The cheapest food is among the nutritionally worst food you can buy. That is of course just one of the causes.

http://seriecientifica.org/sites/default/files/povertyobesit...

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3198075/

http://news.nationalgeographic.com/news/2014/09/140901-ameri...