Good point, but I think a lot of what, in the modern diet, is bad for us doesn't necessarily cause immediate negative effects.

I had been eating the same way for more than a decade (very conventionally healthy, no processed foods, lots of fruits and veggies, lots of whole grains, some meat but not too much). And I feel very noticeably more energetic with a meat/egg heavy diet. But it wasn't a specific negative in my former diet that was the problem.

I just listened to a Plant Free MD podcast (episode 47) where the topic of adding fruits to the diet comes up. The day after he ate a banana, he has a strong negative side effect (or something along those lines). What I'm thinking about are the more subtle effects.

(Also in case anyone is interested, going very heavy on meat/eggs leads to a VERY noticeable reduction in gas. And having even half of a normal size carb serving, like some mashed potatoes or something, causes it to flare up again).