Monday March 17 2008
“Big C risk is worse if you’re fat” reads the headline in The Sun today. The news story it refers to goes on to say that fat women are “less likely to get low-risk breast cancer – but more prone to life-threatening versions”. The researchers have “discovered a link between the fiercest types [of breast cancer] and high blood sugar”, the newspaper adds.
The newspaper report is based on a Swedish study investigating metabolic factors and breast cancer risk. There were few results of statistical significance this study so it is impossible to reach firm conclusions. Although this study adds evidence to previous research which suggests a complex link between metabolism and breast cancer, more studies are needed to identify what this risk is. This study is not conclusive and The Sun and other news sources have overstated its significance.
Where did the story come from?
Dr Anne Cust, Tanja Stocks and colleagues from the University of Melbourne, the University of Sydney, the International Agency for Research on Cancer (France), Umeå University in Sweden and the German Cancer Research Centre carried out this research. The study was funded by the World Cancer Research Fund, the Swedish Cancer Society and the council of Västerbotten county in Sweden. It was published in Breast Cancer Research and Treatment, a peer-reviewed medical journal.
What kind of scientific study was this?
The study was a nested case-control study designed to explore the relationship between body mass index (BMI), the hormones involved in metabolism (leptin and adiponectin), some of those involved in controlling blood-sugar levels (C-peptide and glycated haemoglobin) and breast cancer risk among women in northern Sweden.
The researchers had access to data from several different groups of women who were involved in the Northern Sweden Health and Disease Cohort (NSHDC). One part of the NSHDC ran from 1985 to 1996 and another part has taken place since 1995. In September 2005, they linked all women for whom they had blood samples to the regional cancer register (which records 99% of breast cancer diagnoses). Of these women, 561 had a diagnosis of breast cancer. From the same population (i.e. women who came from the original groups and had blood sample records available), they selected one control for each case. The case-control pairs were matched on age at baseline and the date when their blood samples were taken.
The researchers looked at the blood samples from the women who had breast cancer and compared them with those who did not. They were particularly interested in whether the levels of particular hormones that regulate metabolism (leptin and adiponectin) were different between the groups. They also compared the levels of chemicals involved in regulating blood sugar: C-peptide and glycated haemoglobin.
What were the results of the study?
Overall, the researchers found that BMI, leptin, adiponectin, C-peptide and glycated haemoglobin had no effect on the levels of risk of any type of breast cancer (stages I–IV). When the researchers divided the women into two groups (those with stage I tumours and those with stage II–IV tumours), they found a slightly different pattern of results: obese women were much less likely than normal weight women to have stage one breast cancer.
Women with higher levels of glycated haemoglobin were also less likely to have stage I breast cancer than those with lower levels. The researchers acknowledge that the mechanisms underlying this decreased risk are unclear.
For breast cancer stages II-IV, there were no statistically significant patterns. That is, although a greater number of obese women had stage II-IV breast cancer than normal weight women, this was not statistically significant.
In overweight or obese women, higher levels of glycated haemoglobin had a borderline significant association with risk of more severe tumours.
What interpretations did the researchers draw from these results?
The researchers conclude that their study has found an inexplicable reduction in risk of stage I breast cancer among obese women compared with normal weight women. They also found a reduced risk of stage one breast cancer among women with high “blood sugar” compared with those with normal blood sugar. Furthermore, the study found that higher levels of leptin and glycated haemoglobin together with higher BMI had “a suggestion of an increased risk” of stage II–IV breast cancer.
What does the NHS Knowledge Service make of this study?
- In isolation, the lack of statistical significance in the results linking BMI and other markers of metabolism with risk of more severe breast cancer mean that this study is not conclusive. The claim in The Sun that “high blood sugar in overweight women hugely increases the risk of aggressive tumours” is an overstatement of these results. The authors discuss other evidence that links a particular metabolic profile (overweight, insulin resistance) to progression of tumours. However, they are cautious about their conclusions from this study, saying that there is only a “suggestion of an increased risk”.
- Other limitations that the authors raise include the study’s reliance on the results from only one blood sample, which is unlikely to represent metabolism over the long term. They were also unable to explore in detail the contribution of age differences between the women to the differences in risk.
This research is inconclusive, though it may add some evidence to other research into the relationship between metabolism and breast cancer. Until further studies replicate these findings with statistical significance, this relationship will remain unclear.
Sir Muir Gray adds...
The evidence linking obesity and cancer, perhaps through hormone changes, is getting stronger, year by year. Yet another reason for increasing walking.