The only advice that makes sense, and for the past half century or so has been constant, albeit hiding in plain sight under whatever the fads of the day are:
Exercise regularly. Avoid extremes in diet - but eat a bit of variety. Get the minimum recommended amounts of vitamins and minerals. Keep your calories and your body fat low.
And if you have bad genetics, you're probably just screwed, and no amount of exercise or diet will fix it.
There is no evidence (known to me) that variety is actually beneficial. (Past the obvious "get known required amounts of vitamins and minerals")
There is some evidence that certain kinds of atypical diet are slightly less beneficial in terms of mortality.
(Low carbohydrate long term. Cause unknown, not CVD or cancer. Very good review, thoroughly recommended. The key word is "all cause mortality and morbidity")
And the varied (in certain ways) Western diet is also shown to be quite bad.
Please correct me if I'm wrong, I like to review nutrition papers fairly and thoroughly but have never seen one on this...
In the absence of hard evidence, by avoiding variety, you're probably more likely to miss consuming an essential or beneficial food than you are to eat something that is bad for you in large amounts.
Variety in moderation seems to be the only effective proxy to smooth over the constant "used to be good for you - nope, not any more - wait we were wrong it is good" oscillations.
I mean a variety of plant and animal sources, and a variety of macronutrients - to cover all the bases. With the exception of trans fats, and possibly red meat, nearly every food seems to be safe in moderate doses, so there's no reason to avoid or load up on anything in particular.
And when they find out that something like, for example, oily fish, is beneficial - you can look back and say hey, I ate a bit of that too.
I agree that not all forms of variation are good, but variation within what mainstream nutrition science recommends at this point probably mostly is.
tl;dr - no one really knows anything, so play the odds.
It’s like financial advice stating that you shouldn’t spend more than you earn. If you are in debt, spend less money. If you are overweight, eat less food.
Of course both of these advices are easier to follow for some people than for others.
Exercise regularly. Avoid extremes in diet - but eat a bit of variety. Get the minimum recommended amounts of vitamins and minerals. Keep your calories and your body fat low.
And if you have bad genetics, you're probably just screwed, and no amount of exercise or diet will fix it.