In Poor Economics, the authors Abhijit V. Banerjee and Esther Duflo suggested that often it is not the case that poor people cannot afford an adequate and nutritious meal, but they tend to spend their limited resources on comfort food such as meat. I wonder if there has been any research in how to design a low-budget, nutritious and actually appealing diet for the world's extremely poor people?
Robert Jensen and Nolan Miller found a particularly striking example of the “flight to quality” in food consumption.7 In two regions of China, they offered randomly selected poor households a large subsidy on the price of the basic staple (wheat noodles in one region, rice in the other). We usually expect that when the price of something goes down, people buy more of it. The opposite happened. Households that received subsidies for rice or wheat consumed less of those two items and ate more shrimp and meat, even though their staples now cost less. Remarkably, overall, the caloric intake of those who received the subsidy did not increase (and may even have decreased), despite the fact that their purchasing power had increased. Neither did the nutritional content improve in any other sense.
I doubt that that study was able to tell whether the dietary changes improved nutrition. They don't appear to have looked at many nutrients, or figured out which nutrients the subjects were most deficient in. Even if they had quantified all important nutrients in the diet, nutrients in seeds are less bioavailable than nutrients in animal products (and that varies depending on how the seeds are prepared).
There's lots of somewhat relevant research, but it's hard to tell which of it is important, and maybe hard for the poor to figure out whether they ought to trust the information that comes from foreigners who claim to be trying to help.
I'll guess that that more sweet potatoes ought to be high on any list of cheap improvements, and also suggest that small increases in fruit and seafood are usually valuable. But there will be lots of local variation in what's best.