THE strong link between diet and cardiovascular disease, including diabetes, has led to preventive programs at the community and individual level in many Western countries.
Analyses of declining death rates from cardiovascular disease in Australia, the US, New Zealand, Canada and the UK conclude that about half of the decline is attributable to improved medical and surgical care and half to changes in lifestyle, especially reduced tobacco consumption but also to changing nutritional patterns.
With increasing knowledge about the epidemiology of cancer, the importance of diet in the development of several cancers is also becoming better understood.
Some early studies on the link between diet and cancer were conducted in Seventh-day Adventist populations as they presented a unique opportunity to examine the effects of diet given their abstinence from smoking and drinking and their propensity towards a vegetarian diet. Colon and prostate cancers were found to be significantly more likely in Seventh-day Adventist non-vegetarians.
In the past few decades, many “associations” between diet and cancers of the colon, bowel, prostate, stomach and breast have been made.
However, the evidence has been far from consistent and, in many cases, is far from convincing. The World Cancer Research Fund (WCRF) continuously updates its 2007 report on the links between diet and cancer prevention. It recommends eating a plant-based diet low in red meat and avoiding processed meat consumption, and says that foods high in lycopene (ie, tomatoes) “probably protect against prostate cancer”.
The main epidemiological evidence for the link between diets and cancer has been drawn from two principal sources — long-term studies of populations and the comparison of dietary behaviour in people with and without cancer.
The difficulties with the second method are that findings in comparative studies can be overly simplistic and include the inability to define retrospectively what diets people followed. Memory is not only faulty, but biases creep in, meaning that those with cancer will recall the past, including their dietary behaviour, differently.
Nevertheless, much knowledge has been gained and, in essence, the kind of diet that we associate with heart health more or less applies to cancer prevention.
Another question now arises, especially as cancer has become a chronic disease with early detection and treatment substantially extending the life expectancy. If a patient has, say, non-metastatic prostate cancer that has been treated and the outlook is good, should he do anything about his diet?
One of the brilliant things that Walter Willett and his colleagues at Harvard did in the 1980s, and even earlier with a massive long-term study of nurses, was to enrol health professionals in prospective surveys of their health and lifestyle. By enrolling them when they were well, Willett et al were able to obtain lifestyle information that was unclouded by the selective recall that applies when people with an illness try to remember what they did and what they ate.
In a recent study researchers, including Willett, prospectively studied 4577 men with non-metastatic prostate cancer in the Health Professionals Follow-up Study (1986‒2010). Specifically, they set out “to examine postdiagnostic fat intake in relation to lethal prostate cancer and all-cause mortality”.
They found that “among 4577 men with non-metastatic prostate cancer, 315 died of lethal prostate cancer” out of the total 1064 deaths during the 8-year follow-up. “The primary causes of death were cardiovascular disease (31.2%), prostate cancer (21.3%), and other cancers (20.6%).”
After dizzying and elegant statistical analyses, the research workers concluded that “among men with nonmetastatic prostate cancer, replacing carbohydrates and animal fat with vegetable fat may reduce the risk of all-cause mortality. The potential benefit of vegetable fat consumption for prostate cancer-specific outcomes merits further research”.
In an accompanying editorial, the author writes: “When counseling patients, I remind them that obesity is the only known modifiable risk factor linked with prostate cancer mortality to date. Thus, avoiding obesity is essential. Exactly how this should be done remains unclear, although the data by Richman et al suggest that substituting healthy foods (i.e., vegetable fats) for unhealthy foods (i.e., carbohydrates) may have a benefit. Determining whether this benefit is due to reduced consumption of carbohydrates or greater intake of vegetables will require future prospective randomized trials.”
This echoes the conclusions of the WCRF review of diet and cancer survivor outcomes that “large-scale trials with longer follow-up are required”.
So, for the moment we are left with the challenge — but perhaps another incentive for those with stable prostate cancer — to follow a prudent diet that emphasises fruit, vegetables and vegetable oils, and limits consumption of animal fat.
Perhaps it should be along the lines of the traditional Mediterranean diet — that way, you can get your lycopene-rich tomatoes too!
Professor Stephen Leeder is the editor-in-chief of the MJA and professor of public health and community medicine at the University of Sydney. Ms Shauna Downs is a PhD student in nutrition policy at the Menzies Centre for Health Policy.
Jane McCredie is on leave.