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Comment by potatochup | Hacker News Reader

I very humanizing account. I've had the blessing of having a very fast metabolism, even in my thirties I'm in the "underweight" BMI category despite eating whatever I want. I'm petrified it'll slow down at some point, and I'll get fat because I have no self control, as I've never needed it before.

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This is a valid fear. I was in the same boat. Getting into great shape at 30 and having an extremely active life did me little favors when I simultaneously turned 40, went to school, and spent a lot more time in front of the computer. At this point, maintaining a healthy weight means eating less than half of the amount of food I ate at a healthy weight at 38, and it needs to be much more healthy. I'm fortunate in that I learned to cook and eat whole, healthy food early in life but despite being delicious and feasible, my diet is much, much more limited.

Basal metabolic rate changes only a few percent with age (unless there's an actual disease involved). Most people who blame their weight on a "slow metabolism" actually have a perfectly normal metabolism. They just consume too many calories.

If anyone wants to actually measure their metabolism it's cheap and easy to get a resting metabolic rate (RMR) test in most any sports medicine lab. You just sit in a chair for a few minutes while they measure your inhaled and exhaled gasses.

This is correct. Resting metabolic rate doesn't vary by more than a couple hundred calories per day among humans of the same weight. And even that difference can be predicted by things like muscle vs fat composition.

Nobody has a metabolism that is defying thermodynamics and creating mass that you didn't intake as food.

Body composition has a rather large effect on BMR (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3661116).

This is untrue, you have to take into account the environment. If you used to live in a cold area like Quebec and move to let's say LA or Houston, without changing your diet, your metabolism will change a lot.

If you compare two neighbors, you're right.

No that's not how it works. Unless you're running around naked in the snow like Wim Hof, your environment has very little impact on daily energy expenditure. People in Quebec and Houston both spend most of their time indoors.

You say that as if "a couple hundred calories a day" is not much!

Have you ever done the math?

200 calories can mean the difference of two people eating the exact same calories every day where one maintains the same weight, and the other gains 2 lbs a month, or 24 lbs a year...nearly 100 lbs in just 4 years!

You are proving the point that variance in metabolism can be a HUGE factor.

Resting metabolic rate increases along with weight. Just keeping adipose tissue alive takes a significant amount of energy. People don't keep gaining weight indefinitely unless they also keep continuously increasing intake. Most people eventually reach an equilibrium, although that equilibrium is often much heavier than optimal.

(I have done the math.)

You will reach an equilibrium after gaining a not insignificant amount of weight, but of course hunger hormones tend to increase as well.

But my point stands that a few percentage points difference in metabolic rate makes a big difference. And likely could lead to one person becoming overweight while another person with identical habits maintains a healthy weight.

That is incorrect. Many things can cause the human body to burn fewer calories than height, weight, and activity would suggest, and one of the easiest ways to determine if someone is affected by one of those metabolic dysregulations is to take their temperature.

If their temperature isn't 98.6 degrees Fahrenheit or very close to it, they are metabolically compromised (note, of course, that people will practically never have a resting body temperature of greater than 98.6)

I had a full dexa scan and calorimetric exam done and my body was burning approximately 300 calories a day under what it was supposed to. My resting body temperature was 97.3. If I ate what my TDEE should have been I would have been putting on half a pound a week or more.

Eating more and exercising to rebuild muscle lost to over dieting has corrected that issue.

But saying that it doesn't exist is completely wrong. The math is still "calories in, calories out" but you can't assume that "calories out" is static and inflexible. There are many variables at play in the human body.

You are technically correct in that your metabolic rate only varies by a few percentage points. But where you are way off is how much of a difference "a few percentage points" can make.

If you used to burn 2500 calories at rest and then that drops 3% to 2425, that is 75 calories a day. Which amounts to around a pound of excess weight every 5-6 weeks. Or 10 lbs a year. After a quick 5 years you look up and you are suddenly 50 lbs over weight just because your BMR dropped 3% and you kept eating the exact same number of calories.

There's no such thing as a "fast metabolism." How much energy you burn is strictly a function of how much energy your body needs to keep itself at ~98.6 degrees and how much work your body has to do.

Right. So two furnaces run at 98.6. Is it not possible that one is more efficient at maintaining that temp (slow metabolism) versus one that is inefficient(fast metabolism). That’s why people suggest finding your basal metabolic rate (BMR) and from there everything is CI<CO.

Expenditure will adapt to your average daily burn though — I ran a half-marathon every single day for two years and would still consume calories of a comparatively sedentary person. No big swings in either direction.

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