Epidemiologic and Clinical Studies

Most of the available clinical evidence regarding the effect of dietary fat intake on prostate cancer comes from observational rather than interven-tional studies. A close correlation exists between average per capita fat intake and prostate cancer mortality in numerous countries round the world [9]. Japanese and Chinese men who migrate to the United States experience dramatic increases in prostate cancer risk within one generation compared to their Caucasian neighbors [10]. Numerous case-control studies over the past 25 years have demonstrated a positive correlation between prostate cancer and increased fat or fat-type food consumption. Such studies include an analysis of 384 men diagnosed with prostate cancer between 1990 and 1992 in Quebec, Canada [11]. On average, after controlling for age, grade, clinical stage, initial treatment, and total energy intake, saturated fat consumption was significantly associated with disease-specific survival. Compared with men in the lower tercile of saturated fat intake, those in the upper tercile had three times the risk of dying from prostate cancer. Another study has reported an attributable risk of 13% for saturated fat intake in excess of 26g per day as compared to diets with less than 13g per day [12].

However, other studies have failed to show an association between prostate cancer and total fat or total saturated fat. A prospective study of 6763 white male Seventh-Day Adventists observed for 21 years failed to find a significant relation between prostate cancer risk and fat-associated food—meat or poultry, milk, cheese, and eggs. There was a suggested positive association that was stronger when an individual consumed all four of these dietary items, but overall there was no significant relation between diet and prostate cancer [13]. A further finding of this study was that overweight men had a significantly higher risk of dying from prostate cancer compared with nonobese men (relative risk 2.5). Another cohort study of 7999 men of Japanese ancestry living in Hawaii also found no association between fat and prostate cancer risk [14].

The largest cohort study examining the relation between fat and prostate cancer risk was the Netherlands Cohort Study [15]. For over 6 years, approximately 58,000 men were observed and 642 cases of prostate cancer were documented. An extensive 150-item food frequency questionnaire was used. No significant association was found between total fat or subtypes of fat and prostate cancer risk. The average intake of fat as a percentage of total calories in this study was

Diet and Prostate Cancer high (40%), so any influence of extremely low levels of fat or reduced calorie intake and prostate cancer risk was not mentioned. Another large cohort study involved 47,855 men who were sent biannually an expanded food frequency questionnaire with 131 items. There was a positive association of fat but no statistically significant difference was found [16]. Linoleic acid, which is the major polyunsaturated fat in most diets, has been associated with an increased risk of prostate carcinoma in some studies [17,18]. It is difficult, though, for adequate conclusions to be drawn from these investigations because of the potential for recall bias and confounding. Omega-3 fatty acids, obtained mainly from fatty fish, have been shown to inhibit prostate cancer cell lines in laboratory experiments [19].The Netherlands Cohort Study found a potential protective effect of omega-3 fatty acids, but this was not statistically significant [15].

Only one large prospective study has demonstrated a statistically significant finding between high-fat foods and prostate cancer [20]. This study involved a cohort of 20,316 men of multiethnic backgrounds living in Hawaii. There were 198 cases of prostate cancer documented during follow-up that ranged from 9 to 14 years. The relative risks for consuming beef, milk, and high-fat foods were 1.6, 1.4, and 1.6, respectively. However, a closer look at the study showed that the food questionnaire assessed only 13 dietary items. In addition, height was demonstrated as being the largest risk factor for prostate cancer with the most significance (p < .01), compared with food associations with fat (p < .05). Furthermore, high intakes of milk and beef were not found for this cohort, so these could not be compared with the lowest intakes to determine whether there was a consistent trend with these dietary variables. Another cohort study, the Physicians' Health Study, also found an association between red meat consumption and prostate cancer risk, but this association was not statistically significant [21].

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