Zinc is an essential mineral that is found in almost every cell of the body. It is necessary for proper growth and immune function. Oysters are well known for their zinc content, but other animal foods are excellent sources of zinc as well. You can also get zinc from plant-based foods, but this zinc is not as well absorbed.

Functions

Zinc's functions include:

• Supporting normal growth and development during pregnancy, childhood, and adolescence
• Playing a role in tissue repair
• Helping the body use carbohydrate, protein, and fat
• Helping to maintain proper immune function, including promoting wound healing
• Serving as part of more than 70 enzymes that control body processes
• Maintaining a sense of taste and smell
• Maintaining normal vitamin A levels and usage

Recommended Intake

Zinc Deficiency

The human body is able to adapt to a short-term mild zinc deficiency by absorbing greater amounts from the foods you eat, and excreting less. However, sustained inadequate zinc intake will affect bodily functions. A zinc deficiency can have the following effects (but is relatively rare in the US except among people with general malnutrition):

• Poor growth
• Acne-like rash
• Hair loss
• Diarrhea
• Delayed sexual maturation
• Impotence
• Sterility
• Eye lesions
• Loss of appetite
• Reduced sense of taste and smell
• Skin lesions and inflammation
• Reduced resistance to infections
• Poor wound healing
• Mental confusion
• Poor learning ability
• Changes in hair and nails
• Anemia

People who may be at risk for a zinc deficiency include:

• People who eat very low calorie diets
• Pregnant women
• Elderly people
• Vegetarians
• Athletes
• Alcoholics or others with chronic liver disease
• People with malnutrition from a variety of causes including cancer and digestive diseases that cause malabsorption and/or diarrhea, for example:
  • Celiac sprue
  • Crohn's disease
  • Short bowel syndrome
  • Other conditions treated with nutrition given by vein (parenteral alimentation)

Zinc Toxicity

Zinc toxicity is rare in the US. However, people who take zinc supplements may reach toxic levels. Oversupplementation with zinc can cause a copper deficiency. Excess zinc may also cause the following:

• Diarrhea
• Cramps
• Nausea
• Vomiting
• Suppressed immune function
• Impaired formation of red blood cells
• Reduced levels of HDL ("good") cholesterol

Major Food Sources

Good sources of zinc include foods of animal origin, such as meat, seafood, and liver. Eggs and milk supply zinc in smaller amounts.

Some plant-based foods also contain zinc, but it is not absorbed as well by the body. Phytates found in plants can bind the zinc and decrease its absorption. Persons who eat large amounts of whole grain bread, especially those who eat few zinc containing foods listed below, may be at special risk. The zinc content of the soil in which these foods are grown also affects the zinc content of plant foods. In addition, many whole-grain foods contain zinc, but this zinc is removed when whole grains are refined.

Health Implications

Infections and Wound Healing

Sufficient levels of zinc are essential for a properly functioning immune system. For example, zinc is required for the development and activity of T-lymphocytes, which are a type of white blood cell that helps fight infection.

When people deficient in zinc are given zinc supplements, biological markers of immunity improve. These effects are most clearly seen among children in emerging nations—when given zinc supplementation, they experience shorter courses of infectious diarrhea and may have less serious problems from pneumonia.

However, if someone is not deficient in zinc, taking zinc supplements does not improve wound healing or other functions of the immune system.

The Common Cold

Zinc lozenges and nasal sprays may help to decrease the duration and severity of cold symptoms. They are believed to directly inhibit viruses in the nose and throat. However, this topic is still controversial, since the findings from scientific studies have been mixed.

Those studies that have found a positive effect suggest beginning zinc at the first sign of a cold. The recommended dose is 13 to 23 mg of zinc as zinc gluconate or zinc acetate every 2 hours. This treatment should continue until symptoms subside, but never for longer than two weeks.

Also, lozenges that have citric acid or tartaric acid should be avoided. These acids are often added to improve flavor, but they can block zinc's antiviral action.

Absorption of Calcium, Iron, and Copper

Minerals can compete with one another for absorption in the body. This is especially a concern when they are taken at high doses, such as those in supplements.

If you take calcium supplements but consume little or no zinc, you might need to take a multivitamin/mineral containing zinc. Most multivitamin/mineral pills contain 100% of the RDA for zinc.

Large doses of zinc can interfere with the body's absorption of copper and iron, other minerals that are essential to immune function. If you are taking a zinc supplement, talk to your doctor about your need for other supplements.

Resources:

American Dietetic Association
http://www.eatright.org/

National Institutes of Health
http://www.nih.gov/

References:

The American Dietetic Association's Complete Food & Nutrition Guide. Chronimed Publishing; 1998.

Facts about dietary supplements. National Institutes of Health website. Available at: http://www.nih.gov/.

The Nutrition Desk Reference . Keats Publishing; 1995.

Perspectives in Nutrition. 2nd ed. Mosby; 1993.

ThirdAge WWW