Comparison of complementary feeding strategies to meet zinc requirements of older breastfed infants

BACKGROUND

The low zinc intake from human milk at ∼6 mo of age predicts the dependence on complementary foods (CF) to meet the zinc requirements of older breastfed-only infants.

OBJECTIVE

The objective of this study was to compare major variables of zinc homeostasis and zinc status in 9-mo-old breastfed infants who were randomly assigned to different complementary food regimens.

DESIGN

Forty-five exclusively breastfed 5-mo-old infants were randomly assigned to receive commercially available pureed meats, iron-and-zinc-fortified infant cereal (IZFC), or whole-grain, iron-only-fortified infant cereal (IFC) as the first and primary CF until completion of zinc metabolic studies between 9 and 10 mo of age. A zinc stable-isotope methodology was used to measure the fractional absorption of zinc (FAZ) in human milk and CF by dual-isotope ratios in urine. Calculated variables included the dietary intake from duplicate diets and 4-d test weighing, the total absorbed zinc (TAZ) from FAZ × diet zinc, and the exchangeable zinc pool size (EZP) from isotope enrichment in urine.

RESULTS

Mean daily zinc intakes were significantly greater for the meat and IZFC groups than for the IFC group (P < 0.001); only intakes in meat and IZFC groups met estimated average requirements. Mean (±SEM) TAZ amounts were 0.80 ± 0.08, 0.71 ± 0.09, and 0.52 ± 0.05 mg/d for the meat, IZFC, and IFC groups, respectively (P = 0.027). Zinc from human milk contributed <25% of TAZ for all groups. The EZP correlated with both zinc intake (r = 0.43, P < 0.01) and TAZ (r = 0.54, P < 0.001).

CONCLUSION

Zinc requirements for older breastfed-only infants are unlikely to be met without the regular consumption of either meats or zinc-fortified foods.

Food choices to meet nutritional needs of breast-fed infants and toddlers on mixed diets

The primary focus of this review is considerations for complementary feedings to meet micronutrient needs of infants aged 6-24 mo who are continuing with breast-feeding and minimal or no formula. The World Health Organization recommends initiation of complementary feeding to breast-fed infants at approximately 6 mo of age. Whether complementary foods will meet nutrient needs will depend on the types of food selected. One criterion for the selection of complementary foods is that they be rich sources of zinc and iron because both of these essential micronutrients are critical for normal growth and development, and requirements are not met by exclusive breast-feeding after approximately 6 mo. For an exclusively breast-fed 7-mo-old infant, human milk provides approximately 0.5 mg of zinc, and a little over half of that is absorbed. Adding some cereal can increase zinc intake modestly but will fall short of providing the estimated physiologic requirement because adaptive mechanisms are inadequate to compensate for moderately low zinc intake. Maize, wheat, rice, and roots are also relatively low in zinc and have the added factor of a high phytate-to-zinc molar ratio, which makes the zinc less bioavailable. Meats and liver have greater zinc and iron concentrations than unfortified plant foods and have been shown to have good acceptance by 7-mo-old infants. In contrast to current practices in both developed and developing countries, meats should be considered as an early complementary food for breast-fed infants to provide essential micronutrients.

Complementary feeding: clinically relevant factors affecting timing and composition

Exclusive breastfeeding for the first 6 mo of life followed by optimal complementary feeding are critical public health measures for reducing and preventing morbidity and mortality in young children. Clinical factors, such as birth weight, prematurity, and illness, that affect the iron and zinc requirements of younger infants are discussed. Maternal diet and nutritional status do not have a strong effect on the mineral content of human milk, but physiologic changes in milk and the infants' status determine the dependence of the infant on complementary foods in addition to human milk to meet iron and zinc requirements after 6 mo. The nature of zinc absorption, which is suitably characterized by saturation response modeling, dictates that plant-based diets, which are low in zinc, are associated with low absolute daily absorbed zinc, which is inadequate to meet requirements. Foods with a higher zinc content, such as meats, are much more likely to be sufficient to meet dietary requirements. Current plant-based complementary feeding patterns for older fully breastfed infants in both developed and developing countries pose a risk of zinc deficiency. The strong rationale for the potential benefits of providing meat as an early complementary food, and the examples of successful intervention programs, provide potent incentives to pursue broader implementation programs, with concurrent rigorous evaluation of both efficacy and effectiveness.