Zinc, vital for immunity

Zinc is a common metal that was first discovered in ancient times. Most metallic zinc nowadays is used to galvanise steel. When mixed with copper it produces brass. 

Zinc oxide is used in the rubber industry and as pigment in plastics, cosmetics and photocopier paper. It can also be used as a sunblock, commonly seen with test cricketers. Calamine lotion which is used to calm skin irritation is a mixture of zinc and iron oxides.

What zinc does for us

Zinc is essential for all humans. It plays a major role in the cells of our body, where it is involved in multitude of roles. One of the key roles is in allowing our cells to grow and develop. As such it is especially needed where there is a high turnover of cells such as in the intestines, skin, eyes and the cells of the immune system. It also activates parts of the brain involved in taste and smell.

Deficiency in zinc can lead to problems including night blindness, reduced fertility, poor immunity, diarrhoea and skin rashes.

Do we need to worry about zinc

Zinc is probably the most deficient mineral in the modern diet. This is likely due to the large intake of processed foods that replaces natural foods that would contain more zinc.

Anyone consuming a lot of alcohol, sweating excessively or eating a poor diet is likely to be short on zinc. Given zincs major role on health, it is worth being aware where you can get zinc from in your diet.  

Which foods provide most zinc

Unfortunately for vegans zinc is best absorbed from foods such as meat, seafood and eggs. These foods contain sulphur containing amino acids that help with zinc absorption. The zinc found in wholegrains, nuts and seeds is less bioavailable than that found in meat, fish and eggs as it is tightly bound by phytic acid, preventing much absorption taking place in the gut.

When to supplement

There are a number of health conditions which merit consideration of supplemental zinc.

Common cold

There is quite strong evidence that zinc lozenges can reduce the duration of cold symptoms. It is thought that zinc ions bind to sites on the surface of the rhinovirus that causes the common cold. These sites are used to help the virus stick to the surfaces lining our mouth and throat. By preventing the rhinovirus sticking to surfaces in our mouth and throat zinc can stop colds getting established(1).

The most convincing effects were found for adults, with a typical reduction in cold duration from 5-9 days of symptoms for those taking a placebo compared to 3-5 days for those taking zinc. In contrast children did not seem to benefit very much, if at all(2).

The types of zinc lozenge tested have included zinc gluconate, zinc acetate and zinc sulphate. I myself take zinc citrate and feel it has been beneficial. It is hard to say whether one formulation is better than another. Reported differences between trials may well represent greater usage of one type with children than another(2). My own take is that there is probably not much between them.

Don't take zinc lozenges in large doses (typically >5 tablets per day) for periods of greater than 1 week. There are a few reasons for this. Firstly the effects are not proven for extended durations of intake. Secondly most zinc tablets have sugar as the main ingredient. Thirdly many zinc tablets also contain vitamin C, which is not anywhere near as effective at reducing colds as zinc, and being acidic, can irritate your tongue and throat.

An alternative to zinc lozenges is a zinc nasal spray. However, I do not recommend its use as it can lead to permanent loss of smell (anosmia).

Depression

Zinc plays important roles within the brain as is demonstrated by the fact that children deficient in zinc may experience reduced cognitive deveolopment and attention disorders. 

People with depression generally have much lower levels of zinc in their blood than other people. This is particularly so for women(3). There is some evidence that zinc supplementation at 25mg per day for 3 months may significantly help reduce subjective measures of depression after a period of about 3 weeks(4).

Age Related Macular Degeneration (ARMD)

Age related macular degeneration is thought to affect about 1.5% of people over the age of 40. It is indicated by the presence of increasing amounts of drusen (white/yellow, extracellular deposits in the macula). The macula is a circle of roughly 6mm in diameter that surrounds the fovea, the part of the retina that is responsible for our central, high resolution vision. ARMD leads to an obvious degeneration in our vision. The prevalence of ARMD increases as we age and it is present in 15% of women over the age of 80(5). Anything that can reduce the likelihood and severity of this condition then is therefore well worthwhile.

There is reasonable evidence that supplementation with zinc and a range of anti-oxidants such as vitamin A, can reduce ARMD(6). Vitamin A is vital for eye function and it is zinc that helps convert the retinol form of vitamin A into the retinal form that helps us see.

Fertility and prostate cancer

There is reason to believe that having reasonably high intakes of zinc can be protective against prostate cancer, as this type of cancer involves cells that cannot transport enough zinc through their plasma emmbranes(7). However, really excessive amounts of zinc supplementation (>100mg) for 5years+ could make prostate cancer worse.

How much zinc should I consume

The RDA - recommended daily allowance of zinc is 8-11mg/day. The amount you actually need is determined by a number of factors: 1) your health, 2) what % you can absorb, 3) your activity level, 4) what type of foods you eat and 5) your size. Some people may get by on as little as 5mg/day, while others may need at least 25mg/day to be at full health. 

People who consume large amounts of zinc such as 150mg per day may well suffer from reversible gastrointestinal symptoms such as nausea and abdominal pain (8). Smaller amounts such as 40mg/day over a long period of months-years may experience copper deficiency. This can lead to aneamia and impaired function of the heart.

References

1) http://jn.nutrition.org/content/130/5/1512S.long

2) http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3394849/#!po=35.0000

3) http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3272121/

4) http://www.if-pan.krakow.pl/pjp/pdf/2003/6_1143.pdf

5) http://www.nei.nih.gov/eyedata/pbd4.asp

6) http://www.ncbi.nlm.nih.gov/pubmed/11594942

7) http://archopht.jamanetwork.com/article.aspx?articleid=416806

8) http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2872358/#!po=6.52174