From The July 2001 Issue of Nutrition Science News
By Marilyn Sterling, R.D.
Zinc is one of the nutrients most essential to health. In the last year, research has brought further understanding of zinc's critical role in immune system health. Zinc is now understood to maintain proper functioning of the epithelial cells lining the intestines and blood vessels. In addition, some studies have yielded intriguing results suggesting that zinc may protect against atherosclerosis and diabetes, as well as opportunistic human immunodeficiency virus (HIV) infections, and even the common cold.
Zinc and the Immune System
Zinc deficiency has been noted in many illnesses, including sickle cell anemia, kidney disease, and gastrointestinal disorders. It can also contribute to disruptions in the immune system that accompany these diseases. In fact, a recent study at Michigan State University in East Lansing shows that the immune systems of animals given inadequate zinc for a month decreased in effectiveness by 30 to 80 percent. 
Recent research has helped explain why zinc is so necessary to the immune system. The warriors in the immune system's army are white blood cells called lymphocytes. Classes of lymphocytes include B cells, T cells, and natural killer cells. B cells secrete antibodies into body fluids. T cells regulate the immune system and destroy both damaged cells and invaders when antibodies recognize a specific antigen. Natural killer and other disease-fighting cells attack and kill cells indiscriminately without the stimulus of a particular antigen. Supplemental zinc, as well as vitamin C, may enhance the activity of natural killer cells. These same researchers, in the department of cell biology and human anatomy at the University of California-Davis School of Medicine, also found that deficiencies of zinc, as well as vitamins A and D, may conversely reduce natural killer cell function. 
All lymphocytes develop in the bone marrow and require zinc for normal levels of production. T cells complete development in the thymus gland. Lymphocytes cannot function without thymulin, a hormone produced in the thymus that binds to zinc. People with HIV have low thymulin levels, which inhibits their ability to fight off infections. Zinc supplementation of 45 mg/day for one month in HIV patients on AZT drug therapy was found to boost body defenses against such opportunistic invaders as candida (a fungus) and pneumocystis carinii (a parasite that causes pneumocystis pneumonia). 
Certain large lymphocytes kill bacteria and viruses by engulfing them. One type, the neutrophils, also contains granules of toxic chemicals. Zinc is required for proper neutrophil function. A Japanese in vitro study found both a deficiency or excessive amount of zinc can inhibit this normal functioning. 
Since zinc is so critical to immune function, it is no wonder that zinc supplementation has been found beneficial in fighting infection. Diarrhea is the biggest childhood killer in developing countries. A recent analysis of studies on the subject determined that children receiving zinc supplementation had a 42 percent lower death rate from persistent diarrhea, particularly among males under age 12, those who had wasting, or those with lower baseline plasma zinc concentrations.  Zinc supplementation also has been found to reduce attacks of malaria, a major cause of disability in much of the world. 
Zinc is also beneficial to the average American. Older people have less efficient immune systems, are more vulnerable to infections, and are less apt to survive them. New evidence shows that supplementing with zinc at the RDA dosage15 mg/day for men, 12 mg/day for womenhelps maintain immune function in the elderly. Within two months of zinc supplementation, resistance improves, and the chances of surviving an infection increases. 
Zinc was recently found to dramatically relieve suffering from the common cold. An over-the-counter homeopathic zinc nasal gel formulation was given to 108 patients; another 105 received placebo. When given within 24 hours of symptom onset, the zinc nasal medicine shortened cold duration from the usual 9 days to 2.3 days. 
Zinc and Growth
Severe zinc deficiency has long been known to impair growth and sexual development in children. Inadequate zinc may also inhibit childhood development in more subtle ways. In a recent study, babies in Chile were randomly assigned to placebo or 5 mg/day zinc until they were 1 year old. Although the growth rate and size of the children in the two groups were the same, babies taking the placebo were more apt to lag in psychomotor or mental development than those taking zinc. 
Another study conducted in Brazil on short children aged 7 to 10 found zinc supplementation increased their growth rate by 5.99+/-0.80 cm/year compared with 5.05 +/-0.85 cm/year for placebo. When zinc supplementation was discontinued, the growth rate slowed. 
In the United States, children between 1 and 3 years of age are at risk of zinc deficiency, according to the National Health and Nutritional Examination Survey III. 
Most pregnant women are given iron supplements, which can deplete zinc.  Thus, some experts, such as Fernando Viteri, M.D., at the University of CaliforniaBerkeley, are concerned about the routine use of iron supplements among pregnant women and recommend taking such supplements only once a week. 
Zinc and Obesity
Although zinc is necessary for normal growth, zinc supplementation does not lead to obesity. In fact, the reverse may be true. Leptin, a hormone that regulates appetite, causes satiety. In developed countries, a high percentage of calories comes from table sugar, or sucrose. Animal experiments find that sucrose increases circulating levels of insulin, decreases leptin, and causes obesity. However, rats given zinc supplements with sugar showed normal leptin levels. 
A recent study found that children with higher zinc levels were less apt to be obese than children with low levels. 
These results may be especially important for diabetics, who have high blood sugar and disrupted leptin regulation. In an animal model of diabetes, zinc supplementation normalized blood sugar. Despite lowering blood sugar impressively, supplementation did not affect weight gain, fat content or food intake. 
Zinc and Cellular Health
Researchers are beginning to understand zinc's role in maintaining the structural integrity of the endothelia, which line the blood vessels, and the epithelia, which line the gastrointestinal tract. In atherosclerosis, arterial endothelial cells are destroyed by oxidating fatty acids and inflammatory immune factors. Zinc deficiency magnifies the problem. Supplementation with zinc has recently been found to protect the integrity of the blood vessel cell lining. 
Zinc is also necessary to maintain the health and integrity of epithelial cells that line the intestines. Cell studies have found zinc is required for intestinal wound healing. Without adequate zinc, wounds do not heal properly. Conversely, supplementation with 12.5-50 microM zinc caused an enhancement of epithelial cell restitution, the initial step of wound healing. Inflammatory bowel disease, a serious intestinal disorder, causes intestinal lesions, which zinc has been found to help heal. Parasites and other intestinal problems can also damage the intestines. Adequate absorption of nutrients depends on an intact intestine, so intestinal injuries can lead to zinc deficiency. This may cause appetite loss and diarrhea, speeding the downward spiral of zinc loss and tissue damage. 
Do We Get Enough?
Inadequate zinc intake has been implicated in many diseases; however, no laboratory test can clearly distinguish zinc deficiences.  The National Health and Nutrition Examination Survey III found that only half the U.S. population gets the RDA of zinc. Most at risk were young children, teenage girls, and people over age 71. Other studies have found that elderly people, those with lower incomes, and those with less education are apt to consume inadequate amounts of zinc. 
The RDA for zinc is 15 mg/day for men and 12 mg/day for women. The average intake of zinc from diet alone is 9.5 to 10.5 mg/day. Foods highest in zinc include seafood (especially oysters), beef, poultry, legumes, hot and cold cereals and dairy products. Vegetarians are at risk of deficiency. Supplements add, on average, 2.5 to 3.5 mg extra zinc to daily intake and thereby help protect against more severe deficiencies. 
In the past year, science has provided more evidence of zinc's crucial role in maintaining immunity, regulating appetite, and safeguarding epithelial cell function. Supplements can help the half of all Americans presently zinc deficient.
Marilyn Sterling, R.D., M.P.H., is a consultant to the natural products industry, a freelance health writer, and a contributing editor to Nutrition Science News. She is also a clinical nutritionist in Trinidad, Calif.
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