Philip G. Reeves
When we hear the word "zinc", we older folks more than likely think of it in terms of galvanized water pipes for our houses and watering bins for our animals, not as a nutrient. Indeed, zinc is an essential nutrient that humans and animals cannot live without.
Our bodies contain about 2 g (0.07 oz) of zinc, a little less than the amount of iron. Everybody knows about iron and what it can do to us if we don't get enough in our diet, but hardly anyone knows about the importance of zinc, especially in growth and development.
Part of the zinc in our bodies is incorporated into proteins called metallo-enzymes. If we remove zinc from these enzymes, they don't work. At last count, there were about 50 of these enzymes that keep our system in working order. Some of the important ones are: 1) polymerases that help regulate the passing on of information in our genes (Genes are molecules called DNA); 2) digestive enzymes that help break down the food we eat; 3) a class of peptidases that help regulate our blood pressure and our ability to reproduce; 4) another class that helps regulate our appetite; and 5) still another class of peptidases that aid in wound healing.
Zinc is also important in another kind of proteins that are not enzymes, and are called zinc-fingers. These are proteins that have a specific structure that holds zinc in a configuration that looks like the fingers on your hand. These proteins help find places on our DNA so that molecules called RNA can be made that produce the proteins our cells need to maintain good health. Zinc is also important in maintaining a healthy immune system, although we don't know how it does this yet. There are also reports that zinc may reduce symptoms of the common cold, but these reports need to be confirmed.
Zinc is extremely important in reproduction. If the female doesn't get enough zinc during pregnancy, the offspring may be malnourished and have a high risk of malformations. If the male doesn't get enough zinc in the diet when young, he may have delayed puberty and have a high incidence of infertility. If older, he may become infertile because sperm cannot develop properly.
In animal studies at the Grand Forks Human Nutrition Research Center, we found that an enzyme specific to the testes, called ACE, depends on zinc for activity. If zinc is left out of the diet of these animals, the number of sperm cells are dramatically reduced. Part of the reason for this is that the RNA needed to produce ACE is not transcribed efficiently, showing that without sufficient zinc, our genes do not function properly.
If a little is good, is a lot better? No! Zinc is one of those Goldilocks nutrients that needs to be properly balanced; not too much, not too little, but just right. Because it competes with copper for absorption sites in the intestine, too much zinc (3 to 4 times the RDA; RDA = 12 mg/day) can lead to copper deficiency. Therefore, we advise against taking vitamin and mineral supplements that contain a lot of zinc. However, If you prefer to take a vitamin/mineral supplement once a day, it should contain no more than 12 mg of zinc or 100% of the RDA.
Not all is as gloomy as it sounds. Although most of us get only about 80% of our RDA for zinc, our bodies become very efficient in absorbing the zinc we do get. However, we shouldn't force our bodies to work overtime. Even though it sounds boring, a balanced diet containing a variety of foods is always our best bet. Foods high in zinc include, oysters, meats, nuts and seeds such as sunflower kernels, and whole grain breads.