Midwinter is traditionally the time for feasting; enjoyed, all too often, to a counterpoint of coughs and sneezes. And seasonal flu often peaks in the final week of the year.

Might there be a connection, then, between the feasts of the winter solstice and the desire to protect against the infections it brings? In the days before antibiotics and flu vaccines, avoiding infections could have been life-preserving. And it is certainly true that the immune system can be fortified by diet.

Serious malnutrition depresses immune function, says Professor Philip Calder, of Southampton University, one of Britain’s leading experts on diet and immunity. “It’s clear that people who are malnourished or suffer nutrient deficiencies have impaired immunity and increased suceptibility to infection,” he says. “If the malnutrition is reversed, the immune system returns to more like normal.”

Last month Professor Calder was awarded the prestigious Nutricia International Award, by the charity the Nutricia Research Foundation, for his pioneering research into nutritional immunology or, put more simply, how diet affects our immune system.

So how much does what we eat affect our response to winter bugs? Quite a lot, according to Professor Calder. “Vitamins and minerals are essential for the immune system, which is particularly sensitive to deficiencies,” he says. In the past few years his work has revealed that the key dietary ingredients for boosting the immune system are zinc, selenium, vitamin D, and the long-chain fatty acids known as omega3s and 6s.

ZINC

“If people are made zinc-deficient through their diet, then their immune system declines,” Professor Calder says. “Give them zinc and it improves again.” Zinc increases the number of infection-fighting T cells, and there is some evidence that it can slow the growth of cancer. But few doctors recommend taking zinc supplements to boost the immune response. A healthy, normal diet should contain enough. As it happens, the dark meat of turkey is a good source, as are oysters, crab, red meat, beans (including baked beans), wholegrain cereals and nuts.

SELENIUM

Another mineral with an immune-enhancing role is selenium, and it is one that our diets may lack. Turkey is also a good source of selenium. The trouble is that pigging out on it once a year is not really going to make up for deficiencies during the rest of the year. But there are other food sources of this important mineral, including seafood, vegetables, brown rice, egg yolks, lamb chops, garlic and brazil nuts.

VITAMIN D

Another key nutrient that Professor Calder suspects we may lack is vitamin D. Our normal diets do provide some vitamin D, but much of it is made in the skin after exposure to sunlight. In the midwinter, it is quite possible that some people are deficient. Cod-liver oil is a cracking source of vitamin D, but all oily fish are good. So are eggs, and fortified foods such as margarine and breakfast cereals.

The actual role of vitamin D in the immune system is not as a simple booster. The secret of successful immunity is not to try to erect an impregnable barrier against infection, but to achieve a balance. The reason is that an overactive or wrongly programmed immune system can turn on its host. While the immune system is designed to overlook some foreign elements – such as the bacteria that teem in our guts, for example – its ability to do so declines with age. The result is a reduced ability to distinguish self from outsider, and the consequence is autoimmune disease, which can happen at any age but tends to increase in the elderly. The immune system attacks the body it is supposed to defend, causing inflammation and damage. One example is rheumatoid arthritis.

Vitamin D seems to play a part in achieving a good balance in the immune system: alert to the invader, but gentle to the host. A study in Finland, reported in The Lancet in 2001, found that children who were given vitamin D supplements during their first year of life were 80 per cent less likely to develop type 1 diabetes, which is caused by the immune system destroying the cells of the pancreas that make insulin.

OMEGA3s AND 6s

The omega3 and omega6 fatty acids, which have been the main focus of Professor Calder’s research, have a similar balancing role. Immune system cells typically contain high levels of the omega6, and low levels of the omega3 acid.

In diseases such as rheumatoid arthritis, high levels of omega3 in the diet – from oily fish, for example – can reduce symptoms by moderating the misdirected inflammatory response.

Professor Calder’s research suggests that the omega3 acid achieves this effect by improving the physical structure of the membrane that surrounds all cells, including those of the immune system. This membrane consists of fatty acids, so the explanation makes sense.

So the key to good health is not boosting the immune system at all costs, but ensuring that it is in balance. For that, all the usual and familiar dietary rules apply: a balanced diet, with plenty of fruit and vegetables.

But will Christmas dinner boost your immune system ready for the return to work in the grim new year? Not so you’d notice, I suspect. But all that selenium and zinc can’t do a lot of harm.

Nigel Hawkes is the Health Editor of The Times

Give your body a boost . . .

Blueberry and grapefruit juice University of Sydney scientists announced this week that experiments on mice suggest that the juice may help fight prostate cancer.

Extra-virgin olive oil Studies say its monounsaturated fat might protect against age-related cognitive decline.

Hot chocolate Plant chemicals called flavanols in cocoa increase blood flow to the thinking parts. May be especially beneficial for older brains.

Tomatoes Linked to reducing the risk of heart disease and prostate problems, probably due to their many nutrients, including lycopene.

PROF OF THE WEEK

Who? Peter Sandercock (it’s pronounced Sander-coke), professor of medical neurology at Edinburgh University.

Why? The Government promised quicker care for strokes this week, and Sandercock is leading a global effort to improve rapid treatment. He’s in charge of the Third International Stroke trial, testing a clot-busting drug called Actilyse. “It is injected into the vein, like a chemical Dyno-Rod that unblocks the artery. If it is done within two or three hours of a stroke, the brain can actually recover,” he explains. “At the moment, less than 0.2 per cent of people get treated by the NHS this way, even though there are 150,000 strokes in Britain a year.”

His mission He aims to make sure that doctors all around the world adopt this approach, by showing in a huge trial that it works. He’s seeking to recruit about 3,000 people across the globe with strokes that can be treated within six hours. That takes someone with great organisational skills. “To the outside world I might look organised and tidy. You have to, if you want people to collaborate with you across the world, free of charge. They must feel that they are involved in something very professional, even if you are sometimes plagued by self-doubt and problems are looming.”

Life The father of four loves walking and skiiing, especially hill-walking in Scotland. “Do I bag Monros? It’s a bit nerdy for me. I like to get to the summit, but I don’t record it.”

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