Significant delays in linear growth and weight gain (‘growth retardation’) are common features of mild zinc deficiency in children.
A meta-analysis of growth data from zinc intervention trials confirmed the widespread occurrence of growth-limiting zinc deficiency in young children, especially in developing countries (15).
Although the exact mechanism for the growth-limiting effects of zinc deficiency are not known, research indicates that zinc availability affects cell-signaling systems that coordinate the response to the growth-regulating hormone, insulin-like growth factor-1 (IGF-1) (16).
Delayed neurological and behavioral development
Low maternal zinc nutritional status has been associated with diminished attention in newborn infants and poorer movement (‘motor’) function at six months of age. Zinc supplementation has been associated with improved motor development in very low birth weight infants, and more vigorous activity in infants and toddlers (17).
Additionally, zinc supplementation was associated with better ‘neuropsychological’ functioning (e.g., attention) in Chinese first grade students, but this was observed only when zinc was provided with other micronutrients (18).
Two other studies failed to find an association between zinc supplementation and measures of attention in children diagnosed with growth retardation.
Although initial studies suggest that zinc deficiency may depress cognitive development in young children, more controlled research is required to determine the nature of the effect and whether zinc supplementation is beneficial (19).
Impaired immune system function
Adequate zinc intake is essential in maintaining the integrity of the immune system (20); thus, zinc-deficient individuals can experience increased susceptibility to a variety of infectious agents (21, 22).
Diarrhea
The adverse effects of zinc deficiency on immune system function are likely to increase the susceptibility of children to infectious diarrhea; persistent diarrhea contributes to zinc deficiency and malnutrition.
In a number of randomized controlled trials, zinc supplementation in combination with oral replenishment of water (’rehydration’) has been shown to significantly reduce the duration and severity of acute and persistent childhood diarrhea and to increase survival (23, 24). A meta-analysis of randomized controlled trials concluded that zinc supplementation reduces the frequency, severity, and duration of diarrhea episodes in children less than five years of age (25).
The World Health Organization and the United Nations Children's Fund recommend zinc supplementation as part of the treatment for diarrhea diseases in young children (26).
Pneumonia
Zinc supplementation may also reduce the incidence of lower respiratory infections, such as inflammation of the lungs (‘pneumonia’).
A pooled analysis of a number of studies in developing countries demonstrated a substantial reduction in the total number of cases of pneumonia in children supplemented with zinc (27). A meta-analysis found that zinc supplementation reduced the incidence but not duration of pneumonia or respiratory tract illnesses in children less than five years of age (25).
Malaria
Some studies have indicated that zinc supplementation may reduce the incidence of clinical attacks of malaria in children (28).
A randomized controlled trial in preschool-aged children in Papua New Guinea found that zinc supplementation reduced the frequency of health center attendance due to malaria by 38% (29). Additionally, the number of malaria episodes accompanied by high blood levels of the malaria-causing parasite was reduced by 68%, suggesting that zinc supplementation may be of benefit in preventing more severe episodes of malaria.
However, a 6-month trial in more than 700 West African children did not find the frequency or severity of malaria episodes (30). Additionally, a randomized controlled trial in over 42,000 children aged one to 48 months found that zinc supplementation did not significantly reduce mortality associated with malaria and other infections (31).
Due to conflicting reports, it is not yet clear whether zinc supplementation can be used in treating childhood malaria.
Immune response in the elderly
Age-related declines in immune function have been associated with the vulnerability of the elderly to mild zinc deficiency. However, the results of zinc supplementation trials on immune function in the elderly have been mixed.
In randomized controlled trials, certain aspects of immune function (e.g., increased levels of immune cells) in men and women over 65 years of age have been found to improve with zinc supplementation (32, 33).
However, other studies have reported that zinc supplementation does not improve parameters of immune function, indicating that more research is required before any recommendations can be made regarding zinc and immune system response in the elderly.
Pregnancy complications
Poor maternal zinc nutritional status has been associated with a number of adverse outcomes of pregnancy, including low birth weight, premature delivery, labor and delivery complications, and anomalies in developing fetuses (34).
However, the results of maternal zinc supplementation trials in the U.S. and developing countries have been mixed (17).
Although some studies have found maternal zinc supplementation increases birth weight and decreases the likelihood of premature delivery, two randomized controlled studies in Peruvian and Bangladeshi women found that zinc supplementation did not affect the incidence of low birth weight or premature delivery (35, 36).
Supplementation studies designed to examine the effect of zinc supplementation on labor and delivery complications have also generated mixed results, though few have been conducted in zinc-deficient populations (17).
A review of 17 randomized controlled trials found that zinc supplementation during pregnancy was associated with a 14% reduction in premature deliveries; the lower incidence of preterm births was observed mainly in low-income women (37).