REED MANGELS, PhD, RD
This issue's Nutrition Hotline reviews whether there are nutritional differences between canned and cooked beans and addresses the myths surrounding soy.
QUESTION: "I use canned beans most of the time because I just don't have time to cook beans and I never seem to remember to soak them ahead of time. What is the difference between canned and cooked beans, nutritionally speaking?"
ANSWER: There's not a big difference between canned and cooked beans. The effects of heating and cooking on nutrients would be similar whether the beans are cooked or canned. Folate appears to be lower in canned beans compared to cooked beans, although a cup of canned beans still provides close to 1/3 the folate RDA for adults. Certainly, sodium will be higher in canned beans compared to beans cooked without adding salt. You can reduce the sodium in canned beans by placing them in a strainer and rinsing them well with cold water. This has been shown to reduce the sodium by as much as 40 percent. Beans are a good source of fiber, protein, iron, and zinc. Don't avoid them just because you don't have time to cook them from scratch. Canned beans are an acceptable substitute.
QUESTION: "What's going on with soy? First I heard that eating soy would help with hot flashes and would be good for my heart, now I've heard that eating soy is dangerous. What should I believe?"
ANSWER: You're right to be confused. New results of scientific studies showing the benefits of soy products appear almost daily. Possible reasons to use soy products are to fight heart disease,1-4 promote stronger bones,5-7 reduce risk of some kinds of cancer,8-10 and to lose weight.11 However, a number of websites and brochures have appeared saying that soy isn't good for people at all. Reasons for avoiding soy are often loosely based on scientific studies, but a lot of times, the results have been twisted quite a bit to make the case that soy is harmful. In reality, soy is neither the cure for all of the chronic diseases that plague an affluent society, nor is it a food that should be avoided. Soy foods can certainly add variety to a vegetarian diet, and they do offer some health benefits, but they should be a part of the diet— not the foundation for it.
Soybeans contain relatively large amounts of isoflavones. Isoflavones are a type of phyto-estrogens, which are substances found in plants that have properties like the hormone estrogen. The levels of isoflavones in soy products are the most common reason for concerns about soy's effects on health. Recently a group of scientists from several different countries looked at more than 200 studies on soy safety and concluded that "the available scientific evidence supports the safety of isoflavones as typically consumed in diets based on soy, or containing soy products."8
If we look at the amount of soy isoflavones used in countries where soy is a regular part of the diet and where no harmful effects have been documented, perhaps this can give us some idea of a reasonable amount of soy. The average daily soy intake in Japan is about 65 grams per person,12 and the average isoflavone intake is about 20-32 milligrams per day.12-14 Higher intakes have been reported in China, where women's median isoflavone intake was 39 milligrams per day, and in Singapore, where the median intake was 35 milligrams per day.15-16 To find out the isoflavone level of your diet, use the USDA's isoflavone database,17 or look on packages of soy foods that you eat. Choosing 2-3 servings of soy per day will generally lead to an isoflavone intake similar to that seen in countries where soy is a regular part of the diet.
Here are some common areas of concern:
Soy and Infants, Children, Pregnancy
During pregnancy, isoflavones from the mother's diet appear to be passed on to the fetus. High levels of isoflavones were found in healthy Japanese infants whose mothers also had high blood levels of isoflavones, probably due to high intakes of soy.18 These levels of isoflavones have not been associated with any health problems in infants. One report has found that a birth defect of the penis called hypospadias occurred more frequently in infants whose mothers followed a vegetarian diet during pregnancy.19 Although some have attributed these results to use of soy, there was no significant association between use of soymilk and other soy products and development of hypospadias.19 Isoflavones from the mother's diet also appear in breast milk, although the daily isoflavone intake of breastfed infants remains negligible,20 even when breast milk levels are increased as much as tenfold by the mother's use of soy foods.21 At this point there is no scientific evidence of a need to avoid soy foods in pregnancy or during breastfeeding; 2-3 servings a day is a reasonable amount.
Ideally, all infants would be breast-fed. There are circumstances, however, where soy formula is the next-best alternative. Researchers have concluded that use of soy formula appears to have no effect on fertility, miscarriage rate, birth defects in offspring, or maturation.22 Based on the results of this study, and calculating isoflavone intake on a body weight basis, and assuming that older children absorb and metabolize soy isoflavones similarly to infants, a daily soy intake of 2-3 servings per day appears reasonable for children.
