Friday September 30 2011
The men who ate the most eggs were at a higher risk
“Eating just three eggs a week increases chance of men getting prostate cancer,” reported the Daily Mail. The story went on to say: “Experts in the US claimed that men who consume more than two-and-a-half eggs on a weekly basis were up to 81% more likely to be killed by the disease.”
This research examined the association between eating red meat, poultry and eggs and the risk of developing lethal prostate cancer (which the researchers defined as either dying from the disease or having metastatic disease that had spread to other organs). The study was in a large group of 27,607 healthy men, of whom 199 developed lethal prostate cancer over 14 years of follow-up. The researchers calculated that the men who ate the most eggs were at a significantly higher risk than those who ate fewer eggs. No significant association was found with any other food item.
This large cohort study has some strengths, such as its large size and the fact that information on the participants’ diet was continually updated over the course of the study. However it also has several limitations, and only a small number of lethal cancers actually occurred, which could suggest that this association is due to chance. Furthermore, these results are inconsistent with previous research, which found no significant association between eggs and prostate cancer. The findings will need to be confirmed in more robust studies before any firm conclusions can be drawn.
Where did the story come from?
The study was carried out by researchers from the Harvard School of Public Health, the University of California in San Francisco, Brigham and Women’s Hospital and Harvard Medical School. Funding was provided by the US National Institute of Health.
The study was published in the peer-reviewed medical journal, Cancer Prevention Research.
The media generally reported the study accurately. However, the Daily Mirror’s suggestion that “a clear link between eggs and prostate cancer” has been found may be misleading, as the researchers say that their results contradict previous findings into the association and that more research is needed. But the Mirror does point out that men in the study who ate the most eggs differed from the rest of the participants in important ways, such as weight and smoking status.
What kind of research was this?
This was a prospective cohort study that investigated whether there is an association between eating red meat, poultry and eggs and the risk of developing lethal prostate cancer in healthy men. A subgroup analysis was carried out afterwards in the men from this cohort who went on to develop prostate cancer. The researchers wanted to see whether eating habits after a prostate cancer diagnosis were associated with the risk of the disease progressing and becoming fatal.
The researchers’ theory was based on the findings from previous research, which found:
- an increased risk of developing lethal prostate cancer in healthy men who ate red meat
- an increased risk of progression to lethal disease in men with prostate cancer who ate eggs and skin-on poultry after their diagnosis
Participants were recruited from an ongoing cohort study that began in 1986. This study was comprised of American male health professionals who were between the ages of 40 and 75 in 1986. Men in this study completed a questionnaire every two years with information on their medical conditions, physical activity, weight, medications and smoking status. They provided information regarding their eating habits every four years.
Prospective cohort studies are an appropriate design for answering this type of research question. Assessing eating habits at the beginning of a study reduces the risk that people will inaccurately recall their dietary habits, which can arise when you ask people to remember what they ate over a long period of time. It also ensures that the exposure (eating certain foods) precedes the outcome (developing and dying of prostate cancer).
What did the research involve?
In 1994, the researchers recruited 27,607 men from the existing cohort study in the US. The men did not have prostate or other forms of cancer (except non-melanoma skin cancers, which are rarely aggressove). They had also had a prostate specific antigen (PSA) test (PSA screening is not performed in the UK, as higher PSA levels can indicate cancer but are not specific for it. For example, raised levels can also occur with benign enlargement, infection or inflammation).
In this study:
- Information on the men’s eating habits was collected every four years.
- Information regarding prostate cancer diagnosis was collected every two years.
- From men who had been diagnosed with prostate cancer, information of treatment and disease progression was collected every two years.
The researchers defined lethal prostate cancer as disease that had spread to distant organs (metastatic cancer) or death due to prostate cancer during the study’s follow-up period (1994 to 2008).
