The varicella zoster virus that can trigger shingles, also called herpes zoster, is found in everyone who has had chicken pox, a disease that has affected approximately 90 per cent of any population. The virus lies dormant in the spinal cord until triggered.
If the immune system is weakened, the virus can reactivate and cause shingles. The risk of the disease increases with age. It is estimated that 60 per cent of 85 year olds will have suffered from shingles during their life.
The new study, published on-line in the International Journal of Epidemiology (doi: 10.1093/ije/dyi270), examined the association between the risk of shingles and the dietary and supplementary intake of a micronutrients, and also with respect to fruit and vegetable consumption.
The micronutrients studied were vitamins A, B6, C, E, folic acid, zinc and iron. All of these have been linked to good immune system health.
Intake of fruit and vegetables, and micronutrients for the 243 case (people with shingles) and 488 control (healthy) participants was recorded using the validated British version of the Nurses' Health Study food frequency questionnaire (FFQ). The age of the volunteers ranged from 16.5 to 91.2, with an average age of 57.1.
"In this population, we found a strong dose-response relationship between decreasing food intake and increasing zoster [shingles] risk amongst both younger and older individuals," reported lead researcher Sara Thomas from the London School of Hygiene and Tropical Medicine.
Eating less than one piece of fruit per day was associated with a three-fold increased in the risk of shingles, compared to those who ate more than three portions per day. People with decreased micronutrient availability had a five-fold increased risk of the disease.
Interestingly, none of the micronutrients on their own had a significant effect on the risk of shingles, but when taken together appeared to be protective.
"These results suggest that a mix of nutrients such as those found in fruit and vegetables act together to maintain immune health," said Thomas.
When the results were analysed in relation to non-prescription supplements no protective effect was observed.
"Perhaps because supplements do not replicate the full mix of nutrients in fruit and vegetables, or because some participants with poor immune function took micronutrients to compensate for poor diet or for feeling unwell," explained the researchers.
This observation may help to explain why Scottish researchers saw no effect of multivitamin supplements on the immune system of elderly people (British Medical Journal, 2005, Vol. 331, pp. 324-327). It may also be that the micronutrient dose in the supplements was too small to initiate a response.
Miscalculation of nutrient intake, a problem commonly associated with using a FFQ, may have occurred, admit the researchers, since the choice for fruit and vegetables items was narrow. Indeed, younger participants are likely to have a more varied diet, which would have increased the misclassification.
Also, the researchers did not measure cell-mediated immune function, so no investigation of the effects on the micronutrients on the aging of the immune system (immunosenescence).
The results may not be too surprising, however, since shingles are linked to weakened immune systems, and the micronutrients studied have previously been associated with immune health. It is further support for a diet rich in fruit and vegetables.
The researchers proposed that existing cohort studies could easily integrate a measure of biochemical markers to test for immune system health.
In Europe, most countries do not vaccinate against chickenpox. A routine vaccination program was launched in the US in 1995.