Analysis of the AHS-1 data has been ongoing for about a decade. As investigators have pieced together answers to their questions, new directions for investigation have been suggested by the outcomes.
Participant Profiles
A basic profile of the study population showed an average age of 51 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.
Other profile information includes:
- A large proportion claimed to exercise with at least moderate frequency.
- The population, made up of nearly two-thirds women, tended to be well-educated.
- More than 50% of respondents said they ate meat less than once a week.
Link to Cancer Prevention
One area investigated was a possible link between diet and cancer. Data from the original Adventist Mortality Study provided good evidence that Adventists enjoy moderately lower rates of many 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 intra-luminal 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. Reasons for this include:
- 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.
- Although specific nutrients have shown to be active agents, foods are literally comprised of hundreds of such chemicals. Therefore, it seems presumptuous to focus on the few nutrients commonly tabulated in dietary tables.
- Because each food is a peculiar mix of nutrients and chemicals, it is possible that a particular food with its unique 'soup' of chemicals might have properties which either promote or prevent cancer.
Findings from the Adventist Health Study have shown certain 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.)
Specific Findings on Cancer
Lung Cancer
63 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 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. Although this is independent of smoking status, the expected associations between past and current smoking and lung cancer were also evident in this population.
Stomach Cancer
17 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 much greater risk than those who consumed fruit frequently.
Pancreatic Cancer
40 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. Those who consumed legumes more than twice 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, 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 twice 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 (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
46 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
179 new cases of prostate cancer were found during the follow-up. The only statistically significant trend indicated that consuming dried fruit three to five times per week may decrease the risk of prostate cancer by 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
212 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.
Other Cancers
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.
After adjusting for age and sex alone, researchers found that vegetarians in the study had a lower risk than non-vegetarians for every one of the cancers mentioned. Thus, vegetarians are an interesting group with respect to cancer risk. Is the active principle the diet or some other attribute of vegetarians? There is clear evidence that vegetarians tend to differ from non-vegetarians in many ways; vegetarians tend to be less obese, drink less coffee, exercise more regularly and eat more legumes and vegetarian protein products.
Consequently, it is possible that other vegetarian principles (aside from the absence of dietary meat) are the lifestyle and dietary attributes reducing the risk of cancer among vegetarians. Staples of the vegetarian diet such as fruit, legumes and vegetarian protein products are likely anticarcinogenic. Meat consumption may have some carcinogenic influence; evidence of this may have been found for at least bladder cancer.