The researchers followed up the cohort for 14 years and analysed the associations between eating different amounts of red meat, poultry and eggs and the risk of developing lethal prostate cancer. The researchers grouped each participant according to the average amounts of each type of food they ate per week. For red meat, the subgroups included (per week):
- less than three servings
- 3 to 4 servings
- 5 to 7 servings
- over 8 servings
For poultry, the subgroups were defined as (per week):
- less than 1.5 servings
- 1.5 to 2.5 servings
- 2.5 to 3.5 servings
- over 3.5 servings for week
For eggs, the subgroups were:
- less than half an egg
- 0.5 to 1.5 eggs
- 1.5 to 2.5 eggs
- over 2.5 eggs
To determine which subgroup each participant would be allocated to, the researchers averaged their responses from all of the dietary questionnaires the participants had completed up until their diagnosis, or until the end of the study (for those who were not diagnosed).
To determine the amount of each food eaten, the researchers averaged the reported amounts over all of the questionnaires that were completed before diagnosis. During the analysis, the researchers controlled for possible confounding factors such as age, amount of food eaten, body mass index (BMI, which is an indicator of obesity), smoking status and physical activity levels.
The researchers also analysed the risk of dying from prostate cancer in the men who were diagnosed with it during the course of the study, based on their eating habits after diagnosis. The researchers only included men who were diagnosed with localised cancer (cancer that had not spread beyond the prostate). During the analysis, they controlled for possible confounding factors such as age at diagnosis, time since diagnosis, disease stage, treatment type, BMI, activity level, smoking status and pre-diagnosis diet.
What were the basic results?
Of the 27,607 men included, 199 died of prostate cancer during the study. When the researchers analysed the association between eating habits and risk of lethal prostate cancer when using data up to the point of initial diagnosis, they found that:
- Men who ate an average of 2.5 or more eggs per week had an 81% higher risk of lethal prostate cancer compared to those who ate an average of less than half an egg per week (Hazard Ratio [HR] 1.81, 95% CI 1.13 to 2.89, p=0.01).
- The association between average amount of eggs eaten per week and risk of lethal prostate cancer became non-significant when the researchers analysed data collected up to the point of development of a lethal form of the disease (that is, disease progression or death).
- There was no significant association between the average amount of red meat eaten and the risk of lethal prostate cancer.
- Men who consumed more red meat or eggs tended to exercise less and have a higher BMI, and were more likely to smoke and have a family history of prostate cancer.
Of the 3,127 men who developed prostate cancer during the course of the study, 123 died of it during follow-up. Further analysis of the men who died found no significant association between eating habits after diagnosis and risk of the disease progressing from localised prostate cancer to lethal prostate cancer.
How did the researchers interpret the results?
The researchers conclude that: “Eating eggs may increase risk of developing a lethal form of prostate cancer among healthy men,” and that although “additional large prospective studies are needed, caution in egg intake may be warranted for adult men”.
This was a large prospective cohort study that examined the impact of lifestyle on the risk of developing and dying of advanced prostate cancer.
In addition to its large size, another strength of the study is that the information regarding exposure (eating habits) and possible confounders (medical conditions, activity levels, weight, medications and smoking status) were continually updated over the study’s course. However, updating information on eating habits every four years may still introduce a significant level of recall bias, and accurately remembering what you ate over the previous four years is likely to be difficult.
The study and data analysis also has several limitations. First, the number of deaths and cases of lethal prostate cancer were small (only 199 out of 27,607 men in the whole cohort, and 123 out of 3,127 in the case-only cohort [those who initially developed localised disease]). This small number increases the likelihood that the results are due to chance. Second, the researchers say that the group of men included in the study generally ate low amounts of the foods of interest, which limits the "power" (or ability to detect a difference) of the analysis.
Furthermore, while the researchers controlled statistically for a number of possible confounders, it is difficult to say whether other factors could account for this relationship. The researchers say that men in the study who consumed more red meat or eggs tended to have a higher BMI, exercise less and were more likely to smoke and have a family history of prostate cancer. Additionally, it is probably difficult to control completely for other dietary effects and focus the analysis on a single component of a person’s diet.
This study points to possible associations between diet and risk of prostate cancer. The aforementioned limitations, however, weaken the strength of these conclusions, along with the fact that previous research has looked at this question and found no association. While an 81% increased risk sounds like a high and definitive figure, it is probably best to wait for more conclusive research before cutting eggs out of your diet. There are existing dietary and lifestyle guidelines for reducing cancer risk, such as limiting your consumption of energy-dense foods such as meat and increasing your consumption of fruits, vegetables and wholegrains.