Wednesday May 25 2011
Brisk walking may reduce the growth of cancer cells by reducing insulin resistance
“Power walks could be a life-saver for men newly diagnosed with prostate cancer”, according to the Daily Express. Brisk walking for at least three hours a week could improve outcomes and even prevent the cancer’s progression, it added.
The story comes from a study of nearly 1,500 men diagnosed with early prostate cancer (cancer that has not spread), which looked at whether vigorous activities and brisk walking had any effect on the progress of the disease. It found that men who walked briskly for three hours a week or more had a 57% lower risk of the disease spreading or recurring compared to men who walked at an easier pace for less than three hours a week.
While this study suggests that brisk walking and intense exercise may slow down the progression of early prostate cancer, the results should be viewed with caution: the results were based on a very small number of cases in which the disease had spread, including only three prostate cancer deaths. Also, very few men took part in vigorous activity, which makes the findings in this area less reliable. Furthermore, the study also relied on men estimating their levels of activity over a year, which might introduce inaccuracy.
Walking and similar activities has many health benefits, and keeping active can help people recover more quickly after receiving cancer treatment.
Where did the story come from?
The study was carried out by researchers from Harvard School of Public Health, Brigham and Women’s Hospital, Harvard Medical School and the University of California. The study was published in the peer-reviewed journal, Cancer Research. It was funded by the US National Institutes for Health, the Prostate Cancer Foundation and Abbott Laboratories.
The study was generally covered accurately, if uncritically, by the press. Helpfully, the BBC included comments from independent experts who placed the results into context.
What kind of research was this?
This was a prospective cohort study which aimed to find out whether vigorous activity and brisk walking after a diagnosis of localised prostate cancer would delay or stop the disease from growing and spreading.
The researchers point out that a previous study had found that among men diagnosed with non-metastatic prostate cancer (cancer which has not spread to other parts of the body), vigorous activity after diagnosis was associated with a 61% reduction in the risk of death from the disease. However, they point out that the associations in this study could reflect the possibility that the progression of the disease may have lead men to become less active, rather than activity blocking the progression of their cancer.
This current study looked at progression of the disease by measuring biochemical markers, rather than prostate cancer death, which the researchers say reduces the possibility that ‘reverse causation’ lies behind their results.
Vigorous activity and, to some extent, the moderate activity of brisk walking are associated with various factors that might inhibit the growth of prostate cancer cells, including lower levels of insulin. Given this association, the researchers were particularly interested in the link between recurrence and the intensity of any physical activity.
What did the research involve?
The analysis was based on a study of 2,134 men diagnosed with prostate cancer, who were drawn from a larger ongoing study. The men all completed questionnaires about their diet and physical activity during 2004-5. They were asked how often on average they had participated in walking or hiking, running, aerobics, rowing, cycling and squash over the past year. Their answers were recorded as ranging from ‘never’ to ‘more than 11 hours a week’. The men were also asked about their usual walking pace and how many flights of stairs they climbed daily.
The researchers calculated a metabolic equivalent task value (MET) to each activity on the basis of the energy required by that activity, relative to the resting metabolic rate. Normal walking is equivalent to a MET score of approximately three. Depending on the energy required, activities were classified in terms of how vigorous they were: vigorous, non-vigorous and walking pace.
The researchers followed up the men using clinical data from the urology clinics they attended and mortality data from national registries. They looked at the data on prostate cancer progression and recurrence, as measured by blood tests, secondary bone cancers, secondary treatment and death from the disease.
The researchers’ analysis excluded men whose disease was more advanced at diagnosis, men whose disease had progressed before they completed the questionnaire, and men who had not completed their treatment before their questionnaire. They also excluded other cases where important information in the questionnaire was missing or unreliable. This left the data on 1,455 men for their analysis.
The researchers used validated statistical methods to evaluate the possible association between activity and prostate cancer progression. They adjusted their results for the grade of the disease (which may indicate its aggressiveness) and for various other clinical factors. Results were also adjusted for risk factors including race, family history, smoking, education, income and diet.
What were the basic results?
A total of 117 men out of the 2,134 experienced spreading or progression of their prostate cancer:
- 45 had a recurrence of disease
- 66 needed further treatment
- 3 had bone tumours
- 3 died from prostate cancer
Walking accounted for nearly half of all activity. Few of the men engaged in vigorous activity.
- Men who walked briskly for three hours a week or more had a 57% lower rate of disease progression than men who walked at an easy pace for less than three hours a week (Hazard Ratio [HR] = 0.43; 95% Confidence interval [CI] 0.21 to 0.91)
- A faster walking pace was associated with a reduced risk of the disease progressing, independent of how long men walked for (brisk vs. easy pace HR 0.52; 95% CI 0.29 to 0.91)
- The link between vigorous activity and lower risk of disease progression was not significant (HR ≥3 h/wk vs. none HR 0.63; 95% CI 0.32 to 1.23)
- How long the men walked was not associated with a lower risk once pace was disregarded.
- Time engaged in activity (independent of metabolic rate) was not associated with a lower risk.
How did the researchers interpret the results?
The researchers conclude that brisk walking after diagnosis may inhibit or delay prostate cancer progression among men with clinically localised prostate cancer. They argue that it may affect the proliferation of prostate cancer cells in various ways, such as reducing insulin resistance and by reducing inflammation.
This study had a number of limitations and the results should be viewed in this context:
- There were only a small number of cases where the disease had spread or recurred. This makes the results less reliable.
- It is possible that “reverse causation” was a factor – i.e. that men whose cancer progressed were less likely to do regular brisk walking because of the effects of the disease. The researchers argue that using biochemical indicators of recurrence as a measure of progression makes this less likely since physical symptoms that may cause a reduction in activity are unlikely to occur before biochemical recurrence would be detected.
- Although the researchers adjusted for confounding factors, it is still possible that other factors affected the men’s risk of disease spreading or recurring.
- The study relied on men recalling their levels of activity over the previous year and trying to estimate their walking pace. The reliance of participant estimates leaves the findings open to error.
- One quarter of the men who had completed the physical activity questionnaire were not followed up (although the researchers argue that this is unlikely to have biased their results).
- Men who volunteered for the study were on average younger at diagnosis, more likely to be white and had a better prognosis compared with the average man with prostate cancer. Therefore the findings may not apply to older men or to those with more severe local disease.
Walking and similar activities have many health benefits and can help people recover more quickly after cancer treatment.
Dr Helen Rippon, head of research management at The Prostate Cancer Charity, reportedly said: "Although this research will need to be repeated to make sure the results can be applied to all men with prostate cancer, we would certainly advise men diagnosed with prostate cancer to ensure that their lifestyle includes a good amount of physical activity - and walking is often the easiest way of achieving this."