Getting proper nutrition is imperative for good cognitive development! Poverty often goes hand-in-hand with poor nutrition and even vitamin deficiency. Even if starvation isn't a factor, fast foods with low nutritional value are often in play.
Malnutrition develops when the body does not get the proper amount of energy (calories), proteins, carbohydrates, fats, vitamins, minerals and other nutrients required to keep the organs and tissues healthy and functioning well. A child or adult can be malnourished by being undernourished or overnourished. http://www.factsforlifeglobal.org/05/
Further, this same malnutrition can lead to other factors that can, in turn, lead to cognitive development problems. Illness for example is a general blanket term for so many things that can happen in the body due to malnutrition, ... and any number of illnesses (apart from teh manutrition that caused them) can cause a lag in cognitive development. In this way, malnutrition can also be an indirect cause of delay.