What got me thinking about this (again) was an article entitled 12 Workout Myths That Just Need To Die. It's not a bad article, but Myth #2 includes this: "...having more lean muscle will help your body burn more calories at rest."
Of course, right? Well, sort of.
It used to be commonly thought that a pound of muscle would burn 30-50 calories a day doing nothing. Not sure why. But the 50-calorie-per-day number is definitely still floating around out there. For example, here and here.
These days, most of what you find online regarding this subject is that no, that's wrong. While there doesn't seem to be any conclusive answer as to the number of calories burned by muscle at rest, current estimates are 6-10 calories per pound per day. (I can't find any well-sourced authoritative sources for this range, but work with me here.)
Could the 30-50 number be right? Let's think it through.
Assuming (based on this study) 30% of body weight is skeletal muscle in a 150-pound female, that gives us 45 pounds. If one pound of muscle burns 30 calories per day at rest, that would mean 1350 calories per day just from inactive muscle tissue. If our mythical subject is five-feet-five-inches tall, 1350 is almost exactly her estimated basal metabolic rate (BMR) -- the number of calories burned per day at rest in a temperate environment with no digestive activity. This leaves no calories left for anything else, including reasonably important things like heart, lungs, kidneys, intestines, liver, and brain. Clearly, that ain't right!
Meanwhile, fat requires energy for maintenance, too -- an estimated 2-3 calories per pound per day. Using these numbers conservatively, replacing 10 pounds of fat with 10 pounds of muscle results in up to 80 more calories burned at rest per day. It's not nothing, but it's not much. And adding 10 pounds of muscle (especially when over 40) is not easy.
So yes, adding muscle burns more calories at rest, but not enough to be compelling for fat-loss.
(Note: Before someone claims that 80 calories times 365 days a year divided by the supposed 3500 calories in a pound of fat means losing more than 8 pounds a year, let me smugly point out that the human body is not a closed system and this kind of calorie summation never works.)
However, that's at rest. Muscles are designed to move and that's where adding more muscle can really help with burning calories. Having more muscle means you can do more work -- in the physics sense -- and work takes energy. (Caloric expenditures in human movement is much more complicated than basic physics, but it's the same idea.) If you go from pressing 50 pounds to pressing 100 pounds, you're doing more work and that takes more energy...measured here in calories.
This effect is not limited to workouts. When you're stronger, you can use that strength in everyday tasks. You might lift a piece of furniture to move it rather than simply sliding it. You can turn a screwdriver with more force. There are countless ways you can burn more calories with more muscle throughout the day without working out (known as Non-Exercise Activity Thermogenesis or NEAT).
But really, adding muscle as a way to burn more calories is missing the point. Yes, strength training can help you burn calories and shed fat, but it can also:
- Strengthen your cardiovascular system reducing risk of stroke or heart attack
- Improve cognitive function
- Improve bone density
- Reduce stress and depression
- Makes you stronger (duh) which makes many daily activities easier (or possible)
- Boost energy levels and improve mood
- Sleep better
- Look more muscular (or "toned" if you prefer)
So yeah, definitely, lift weights, get stronger, and build some muscle. But not to burn more calories at rest. Anyway, when you're training regularly, who wants to be sitting around doing nothing anyway?
Be seeing you.