Sunday, March 9, 2014

You're probably hot if you have a fast metabolism - rudimentary thoughts on "fast" and "slow" metabolism.

- Gautham

(An update of something I posted on facebook many years ago on applying some simple thoughts related to machine efficiency to the topic of human weight loss and gain and "slow/fast" metabolism).

I sometimes run into people (usually young males) who want to gain weight but claim they can't because they feel they have a fast metabolism. The first question I ask is what did they have for breakfast. The answer was nothing or not much, which is typical. Literally >80% of people that I've run into that say they can't gain because of metabolism don't eat a substantial breakfast. It is extremely hard to make it over the bar with two meals a day rather than with three or four. I have grown very suspicious of this talk of "metabolism" differences. They may be there but my guess is that they are overstated.
 

What would it mean for someone to have a fast metabolism?

 
If your body composition and weight is at steady state, the following must be true, because a fuel that is not stored must be either burned, used for work (which also produces some heat), or thrown out (can you think of another option?):
 
Food in = Food out (excreted) + Work done + Heat produced.
 
In this scheme, the only way you can eat more than someone of identical mass but remain at steady weight is if: 
  1. You excrete more.
  2. You do more work. Maybe you run more than someone, or you run the same but inefficiently. Maybe you are a fidgeter as opposed to a laid-back relaxed type. This could include having a more muscular composition, which is said to require greater work to maintain.
  3. You expel more heat. This could be some combination of the following factors:
  • You have a harder time keeping your body temperature because of higher surface area, less clothing, or colder ambient temperature.
  • You have a substantially higher body temperature. 


Considering that heat loss is proportional to the temperature difference between you and your surroundings, a 10% faster metabolism that doesn't involve more excretion, more work or higher surface area means that your temperature difference with the environment is at least 10% larger. For a normal human being at room temperature (25C), this would mean your body temperature is at least a full degree celsius warmer: from 37C to 38C. And that is just for a 10% faster metabolism.
So if you want to claim that you have a fast metabolism, an implication is that you are likely running hot. The other option is a high heat transfer coefficient.

More likely, a close examination of total food intake and activity will show that systemic differences are smaller than they seem.

Metabolism slowing down when you get older 


Looking at things this way, it reduces the plausibility that anybody's metabolism 'slows' down when they grow older. That used to make sense - oh it slows down, like cars or other devices with age. 

However, this way of thinking implies that the design goal of human physiology is to be able to eat more without increasing in weight. Our long-ago ancestors would strongly disagree that this was the main problem they were facing.
 
A 'slowdown' in metabolism in the sense people usually seem to mean (I am doing everything the same as when I was younger but I am gaining weight) actually would imply an increase in work efficiency of the body. You work more efficiently or you expel less heat. In this sense, a person's metabolism slowing down with age is like a car's gas mileage increasing the longer you have it.

More likely, as a friend pointed out, an older person with a "slower" metabolism is simply doing less physical work (perhaps due to learning how to do things more efficiently), not exercising, not fidgeting, living in comfortable climate-controlled surroundings, etc. 

So in both cases: decline with age and apparent variation between people, perhaps best to first carefully look at what one eats and what one does before concluding that the difference is in one's own system.

4 comments:

  1. You are skeptical of large differences in metabolism partially because of a lack of large differences in internal body temperature. I share your skepticism
    generally about these metabolism arguments but am also skeptical about this aspect of your argument. The relevant thing for heat loss is skin temperature, which varies a lot, especially at the extremities. You also sort of dismiss the heat transfer coefficient idea, but I'd argue that that can vary a lot too depending on what you wear.

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  2. One thing I never see considered when thinking whether or not some people generate more heat is where the heat is generated. I think we all have encountered people who's extremities seem to always be cold. While internal temperature is similar amongst everyone, how much heat produced in all other areas seems highly variable. A tall skinny person would probably expend much more energy thermo-regulating than a short fat person.

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  3. I’ve often wondered about heat as a factor. I had a pretty bad accident 6 years ago. Head injury. Left me with a permanent migraine. I was unable to exercise and gained 45 pounds. 5 months ago my dr asked me to take phentermine to help lose weight because I was unable to workout as hard as I used to due to the migraine. I lost the 45 pounds in 4 months. It also somehow helped my migraine. So I’ve stayed on a half dose of phentermine since. Here’s the interesting point of all this- my body temp has gone up. I used to be around 96 degrees and now I’m consistently a degree higher. It’s still lower than the normal 98 for most people. But I’m finding that with this 1degree difference I’m maintaining weight much easier. Just thought I’d throw this out there. I know there are other factors involved with phentermine. My blood pressure has always been low (about 107/67). This medication has not increased it. In fact, with the weight loss I’m hovering closer to my pre-accident BP of 98/64.

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  4. Interesting as a person who thinks he has a high metabolism (an always have a big breakfast ;)). I think you miss some other potential aspects: a 4th point would be that your food/calorie uptake is less. Your inestines might be less efficient (or the microbes in you inestine) and therefore the calorie uptake is less for the same amount of food compared to another person. Your nody temperature normally don't fluctuate much due to homeostasis. So it implicates that when you produce more heat, your body will try find ways to get rid of the excess of heat by sweating, getting more blood to your skin etc. If the room temperature is lower than your body temperatures there are still possibilities to cool ;)

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