Brisk walking is all that is needed, and the longer that you continue to exercise daily the greater the benefit.

The trial recruited 115 older women from Seattle, Washington state. All were sedentary and either overweight or obese. For a year they were asked either to exercise daily or take part in a 45-minute stretching class once a week.

The women were randomly allocated to one or other of these groups, and every quarter were asked to fill in questionnaires asking them whether they had suffered colds or other upper respiratory infections in the previous three months.

The results, published in The American Journal of Medicine, show that the daily exercise group had only half as many colds as the weekly stretch-class group. Over the final three months of the study the gap was even wider, with the stretchers suffering three times as many colds as the exercisers.

“This adds another good reason to put exercise on your to-do list,” said Cornelia Ulrich, the paper’s senior author and an associate member of the Fred Hutchinson Cancer Research Centre’s public health sciences division in Seattle.

But moderation was the key. Other studies had shown that excessive exercise could increase the risk of colds. She said that the likeliest cause of the benefit, if it proved to be real, was the enhancement of the immune system.

“It has been shown that just a 30-minute walk can increase levels of leukocytes, which are part of the family of immune cells that fight infection,” she said.

The exercise group was asked to do 45 minutes of exercise a day, five days a week, but managed only 30 minutes, mainly brisk walking. That, however, seems to have been enough to achieve a benefit that increased over time.

“The enhanced immunity was strongest in the final quarter of the year,” Dr Ulrich said. “This suggests that, when it comes to preventing colds, it’s important to stick with the exercise.”

But the findings are by no means definitive. If all upper respiratory infections, rather than just colds, were counted there was no significant difference between the two groups.

Jessica Chubak, a research associate at the Hutchinson centre, said that, although the study was larger and longer than earlier ones, it still may not have been extensive enough. “It would be interesting to see what would happen over the course of a longer exercise intervention,” she said.

An editorial in the journal by Richard Ruffin and Paul Thompson, of Hartford Hospital, in Connecticut, raises further doubts. They ask whether the stretching group’s higher number of colds could have been caused by a spread of the infection during the weekly session and say that more evidence is required.

The main objective of the study, funded by the US National Cancer Institute, was to assess the impact of physical activity on the risk of breast cancer. As well as reducing their incidence of colds, the women in the exercise group reduced their weight, total-body fat and intra-abdominal fat, which should reduce their risk of cancer.

. . . but overdoing it can be risky

Dr Thomas Stuttaford Medical Briefing

FEW doubt the value of steady exercise in helping to maintain good health. Part of its contribution is its ability to preserve an efficient immune system.

But excessive exercise, such as that which sportsmen may do, has long been seen as one of the factors that undermine the immune system. Too strenuous a workout that repeatedly leaves athletes exhausted is thought to reduce the immune system’s efficiency.

A mild immunodeficiency state after over-training is thought to induce symptoms such as recurrent minor infections, the coughs and colds of winter, and a vulnerability to the viruses and bacteria responsible for minor gastrointestinal infections throughout the year. Other manifestations of a substandard immune system include an increased liability to complications in otherwise minor diseases, especially respiratory infections, and the occurrence among overtrained athletes of uncommon infections.

Athletes also need to avoid violent exercise when suffering from a severe cold or undiagnosed viral infection. The incidence of myocarditis, inflammation of the heart muscle, has increased. Other factors are probably involved, and may be of greater importance, in this change in the disease pattern of myocarditis in this country, but one of its recognised causes is violent heavy exercise when one is suffering from a viral infection.

Myocarditis is an occasional cause of death in young footballers who may even drop dead on the pitch, or who may die a few days after playing when having been thought to be suffering from nothing more than a heavy cold or other winter respiratory virus.

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