My guess is that his strength loss came because of “how” he cut his calories. He didn’t eat enough protein and he didn’t do any weight training. His weight loss plan didn’t factor in or compensate for strength and muscle loss.

This is pretty typical on both sides. For many real men who continue to overeat as they age while still weight training, with fat gain comes some strength gains. This is especially true if that fat gain is a product of a high protein diet, i.e. steaks, burgers, chicken fingers, etc. They’re not good for you but they do have a lot of protein! For my friend, with fat loss came some strength loss. This is pretty common too if in your dieting you cut out fatty meats and don’t add back the protein with lean protein sources.

How can I build muscle without getting fat?

For guys over 40 it’s challenging but not impossible. Staying lean in general is difficult for most of us as we age, especially if we’ve always been on the heavy side. But in order to gain muscle, in general, you should do 3 things:

  1. Eat a healthy calorie surplus (that is, more calories than you need to merely maintain your current weight at your current level of activity). It doesn’t have to be a huge surplus – maybe 200 calories of lean meats, fruits, vegetables, and nuts.
  2. Eat enough protein. What “enough” means has and is being debated furiously. I highly recommend reading this article from Menno Henselmans which cites several studies and their results regarding protein intake. According to those studies, in order to preserve muscle and/or gain muscle mass, consuming .7 to .8 grams of protein a day per pound of bodyweight should be plenty. That means for a 200lbs man, he needs to eat 140g to 160g of protein per day. I can verify this with my own training – it’s enough! Assuming you’re training naturally, eating more protein than that simply won’t make much of a difference.
  3. Train with weights with progressively increase intensity and effort (man, that’s a loaded statement!) There’s much more to explain here but in general you should strive to gradually increase the amount of weight you lift over time either by increasing the weight or the number of reps and sets you perform per exercise.

How can I lose fat without losing muscle?

Surprisingly, by doing numbers 2 & 3 listed above (enough protein + progressive weight training) you’ll get good results losing fat while maintaining muscle but instead of eating a calorie surplus you should eat at a healthy calorie deficit. Depending on how much you need to lose and how aggressively you want to lose fat you can create a daily calorie deficit of varying amounts. 200 a day? 500 a day? 1,000 a day? Again, it really just depends on how much you’re currently eating and how much you need to lose.

It’s important to figure out your maintenance calories so you know what kind of deficit you’re creating. Be sure to read the article Find Your Maintenance Calories Through Experimentation. This article lays out the process I used to find mine and it’s worked really well. In any case, it’s usually best not to eat less than 1,500 calories a day if you’re training with weights or doing high intensity cardio. As always, check with your doctor if you have any doubts at all. There may be other factors at play regarding your overall health and calorie restriction.

Know Your Calories

Finally, remember that all calories aren’t created equal. By cutting 500 calories while maintaining a sugary, high fat diet you won’t lose much fat. Instead, make sure the calories you cut ARE the sugary, high fat calories. Keep it simple, eat vegetables first – broccoli, spinach, kale, brussels sprouts, kale, carrots, etc. Then eat lean meats like chicken, turkey, and lean beef. Add in some nuts like almonds and walnuts to round out your fat intake. Get the bulk of your calories from these foods.

What about carbs? Again, keep it simple and choose a variety of nutrient dense carbs like sweet potatoes, oats, quinoa and bananas to reach your calorie goals. For the quickest fat loss, keep your carb intake low (i.e. 100 – 150 per day). You should get the fewest amount of daily calories from carbs if you’re trying to lose fat.

I can’t emphasize how important it is to increase your activity level through an intelligently designed workout plan in order to lose fat and preserve or build muscle. Maintaining a well-designed exercise program is key to losing fat for most people. Though fat loss is possible with calorie restriction alone, often people’s progress gets slowed or stunted altogether without incorporating a consistent increase in activity through regular exercise.