Wikipedia defines common sense as “knowledge, judgement, and taste which is more or less universal and which is held more or less without reflection or argument”
Try to avoid using this topic to express niche or unpopular opinions (they’re a dime a dozen) but instead consider provable intuitive facts.
Is this not true?
No. At least, it’s not the general cause of ‘middle age spread’.
The base metabolic rate refers to how your individual cells respire when at rest. And a brain cell in 20 year old respires much the same way as a brain cell in a 45 year old. Same for all other organs. There is a gradual decline but it’s on the order a single percents.
Organs and tissue at rest respire at different rates, so some of the change people notice is due to change in body composition. Muscle at rest burns twice the calories as fat however this is still only a minor contribution.
Base metabolic rate doesn’t vary much at all. The vast difference in daily calories consumed as one ages is general activity level.
Overall metabolic rate = base rate (varies a little on body composition) + calories burned in general activity (varies a lot)
People typically are less active between 20 and 40. This is not just sport but also lifestyle. People become more efficient in their habits as they age. They drive instead of biking or walking. They sit in the sun on holiday with nice food and wine rather than dancing all night. Etc
Lifestyle choice is the primary cause of excess calorie intake and ‘middle age spread’. Not “my metabolism that I can’t do anything about”.