Iron deficiency is thought to be the most common nutrient deficiency in the world, and iron deficiency anemia slows mental and motor development and reduces work performance and physical activity. Progress in combating iron deficiency has been hindered by technological limits on iron fortification of staple foods and the need to supply iron supplements on an almost daily basis for them to be effective. But there have been relatively few studies of the prevalence of these micronutrient deficiencies or of how they cluster in specific populations. Limited data show that zinc, riboflavin, and vitamin B-12 deficiencies tend to cluster with vitamin A and iron deficiencies. Many investigators then mounted zinc intervention trials with children, because intakes of absorbable zinc are often low and growth-stunting occurs nearly universally during the first two years of life in underprivileged populations.
Diarrhea also causes the body to lose zinc.
An analysis of 27 such trials concluded that zinc supplements are likely to improve the height gain of the most stunted children and to improve the weight gain of those with low plasma zinc concentrations. In recent intervention trials, zinc supplements reduced the prevalence of pneumonia and malaria in children, shortened the duration of acute and persistent diarrhea, and improved neuropsychological performance. Zinc supplements improved birth weight in one U.S. study, although in Bangladesh and Peru they did not. In the relatively few studies of riboflavin the prevalence of deficiency has been alarmingly high. Almost all the pregnant and lactating women studied in The Gambia, the majority of lactating women and elderly in Guatemala, and most Chinese adults were reported to be deficient.