SCIENTIFIC UPDATE.

A REVIEW OF RECENT SCIENTIFIC PAPERS RELATED TO VEGETARIANISM 

Is the Calcium in Calcium-Fortified Soymilk Absorbed?

Soymilk is frequently fortified with calcium in the form of tricalcium phosphate. Many vegans and others who do not use dairy products rely on fortified soymilk as a concentrated source of calcium, since fortified soymilk has between 80 and 500 mg of calcium per serving. Just because a product has calcium in it does not mean that the calcium is well absorbed. For example, food composition tables make it appear that spinach contains generous amounts of calcium. However, spinach also contains oxalic acid, which binds with calcium and keeps it from being absorbed. Is the calcium in fortified soymilk well absorbed or does something in soy interfere with absorption? Recent research suggests that calcium in fortified soymilk is absorbed about 75% as well as is the calcium in cow's milk. This means that soymilk containing 500 mg of calcium per serving would supply approximately as much usable calcium as does a serving of cow's milk containing 300 mg of calcium. These findings are reassuring to many of us who have wondered whether or not fortified soymilk is truly a good source of calcium: it is.

Heaney RP, Dowell MS, Rafferty K, Bierman J. 2000. Bioavailability of the calcium in fortified soy imitation milk, with some observations on method. Am J Clin Nutr 71:1166-1169.

New Dietary Recommendations for Vitamin C, Vitamin E, Selenium, and Carotenoids 

The Institute of Medicine recently issued new guidelines for intakes of dietary antioxidants (nutrients which help protect cells from oxidative stress). This report recommends intake levels for those in the US and Canada. These recommendations take the place of the Recommended Dietary Allowances (RDAs), which were last updated in 1989.

Recommended intakes of both vitamin C and vitamin E were increased, with a recommendation to smokers to consume more vitamin C than non-smokers. Women should consume 75 mg of vitamin C daily; men should consume 90 mg; smokers should consume an additional 35 mg. Food sources of vitamin C include citrus fruit, potatoes, strawberries, broccoli, and green leafy vegetables. Both men and women should consume 15 milligrams of vitamin E daily from food sources such as vegetable oils, nuts, seeds, and green leafy vegetables. The selenium level in plant food depends on the selenium level in the soil in which the food was grown. Cereals and grains are a good source of selenium for vegetarians. The RDA for selenium is 55 mcg for men and women. Recommended intake levels can be easily attained through foods, so supplements of these nutrients do not appear to be necessary. This report did not recommend a specific daily intake level for carotenoids. Upper intake levels were set for vitamins C and E and selenium. This level is the maximum intake of a nutrient likely to pose no risk of adverse health effects in most people.

Food and Nutrition Board, Institute of Medicine. 2000. Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. Washington, DC: National Academy Press, <http://national-academies.org>.

Possible Link Between a Birth Defect and Vegetarian Diet in Pregnancy?

Hypospadias is a birth defect of the penis which occurs in about 1 out of 300 newborn male babies. While the cause of this defect has not been determined, it is seen more commonly in sons of women who became pregnant later in life and in sons of women who used hormone-containing medicines during pregnancy. A study in Britain examined the role of maternal nutrition in the development of hypospadias. Close to 8000 boys were studied, 51 of whom had hypospadias. The results were surprising. Mothers who followed a vegetarian diet during pregnancy had a higher risk of giving birth to a boy with hypospadias. Was this simply coincidence? Possibly. This is just one study, involving a relatively small number of vegetarians (321), of whom 7 had sons with hypospadias. Was there something unique about the women who were vegetarians and whose sons had hypospadias? We don't know. Of the boys who had hypospadias, 44 were sons of nonvegetarians. What factors caused hypospadias in these boys? Although iron supplements and influenza early in pregnancy may have had some effect, there are many unanswered questions.

