There are only 3 macro-nutrients: fat, proteins and carbohydrates.

If you go low fat, you necessarily go low protein too, so you need to replace that with carbohydrates. Even more, you now need "fortified" carbohydrates to get the needed B vitamins and other nutrients usually found in fatty foods, and those will be refined carbohydrates; which is what the American Health Association now recommends. Yes, they actually recommend that people should eat enriched cereals and other bullshit like that.

A high carbs diet made of "healthy, complex carbs" is a myth ;-)

> he gets furious when he sees doctors advocating the opposite, a trendy practice he believes speeds aging.

You cannot claim that without evidence. Nutrition is at this point like alchemy and I won't trust furious alchemists.