Analysis of the data collected has been ongoing for about a decade. As the investigators for the Adventist Health Study have pieced together the answers to their questions, new directions for investigation have been suggested by the outcomes.
A basic profile of the study population showed an average age of 51 years for men and 53 for women. The proportion of subjects who had been diagnosed by a physician as hypertensive was close to that expected for an adult population. Although a modest number of subjects admitted to past cigarette smoking--usually before joining the Adventist Church--there were virtually no current smokers in the population. A relatively large proportion claimed to exercise with at least moderate frequency. The population, made up of nearly two-thirds women, tended to be well-educated. And a little more than half of the respondents said they ate meat less than once a week.
One area investigated by the Adventist Health Study has been a possible link between diet and cancer. Data from the original Adventist Mortality Study provided good evidence that Adventists do enjoy moderately lower rates of a number of cancers. The differences in risk could be due to dietary habits.
Investigators have suggested a number of mechanisms that might explain the way certain foods could alter the risk of contracting various types of cancer. First, it is possible that certain nutrients alter the cellular environment in important ways. Some nutrients, including Vitamin E, ß-carotene (a form of vitamin A), and Vitamin C have antioxidant properties. Since there is evidence that oxidation of important intracellular chemicals may alter the control of cell differentiation and proliferation, such antioxidants may influence carcinogenesis.
Second, it has been suggested that foods we eat may alter hormone production, though little direct evidence exists for this idea. However, it is well-known that certain tumors, particularly those involving genital organs, respond to hormone levels and may actually be promoted by them. Interestingly, there is some evidence that soybeans alter sterol metabolism in ways that may be important for cancers such as breast cancer.
Third, it is clear that certain dietary articles can influence the rate that materials pass through the gastrointestinal tract and, by this action, alter the nature and duration of contact between the surface epithelium and certain intraluminal chemicals. This may influence the risk for bowel cancer.
Commonly, researchers tend to associate consumption of specific nutrients with the risk of cancer. Investigators for the Adventist Health Study have initially followed a different course, preferring to look at individual foods or food groups rather than specific nutrients.
From a public health perspective,
the examination of foods,
rather than nutrients by themselves,
has attraction because people tend to shop
for foods rather than nutrients.
Even though specific nutrients have been shown to be active agents, foods are literally comprised of hundreds of such chemicals and it seems presumptuous to focus on the few nutrients commonly tabulated in dietary tables.
Each food is a peculiar mix of nutrients and chemicals and it is possible that a particular food with its unique 'soup' of chemicals might have properties which either promote or prevent cancer. From a public health perspective, the examination of foods, rather than nutrients by themselves, has attraction because people tend to shop for foods rather than nutrients.
Findings from the Adventist Health Study have shown certain fairly clear indications of association between diet and cancer, though these relationships are found mainly in tissues of endodermal origin. These include the lungs, stomach, pancreas, colon, and bladder. (See a PDF summarizing cancer findings.)
Lung Cancer: Sixty-three new cases of lung cancer were detected during the follow-up period. After adjusting for smoking habits, age, and sex, it appeared that consuming fruit is protective for both squamous carcinoma and adenocarcinoma of the lung. The relative risk for lung cancer was only about 25 percent for those who consumed fruit more than once a day, as compared to those who consumed fruit less than three times a week, this being independent of past smoking status. However, the expected associations between past and current smoking and lung cancer were also evident in this population.
Stomach Cancer: Seventeen new cases of stomach cancer were observed during the follow-up. Consequently, finding statistically significant associations was particularly difficult. However, those who consumed fruit less than three times a week showed a relative risk that was many-fold greater than that of frequent consumers of fruit.
Pancreatic Cancer: Forty new cases of pancreatic cancer were detected during the follow-up. After adjusting for past or present smoking habits, researchers found strikingly protective effects for those who frequently consumed legumes, vegetarian protein products, dates, raisins, or other dried fruits. For instance, those who consumed legumes more than two times a week had only 1/30th the risk of those who consumed legumes rarely or less than once a week. Similarly, those consuming dates, raisins, and dried fruit on a frequent basis had only 1/5th the risk of developing pancreatic cancer as compared to those who consumed these foods rarely or not at all.
Colon Cancer: The researchers looked at the relationship between diet and colon cancer. They found that individuals who ate beans at least two times a week had a 42 percent lower risk of developing colon cancer than those who said they ate beans less than once a week. They also discovered that individuals who ate flesh foods--defined as meat, fowl, and fish--several times each week, had a somewhat higher risk for colon cancer. On the other hand, those who ate more fiber, defined as indigestible carbohydrates found only in fruits and vegetables, experienced a 40 percent reduction in their risk of colon cancer.
Bladder Cancer: Forty-six new cases of bladder cancer were found during the follow-up. As was expected, cigarette smoking was a significant risk factor. After adjusting for age, sex, and smoking history, researchers discovered that frequent consumption of beef was associated with a more than two-fold risk for cancer of the bladder.
Prostate Cancer: Cancer of the prostate was found in 179 new cases during the follow-up. The only statistically significant trend indicated that consuming dried fruit three to five times a week may decrease the risk of prostate cancer about 40 percent. However, associations of borderline significance suggested that frequent consumption of tomatoes and beans may also be protective, while consuming fish more than once a week may increase the risk of prostate cancer by 50 percent. Further evidence is needed regarding these last factors.
Breast Cancer: Two hundred and twelve new cases of incident breast cancer were reported during the follow-up. No clear associations between dietary factors and breast cancer were found. However, the researchers noted that all of the expected associations with many of the well-established risk factors held true for the Adventist population, including socio-economic status, maternal history of breast cancer, age at the birth of the first child, and years of menstruation. This was important, indicating that the data from the study had not been distorted in some way by the unusual population of the study.
Cancer of the uterus and cervix have not yet been fully analyzed by the researchers, though 140 new cases of uterine cancer and 28 new cases of cervical cancer were observed.
It is interesting to note that, adjusting for age and sex alone, vegetarians in the study population had a lower risk than non-vegetarians for every one of the cancers mentioned. In some instances, the differences were relatively slight, but in every case the risk for vegetarians was lower. Thus, vegetarians are an interesting group with respect to cancer risk. Is the active principle in this case the diet or some other attribute of vegetarians? There is clear evidence that vegetarians tend to differ from non-vegetarians in many ways aside from the consumption of flesh foods. For instance, vegetarians tend to be less obese, drink less coffee, and eat more legumes and vegetarian protein products. In addition, they exercise more regularly.
Consequently, it is entirely possible that some of these other lifestyle and dietary attributes are the active principles in vegetarianism rather than just the absence of flesh foods. The previous analyses seem to show that, with the exception of bladder and perhaps colon cancer, dietary variables other than the absence of meat are more likely to be the active principles in reducing the risk of cancer. For instance, vegetarians tend to eat more fruit, legumes, and vegetarian protein products, and these foods are probably anticarcinogenic in and of themselves. However, the consumption of meat may also have some carcinogenic influence, and evidence of this may have been found for at least bladder cancer.