Can You Explain The Science Behind "Fast" vs. "Slow" Metabolisms?

[fattytca]

I have accepted my Doctor's "Slowing Metabolism" reasoning behind my fat gain as I've gotten older without much questioning.

But as I think about it, no one has ever been able to explain the science behind fast vs. slow metabolisms to me.

Namely - I have worked out consistently since I was 15, but why do I look like 12 kinds of $#!t compared to myself as a 23 year old when:

1) I'm 20% stronger than I was.

2) I can now run 50% farther than I could when I was 23 due to increased cardio work.

3) My resting heart rate is the same as it was when I was 23.

4) I now watch what I eat every day - I consume HUNDREDS of fewer calories per day, with far lower fat content, and higher protein intake.

Is there a science behind metabolism?

Why does it seem that the young can work out half as hard with twice the results?

Why can the young seemingly defy the bodybuilding "science" that one cannot both gain muscle and lose fat at the same time?

Re-reading this I know it sounds cynical but I'm truly looking for some science behind this... hormones? If so, which ones? Testosterone alone can't be the answer to a fast metabolism... my T levels are fine and I still look like garbage! Muscle mass is up, resting heart rate is the same, fat is sky high.

Any help with links to studies behind fast vs. slow metabolism are much appreciated!

[HealthResearch]

There are generally 2 factors:

1) Loss of muscle. Which it does not sound like you fall into this category.

2) Activity level overall. Most people get day jobs, work at a desk all day, maybe work out at the gym for an hour, then go home and relax, eat a meal, watch TV. Compared to a teenager or someone in their 20s who are going out, walking to class, participating in sports, activities, dancing, always moving etc, etc. It's pretty easy for a student to expend an extra 1000 calories a day compared to someone with a desk job.

There is a small hormonal adjustment that occurs, but I haven't seen evidence that this causes more than a couple hundred calorie difference at most. If anyone reading this has seen studies suggesting otherwise, I would love to see them.

[Aes_RocK]

While doing my undergrad my proff mentioned that your metabolism may or may not be "slowing down", but you definitely are producing less hormones. At 23 you're producing the most testosterone and other bodybuilding associated hormones at peak levels. By the time you're in your 30's your tapering off. This may be the answer to your question. The whole metabolism slowing down or stoking your metabolic fire is not widely believed or taught anymore

Edit: its not just testosterone, t3 and t4 hormones are being tapered in addition to other hormones (think GH, Insulin, Cortisol... testosterone alone isnt the reason why we're able to build muscle). Also, more free radicals and telomere splicing as you age (causes people to look older [wrinkles and sh it])