The present study showed that substituting 3 oz of walnuts per day for fat sources in an Average American Diet significantly increased intake of γ-T and mono- and polyunsaturated fats and resulted in a significant decrease in serum α-T:γ-T, with a trend towards an increase in serum γ-T and PFP, in this population of older men at risk for prostate cancer. The ratio of serum α-T:γ-T, which decreased almost four-fold, is of particular interest because it specifically contrasts changes in the two forms of vitamin E and also because it should be less affected by confounding variables such as lipids [37
]. These results are consistent with a study by Lemcke-Norojarvi et al [38
], which reported that feeding oils containing differing amounts of α- and γ-T resulted in an increase in serum γ-T but not α-T, and a decrease in α-T:γ-T. A previous study identified a higher ratio of serum α-T:γ-T as an important difference between patients with heart disease and healthy controls [39
]. Thus, a lower ratio of serum α-T:γ-T may be protective against heart disease, and walnuts may be beneficial in lowering this ratio. Further research is needed to explore the effects of mixed tocopherols on markers of heart disease risk, such as platelet function. A recent study conducted in subjects with type II diabetes showed that, although supplementation with mixed tocopherols (500 mg/d total: 315 mg of γ-T, 75 mg of α-T and 110 mg δ-tocopherol) increased both serum α-T and γ-T and erythrocyte and platelet γ-T four-fold, it did not improve platelet activation and endothelial function [40
]. The lack of an increase in α-T and the trend for a decrease in γ-T in the present study may be explained by the fact that we fed a much lower "dose" of tocopherols as walnuts in different proportions (i.e. 0.52 mg/d of α-T and 15.6 mg/d of γ-T) than did Clarke et al [40
Our timecourse experiment confirmed that tocopherol absorption from walnuts, a whole food source of vitamin E, is similar to that which has been reported following a vitamin E supplement [27
]. Our experiment also showed a significant increase in the ratio of γ-T to triglycerides. Although the average concentration of serum triglycerides decreased between 4 and 8 h, serum γ-T increased between 1 and 8 h (Figure ), suggesting that the two do not move in parallel. While the action of lipoprotein lipase and the hepatic uptake of chylomicron remnants causes serum triglyceride concentrations to drop, transfer of γ-T to longer-lived lipoprotein particles may cause serum γ-T to rise over a longer period and may explain the significant increase seen in triglyceride – normalized serum γ-T. Further work is needed to determine if serum γ-T would have continued to rise beyond 8 h in our experiment. Although 8 h is sufficient for measuring maximal postprandial changes in serum lipids, our data suggest that a period of at least 8 h, and possibly longer, is needed for a maximal increase in postprandial serum γ-T from a whole food source, as has been demonstrated previously in vitamin E supplementation studies [42
]. Nonetheless, our timecourse experiment does show an increase in serum γ-T over an 8 h time period indicating its bioavailability in the walnut supplement study.
Previous research suggests that walnut consumption is not associated with a higher body weight unless, as would be expected, energy intake exceeds energy expenditure [43
]. In our study, mean weight decreased only during the usual diet period (net difference in weight change was -0.85 ± 0.37 kg between the two diets). Reported caloric intake was significantly less on the usual diet than during the walnut supplement diet, which likely explains the very modest weight loss observed. During the walnut supplement diet, even though subjects reported a higher energy intake, they maintained their body weight. Further, adjusting our results for weight as described above showed the changes in tocopherols persisted even following weight adjustment.
For several reasons, we suspect that the relative good health of our participants (aside from their prostate symptoms) may have contributed to the lack of significant treatment effects for many of our serum hormone and lipid variables. Our participants learned about the study through a newspaper advertisement targeting men concerned about elevated PSA levels and interested in using a dietary intervention to possibly improve prostate health. Consequently, they were "self-selected" and might have been more health-conscious, and thus healthier, than males of the same age in the general population. Although they had elevated PSA levels, their IGF-1 and total cholesterol levels were lower and their HDL cholesterol was higher than observed in other studies [7
], indicating that they might have been healthier than their peers.
