Wednesday June 24 2009
‘Gastric bands can cut cancer risk by one third,’ The Daily Telegraph has said. The newspaper says that research following people for 11 years after anti-obesity surgery found that women’s risk of developing cancer fell even further, by 42%, which may be because the risk of breast cancer is reduced after surgery.
The research behind this report followed 2010 obese people over 11 years following gastric surgery for weight loss. It compared their rates of cancer with the rates seen in 2037 obese people who received non-surgical weight management and found there were 117 cancer cases in the surgery group and 169 in the comparison group. Across men and women, this was a decrease in the risk of cancer in the surgery group of around a third, but separate analysis by gender found that the decreased risk only applied to women.
There are important limitations to these findings, principally the small number of cancers observed, which makes it hard to draw firm conclusions from the associations seen. Also, the reasons why there may be a decreased risk were not explored and this will require further study. However, despite having limitations this study confirms the idea that reducing your weight if you are obese does have health benefits.
Where did the story come from?
Lars Sjöström and colleagues of the Swedish Obese Subjects Study carried out this research. Sources of funding included the Swedish Research Council, Swedish Foundation for Strategic Research, Swedish Federal Government, Hoffmann La Roche, Cederoths, AstraZeneca, Sanofi Aventis and Ethicon Endosurgery. The study was published in the peer-reviewed medical journal The Lancet.
What kind of scientific study was this?
This study compared cancer incidences in people who were given anti-obesity surgery and obese individuals who were given normal weight controls.
The Swedish Obese Subjects (SOS) study is reportedly one of the first pieces of research to investigate the effects that intentional weight loss in the obese may have upon cancer incidence. This cohort study started in 1987 and involved 2010 obese patients (BMI of 34 kg/m² or above in men and 38 kg/m² or above in women), aged 37 to 60, who were scheduled for weight-loss surgery. They were matched, according to body measurements and a variety of medical and lifestyle factors, to 2037 obese control subjects. These controls received conventional treatment without surgery.
Participants could not be considered for surgery, if they had:
- a stomach ulcer, or had received surgery for one,
- earlier weight-loss surgery,
- cancer within the past 5 years,
- a heart attack,
- drug or alcohol abuse, or
- a psychiatric illness or other medical condition which would make the surgery unsuitable.
The surgical procedures included adjustable and non-adjustable gastric banding (received by 376 participants), vertical banded gastroplasty (received by 1369) and gastric bypass (received by 265). Controls received “variable management”, which was not reported on further in the study.
The SOS study assessed overall mortality following surgery, but for this particular study the researchers were interested in cancer incidence up to the end of December 2005. Cancer follow-up data was available for 99.9% of participants, with an average duration of follow-up of 10.9 years.
What were the results of the study?
Over the 11-year follow-up period, weight-loss surgery resulted in an average weight reduction of 19.9kg. During this time, members of the control group gained 1.3kg on average. During follow-up, there were 117 new diagnoses of cancer in those who received surgery compared to 169 new cancer diagnoses among the controls. This equated to a 33% decreased risk of cancer following weight-loss surgery (hazard ratio 0.67, 95% confidence interval 0.3 to 0.85).
The researchers found that gender had a significant effect upon the association. The number of cases of new cancers in women who had surgery was lower than among women in the control group (79 cases in the surgery group to 130 in the controls). Surgery had no apparent effect on cancer risk in men (38 cases in the surgery group to 39 in the controls).
When analyzing their data, the researchers excluded cancers that developed during the first three years after surgery to take into account any cancer that may have developed around the time of surgery. This did not alter the observed effect.
What interpretations did the researchers draw from these results?
The authors concluded that weight-loss surgery is associated with reduced cancer incidence in obese women but not men.
What does the NHS Knowledge Service make of this study?
This study has found that, over the course of an 11-year follow-up, there were more cases of cancer among those who did not receive surgery. This equated to a decrease in risk of cancer of a third. Separate analysis of women and men found that the decreased risk occured in women but not in men.
There are several points to note:
- The actual number of cancer cases that developed during follow-up was relatively small, particularly when a separate analysis of males and females was conducted. This small number of cases decreases the chance of showing an association within subgroups. For example, as men made up only 29% of the total sample, the conclusion that weight-loss surgery decreases cancer risk for women but not men must be made very cautiously.
- The primary outcome when this study was designed was to assess the effect of obesity surgery upon overall mortality. Cancer incidence was not considered in any outcome and, therefore, the study is unlikely to have adequate power to assess the rates of cancer, either overall or by type.
- There are many health factors that may affect the risk of cancer, both related and unrelated to obesity. For example, this study observed that not having diabetes and not smoking were both found to decrease risk of cancer during follow-up. Further lifestyle and medical health factors that were not taken into account may also influence the risk of getting cancer.
- Although it is suggested that decreased risk of breast cancer may account for the associations seen, this was not examined by this study and it is not possible to make conclusions about different types of cancer.
- This was not a randomised study and although the groups were well matched on most measures, it is not possible to say for sure that those who had surgery did not differ from the control group in other unmeasured ways.
The decision to perform weight-loss surgery is not made lightly. It is usually only considered as a last resort if a person remains morbidly obese after stringent attempts to lose weight and where other medical factors make obesity a risk to health. Therefore, this study should not be taken as promotion for weight-loss surgery. However, despite its limitations, this study supports the notion that, regardless of method, weight loss in an obese person does have health benefits.