Zinc

Zinc has been recognized as an essential trace mineral for plants, animals and humans since the 1930s. The average adult body contains between 1.5 and 3 g of zinc with approximately 60 per cent of this in the muscles, 30 per cent in the bones and 6 per cent in the skin. The highest concentrations of zinc are in the prostate gland and sperm in men, and in red and white blood cells. The retina of the eye, liver and kidneys also have high concentrations and there is some zinc in hair.

What it does in the body

Metabolism

Zinc functions in over 200 enzymatic reactions in the body. It plays a key role in the synthesis and stabilization of genetic material. It is necessary for cell division and the synthesis and degradation of carbohydrates, lipids and proteins, and is therefore essential for the growth and repair of tissue.

Antioxidant function

As part of the enzyme copper-zinc superoxide dismutase, zinc helps to protect cells and other compounds against the effects of free radicals.

Cells and tissues

Zinc is vital for the normal structure and function of cell membranes. It is vital for the formation of connective tissue, teeth, bone, nails, hair and skin. Zinc may play a role in calcium uptake in bone and modulate the effects of growth hormones.

Immunity

Zinc is considered one of the most important nutrients for the immune system as it is necessary for healthy antibody, white blood cell, thymus gland and hormone function. It is therefore vital in maintaining resistance to infection and in wound-healing.

Hormones

Zinc is necessary for the secretion, synthesis and utilization of insulin. It also protects the insulin-producing pancreatic beta cells against destruction. Zinc is also involved in the metabolism of the pituitary, thyroid and adrenal glands, the ovaries and the testes. It is vital for healthy male sex hormone and prostate function.

Skin

Normal skin function requires zinc. It is involved in oil gland function, local hormone activation, vitamin A binding protein formation, wound-healing, inflammation control and tissue regeneration.

Pregnancy

Zinc is essential for normal fetal growth and development, and for milk production during lactation. Maternal zinc levels are linked to proper formation of the palate and lip, brain, eyes, heart, bones, lungs and urogenital system of the baby. Adequate zinc is necessary for normal growth, birth weight and completion of full term pregnancy.

Other functions

Zinc is necessary for the production of brain neurotransmitters. Healthy liver function and release of vitamin A from the liver both require zinc. Zinc is also necessary for maintenance of vision, taste and smell and is the most abundant trace mineral in the eye. It is involved in the production of hydrochloric acid in the stomach and in the conversion of fatty acids to prostaglandins, which regulate body processes such as heart rate and blood pressure. Zinc is necessary for muscle contraction and maintaining acid-alkali balance. It also helps detoxify alcohol.

Absorption and metabolism

On average, absorption of zinc is around 20 to 40 per cent of dietary intake, improving when zinc intake is low. Absorption also depends on the food source. More zinc is available from animal and fish sources as these high protein foods contain amino acids which bind to zinc and make it more soluble. Zinc from vegetables, fruits and cereals is less well absorbed as these foods contain compounds such as phytates and oxalates which binds zinc and reduces the amount available for absorption. Food additives and chemicals such as EDTA, which are used in food processing, can also reduce zinc absorption as can large amounts of textured vegetable protein. Zinc absorption decreases with age. People over 65 may absorb half as much zinc as those between 25 and 30 years old.

Zinc is combined in the intestines with picolinic acid which is secreted by the pancreas. This compound requires vitamin B6 for production. The zinc picolinate complex is transported across the absorptive cells of the intestine, then to the liver where some is stored. Vitamin B6 deficiency or a decrease in pancreatic secretion, which is often seen in elderly people, will therefore affect zinc absorption.

Excretion of zinc is mainly via the feces but some is lost in the urine. Excessive sweating can cause losses of up to 3 mg per day. Zinc is not well stored in the body and a reduction in dietary intake leads to deficiency fairly quickly. Plasma or serum zinc levels may not reflect body levels. Red or white blood cell measures of zinc may be the most accurate way to assess body stores.

Zinc absorption does not seem to increase during pregnancy but, according to a 1997 study, can increase nearly two-fold during lactation, presumably in response to the demand for zinc to synthesize breast milk.1

Hormone replacement therapy has been shown to decrease zinc excretion. In a study done in 1996, Israeli researchers assessed the effect of estrogen treatment on the excretion of several minerals, including zinc in 37 postmenopausal women. They found that zinc excretion decreased 35 per cent after three months and 26 per cent after one year of treatment.2

Deficiency

Symptoms of zinc deficiency include eczema on the face and hands, hair loss, mental apathy, defects in the reproductive organs, delayed sexual maturation, menstrual irregularities, decreased growth rate and impaired mental development. Deficiency may also lead to postnatal depression, loss of the senses of taste and smell, anemia, poor appetite, impaired conduction and nerve damage, white spots on the nails, mental disorders, susceptibility to infections, delayed wound-healing and impotence in men.

Zinc deficiency was first identified in the Middle East in adolescent male dwarfs with poor development of sexual organs. This was caused by high consumption of unleavened bread, which contains zinc-binding phytates.

