Thursday November 15 2007
Obesity raises the risk of many diseases
“Putting on pounds ‘doubles prostate cancer death risk’” reported the Daily Mail today.
The newspaper went on to say that new research had found that men who have prostate cancer double their risk of dying from it if they are overweight. The report described a study that compared the chances of survival from advanced prostate cancer in normal weight, overweight and obese men.
Although this is reliable, well-conducted research, overly simple conclusions should not be drawn from it. The main things to bear in mind are that the study design is unable to prove that obesity causes prostate cancer or that, by losing weight, men may be able to avoid developing prostate cancer.
However, this study does reinforce existing advice that men who have developed prostate cancer should aim to lead healthy lifestyles. There is large body of evidence that indicates that maintaining a healthy weight and diet increases a cancer survivor's chances of living longer.
Where did the story come from?
Dr Efstathiou from the Massachusetts General Hospital in Boston US and colleagues from other radiation departments in the US conducted this research. The study was funded by a grant from the National Cancer Institute. The study was published in the (peer-reviewed) medical Journal of the American Cancer Society: Cancer.
What kind of scientific study was this?
This was a multivariate analysis of data collected in a randomised controlled trial (RCT) of a new treatment for locally advanced prostate cancer (cancer which has spread beyond the prostate).
The original randomised clinical trial was carried out between 1987 and 1992, on 945 men who were undergoing, or had already undergone, radiotherapy for locally advanced prostate cancer.
The subjects were randomised to receive goserelin (a drug that blocks the production of testosterone and oestrogen) either in the last week of their radiotherapy treatment, or if the cancer recurred, during their subsequent treatment.
They followed the men for an average of 8.1 years (and up to 15 years in some cases) and recorded their cause of death and whether it was related to prostate cancer or not.
The multivariate analysis from this latest research focused on height and weight data, which were collected for only 788 of the 945 participants. The analysis is therefore based on this subset (83%) of the total participants.
What were the results of the study?
The original trial had found that the men who received radiotherapy and the new drug together at the end of their first course of treatment were less likely to die of prostate cancer or from any other cause than those who received the drug only if they relapsed following the radiotherapy. Only 169 of the total 476 deaths over the whole study were prostate cancer related.
This data analysis looked for an association between weight at enrolment into the clinical trial and time to death. When the researchers looked at the data on weight (measured by Body Mass Index (BMI)) they found that this was linked to death from prostate cancer. Fewer men who were a normal weight died of prostate cancer in the five years following the trial, compared to the proportion who died and were overweight or obese. This difference in death rate was about double; 13.1% in the overweight group and 12.2% in the obese group, compared to 6.5% in the normal weight group.
The authors adjusted for other factors that they thought might also influence survival, such as age, race, treatment received, whether the patient had had prostatectomy or had lymph nodes involved and the histological and clinical stage of the cancer. They found that after these adjustments were made the association between weight and death from prostate cancer was reduced but still remained significant.
What interpretations did the researchers draw from these results?
The researchers concluded that a “greater baseline BMI is independently associated with higher cancer specific mortality in men with advanced prostate cancer”, meaning that men who had a greater BMI at the time of their treatment had a higher mortality rate from advanced prostate cancer.
What does the NHS Knowledge Service make of this study?
The link between being overweight and death from prostate cancer has been found in other studies and several mechanisms have been suggested to account for this association. The authors mention that the changes in a number of hormones such as oestradiol, testosterone, insulin and leptin, have been implicated in the aggressiveness of prostate cancer and may account for some of the differences observed.
However, there are factors other than weight that could also account for the differences in survival. In particular, there is the possibility that obesity may interfere with the treatment of prostate cancer, for example by making the examinations, the radiotherapy planning or the accuracy of radiotherapy delivery more awkward. There may also be other contributory factors such as diet, physical activity or smoking, which may be acting alongside weight and partly contributing to the effect observed in this study.
Overall this study reinforces existing advice that men who have developed prostate cancer should aim to lead healthy lifestyles, but on its own is probably not enough to tell us which aspect of the healthy lifestyle is linked to the benefit. However, there is ample evidence indicating that maintaining a healthy weight and diet increases a cancer survivors chances of living longer.
Sir Muir Gray adds...
Obesity affects health not only by increasing the risk of disease, but also by complicating the treatment of disease, and not only diseases caused by obesity.