Without any warning at all, you're suddenly a fat bastard.

A reader writes:

How does muscle turn to fat?

Aren't muscle cells and fat... cells... (is there even such a thing as a fat cell?) different? But boxers and bricklayers and weightlifters and other huge muscly men, when they retire, are famous for turning into giant lardasses and then maybe putting their name on a fat-free grilling machine. How does this occur, biologically?


Muscle doesn't turn into fat. As you say, they're different kinds of tissue; muscle is one kind of cells (subdivided into further categories), whereas fat, or adipose adipose, tissue is composed of quite different cells, chiefly adipocytes.

Also as you say, though, there's a common phenomenon in which big strong men, and the somewhat less common big strong women, turn into big fat men or women as soon as they, for whatever reason, stop exercising all day. Their muscles give up, they wave a little white flag, and without any warning at all they're suddenly a fat bastard.

The reason for this is simple enough: They've stopped exercising, but they haven't changed their eating habits. Or, at least, they haven't changed them enough.

As I mentioned in the ice-cube diet post, it's quite difficult to burn enough calories in exercise to make up for a rich diet. It's possible, though. Fairly strenuous work can easily burn about 500 calories, or about 2100 kilojoules, per hour. Very strenuous exercise can double that, but even if your job involves digging ditches, carrying couches or running after teenagers while waving rusty gardening tools you're unlikely to actually manage a thousand calories an hour for very long.

Even 500 calories an hour, though, means you can eat one standard meat pie, or one Big Mac or large fries (but not both!) per hour, and more or less break even.

If you suddenly transition to a fairly sedentary life, though, you'll now be burning far less energy. An average desk job, for instance, uses only about a hundred calories an hour. So even if the retired boxer halves his food intake, he'll still end up with a big energy surplus, which will in due course make itself visible as fat, even as his muscles atrophy from lack of exercise.

(The quote from the title.)

Psycho Science is a regular feature here. Ask me your science questions, and I'll answer them. Probably.

And then commenters will, I hope, correct at least the most obvious flaws in my answer.

4 Responses to “Without any warning at all, you're suddenly a fat bastard.”

  1. Shivoa Says:

    I'd say if your career / job is exercise then the 'run yourself thin is worse than changing diet' advice (which is the standard sane advice, cutting 600kcal from your diet is the best way to start losing weight as it's a lot easier than adding over an hour of strenuous exercise to your day) goes out of the window.

    As you say, 500kcal an hour and if you try to get 5+ hours of heavy exercise into the day beyond the normal sedentary lifestyle of the average male then you've got every reason to be eating double the average (obviously roughly, but you've got 2500kcal of work on top of the 2500kcal 'typical male' stable weight food intake).

  2. kai Says:

    +1 for the Red Dwarf quote...

  3. cthulhubert Says:

    As I understand it there are two more facets to this explanation.

    The first is that it's not just that people who've stopped exercising are going to rapidly gain fat. Especially for the sorts of people with big, bulgy muscles, when they stop exercising, the muscle doesn't atrophy instantly, but it does rapidly lose tone. So that once heroic slab of pecs is now kinda droopy, and adding a layer of fat on top of that will often make an injured body build look like they've done nothing but eat cakes for the past three years.

    The second is just an extension to the issue of calorie requirements for people that exercise a lot. It's not just the physical activity itself that requires calories. In fact, a more significant impact can be just from your body maintaining its state of athletic readiness. More exercise typically means larger muscles, with proportionally greater internal stores of oxygen; more red blood cells; even heartier organs to handle all the other demands exercise makes on the body. So the calorie surplus of an over-eating ex-power-lifter may actually be even more extreme than just the cessation of exercise may imply.

  4. dmanuel Says:

    It should also be noted that recovery is a significant source of caloric expenditure from training. The training itself might not take too many calories, but the recovery and ensuing adaptation might.

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