First, the men in our study were in the lowest quartile of serum IGF-1 when compared to individuals in the Physician's Health Study, another study with participants who were more likely to be health-conscious [7
]. However, the effect size was small to moderate (0.43) for our serum IGF-1 data, indicating that a significant decrease in IGF-1 might have been detected if the sample size had been larger.
Second, among our participants, mean serum total cholesterol was lower (5.00 mmol/L, usual diet and 4.82 mmol/L, walnut supplement diet) and mean serum HDL cholesterol was higher (approximately 1.32 mmol/L, both diets) when compared to cholesterol levels of men in a large heart-health study (5.33 mmol/L, total cholesterol and 1.11 mmol/L, HDL cholesterol) [45
]. Many of the previous feeding studies with walnuts that have reported favorable improvements in serum lipids have studied subjects with hypercholesterolemia [24
]. Further, whereas these previous studies employed a controlled feeding design, in which participants followed a prescribed, low-fat diet, the present study employed a free-living design, in which participants followed their typical diet with or without walnuts, depending on the diet period. By nature of the design, a free-living feeding study cannot minimize dietary variation as well as a controlled feeding study, and this can make producing significant dietary change more difficult, especially given the small sample studied.
Previous research on the role of dietary γ-T in prostate cancer and CVD is limited [47
]. To our knowledge, this is one of the first studies to assess the effects of walnut intake on markers of prostate health, to demonstrate the bioavailability of γ-T following walnut consumption and to contrast the postprandial timecourse of dietary lipids and tocopherols. Although the tocopherol ratio significantly improved with walnut consumption, the primary markers of prostate health (PSA and urinary symptoms) were not changed significantly by this 8-week dietary intervention. However, there was a nonsignificant increase in PFP (the ratio of free PSA: total PSA) during both diets, and research has indicated that the use of PFP in combination with total PSA may improve specificity of diagnosis and decrease false-positives [5
]. Future work is needed to determine whether dietary tocopherols must be administered over longer periods of time to have a meaningful effect on prostate health and if they are effective both as a preventative measure against the development of prostate disease and as a secondary prevention strategy.
Nevertheless, the significant decrease in α-T: γ-T (without changes in body weight) in response to incorporation of 75 g of walnut/d in the diet offers the promise of significant benefits for both vascular and prostate health.
Our results are also important in light of recent research on the relationship between α-linolenic acid and prostate cancer, since walnuts are a rich source of α-linolenic acid (6.81 g/75 g walnuts) [19
]. A meta-analysis of nine observational studies (four prospective studies and five nonprospective studies) assessed the relationship between prostate cancer incidence or prevalence and intake or blood levels of α-linolenic acid [48
]. This meta-analysis reported that α-linolenic acid increased risk of prostate cancer (1.70; 95% CI 1.12–2.58). For the prospective studies, however, the combined estimate of relative risk for prostate cancer incidence was 1.32 (95% CI 0.80–2.18). More recent evidence reports no association between α-linolenic acid intake and risk of prostate cancer [49
] (also, Simon JA, Tanzman JS, Sabaté J: Lack of effect of walnuts on serum levels of prostate specific antigen: a brief report, submitted). The Lyon Diet Heart Study, a randomized secondary prevention trial, found that subjects consuming a Mediterranean diet with 0.8% of calories from α-linolenic acid were not at increased risk of prostate cancer compared to those consuming a diet that met the criteria of an American Heart Association Step I Diet [49
]. The prospective study conducted by Koralek et al [50
] with 29,592 participants (mostly Caucasian) of the Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial reported no association between dietary intake of total α-linolenic acid (multivariate relative risk for highest vs. lowest quintile of intake/d [1.75 g/d vs. 1.38 g/d] was 0.94, 95% CI = 0.81–1.09) and α-linolenic acid from specific food sources and risk of total prostate cancer or prostate tumors that were defined by stage and grade. Consistent with our study results, Simon et al (Simon JA, Tanzman JS, Sabaté J: Lack of effect of walnuts on serum levels of prostate specific antigen: a brief report, submitted) reported that short term consumption of walnuts did not affect PSA levels adversely in healthy men.