Though studies continue to support the understanding that vegetarians have a lower cancer risk than meat eaters, colorectal cancer (CRC) risk is virtually the same among both groups in the United Kingdom. A British literature review, published in the European J of Nutrition, suggests that selenium status may influence CRC risk (click here for the full test of the study).
Vegetarians in the UK and other low-Se areas were found to have low Se intakes and status compared to non-vegetarians. There was some evidence of a reverse J-shaped curve of Se intakes and status in the UK throughout the last three decades. These presumed patterns were followed by the changes in CRC mortality or incidence in British vegetarians during this period.” ~ Vegetarianism and colorectal cancer risk in a low-selenium environment: effect modification by selenium status? A possible factor contributing to the null results in British vegetarians
The World Cancer Research Fund (WCRF) shows convincing evidence that some foods are causal risk factor for CRC, including red meats and processed meats, preserved by smoking, curing or salting, or addition of chemical preservatives. It was presumed then that a vegetarian diet, which emphasizes minimally processed plant-based foods, would serve as a model for risk reduction of CRC. In America, the Adventist Health Study data shows a reduction in CRC for pesco-vegetarians and a 22% reduction for all diet groups with reduced or no meat intakes. But for British meat eaters and vegetarians, there was no significant difference in CRC rates.
Despite the consistent findings of lower total cancer incidence in vegetarians than in meat-eaters in the UK, the results of studies of colorectal cancer (CRC) risk in British vegetarians have largely been null. This was in contrast to the hypothesis of a decreased risk of CRC in this population due to null intake of red and processed meats and increased intake of fibre. This study suggests that for vegetarians selenium (Se) status may influence CRC risk. See Figure 1
Data from three Oxford studies, with a total cohort of more than 87,000, the CRC mortality risk was insignificantly lower in vegetarians than in non-vegetarians at 0.85 (95% CI: 0.52–1.39) and 0.85 (95% CI: 0.55–1.32), respectively, after a mean follow-up of 21 and 17 years, respectively. And notably, in one Oxford study, the relative rate of CRC incidence was higher in vegetarians than in meat-eaters after a mean follow-up of 10.7 years (RR 1.39; 95% CI: 1.01–1.91), despite the total cancer incidence being borderline significantly lower (P = 0.052) by 11% in vegetarians.
Selenium as a Dietary Factor in Cancer Risk
In reviewing the data, the researchers contrasted dietary factors in vegetarians residing in the USA and the UK, as well as most of Europe. One influential factor and differentiating point was low selenium. “To illustrate the magnitude of the difference, mean daily Se intakes are approximately 40μg per day in Europe, and 93μg in American women and 134μg in American men. For reference, the daily level of intake currently recommended in the UK for individuals is 1μg of Se per kg of body weight, which in practical terms translates to recommended intakes of 60 and 75μg/day for females and males, respectively,” the researchers wrote.
Given these notable differences in Se intakes and that Se status has been implicated in cancer risk in humans, the researcher felt it wise to “explore the speculative hypothesis that it may have been involved in the CRC risk in the studies of British vegetarians.”
The following are excerpts of key takeaways from the study:
- The results of studies of Se status in European vegetarians show that Se status as assessed by plasma or serum is ca. 10–20% lower in vegetarians than in non-vegetarians (but suboptimal in both diet groups), and highlight the considerable contribution of animal products to total Se intake in low-Se areas.
- Overall, the results of European including some British studies convincingly suggest that lower Se status in British vegetarians than that of meat-eaters is very likely.
- The decrease in Se status in the UK was due to changes in Se content of food products available in the British market, largely resulting from the decline in imports of selenium rich wheat for breadmaking flour from North America.
- Data on Se intakes in British vegans estimated from food records suggest insufficient intakes, however, unlike for many other nutrients, this is not a reliable measure of Se dietary intake due to large within-food variation of Se content. Imported foods may present varying levels of Se depending on where they are grown.
- Analysis showed 34% lower CRC incidence in fish-eaters (pesco-vegetarians) than in meat-eaters, … Fish are a rich source of Se; therefore, the lower CRC risk in British fish-eaters than in vegetarians is consistent with the proposed hypothesis of secular changes in Se status/intake modulating CRC risk, … It is possible that other nutrients from fish (e.g. vitamin D, omega-3 fatty acids), their synergistic effect (i.e. fish itself), or some other aspect of diet or lifestyle common to American and British fish-eaters/pesco-vegetarians reduces the CRC incidence in these diet groups.
- Suggestions to use Brazil nuts as a food-based source of Se may not be reliable because the Se content varies significantly, “ranging from 8 mcg/nut (5 g) in nuts grown in Bolivia, through 18 mcg/nut in Brazil and 33mcg/nut in Peru, to 130mcg/nut when grown in northern countries of South America.” This suggests that country of origin-dependant, “dosing” of Brazil nuts could be used, but that standardized dietary supplements are a reliable option for vegetarians in low-Se areas.
- Se status represents the most pronounced difference in nutritional status between British (likely low Se status) and American vegetarians (likely adequate Se status), in whom significantly decreased CRC risk has been observed.
Available data on Se intake and status in British vegetarians, as well as the relationship between their secular changes in the UK and changes in CRC risk in this dietary group, are compatible with the hypothesis that low Se status may contribute to the largely null results of studies of CRC risk in vegetarians in the UK.
The study made a specific point that future Se trials should be done in geographical areas, such as Europe, where Se status and baseline levels are known to be lower than the United States. “In the context of observational studies, it is reasonable to assume that revealing a ‘true’ Se-cancer risk association may only be possible, when the range of Se statuses in the study sample encompasses both deficient or suboptimal, as well as optimal concentrations of selenoproteins relevant to a particular type of cancer,” they write.
Click Here for Full Text of the Study, Vegetarianism and colorectal cancer risk in a low-selenium environment: effect modification by selenium status? A possible factor contributing to the null results in British vegetarians