2.11 Zinc (Zn)
Zinc is involved in many metabolic processes in the body, due to its importance in the functioning of more than 100 enzymes. These control, amongst other things, metabolism of foods, production of energy, cell division and protein synthesis. The body contains 2 to 3 g of zinc in total. Zinc is most commonly found in protein-rich foods, such as meat, and also in peanuts and pulses (peas and beans). Generally sufficient is available in Western diets, although there is some concern that amounts are declining due to increased processing of food and there may be insufficient reserves in some individuals to cope with increasing demands such as during periods of increased growth in children, during pregnancy and during wound healing after injury. Vegetarians who consume a variety of legumes and nuts will probably take in sufficient zinc, but other vegetarian diets may not contain enough, especially since zinc from plant sources is absorbed less readily than that from animal foods.
The clearest evidence of zinc deficiency was seen in the 1960s, when Iranian military officials noted that an unusual number of young men eligible for army duty were short in stature and showed delayed sexual maturation, as well as a number of other symptoms. Earlier research had shown that similar symptoms developed in animals if they were deprived of zinc in the diet. When the zinc levels of the army recruits were tested, they were found to be particularly low. Their normal diet was based almost exclusively on cereals, which although they do contain zinc, also contain chemicals called phytates (see the section on iron) which prevent the zinc from being absorbed. Furthermore, many of the young men indulged in the strange habit of geophagia, or clay eating. Clay binds to zinc in the digestive system and slows down its absorption. After treatment with a well-balanced diet containing adequate amounts of zinc for a year, pubic hair appeared, sexual organs increased in size, and growth in height was resumed, so confirming the vital role of zinc.
Signs of mild zinc deficiency are less dramatic and there is no specific deficiency disease associated with zinc. Instead many general signs appear such as poor appetite, a decrease in the sense of taste and smell, weight loss, poor night vision, delayed healing of wounds and repeated infections. About 25% of people who have an impairment in taste and or smell are suffering from zinc deficiency. Half of people with anorexia nervosa also appear to have a zinc deficiency and there is some evidence that zinc supplements improve the condition. Zinc supplements are sometimes used to treat skin ulcers or bed sores, but they do not increase rates of wound healing when zinc levels are normal. Zinc and castor oil creams are used to prevent nappy rash in babies. There are also health risks if the intake of zinc is too high. Metal fume fever, also called brass-founders’ ague or zinc shakes, is an industrial disease caused by inhaling zinc oxide fumes, which cause damage to the nervous system.