What happens if you lose weight without exercising?

Illustration of an overweight and a skinny guy

What do you think would happen if you lost weight by either:

  • Just dieting
  • Dieting and doing cardio workouts
  • Dieting, cardio, and weight training

Which protocol is going to cause the most weight loss? The most fat loss? And what’s going to happen to your muscle mass?

There are two studies that answer that question. But before we go over the results, try and guess what would happen. I think it could be fun to test your intuition.

How Does Exercise Impact Body Composition While Dieting?

In this 12-week study, the researchers split the participants into three groups: no exercise, a fitness routine, or a lifting routine. Each group was put on a weight-loss diet.

Here are the results:

  • No exercise: 14 pounds of fat loss, 7 pounds of muscle loss, and 21 pounds lost overall.
  • Cardio workout: 16 pounds of fat loss, 5 pounds of muscle loss, and 21 pounds lost overall.
  • Lifting routine: 21 pounds of fat loss, 0 pounds of muscle, and 21 pounds lost overall.

What’s interesting about this study is that it showed equal weight loss between all three groups. The degree of the calorie deficit determines the rate of weight loss. No exercise required.

But as soon as we look at body composition, the story changes. If you lose weight without exercising, about 1/3 of the weight you lose is muscle. In fact, the same is true with weight gain. If you gain weight without exercising, about 1/3 of the weight you gain will be muscle. That’s why overweight men have so much more muscle mass than skinny guys. Simply gaining weight results in muscle gain. 

This is why exercise is so important for body composition. Doing exercise while losing weight allows people to lose a little more fat and retain a bit more muscle.

If people do a proper muscle-building routine while losing weight, though, this study shows that they’re able to maintain all of their muscle, losing only fat. For someone like me, with no muscle to spare, these kinds of details matter.

An even more recent study tried to replicate these results and got even more dramatic findings:

  • Cardio workout: 7 pounds of fat loss, 6 pounds of muscle loss, 13 pounds lost overall.
  • Lifting routine:  22 pounds of fat loss, 4 pounds of muscle gain, 18 pounds lost overall.

This allows us to answer the question we posed at the beginning of the article. You can lose weight perfectly well with just diet alone. However, if you want to lose more fat, then you should add in exercise. And if you want to build muscle while losing even more fat, you should also lift weights.

Should you lift weights while losing weight?

Somehow cardio has gotten itself associated with weight loss, whereas lifting weights is associated with building muscle. And, I mean, there’s good reason for that. Cardio burns more calories than lifting weight (study) and lifting weights is better for building muscle (study). These findings don’t change that.

Thing is, we get emails like this all the time:

Ultimately, I want to build some muscle and become stronger. However, I’m a little skinny-fat, so I’ve been doing some cardio and trying to lose weight. But when I’m done losing weight, I’m going to start lifting weights so that I can build some muscle.

Sincerely, A Skinny-Fat Guy

Most people who are trying to lose weight are trying to lose fat. They aren’t just trying to get smaller, they’re trying to get leaner. It’s not just about burning calories, it’s about improving body composition.

This (fictional) guy is a great example of that. He wants to lose fat and build muscle, so he’s doing cardio to lose the fat, and then he’ll lift weights to build the muscle.

A smarter approach, though, would be to lift weights while losing weight. That way he can gain some muscle while burning more fat. When he finishes losing weight, he’ll not only be leaner, he’ll also be bigger and stronger.

Or Maybe You Should Save the Weights for Later

However, another thing to consider is the fact that this fictional guy is succeeding at losing weight and improving his cardiovascular health. Is it a 100% optimal routine? No. But it’s a whole lot better than nothing.

If someone feels more confident losing weight by doing cardio and then switching to building muscle, more power to them. There’s plenty of time to build muscle later. The important thing is progress, right?

We give skinny guys similar advice. If their main goal is to bulk up, they should start by lifting weights. If they want to lift weights and do cardio, great—cardio is profoundly healthy—but it’s perfectly fine to tackle one goal at a time.

To make that argument even stronger, you don’t ever really lose muscle. Muscle has a certain amount of memory to it. It sounds like pseudoscience, but larger muscles require more nuclei, so as you build bigger muscles, your muscle fibres gain more nuclei. Then, when you lose muscle, those nuclei stick around. It’s a permanent adaptation.

Building muscle is analogous to adding new rubber to a balloon, making the balloon structurally bigger. Then when you lose muscle, your muscles don’t “shrink,” they just “deflate.” You still have that bigger balloon. Blow some air into it and, poof, you’ve got big muscles again.

So even if you lose muscle, it doesn’t matter that much. When you start trying to bulk up, it will spring right back.

Alright, that’s enough for now. If you dug this article, we’ve got a full skinny-fat guide over on Bony to Beastly. I think you’ll dig that article even more.

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