Children can use soy products as a part of a healthy diet. Sometimes, perhaps because of convenience, or perhaps because they look like what other children are eating, vegetarian children become over-dependent on soy products like veggie burgers, veggie dogs, and veggie deli slices. Using seitan or bean burgers, peanut butter sandwiches, or other foods can encourage some dietary variety rather than focusing strictly on soy.
Soy and Breast Cancer
Concerns have been raised about soy food consumption increasing the risk of breast cancer because of soy's phytoestrogen content. However, soy does not appear to have an estrogen-like effect on breast tissue, suggesting that it does not increase breast cancer risk.23 The results of a recent large study of hormone-replacement therapy (HRT) in menopause found that the combination of estrogen and progestin, rather than estrogen alone, increased risk of breast cancer.24 Since soy does not have progestin activity, it should not increase risk of breast cancer.25
Perhaps use of soy foods early in life is protective for breast cancer. One study has shown that Chinese women who had higher intakes of soy foods during adolescence had lower risk of breast cancer as adults.10 Several studies have suggested that the use of soy foods leads to a longer menstrual cycle, which may reduce risk of breast cancer,26-27 although not all studies show an effect on menstrual cycle length.28-31
Soy and Brain Function
Concerns have been raised about soy's effect on brain function based on one study that found an association between eating tofu regularly and dementia in Japanese-American men.32 Other research has contradicted these findings, and the original study's design has been questioned.33-35
Soy and Reproduction
Soy's phytoestrogen content has also led to concerns that soy will have a harmful effect on the ability to reproduce. Certainly, there have been no reports of widespread problems with reproduction in countries like China and Japan, where soy is a regular part of the diet. Other evidence that soy does not impair reproductive performance includes a study showing that two months of soy supplements had no effect on semen quality in men,36 several studies showing that ovulation occurs in women using soy products,8 and a study that found that soy formula had no effect on fertility. 22
Soy and Thyroid
What about thyroid function? As I write, I'm looking at a pamphlet that says, "Soy foods can cause thyroid problems." There's a bit of truth to this statement. Soy can be a problem for people whose diets don't contain enough iodine.8 By using iodized salt, sea vegetables, and other sources of iodine, any detrimental effects of soy on thyroid function can be minimized. In clinical trials where soy is added to people's diets, no harmful effects on thyroid function have been seen.8 One study has suggested that a diet containing higher levels of phytoestrogens is actually associated with a reduced risk of thyroid cancer in women.37
Soy Foods as a Protein Source
Some questions have been raised about the quality of soy protein and whether or not it supplies all of the amino acids that we need. The World Health Organization has identified soy as a high quality protein that can meet all of the essential amino acid requirements of humans.38 In addition, the newest protein recommendations state that soy is a high quality protein, equivalent to animal protein.39
Soy and Mineral Absorption
"Soy foods have high levels of phytic acid that reduce absorption of calcium, iron, and zinc," say soy's critics. Again, there is some truth to this statement. Soy products do contain high levels of phytic acid, which can reduce absorption of some minerals. However, this statement is not entirely true. Calcium is reasonably well-absorbed from soy products. For example, the calcium in fortified soymilk is absorbed about 75% as well as the calcium in cow's milk is.40 In addition, soy isoflavones have been shown to promote bone health.5-7 The phytates in soy products do appear to inhibit iron absorption, although this effect can be overcome, at least partially, by eating a good source of vitamin C along with the soy product.41-42 A recent study also suggests that soybeans can be a good source of iron for people who are marginally iron-deficient.43 However, zinc is poorly absorbed from soy foods. This alone is not a reason to avoid soy foods; rather, other sources of zinc, such as nuts, seeds, whole grains, and fortified foods, should be eaten daily.
The results of the most recent research suggest that it is all right to include soy as a part of a healthy diet, and that, in fact, there are some health advantages to using some soy products. A reasonable amount of soy for most people seems to be about 2-3 servings daily.
- Anderson JW, Johnstone BM, Cook-Newell ME. 1995. Meta-analysis of the effects of soy protein intake on serum lipids. N Engl J Med 333:276-82.
- Hodgson JM, Croft KD, Puddey IB, et al. 1996. Soybean isoflavonoids and their metabolic products inhibit in vitro lipoprotein oxidation in serum. J Nutr Biochem 7:664-9.
- Scheiber MD, Liu JH, Subbiah MT, et al. 2001. Dietary inclusion of whole soy foods results in significant reductions in clinical risk factors for osteoporosis and cardiovascular disease in normal postmenopausal women. Menopause 8:384-92
- Lichtenstein AH. 2001. Got soy? Am J Clin Nutr 73:667-8.
