Why build muscle?

Part 2

Last time I talked about appearance, this time let’s talk about fat loss:

Fat Loss

Here’s the big one, at least for me. Studies show lifting weights while you’re in a Calorie deficit is the most effective form of exercise to make you you lose fat¹. I find the studied referenced in the metanalysis, combined with the success of bodybuilders to be convincing. End of blog post.

But, for those of you who want to know “how” it works, there’s some theory² that I can include.

Increased Metabolism

What people colloquially refer to as metabolism, as in “I have a fast metabolism, I can eat whatever I want and not get fat” is the number of Calories you burn, just living your life, not deliberately exercising. This is more accurately know as a combination of your Resting Metabolic Rate and your natural Non Exercise Activity Thermogenesis. If you tend to pace around a lot more, you have a “fast metabolism,” if you have a lot of metabolically expensive tissue you’ll also have a “fast metabolism.”

There are a lot of metabolically expensive tissues in your body, your brain is probably the most interesting, but I’m here to talk about muscles. Muscle is about twice as metabolically expensive as fat, with a pound of muscle burning ~6 Calories and a pound of fat burning ~3 Calories. A small change of muscle tissue into fat tissue can start a snowball that leads to obesity even if your Calorie intake is constant!

Take a 18 year old freshmen college student who is skinny³ but eats enough Calories to maintain his bodyweight. He finally gets enough freedom from his parents to spend all his time on his computer⁴, but he keeps his diet healthy⁵. After a year, all this extra sitting⁶ causes him to atrophy a pound of muscle⁷, but since he’s eating to maintain his weight he gains a pound of fat. When he looks at his scale, there’s no change, he can’t see any difference in the mirror, so he thinks his body hasn’t changed. This process goes on through his 20s.

After 1 year his RMR is now 3 Calories fewer the first year. After 2 years, 6 Calories fewer the second year. After 3 years 9 Calories. It’s a pretty small change, but he’s net gained a pound of tissue early in the third year. For every pound of muscle lost, your body needs to add two pounds of fat to maintain the same RMR. After 10 years he’s lost 10lbs of muscle and gained 20lbs of fat, his weight has gone up 10lbs with most of the new weight coming at the end of the 10 year period, and this process isn’t slowing down.

This is a spherical cow model of fat gain⁸, but it does track pretty closely to the pattern of weight gain commonly experienced by young adults who can eat whatever they want in their college years without noticeably getting fat but then their “metabolism slows down” in their 20s and 30s and they put on weight without any perceived behavior change.

Lucky for us, opposite also happens. Adding a pound of muscle requires your body to give up two pounds of fat to maintain the same RMR. This means that getting your newb gains⁹ of 10-30lbs of muscle can THEORETICALLY cause you to lose 20lbs to 60lbs of fat while eating the same number of Calories¹⁰.

Other Reasons

There are other reasons that people think lifting helps you lose weight: You burn a few hundred Calories while you’re lifting¹¹, it can be intense enough to cause EPOC¹², there are a theory that glycogen depletion and replenishment helps you insulin sensitivity and thus better regulates hunger¹³, and there’s the idea that building extra muscle is something for your body to do with Calories instead of storing them in fat¹⁴.

Some of these theories might be important parts of the puzzle, or they could all be true but low magnitude or they could just be wrong guesses about how the body works. Personally, given how many credible mechanistic theories have been wrong in this space, I am choosing to be aggressively agnostic on the subject of internal mechanisms.

Again, I want to reiterate, the theories above are interesting, but they’re not what convinced me this was useful and they’re not what convinced me to write all these posts. What convinced me was 1) lifting weights worked for me losing fat, 2) there’s an entire sport¹⁵ based around gaining and losing huge amounts of fat using these methods reliably and on schedule¹⁶, and 3) there is some research that shows its effectiveness.

1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8365736/

2. All statements about mechanisms are pretty speculative in this space. It’s annoying.

3. Low muscle, low fat

4. Like me at 18

5. Not like me at 18

6. Combined with the lack of puberty hormones

7. The rate of muscle loss probably varies a lot relative to this example, it could easily be a lot more or a lot less.

8. Specifically this person’s hunger, NEAT, and diet quality are held constant, which is extremely unrealistic.

9. “Newb Gains” is the slang for the muscle gains that someone new to lifting can get. Since muscle gets harder to grow the more you have, the first couple of years lifting will have the best results.

10. This is assuming no change in hunger, diet, or activity level. Since that’s a crazy assumption to make in the real world, results will differ from this example.

11. Definitely true, probably useful for weight loss, not any different from any other activity.

12. Excess Post-exercise Oxygen Consumption, it basically means that your cells need more energy after extremely energy draining activity. If you do a low intensity 400 Calorie and then sit on the couch for the rest of the day you burn 400 Calories + RMR etc, if you do very high intensity 400 Calorie exercise you burn 400 C + RMR etc + EPOC. When this was discovered, it was theorized that EPOC could be a a large multiple of 400, maybe doubling it or more. This is the theory behind High Intensity Interval Training for weight loss.

After more studies were done, it looks like EPOC gives a ~18% increase in Calories burned with high intensity exercise, but you’ll get tired pretty quick. You can burn more than twice the Calories directly in a low intensity exercise before you get tired enough to quit.

It’s part of the calculation for how many Calories you burn in a workout, but I don’t think it makes lifting special.

13. This might be true and important, but I don’t know and I’m not basing any decisions on it.

14. It’s like bonus NEAT that you can do while watching TV or sleeping.

15. Yes, I’m saying bodybuilding is a sport

16. Although recent research shows that gaining as much weight as you can as fast as you can (ie get really fat while lifting, aka the dreamer bulk) doesn’t result in more muscle gain than gaining weight more slowly, like a pound a week or less.