CORVALLIS, Ore. - Other vitamins and nutrients may get more headlines, but experts say as many as two billion people around the world have diets deficient in zinc - and studies at Oregon State University and elsewhere are raising concerns about the health implications this holds for infectious disease, immune function, DNA damage and cancer.
One new study has found DNA damage in humans caused by only minor zinc deficiency.
Zinc deficiency is quite common in the developing world. Even in the United States, about 12 percent of the population is probably at risk for zinc deficiency, and perhaps as many as 40 percent of the elderly, due to inadequate dietary intake and less absorption of this essential nutrient, experts say. Many or most people have never been tested for zinc status, but existing tests are so poor it might not make much difference if they had been.
"Zinc deficiencies have been somewhat under the radar because we just don't know that much about mechanisms that control its absorption, role, or even how to test for it in people with any accuracy," said Emily Ho, an associate professor with the Linus Pauling Institute at OSU, and international expert on the role of dietary zinc.
However, studies have shown that zinc is essential to protecting against oxidative stress and helping DNA repair - meaning that in the face of zinc deficiency, the body's ability to repair genetic damage may be decreasing even as the amount of damage is going up.
Two studies recently published, in the Journal of Nutrition and the American Journal of Clinical Nutrition, found significant levels of DNA damage both with laboratory animals and in apparently healthy men who have low zinc intake. Zinc depletion caused strands of their DNA to break, and increasing the intake of zinc reversed the damage back to normal levels.
"In one clinical study with men, we were able to see increases in DNA damage from zinc deficiency even before existing tests, like decreased plasma zinc levels, could spot the zinc deficiency," Ho said. "An inadequate level of zinc intake clearly has consequences for cellular health."
Many zinc studies, Ho said, have focused on prostate cancer - the second leading cause of cancer deaths in American men - because the prostate gland has one of the highest concentrations of zinc in the body, for reasons that are not clearly known.
When prostate glands become cancerous, their level of zinc drops precipitously, and some studies have suggested that increasing zinc in the prostate may at least help prevent prostate cancer and could potentially be a therapeutic strategy. There are concerns about the relationship of zinc intake to esophageal, breast, and head and neck cancers. And the reduced zinc status that occurs with aging may also contribute to a higher incidence of infection and autoimmune diseases, researchers said in one study in the Journal of Nutrition.
Zinc is naturally found associated with proteins in such meats as beef and poultry, and in even higher levels in shellfish such as oysters. It's available in plants but poorly absorbed from them, raising additional concerns for vegetarians. And inadequate intake is so prevalent in the elderly, Ho said, that they should usually consider taking a multivitamin to ensure adequate levels.
Zinc is an essential micronutrient for numerous cellular processes. But taking too much zinc can also be a concern, because in excess it can interfere with the absorption of other important nutrients such as iron and copper. The recommended daily allowance is eight milligrams a day for women, 11 for men, and anything over 50 milligrams a day could be considered excessive, Ho said.
"The consequences of zinc deficiency in adults have been understudied despite the recognition of symptoms of zinc deficiency for decades," researchers wrote in one recent report. "A considerable body of evidence suggests that zinc deficiency may increase the risk of some chronic diseases, including cancer. This link may be attributed to the role of zinc in antioxidant defense and DNA damage repair."