[User23]

Good nutrition actually increases the heritability of traits. For example height is roughly 80% heritable in first-world countries but only 60% heritable in third-world countries. Why? Because reducing deleterious environmental effects like malnutrition, neglect, and so on actually increases the relative contribution of genetic factors.

[Mz]

Stunting is very much a thing and there are complex factors that contribute to it, not just good nutrition. They are finding that lack of proper infrastructure for disposing of wastes and keeping people clean contributes as well. In areas where cleanliness is just not attainable due to lack of infrastructure, you can see stunting in even relatively well fed kids.

The incredibly good health that so many modern peoples enjoy which has been generally extending human life arises out of a long human history of progress and development and depends on enormous amounts of infrastructure and public resources that are basically "gifts" to us from the world. People really shouldn't go taking that for granted as some baseline given. It really is not. It doesn't take that much for a person's life to come completely unraveled. Once that happens, putting it back to together is generally a lot harder than taking it apart was.