If you’re into that kind of thing, we thought we would share a link to this podcast on energy expenditure from Barbell Medicine. It is an interview with Herman Pontzer, author of the book Burn.
The TL;DR of the podcast is this:
It may actually be that the body has a set amount of calories it’s OK with burning. When you FIRST start to get into an exercise routine, you’re going to burn more calories.
But over time your body will become more efficient and find ways to burn fewer calories so you get back closer to your “set point” of calories burned.
This is one potential reason people may see a drop in body fat when they start an exercise routine but it stops relatively quickly unless nutrition is addressed.
Through very recent analysis of data, it appears that the idea that metabolism slows in your 40's actually isn’t supported by data.
It appears that in your first couple years of life your metabolism is sky high, slowly starts to drop until your early 20s, then levels out all the way until your 60s. Only then does metabolism seem to decline.
This is very early data analysis, but it certainly is interesting for the “slow metabolism” theory!