ZEROING IN ON ZINC

Zinc is an essential mineral in the human diet serving a number of functions from the formation of essential enzymes to strengthening the immune system.

“Zinc helps in the production of around 100 enzymes in your body, supports the immune system and helps maintain your senses of smell and taste," states Jayesh Chaudhari, senior formulation scientist, Fortitech, Inc., Schenectady, NY. In addition, zinc is essential for protein synthesis and supports normal growth and development. Because the body does not store zinc, a daily intake is required to maintain zinc levels.

Our nutritional requirements for zinc increase with age from birth, when we need 2 mg per day, to age 14, when males need 11 mg per day and females 9 mg per day, with the greatest RDAs indicated for boys and men over age 14 (11 mg per day), and pregnant and lactating women (needs range from 11 to 13 mg per day, depending on age). Though most life stages have an RDA for zinc, the Food and Nutrition Board at the Institute of Medicine of the National Academies, Washington, established an Adequate Intake for infants ages 0 to 6 months that is equivalent to the mean intake of zinc in healthy, breastfed infants. The Tolerable Upper Intake Level for zinc ranges from 4 mg for infants 0 to 6 months of age to 40 mg for adults 9 years of age or older.

Sources and bioavailability of zinc

“Most nonvegetarian foods that are good sources of zinc include beef, lamb, pork, crab, turkey, chicken, lobster, clams and salmon," Chaudhari says. "Vegetarian sources include dairy products, such as milk and cheese, yeast, peanuts, beans, whole-grain cereals, brown rice, whole-wheat bread, potato and yogurt. Additionally, pumpkin seeds are one of the most-concentrated vegetarian sources of zinc." Though the zinc in plant-based foods is less bioavailable, bioavailability can be increased by soaking beans, grains and seeds for several hours and letting them sit until sprouts form prior to cooking (Journal of the American Dietetic Association, 2003;103:748-765). In addition, leavening increases zinc bioavailability by partially breaking down the naturally occuring phytates, compounds that bind zinc, inhibiting its absorption.

Bioavailability refers to the efficiency of absorption and utilization, or retention, of the nutrients that are present in food. Several factors affect bioavailability, including components found in the food itself, food processing, human physiology and a persons’ health status. Also, nutrients compete with other nutrients for absorption. Some nutrients will enhance or reduce the amount of other nutrients being absorbed by the body. Because there are many factors that affect the bioavailability of zinc, it is not possible to accurately compare the bioavailability of zinc from two different sources of the nutrient.

Zinc and health

Zinc deficiency is prevalent in the developing world, though both zinc deficiency and toxicity are rare in the United States (Molecular Medicine, 2008; 14:353-357). Zinc deficiency typically results from inadequate zinc intake or absorption, an increase in zinc requirements or excess zinc losses. Some diseases, certain digestive disorders, gastrointestinal surgeries and chronic diarrhea can all lead to excessive zinc losses. In addition to diseases and conditions that lead to greater zinc losses, some people have increased zinc needs, including vegetarians, who may need 50% more than the RDA, because plant-based sources of zinc are less bioavailable. Additionally, pregnant and lactating women (11 to 13 mg per day), infants ages 7 to 12 months who are exclusively breastfed (3 mg per day; the amount of zinc in breast milk does not meet the zinc needs of this age group), those with sickle cell disease and alcoholics all have increased zinc needs.

Zinc deficiency can lead to growth retardation, hypogonadism, cognitive impairment and immune dysfunction (Journal of the American College of Nutrition, 2009; 28:257-265).

Zinc deficiency also adversely affects several aspects of immune functioning, decreases resistance to pathogens and impairs wound healing (Annual Reviews of Nutrition, 1990;10:415-31; Wound Repair and Regeneration, 2007;15:2-16). A review of 13 randomized, double-blind, placebo-controlled trials found that supplemental intake of zinc within 24 hours of the onset of a cold was associated with a significant reduction in the duration and severity of common cold symptoms in healthy people. In addition, zinc supplementation for at least five months was found to reduce cold incidence, school absenteeism and prescription of antibiotics in children (Cochrane Database of Systematic Reviews, 2011; (2):CD001364). In addition, a meta analysis of 17 randomized, controlled trials of zinc supplementation in infants and children found that zinc supplementation significantly reduced both the frequency and severity of diarrhea and respiratory illnesses in this population (Pediatrics, 2007; 119:1,120-1,130).

Zinc supplementation doesn't just impact immune functioning. The Age-Related Eye Disease Study found that a supplemental combination of zinc, beta-carotene, vitamin C and vitamin E slowed the progression of age-related macular degeneration (Current Opinion in Ophthalmology, 2010; 21:184-189).

Zinc toxicity in humans is rare, although long-term intake at levels above the tolerable upper limit can result in adverse health effects. And, consuming large quantities of zinc can lead to a copper deficiency (Food and Nutrition Board, IOM).

Fortification caveats

Zinc fortification is an effective approach to improving a population’s zinc status, increasing both dietary zinc intake and total daily zinc absorption. In addition, the majority of studies also show that adding zinc to food does not adversely affect the absorption of other minerals (Food Nutrition Bulletin, 2009;30:S79-107).

However, different forms of zinc added to foods can impact the taste and overall sensory quality of the finished product, depending on the amount of zinc added and type used. According to Chaudhari, “most zinc salts do contribute typical “metallic taste," which could be easily minimized with the use of encapsulated or coated forms, along with a combination of the right flavor and sweetener."