Acrodermatitis enteropathica, a rare disease in infancy, is caused by a genetic inability to absorb zinc. Skin rashes appear when a baby is young; and when breastfeeding is stopped, gastrointestinal problems, decreased growth and mental abnormalities are seen. The disorder is treated with zinc supplements.

Alcoholic liver disease, trauma such as burns or surgery, stress, weight loss, chronic infections, viral hepatitis, diabetes and some kidney diseases can increase zinc requirements and increase the risk of deficiency. Athletes often have an increased need for zinc. Diseases of the gastrointestinal system such as inflammatory bowel disease and celiac disease, also reduce zinc absorption and may lead to deficiency symptoms. Zinc levels are low in people suffering from sickle cell disease and fat malabsorption disorders.

Results of the Second National Health and Nutrition Examination Survey, published in 1995, suggest that zinc intakes are declining.3 This is likely to be due to lower meat and higher cereal consumption, food processing methods which reduce zinc content of food and lower soil concentrations of zinc. Those likely to have low intakes include infants; adolescents; women, particularly those who are pregnant; older adults; and those with lower levels of education and higher poverty levels. Pre-school children and vegetarians may also be at risk.

Pregnancy

Zinc deficiency in early pregnancy increases the risk of congenital birth defects, low birth weight, spontaneous abortion, premature delivery, mental retardation and behavior problems in babies; and may also increase the risk of pregnancy induced high blood pressure. Mothers with low zinc intakes may have babies who are more susceptible to infection.

Elderly people

Zinc intakes in older people tend to be much lower than the RDA. This is likely to be due to reduced intake, reduced absorption, the use of medications which affect zinc, and the presence of disease states which alter zinc usage. Zinc deficiency is likely to contribute to clinical conditions commonly seen in elderly people, including poor appetite, slow wound-healing, loss of taste and reduced immune system function.

Alcoholism

Alcoholism increases the risk of zinc deficiency, particularly in those with liver disease. Zinc deficiency in alcoholism is likely to be linked to altered vitamin A metabolism, suppressed immune function, eye problems and sex organ abnormalities. Zinc deficiency may also play a role in fetal alcohol syndrome, birth defects associated with alcohol use by pregnant women.

Diabetes

Alterations in zinc metabolism are seen in people with both Type I and Type II diabetes. Response to insulin may be decreased and excretion in the urine is increased, thus exacerbating the risk of deficiency, with all the associated risks such as poor immune function and increased risk of birth defects.

Zinc deficiency has shown to increase the risk for diabetes in diabetes-prone experimental animals, and low concentrations of zinc have also been shown in the blood of people recently diagnosed with Type I diabetes. The results of a 1995 Swedish study suggest that a low concentration of zinc in drinking water can increase the risk of childhood onset of the disease.4

Cardiovascular disease

Population studies suggest that low blood zinc levels are associated with an increased risk of cardiovascular disease. The results of a recent study done over a period of ten years in Finland, which involved 230 men dying from cardiovascular diseases and 298 controls matched for age, place of residence and smoking, found an increased risk of disease in those with low zinc levels.5 This may be due to an imbalance in the copper-to-zinc ratio.

There is evidence that zinc can protect the inner lining of blood vessels from damage, thus helping to prevent atherosclerosis. This may be due to its membrane-stabilizing, antioxidant and anti-inflammatory properties.6

HIV/AIDS

AIDS patients often suffer from zinc deficiency, which adversely affects the immune system. Studies have shown an increased risk of opportunistic infections in AIDS patients with low zinc levels. In a study done in 1996, researchers at the San Francisco General Hospital found that AIDS patients with zinc deficiency had a higher risk of bacterial infections than patients with normal zinc levels. (See page 616 for more information.)

Eating disorders

Low levels of zinc have been found in sufferers of the eating disorder, anorexia nervosa. And this complex disorder may be exacerbated by zinc deficiency. Initial dieting may lead to deficiency which then reduces the senses of taste and smell, thus exacerbating poor appetite.

Eye problems

Zinc deficiency can lead to loss of eye function as several zinc-dependent enzymes play important roles in eye function. Levels of these enzymes decline with age. Zinc deficiency may contribute to macular degeneration of the central part of the retina. Results from the Beaver Dam Eye Study, published in 1996, suggest a link between low zinc intakes and risk of macular degeneration.7

Premenstrual syndrome

The symptoms of premenstrual syndrome may be exacerbated by zinc deficiency. In a study published in 1994, researchers at Baylor College of Medicine, Houston, Texas assessed copper and zinc levels in ten PMS sufferers and compared these to those in non sufferers. Results showed lower zinc levels in the luteal phase (latter half) of the menstrual cycle in PMS sufferers.8

Male sexual function

Zinc levels are usually lower in infertile men, leading to decreased testosterone levels and low sperm counts. Zinc deficiency in adolescence can delay puberty and zinc seems to play an important role in controlling serum testosterone levels in normal men. In a 1996 study, researchers investigated the relationship between cellular zinc concentrations and serum testosterone in 40 normal men, aged from 20 to 80. Dietary zinc restriction in normal young men was associated with a significant decrease in serum testosterone concentrations after 20 weeks of zinc restriction, and zinc supplementation of marginally zinc-deficient normal elderly men for six months resulted in an increase in blood levels of testosterone.9

Immune system

Immune function is affected by zinc deficiency, which results in a decrease in the numbers of several types of T cells, natural killer cells and other components of the immune response. This leads to increased susceptibility to infection and wound-healing time.

