Introduction

Zinc, an essential mineral, is naturally present in some foods, added to others, and available as a dietary supplement. Zinc is also found in some cold lozenges, over-the-counter drugs sold as cold remedies, and some denture adhesive creams.

Zinc is involved in many aspects of cellular metabolism. It is required for the catalytic activity of hundreds of enzymes, and it plays a role in enhancing immune function, protein and DNA synthesis, wound healing, and cell signaling and division [1-4]. Zinc also supports healthy growth and development during pregnancy, infancy, childhood, and adolescence and is involved in the sense of taste [2,3,5].

The total amount of zinc in the body is approximately 1.5 g in women and 2.5 g in men [2]. Most of this zinc is stored in skeletal muscle and bone [1-3].

The processes that maintain zinc homeostasis are absorption of zinc from the diet, excretion into the gastrointestinal tract, and reabsorption in the gastrointestinal lumen [2,3]. In general, as zinc intakes rise, the amount of zinc absorbed also increases, but its fractional absorption drops [2,3].

Serum or plasma zinc levels are typically used in clinical practice to assess zinc status. In healthy people, the amount of zinc in serum or plasma is 80 to 120 mcg/dL (12 to 18 mcmol/L) [2]. Serum zinc levels below 70 mcg/dL in women and 74 mcg/dL in men indicate inadequate zinc status. However, both serum and plasma measures have important limitations. Zinc concentrations in serum are associated with the patient’s sex and age as well as the time of the blood draw (morning vs. evening) and do not always correlate with dietary or supplemental zinc intakes [6]. Zinc levels also fluctuate in response to other factors, including infections, changes in steroid hormones, and muscle catabolism during weight loss or illness [1,3]. Clinicians consider risk factors (such as inadequate caloric intake, chronic alcohol use, and malabsorptive digestive diseases) and signs of zinc deficiency (such as impaired growth in infants and children) when they assess a patient’s zinc status [1].

Recommended Intakes

Intake recommendations for zinc and other nutrients are provided in the Dietary Reference Intakes (DRIs) developed by the Food and Nutrition Board (FNB) at the National Academies of Sciences, Engineering, and Medicine [1]. DRIs is the general term for a set of reference values used for planning and assessing nutrient intakes of healthy people. These values, which vary by age and sex, include:

Recommended Dietary Allowance (RDA): Average daily level of intake sufficient to meet the nutrient requirements of nearly all (97%–98%) healthy individuals; often used to plan nutritionally adequate diets for individuals.

Adequate Intake (AI): Intake at this level is assumed to ensure nutritional adequacy; established when evidence is insufficient to develop an RDA.

Estimated Average Requirement (EAR): Average daily level of intake estimated to meet the requirements of 50% of healthy individuals; usually used to assess the nutrient intakes of groups of people and to plan nutritionally adequate diets for them; can also be used to assess the nutrient intakes of individuals.

Tolerable Upper Intake Level (UL): Maximum daily intake unlikely to cause adverse health effects.

Sources of Zinc

Food

The richest food sources of zinc include meat, fish, and seafood [3]. Oysters contain more zinc per serving than any other food, but beef contributes 20% of zinc intakes from food in the United States because it is commonly consumed [7]. Eggs and dairy products also contain zinc [3]. Beans, nuts, and whole grains contain zinc, but the bioavailability of zinc from these foods is lower than that from animal foods because these foods contain phytates. Phytates, the storage form of phosphorus in plants, bind some minerals such as zinc in the intestine and form an insoluble complex that inhibits zinc absorption [1-3,8]. Fruits and vegetables contain very little zinc.

Breakfast cereals, which are often fortified with zinc, are a major source of zinc in the U.S. diet [3]. In U.S. children and adolescents, 12.1% to 18.4% of daily zinc intakes come from enriched and fortified foods [9].

The amount of zinc absorbed from food ranges from 5% to more than 50%, depending on the amount of plant-based foods (and thus of phytate) in the diet [3]. The absorption of zinc from mixed meals or diets containing a combination of animal-based and plant-based foods is lower than from diets or meals containing animal-based foods only [10].

Dietary supplements

Zinc is available in supplements containing only zinc; supplements containing zinc in combination with other ingredients; and in many multivitamin/multimineral products [13]. Supplements can contain any of a variety of forms of zinc, including zinc sulfate, zinc acetate, and zinc gluconate [14]. The Supplement Facts panel on a dietary supplement label declares the amount of elemental zinc in the product, not the weight of the entire zinc-containing compound.

Absorption of zinc from supplements containing zinc citrate or zinc gluconate is similar, at approximately 61% in young adults; the absorption from supplements containing zinc oxide is 50% [15]. Taking supplements containing 25 mg elemental iron or more at the same time as zinc supplements can reduce zinc absorption and plasma zinc concentrations [16-19]. However, the iron added to enriched or fortified foods does not interfere with zinc absorption.

Other sources

Products containing zinc include some labeled as homeopathic medications as well as nasal sprays that are sold over the counter to treat or prevent the common cold, manage flu symptoms, and support immune system health.

Some denture adhesive creams contain 17–34 mg/g zinc to enhance their adhesive properties [20]. Use as directed, resulting in intakes of 0.5–1.5 g zinc from these products per day, is not a concern. However, chronic, excessive use can lead to zinc toxicity, which can cause copper deficiency and neurologic diseases. Such toxicity has been reported in individuals who use at least one tube containing a total of 2.4 g or more denture cream each week for several years [21-23]. Many denture creams have been reformulated to omit zinc.

Zinc Intakes and Status

Most people in the United States consume adequate amounts of zinc, but people in certain age groups, especially older adults, might have marginal intakes. The average daily zinc intake from foods is 7.6–9.7 mg/day in children aged 2–11 years, 10.1 mg/day in children and teens aged 12–19 years, 13 mg/day in men older than 19, and 9.2 mg/day in women [24]. The average daily zinc intake from foods and supplements is 8.4–10.4 mg/day in children aged 2–11 years, 10.7 mg/day in children and teens aged 12–19 years, 16.4 mg/day in men, and 12.6 mg/day in women. In pregnant people aged 20–40 years, the mean zinc intake is 12.4 mg/day from foods alone and 22.7 mg/day from foods and supplements [25].

Less than 1% of U.S. children aged 2 to 8 years have zinc intakes from foods and supplements that are lower than the EAR. Among boys aged 14 to 18 years, 1% have intakes below the EAR [9]. But the proportion is much higher, 20.9%, in girls in this age group. National Health and Nutrition Examination Survey (NHANES) data from 2005 to 2016 show that among U.S. adults aged 19 years and older, 15% have zinc intakes below the EAR [26].

In the United States, 3.8% of children younger than 10, 8.6% of males aged 10 and older, and 8.2% of females have serum zinc concentrations below the serum zinc cutoff for sufficiency for their age and sex according to an analysis of 2011–2014 NHANES data [6,8]. The investigators used cutoffs developed by the Biomarkers of Nutrition for Development Zinc Expert Panel on the basis of a statistical definition: 57–65 mcg/dL for children younger than 10, 69–70 mcg/dL for females aged 10 and older, and 61–74 mcg/dL for males aged 10 and older [8].

Zinc deficiency based on serum or plasma concentrations is much more common in low-income and middle-income countries [27]. According to an estimate based on analyses of zinc and phytate available in national food supplies and the prevalence of stunting, approximately 17% of the world’s population is likely to have zinc deficiency [28].