The heavy ones.

Some subject material is "heavier" than others ;)

I would say for me it was back in college, mainly in textbooks for mainstream physics and chemistry, especially the ones about thermodynamics.

I already was pretty familiar with equations beforehand, and these equations do not lie. I was studying hard.

When you do the math, it just plain takes more work and burns more resources, and always takes more concerted effort to keep your weight up than it does to let it slip.

I was putting relatively more effort into academics instead.

Plus this was way before the internet. Laptops, or even PC's didn't exist yet. These books weighed kilos and I was lugging them miles to classes on my solid steel bicycle and up & down numerous staircases within non-air-conditioned classrooms and dwellings all day. Except when there was no class it was worth a trip to the main library where they had A/C.

I can't recommend any true weight-loss classics since that's not what I was pursuing back then, but I sure ended up thinner and in better shape than when I started, after only a few short years, and for me it was the textbooks alone.