Lifestyle and Nutrition

Milk Consumption is a Risk Factor for Prostate Cancer: Meta-Analysis of Case-Control Studies

Nutrition and cancer 48(1): 22-27, 2004
L. Q. Qin, J. Y. Xu, P. Y. Wang, T. Kaneko, K. Hoshi, A. Sato

Study purpose: Prostate cancer is now the most common cancer among men in Europe and North America. After links between diet and the frequency of prostate cancer were studied in 42 nations (D. Ganmaa et al. in the International Journal of Cancer, 2002), milk was shown to be the dietary product most closely linked to prostate cancer. Ten studies published since also suggest a link. However, other studies continue to show mixed results. This Japanese meta-analysis looks at the links between drinking milk and prostate cancer risk. A meta-analysis is a statistical method that combines previously published study results in order to interpret their overall findings.

Study description: The meta-analysis combines 15 case-control studies from eight countries that examine the relationship between milk consumption and prostate cancer risk. Case-control studies compare a group with a disease against a similar but healthy group, and analyze their medical records and lifestyles to learn what factors may be linked with the disease . Published between 1984 and 2003, the studies were statistically analyzed to estimate the total odds ratio of prostate cancer risk and milk consumption (the overall increase in prostate cancer risk due to drinking milk).

Findings: The combined odds ratio was 1.68 in 11 published case-control studies with increases in prostate cancer risk varying from 1.34—2.12 in individual studies. The combined odds ratio varied little when studies were analyzed separately and broken down further for examination. The meta-analysis also evaluated possible risk factors in milk for prostate cancer, such as fat, calcium, hormones, and chemical trace elements.

Conclusions: Study results on risk factors in milk for prostate cancer were open to doubt. However, the researchers’ meta-analysis found a positive link between milk consumption and prostate cancer. They urge further investigation of possible causes for this link, which include fat, calcium, hormones and other factors.

Meta-Analysis of Measures of Sexual Activity and Prostate Cancer

Epidemiology 13(1): 72-79, January 2002
Leslie K. Dennis and Deborah V. Dawson

Study purpose: The authors conducted a meta-analysis of a possible link between prostate cancer and sexual activity. A meta-analysis is a statistical method that combines results from published studies in order to examine their findings overall.

Study description: A search of the medical literature using Internet databases found 36 relevant studies. A meta-analysis pooled overall results and applied them to topics such as prostate cancer relative risk estimates linked to sexual activity.

Findings: Results suggest a higher relative risk of prostate cancer among men with a history of sexually transmitted infections (STIs), especially syphilis and gonorrhea. This suggests a possible link between these infections and the development of prostate cancer, although the effects of infections on prostate cancer are not well understood. A rising risk of prostate cancer was also linked to having more sex, with results suggesting that 16.7% of prostate cancer among men who have sex three or more times a week may be connected to their sexual activity. To explain this, the authors propose a link between frequent sex and hormonal fluctuations (male hormone production can help the growth of prostate cancer cells). Some studies linked rising relative risk to numbers of sexual partners, although this link became statistically significant only when men had more than 30 partners over a decade. The data did not support any link with multiple marriages, age at first intercourse, or age at first marriage.

Conclusions: The authors found a consistent link between prostate cancer risk and sexually transmitted infections that could not be explained by differences in study methods. According to their data, 30.6% of prostate cancers among men with sexually transmitted infections are caused by their infections. After further calculation, 5.8% of all prostate cancers were seen to have links to sexually transmitted infections. However, the way in which frequent sexual activity is linked to prostate cancer remains unclear.

Vasectomy and the Risk of Prostate Cancer: a Meta-Analysis Examining Vasectomy Status, Age at Vasectomy, and Time Since Vasectomy

Prostate Cancer and Prostatic Diseases 5(3):193-203, 2002.
L. K. Dennis , D. V. Dawson and M. I. Resnick

Study purpose: This meta-analysis conducts a review of previously published prostate cancer studies in order to see if undergoing a vasectomy increases the risk of prostate cancer.

Study description: A meta-analysis is a statistical method combining study results in order to re-interpret overall findings. The authors used results from 22 published studies to calculate a “pooled relative risk estimate.” This measured the potentially higher probability of prostate cancer risk for men with a vasectomy.

Findings: For men who had vasectomies, the meta-analysis deduced a pooled relative risk estimate of 1. 37. No differences in risk according to age at the time of vasectomy were noted. The 16 studies that contained data on the time since vasectomy suggested a 10% increase in risk for each additional 10 years since the procedure, or a relative risk of 1.32 for a 30-year period since vasectomy. After disqualifying results from six studies that did not record any long-term data, the relative risk estimate for the 16 studies remaining was 1.07 and 1.23 for 10 years and 30 years since vasectomy, respectively.

Conclusions: These results suggest that men who have had a vasectomy may be at an increased risk of prostate cancer.

Note: The authors caution that the overall link between prostate cancer and vasectomy is small enough to be explained by biased results. Relative risk estimates also varied according to study methods, with 17 case-control studies showing the lowest risk out of the 22 studies examined. Case-control studies compare a group with a disease against a similar but healthy group, and analyze their medical records and lifestyles to learn what factors may be linked with the disease.

Meta-analysis for combining relative risks of alcohol consumption and prostate cancer.

The Prostate 42(1): 56-66, January 2000
L K Dennis

Study purpose: A meta-analysis is a statistical method combining research from different published studies in order to re-examine the overall results. This American meta-analysis examines studies found in the medical literature that look at possible links between prostate cancer and drinking alcohol. It then pools data from the studies to see if there is a link, and if so, how strong it actually is.

Study description: The authors searched computerized medical databases to find 33 studies before 1998. Data was pooled in order to figure out if male drinkers were more at risk for developing prostate cancer than non-drinking counterparts. The meta-analysis developed a statistical method to show jumps in overall “relative risk” estimates caused by a link between prostate cancer and alcohol consumption. The amount men drank and the types of alcohol consumed were taken into account.

Findings: The overall study results showed few increases in prostate cancer risk due to alcohol consumption. The meta-analysis’ overall pooled relative risk estimate was a small 1.05 jump in risk. The highest increase in risk (1.27) was found among beer drinkers, but this was based on only eight studies looking at types of alcohol consumed. Another 15 studies looked at the amount of alcohol consumed. The meta-analysis used the number of average drinks consumed per day in the 15 studies to calculate an overall relative risk estimate that applied to all 33 studies examined. It found a relative risk increase of 1.02 for each additional alcoholic drink per day.

Conclusions: The overall estimated rise in prostate cancer risk that can be linked to drinking alcohol is very small. The author found no real links between drinking alcohol and prostate cancer.

Note: Although some studies linked types of alcohol to a higher increase in prostate cancer risk, the author believes these studies were biased.