[armchairhacker]

From my reading it seems like a rare few people experience major benefits. Furthermore in some cases it's almost certainly placebo. Although I think certain fads like keto do have some real physical effect, but only for people with rare-ish metabolic or autoimmune disorders.

Most people experience minor if any benefit from these things. The things which really cause benefits are healthy eating and exercise in general. If you go from eating fast food and barely walking to eating whole foods and deliberately exercising 5-7 days a week, you'll almost certainly feel a major difference. What exactly caused that difference, where you crossed the line, is unclear; but it doesn't matter, IMO people should eat well and exercise reasonably anyways.