Tuesday October 23 2012
Around 30% of people in England have high blood pressure
‘Men suffering from high blood pressure are at greater risk of prostate cancer’, the Daily Express has warned.
Despite being the most common type of cancer in men, very little is known about what causes the cells of the prostate to become cancerous. Other than age and family history, the risk factors for prostate cancer are also largely unclear. This news is based on a large European study that aimed to see whether so-called “metabolic risk factors” such as increased body mass index (BMI) and high blood pressure are associated with the risk of developing, or dying from, prostate cancer.
Unfortunately, even with its large size and reliable methods of assessment and measuring disease outcomes, this study doesn’t really provide particularly useful information on risk factors. It found no significant association between any of these metabolic risk factors and risk of developing prostate cancer.
The only significant associations found were for BMI and blood pressure and risk of death from prostate cancer. Men with the highest BMI or systolic blood pressure (the upper of the two-figure measurement), were at increased risk of dying from prostate cancer, compared to those with the lowest. Surprisingly, no association was found with diastolic blood pressure (the lower of the two blood pressure figures), and it is not clear why this might be the case.
Overall, limited conclusions can be drawn about metabolic risk factors and prostate cancer. As the study did not look at other factors that may influence these metabolic factors, such as diet and physical activity, we don’t know to what extent these risk factors could be modified.
Where did the story come from?
The study was carried out by researchers from Umea University Hospital, Sweden, and other institutions in Scandinavia, Europe and the US, and was funded by the World Cancer Research Fund and the Swedish Cancer Foundation.
The study was published in the peer-reviewed medical journal, Cancer.
All media sources have taken a rather simplistic view of this research. The Daily Express headline is incorrect because the association between high blood pressure and risk of prostate cancer was not significant.
While the suggested association between high blood pressure, BMI and an increased risk of death from prostate cancer is interesting, it is also to a large extent, somewhat of a mystery. This study cannot tell us if, or why, high blood pressure or BMI is directly linked to an increased risk of death.
What kind of research was this?
The researchers say that the risk factors for prostate cancer in men are largely unknown (age is the most established risk factor, with most cases occuring in men over 50 years old). Other possible risk factors may include family history and ethnicity (the condition is more common in men of African-Caribbean and African descent).
In particular, it is unclear whether there are ‘modifiable’ risk factors for prostate cancer – that is, things we could change, such as diet. The researchers say that the fact that prostate cancer tends to be slightly more common among ‘Western’ countries, suggests that the Western lifestyle may in some way put men at higher risk.
The researchers observe that in Western countries there tends to be higher prevalence of ‘metabolic syndrome’. This is characterised by having a combination of the following factors:
- insulin resistance (meaning you can’t regulate blood sugar well) and high blood sugar
- high blood pressure
- high levels of blood lipids (fats such as cholesterol)
This is a combination of risk factors that puts a person at high risk of cardiovascular diseases such as heart attack and stroke.
This cohort study aimed to look at whether these metabolic risk factors – alone or in combination – increased risk of men developing prostate cancer during the course of follow-up.
A cohort study is the best way of looking at whether certain factors may be associated with disease risk.
However, they cannot definitely prove causation as it is possible that other confounders may still be involved – that is, other factors that are associated with both the person’s likelihood of having the particular risk factor being examined, and their risk of developing the disease outcome.
What did the research involve?
This study involved participants of the Metabolic Syndrome and Cancer project. This large study is made-up of seven smaller cohorts in Norway, Sweden, and Austria. The current study involved a total 289,866 men. At the time of enrollment to the studies, the men (average age 44) had data collected on their weight, height, blood pressure, blood glucose (sugar), and the blood fats cholesterol and triglycerides.
The men were followed for an average 12 years, and those diagnosed with prostate cancer were identified through linkage to the National Cancer Registers. The National Cause of Death Registers for Norway and Sweden were used to identify causes of death (no mention is made for Austria). The researchers calculated relative risks for prostate cancer with increasing levels of the metabolic risk factors of BMI, blood pressure, blood glucose, and blood cholesterol and triglycerides.
