I thought that some childhood diseases are also having clear effect?
Also Iodine-deficiency, though not sure that's covered by nutrition since it's now added to salt.
People seem very attached to the "you can't do anything" approach though. I find it hard to imagine what a society without education would look like though, even if the people all had high IQs, so presumably there's some benefit to passing knowledge onto the next generation?
Contra what you say, people (really, the liberal elite consensi) seem very attached to the "anyone can do anything" approach, but, once you avoid the obvious stuff and give kids enough food, it is simply not the case: raw genetics is responsible for 50-80% of adult intelligence. This is not a popular position, despite the ample evidence, and will get you thrown out of most sociology departments today.
Educational institutions are, generally, an effect of high societal intelligence, rather than a cause.
But even in the developed world (well, the US at least), the poor very often do not live in a non-disastrous environment, so if that is what you assume, your assumption is flawed.
But, if we are serious about the problem, we need to be honest about the large genetic basis of adult intelligence and then figure out how to humanely encourage eugenic reproductive patterns (or at least discourage dysgenic ones) in the poor.
If the problem you are concerned about is "people with a low adult IQ exist" rather than "people are unfairly obstructed by social conditions from realizing their potential", yes, eugenics would seem to be an important part of the solution.
But for many people, the problem is not which people exist, but the degree to which we harm the potential of certain of the people who do exist.
I would bet that we could agree on many social programs to help the poor. Where we probably disagree is on the amount that we think these programs would help fix the long term problem of poverty, and I might be more inclined to look to see if the program has dysgenic effects that would compound that long-term problem.