Bones

Diets low in zinc may slow adolescent bone growth and increase the risk of osteoporosis later in life. In a study published in 1996, researchers at the University of California studied two groups of ten monkeys. Both groups were given nutritionally balanced diets but one group received 50 mcg of zinc per gram of food while the other group only received 2 mcg of zinc per gram of food. Eight of the monkeys were then studied throughout their lives to ages equivalent to that of ages 10 to 16 in human girls. The researchers found that the monkeys on low zinc diets had slower skeletal growth, maturation and less bone mass than the other monkeys, with substantial differences noticed in the lumbar spine. The differences were only apparent during rapid growth phases in the monkeys, especially during pregnancy. 10 Zinc excretion appears to be increased in osteoporosis sufferers, probably as a result of increased bone breakdown.

Other disorders

A 1997 Israeli study found low zinc absorption in patients with low disease activity and high disease activity when compared to people without the disease.11 Zinc levels may also be lower in people with asthma. A study done in 1996 in the Slovak Republic found lower zinc levels and high copper-to-zinc ratio in asthmatics.12 Periodontal disease may also be related to zinc deficiency.

Sources

Good sources of zinc include liver, shellfish, oysters, meat, canned fish, hard cheese, whole grains, nuts, eggs and pulses. Vegetables contain smaller amounts of zinc and also contain compounds such as phytates and oxalates which bind zinc, leaving less available for absorption.

The zinc in grains is found mainly in the germ and bran coverings, so food refining and processing reduce the amount of zinc in food. For example, flour refining causes a 77 per cent loss in zinc, rice refining causes a loss of 83 per cent and processing cereals from whole grains causes an 80 per cent loss.

Oysters, raw 6 oysters 15.6 mg

Crab meat, canned 1 can 5.0 mg

Beef, cooked, lean and fat 100g 4.4 mg

Lamb, cooked, lean and fat 100g 4.4 mg

Lobster, cooked 1 cup 4.2 mg

Salmon, canned 1 can 4.2 mg

Veal, cooked, lean and fat 100g 4.0 mg

Lamb kidney, simmered 100g 3.8 mg

Cashews, salted ½ cup 3.8 mg

Branflakes ¾ cup 3.8 mg

All Bran ½ cup 3.7 mg

Sunflower seeds ½ cup 3.6 mg

Oats ½ cup 3.1 mg

Mixed nuts ¼ cup 2.6 mg

Sausages, grilled 100g 2.5 mg

Lentils, boiled 1cup 2.5 mg

Chickpeas, cooked 1cup 2.5 mg

Peanuts, salted ½ cup 2.4 mg

Pork, cooked, lean and fat 100g 2.4 mg

Plain hamburger 1 serve 2.0 mg

Scallops, heated 6 pieces 1.8 mg

Brown rice, boiled 1 cup 1.3 mg

Tuna, canned 1 can 1.3 mg

Barley, pearl, boiled 1 cup 1.3 mg

Bulgur, boiled 1 cup 1.0 mg

Milk, whole 1 cup 1.0 mg

Fruit yogurt 1 tub 1.0 mg

Cod, cooked 1 fillet 1.0 mg

Peanut butter 2 tbsp 0.9 mg

Recommended dietary allowances

USA

Men 15 mg

Women 12 mg

Pregnancy 15 mg

Lactation 19 mg

UK

Men 9.5 mg

Women 7 mg

Pregnancy 13 mg

Australia

Men 12 mg

Women 12 mg

Pregnancy 16 mg

Lactation 18 mg

Supplements

Zinc supplements are available in various forms such as zinc gluconate, zinc sulfate, zinc picolinate or chelated zinc. Zinc in the form of zinc picolinate may be the best supplement for use in those who do not secrete sufficient picolinate from the pancreas.

Zinc supplements may be best taken first thing in the morning or two hours after meals to avoid the inhibition of absorption by other food constituents. However, taking the supplements with meals helps to reduce nausea which occurs in some people who take zinc on an empty stomach. Supplements should not be taken at the same time as medications, which reduce zinc absorption.

If you regularly take zinc in doses of 25 mg or above it is wise to take 2 to 3 mg of copper to avoid imbalances in the copper-to-zinc ratio.

Toxic effects of excess intake

Toxic effects of zinc are rare as excessive absorption is usually prevented by the abdominal pain, nausea and vomiting that very high doses (around 200 mg) cause. Other symptoms include dehydration, lethargy, anemia and dizziness.

Long-term use of high doses causes secondary deficiency of copper. This has been seen with intakes of zinc as low as 25 mg per day. Long-term use of doses above 150 mg have been reported to cause the suppression of immune function and decreased levels of HDL cholesterol which can lead to heart disease. Excessive use of supplements during pregnancy may be harmful to the fetus.

Therapeutic uses