Each of these measures were categorised into fifths (quintiles) and the risk in people in the higher quintiles was then compared to the lowest.
In order to reduce the chance of reverse causation (that prostate cancer was causing these risk factors, rather than vice versa), they only looked at prostate cancer diagnoses that were made at least one year after the health factors were measured.
They took into account potential confounders of age and smoking history.
BMI was also taken into account for assessment of the other metabolic risk factors (aside from when looking at BMI itself).
What were the basic results?
Over an average 12 years of follow-up, 6,673 men (2% of the cohort) were diagnosed with prostate cancer and 961 died of the disease (0.3% of the cohort). Three-quarters of the men who developed prostate cancer had been aged 45 or over at the time of their baseline assessments. The average age at diagnosis of prostate cancer was 68 years and, among those who died from the disease, the average age at death was 72 years.
Overall, the researchers found no significant associations between any of the metabolic risk factors and risk of developing prostate cancer. There were some significant trends, (for example, for reducing risk with increasing blood glucose or triglyceride levels), but when the individual quintiles were compared, the relationship was not significant.
The only significant associations found were that higher BMI and blood pressure were associated with higher risk of dying from prostate cancer:
- Men in the top quintile (fifth) of BMI (average 30.8 kg/m2 – which would be defined as being clinically obese) had a 36% increased risk of dying from prostate cancer compared to men in the bottom quintile (average 21.5 kg/m2): relative risk.(RR) 1.36, 95% confidence interval (CI) 1.08 to 1.71.
- Men in the top quintile (fifth) of systolic blood pressure (average 157 mmHg) had a 62% increased risk of dying from prostate cancer compared to men in the bottom quintile (average 112 mmHg): RR 1.62, 95% CI 1.07 to 2.45.
There was no significant risk association observed for diastolic blood pressure (the lower of the two-figure blood pressure measurement), blood glucose or blood fats, and risk of death from prostate cancer. However, they did find that each one-unit increase in a composite score, which took into account all of these metabolic variables, was associated with increased risk of death from prostate cancer (RR 1.13, 95% CI, 1.03 to 1.25).
How did the researchers interpret the results?
The researchers conclude that they found no evidence of an association between high levels of metabolic factors and the risk of prostate cancer. However, they did find that high BMI, raised (systolic) blood pressure, and a composite score that took into account all metabolic factors were associated with an increased risk of death from prostate cancer.
This is a good quality cohort study, which benefits from its very large sample size and reliable methods of assessing the risk factors of interest and the disease outcomes. At 12 years, it also had a reasonable follow-up time. However, this study alone does not really bring us any closer to understanding the ‘potentially modifiable’ risk factors for prostate cancer, or demonstrating how or why prostate cancer may be slightly more common in the Western world.
None of the metabolic risk factors examined – BMI, blood pressure, blood glucose, cholesterol or trigylcerides, were associated with risk of developing prostate cancer. The only significant associations found were for higher BMI and higher blood pressure and risk of death – though why they should be associated with risk of dying from, but not developing, prostate cancer is unclear. Also, rather confusingly, no association was found with diastolic blood pressure (the lower of the two-figure blood pressure measurement) – only the upper systolic figure. Both systolic and diastolic blood pressure measures are normally equally relevant when it comes to the health-related risk of high blood pressure.
It is also difficult to know from this study to what extent these metabolic risk factors, and risk of prostate cancer, may be ‘modifiable’. Most notably, diet and physical activity may be involved in a man’s likelihood of having high BMI and high blood pressure, and his risk of prostate cancer, but the study has not looked into these factors.
Overall, limited conclusions can be made about metabolic risk factors for prostate cancer based on this study, though even studies with predominantly negative findings, such as this one, can provide valuable contributions to overall knowledge in the area.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on twitter.