The researchers who conducted this study hypothesized that vegetarians ate more soy foods and thus were exposed to more phytoestrogens and that this led to their increased risk of having a son with hypospadias. There is little basis for this conclusion since there was no significant association between use of soymilk and other soy products and development of hypospadias. The researchers plan to measure levels of nutrients, phytoestrogens, and pesticides in blood from women who participated in this study to try to determine specific factors which could have increased a woman's risk of having a child with hypospadias. At this point, it seems premature to link increased risk of hypospadias to a vegetarian diet in pregnancy.

North K, Golding J, The ALSPAC Study Team. 2000. A maternal vegetarian diet in pregnancy is associated with hypospadias. BJU International 85:107-111.

High-Fiber, Lowfat Diets Had No Effect on Growth of Precancerous Colorectal Polyps in People at High Risk of Colorectal Cancer

High-fiber diets have been recommended as one way of reducing risk of developing colon cancer. Can these diets help keep people at high risk of colon cancer from developing it? Two studies provided no evidence that diet is effective in slowing the growth of colon polyps in people who have had a polyp removed. A colorectal polyp is a growth on the inner wall of the intestine or rectum. Most colorectal cancers develop in polyps, so finding a way to slow the growth, or prevent these polyps from developing, could reduce the risk of colon and rectal cancer.

Researchers studied men and women who had a history of polyps. Half followed a high-fiber, lowfat, high fruit and vegetable diet, and half followed their usual diet for four years. There was no difference in the number of new polyps between the two groups, which suggests that diet did not affect polyp development. Similar findings were seen in another study which used a dietary supplement of wheat-bran fiber instead of the subjects' usual diets.

The dietary changes may not have been effective in preventing polyp recurrence because they were too short-term to make a difference. Additionally, factors early in the participants' lives may have predisposed them to develop colorectal polyps; intervention was not able to overcome these earlier influences. Researchers also noted that the new polyps tended to be small; perhaps dietary factors prevent small polyps from growing into larger polyps or developing into cancer.

Do these results mean that it's fine to eat a typical American low-fiber diet? Absolutely not. Much evidence suggests that a lowfat, high fruit and vegetable, high-fiber diet can reduce the risk of many chronic diseases including heart disease, hypertension, obesity, and diabetes.

Schatzkin A, Lanza E, Corle D, et al. 2000. Lack of effect of a low-fat, high-fiber diet on the recurrence of colorectal adenomas. N Engl J Med 42:1149-1155.

Alberts DS, Martinez ME, Roe DJ, et al. 2000. Lack of effect of a high-fiber cereal supplement on the recurrence of colorectal adenomas. N Engl J Med 342: 1156-1162.

Byers T. 2000. Diet, colorectal adenomas, and colorectal cancer. N Engl J Med 342:1206-1207.

Tofu and Brain Function

A recent study of Japanese men living in Hawaii found that those who ate tofu 2 to 4 times per week when they were around 45-65 years old had higher rates of mental deterioration (low brain weight, Alzheimer's disease, poorer cognitive function) in their later years when compared with men who ate less tofu (White, et al). Results were similar for the wives of the men, although the women were not asked how much tofu they ate; researchers assumed if their husbands ate tofu, the wives did also. Male subjects who ate the most tofu in midlife had a 1.6 to 2-times higher risk of impaired brain function, compared to those who ate less tofu. The authors of this study speculated that isoflavones were responsible for the apparent adverse effect of tofu consumption. It is possible that other unknown factors impacted brain function and that tofu consumption was associated with those factors and was not itself the risk factor. A study of older Japanese women living in the Seattle area found that tofu intake over a two-year period did not affect cognitive function (Rice, et al). However, two years may be too short a time to observe a significant effect on function.

At this point, dietary changes should not be made as a result of the one study showing brain changes associated with tofu consumption. There may be other factors in these subjects or their diets that could have influenced the results. Hopefully, additional research will clarify many of these issues.

1. White LR, Petrovitch H, Ross GW, et al. 2000. Brain aging and midlife tofu consumption. J Am Coll Nutr 19: 242-255.

2. Rice MM, Graves AB, McCurry SM, et al. 2000. Tofu consumption and cognition in older Japanese American men and women. J Nutr 130 (suppl 3): 676S.