According to my dermatologist, nail characteristics are largely genetic. I've never grown strong fingernails no matter which diet changes I make. Buy, ironically enough, very strong toenails. Not exactly the outcome I'd prefer but that's what I got. I do have friends who have seen improved fingernail growth and strength modifying the amount of various B vitamins that they've added to their diet. And there are some trace elements like silica and such that can make a difference but that might be if someone's diet is deficient in them. Now given that there's so much soil depletion in the US, a lot of produce is no longer grown in mineral-rich soil. So as with any of everything in our diet, we are generally not getting the same quality of nutrition out of food as our ancestors did a century or so ago. My rule of thumb is that if the soil can't support earthworms, it's likely not great for growing mineral-rich food either.