Zinc is an essential mineral needed for over 300 metabolic enzymes to work properly. Zinc plays a major role in the sensitivity of the tissues to circulating sex hormones and is involved in switching on genes to synthesise proteins in response to hormone triggers.
Food sources of zinc include red meat, seafood (especially oysters), offal, brewer’s yeast, whole grains (although processing removes most of their mineral zinc), pulses, eggs and cheese.
Zinc health benefits
Zinc is vital for growth, sexual maturity and wound healing. It plays a central role in immunity and helps to protect against viral infections such as the common cold. Low intakes of dietary zinc (less than 5 mg per day) are associated with low testosterone levels and, in some parts of the world, dietary zinc deficiency is common and results in delayed male puberty.
Zinc and taste
Lack of zinc is a common cause of loss of the senses of taste (ageusia) and smell (anosmia). You can test if your reduced sense of taste is due to zinc deficiency by using a solution of zinc sulphate (15 mg/5 ml). Swirl a teaspoonful in your mouth; if the solution seems tasteless, zinc deficiency is likely. If the solution tastes furry, of minerals or slightly sweet, zinc levels are borderline, and if it tastes strongly unpleasant, zinc levels are normal. Some people with pica – an odd craving for non-nutritive substances such as coal, soil or paper, have zinc deficiency, and this should always be checked for.
Loss of taste also occurs in anorexia nervosa and may be related to zinc deficiency. Taking zinc supplements has been shown to stimulate appetite and improve food intake in people with anorexia nervosa, and can double the rate of increase in body mass index compared with placebo.
Zinc and the common cold
Zinc inhibits viral replication and also has antibacterial effects. Sucking lozenges containing zinc gluconate stimulates immune cells in the throat to boost local immunity against throat infections.
The results from three clinical trials found that taking zinc acetate lozenges at a dose of 80mg to 92 mg per day significantly reduced symptoms of the common cold compared with placebo. Zinc acetate lozenges shortened the duration of nasal discharge by 34%, nasal congestion by 37%, sneezing by 22%, scratchy throat by 33%, sore throat by 18%, hoarseness by 43% and cough by 46%. Sucking zinc lozenges also shortened the duration of muscle ache by 54%.
Other researchers have found that zinc gluconate lozenges are also effective, that lozenges should provide at least 14.2mg zinc and should be sucked at least every 2 hours while awake. For best effect, start treatment within 48 hours of the onset of symptoms.
Data from 17 studies also shows that children receiving zinc supplements are 20% less likely to develop lower respiratory tract infections and pneumonia than those not taking them.
Zinc and Herpes simplex
Zinc inhibits the replication of Herpes simplex cold sore viruses by up to 90%. Among 20 people with more than 6 episodes of recurrent herpes cold sores per year, taking zinc supplements (22.5mg twice daily for 4 months) more than halved the average recurrences to 3 per year, and reduced the duration of each episode to an average of 5.7 days. Applying a zinc oxide cream is also effective at shortening the duration of lip cold sores. Starting to use zinc oxide or zinc glycine cream within 24 hours of onset of symptoms significantly reduced the duration of cold sore lesions by an average of 5 days and also reduced the overall severity of blistering, soreness, itching, and tingling.
Zinc and gastroenteritis
Zinc supplements are used to treat acute gastroenteritis. Data from 18 studies involving 11,180 children under the age of 5 years found that, compared with placebo, zinc supplements significantly reduced diarrhoea duration and reduced the risk of diarrhoea lasting for longer than 7 days by 30%. How zinc reduces diarrhoea is not fully understood but probably relates to its ability to inhibit viral replication.
Zinc and fertility
Zinc is so important for sperm health that it is actively concentrated in prostate tissues. Each ejaculate contains around 5mg zinc – between a third and a half of the daily requirement. Men who are sexually active are therefore more at risk of zinc deficiency than women.
Zinc is needed to keep sperm in a quiet state and prevents premature release of the acrosome enzymes needed to drill a hole through the egg during fertilization. Once in the female reproductive tract, where zinc levels are low, sperm become more active and start releasing these enzymes in what is known as the acrosome reaction, so that fertilization can occur. Men who are zinc deficient may be sub-fertile because their sperm have released their egg-boring enzymes too soon.
Zinc is also needed to keep the genetic material tightly packed within the sperm head.
The results from 20 studies involving over 3,500 men show that zinc levels in semen are significantly lower in men experiencing difficulties with conception compared with fertile males. Taking zinc supplements significantly increased semen volume, sperm motility and the percentage of normal sperm.
Zinc and pre-eclampsia
High blood pressure during pregnancy, pre-eclampsia, is associated with increased risks of miscarriage and premature birth. Thirteen studies involving over a thousand pregnant women, found that those who developed preeclampsia had significantly lower zinc levels than those who did not.
Zinc and prostate health
The prostate gland has the highest concentration of zinc in any body tissue. Lack of zinc is associated with an increased risk of prostatitis (inflammation of the prostate gland).
A clinical trial involving 120 men with chronic prostatitis found that those given zinc sulfate 220 mg daily for 12 weeks significantly improved total symptoms and pain score compared with placebo. This was thought to be due to the anti-bacterial and immune regulating effects of zinc.
Zinc and osteoporosis
Some studies have found that men and women with osteoporosis have blood and bone levels of zinc that are up to 30 per cent lower than in those with healthy bones. The results from 8 clinical trials, involving 2,188 people found that low blood levels of zinc, copper and iron are important risk factors for osteoporosis. Including zinc in supplements designed for bone health is therefore a good idea.
Zinc supplement dose
The upper safe level for long-term use from supplements is suggested as 25 mg per day.
Occasionally higher doses are recommended under medical advice – but need to be taken together with copper at a ratio of 10:1 (eg 15mg zinc:1.5mg copper) to avoid disrupting copper metabolism.