Wednesday May 28 2008
Regular exercise is one of the most effective ways of reducing risk of preventable disease
“Men who take regular exercise are less likely to die from cancer than those who do none”, The Daily Telegraph reported today. It said that a study of more than 40,000 men between 45 and 79 found that those who walked or cycled at least 30 minutes a day were 34% less likely to die from cancer. The Daily Mail reported that although this level of exercise only reduced the risk of developing the disease by 5%, a more intensive programme of between an hour and 90 minutes a day, was associated with a 16% lower chance of developing cancer.
As reported in the newspapers, this large group study of 40,708 Swedish men found that those who exercised moderately for at least 60 minutes a day lowered their risk of cancer. It also found that if men who already had cancer did 30 minutes a day of moderate exercise, they increased their chances of survival by 33%.
Although the study has some weaknesses and the researchers conclude that their findings require confirmation, it substantiates what is already known about physical activity – that it is good for you and one of the most effective ways of reducing your risk of preventable disease.
Where did the story come from?
Dr N. Orsini and colleagues from the Karolinska Institutet in Sweden and Harvard Medical School in Boston, USA carried out the research. The study was funded by the Swedish Research Council, the Swedish Cancer Society, the Swedish Council for Working Life and Social Research, the World Cancer Research Fund International and the Swedish Foundation for International Cooperation in Research and Higher Education.
The study was published in the peer-reviewed: British Journal of Cancer.
What kind of scientific study was this?
This prospective cohort study looked at the link between physical activity in middle-aged and elderly Swedish men and their risk of cancer. In 1997/1998, the researchers sent an invitation to participate in the study to all 45-79-year-old men living in two counties in central Sweden. A questionnaire was included with each invitation, and of the 100,303 men who were contacted, 48,645 returned their questionnaires.
The researchers excluded any returned questionnaires that were blank, or were from men who died before January 1 1998. They also excluded those who had had cancer previously and those who were heavy manual workers as they are known to have a higher overall cancer mortality. This left a sample of 40,708 men. The researchers said that this represented “the whole Swedish male population aged 45 to 79 in terms of age, educational level, and prevalence of overweight”.
The questionnaire collected information on the participants’ duration and intensity of physical activity throughout the past year. Specific questions were asked about activity levels relating to occupation, walking/cycling, household work, inactive leisure time (TV/reading), active leisure time (exercising) and time spent sleeping. From these responses, the researchers were able to determine a total activity score for each man. This was calculated using metabolic equivalents, a unit often used to measure the intensity of physical exercise, and which takes into account the fact that people’s metabolic rates are different.
Over the next seven years (until 2004), the researchers determined the date and cause of death of the men by using the Swedish Death Register, the National Swedish Cancer Register, and the Regional Cancer Register.
The researchers then analysed the data from the questionnaire sent out at the beginning of the study, and the cause and date of the men’s deaths during follow up to determine whether there was any link between level of activity and death from cancer. They took into account other factors that could have affected the association between exercise and cancer, including BMI, smoking, alcohol use, education, diabetes and parental cancer history.
In the researcher’s final analysis, there were a total of 28,880 men (some were excluded because of missing data etc). They then analysed only the men who had cancer to see whether exercise was associated with cancer survival.
What were the results of the study?
Over the seven-year follow up, 3,714 (9%) men got cancer. Of these, 1,153 men died from their disease. The researchers found that the men who exercised more were less likely to smoke or drink alcohol, to have a history of diabetes or to have post-secondary school education.
When taking into account the other factors that can affect cancer (smoking, parental history etc.), the researchers determined that each additional hour of moderate activity a person did reduced their risk of cancer by a further 2%, though this was not statistically significant (RR 0.98, 95% CI 0.94 to 1.01). Men who walked or biked between 60 and 90 minutes per day had a 16% reduction in cancer compared to men who hardly ever walked or biked.
They found that overall, total physical activity was “inversely associated” with deaths from cancer, i.e. the more exercise men did, the less likely they were to die from cancer. Compared to men who did the least amount of exercise, men who did the most (in the highest quartile of exercise level) were 29% less likely to die from their cancer. Taking into account other factors that can affect cancer (smoking, BMI etc), daily exercise equivalent to one hour of moderate effort was associated with a 12% reduction in cancer death rate.
In the group of men who were diagnosed with cancer, their walking or biking for an average of 30 minutes per day was associated with a 33% improvement in cancer survival.
What interpretations did the researchers draw from these results?
The researchers concluded that cancer mortality is linked to total daily physical activity, i.e. that it is reduced in men who exercise more. They say that although the association between physical activity and the likelihood of getting cancer is weak, moderate activity (walking/cycling) of at least 60 minutes a day is associated with a 16% lower risk of cancer. This supports the World Cancer Research Fund and the American Institute for Cancer Research recommendations for levels of activity that people should take.
The researchers say that their findings ‘may have major public health implications in the prevention and treatment of malignancies’. However, they say that the results require confirmation.
What does the NHS Knowledge Service make of this study?
This cohort study provides evidence that moderate exercise can reduce the risk of cancer and, in people who have cancer, prolong survival. The researchers discuss some potential problems with their research, and say that other studies should confirm their findings as they may have major public health implications for the prevention and treatment of malignancies. The following points should be kept in mind when interpreting the findings of this study:
- Physical activity was assessed through a self-report questionnaire. There may have been errors in the way that people classified the amount of activity they did. However, the researchers report that the results from ‘validation’ studies (where the questionnaire was tried out on a different population) were ‘reassuring’. This suggests that this may not have been a big problem.
- In addition, the levels of physical activity that the men reported at the beginning of the study may not have been constant throughout the seven-year follow up period.
- Many data were missing from the returned questionnaires and 30% of them were not included in the final analysis. If sufficient numbers of these questionnaires had contained different responses, then the results would be biased. Similarly, of the 100,000 or so men originally approached, less than half returned their questionnaires. The study would again be biased if the men who did not participate were different in some way from those that did, particularly if there was a different link between their activity and cancer.
- As the study was conducted in Sweden, the findings might not be applicable to men in other countries. Additionally, the findings might not be applicable to women.
- The study did not take into account the effect of diet. A person’s diet can influence their risk of many cancers, and it’s possible that the men who exercised more also generally led a healthier lifestyle with better diets. This would confound the beneficial effects of exercise. A study which takes this into account will give a better estimate of how much other factors affect the risk of cancer.
Overall, this study substantiates the knowledge that exercise is good for you. Further studies that confirm the degree of association between exercise and cancer reduction are now needed, as are studies of whether moderate exercise can really improve survival in people who already have cancer. If this is a true association, it will have important implications for treatment.
Sir Muir Gray adds...
The evidence is getting stronger and stronger; if you don’t smoke the single best thing you can do to reduce your risk of a number of common diseases is to take more exercise. Try an extra 3,000 steps a day.