- Lydeking-Olsen E, Jensen JB, Setchell KDR, et al. 2002. Isoflavone-rich soymilk prevents bone loss in the lumbar spine of postmenopausal women. J Nutr 132:581S.
- Arjmandi BH, Khalil DA, Smith BJ, et al. 2003. Soy protein has a greater effect on bone in postmenopausal women not on hormone replacement therapy, as evidenced by reducing bone resorption and urinary calcium excretion. J Clin Endocrinol Metab 88:1048-52.
- Potter SM, Baum J, Teng H, et al. 1998. Soy protein and isoflavones: Their effects on blood lipids and bone density in postmenopausal women. Am J Clin Nutr 68(suppl):1375S-9S.
- Munro IC, Harwood M, Hlywka JJ, et al. 2003. Soy isoflavones: a safety review. Nutr Rev 61:1-33.
- Gardner-Thorpe D, O'Hagen C, Young I, Lewis SJ. 2003. Dietary supplements of soya flour lower serum testosterone concentrations and improve markers of oxidative stress in men. Eur J Clin Nutr 57:100-6.
- Shu XO, Jin F, Dai Q, et al. 2001. Soyfood intake during adolescence and subsequent risk of breast cancer among Chinese women. Cancer Epidemiol Biomark Prev 10:483-8
- Goodman-Gruen D, Kritz-Silverstein D. 2001. Usual dietary isoflavone intake is associated with cardiovascular disease risk factors in postmenopausal women. J Nutr 131:1202-6.
- Nagata C. 2000. Ecological study of the association between soy protein intake and mortality from cancer and heart disease in Japan. Int J Epidemiol 29:832-836.
- Nagata C, Kabuto M, Kurisu Y, Shimizu H. 1997. Decreased serum estradiol concentration associated with high dietary intake of soy products in premenopausal Japanese women. Nutr Cancer 29:228-33.
- Wakai K, Egami I, Kato K, et al. 1999. Dietary intake and sources of isoflavones among Japanese. Nutr Cancer 33:139-45.
- Chen Z, Zheng W, Custer LJ, et al. 1999. Usual dietary consumption of soy foods and its correlation with the excretion rate of isoflavonoids in overnight urine samples among Chinese women in Shanghai. Nutr Cancer 33:82-7.
- Seow A, Shi C, Franke AA, et al. 1998. Isoflavonoid levels in spot urine are associated with frequency of dietary soy intake in a population-based sample of middle-aged and older Chinese in Shanghai. Cancer Epidemiol Biomarkers Prev 7:137-40.
- USDA-Iowa State University Database on the Isoflavone Content of Foods, Release 1.3 - 2002, http://www.nal.usda.gov/fnic/foodcomp/Data/isoflav/isoflav.html
- Adlercreutz H, Yamada T, Wahala K, Watanabe S. Mar 1999. Maternal and neonatal phytoestrogens in Japanese women during birth. Am J Obstet Gynecol 180:737-43
- North K, Golding J, The ALSPAC Study Team. 2000. A maternal vegetarian diet in pregnancy is associated with hypospadias. BJU International 85:107-111.
- Setchell KDR, Zimmer-Nechemias L, Cai J, Heubi J. 1997. Exposure of infants to phytoestrogens from soy infant formulas. Lancet 350: 23-27.
- Franke AA, Custer LJ. 1996. Daidzein and genistein concentrations in human milk after soy consumption. Clin Chem 42:955-964.
- Strom BL, Schinnar R, Ziegler EE, et al. 2001. Exposure to soy-based formula in infancy and endocrinological and reproductive outcomes in young adulthood. JAMA 286:807-14.
- Atkinson C, Warren RML, Dowsett M, et al. 2002. Effects of isoflavones on breast density, estradiol and gonadotrophins: a double-blind, randomized, placebo-controlled trial. J Nutr 132:577S-8S.
- Rossouw JE, Anderson GL, Prentice RL, et al. 2002. Risks and benefits of estrogen plus progestin in healthy postmenopausal women: principal results From the Women's Health Initiative randomized controlled trial. JAMA 288:321-33.
- Messina MJ. 2002. Soy foods and soybean isoflavones and menopausal health. Nutr Clin Care 5:272-82.
- Cassidy A, Bingham S, Setchell KDR. 1994. Biological effects of a diet of soy protein rich in isoflavones on the menstrual cycle of premenopausal women. Am J Clin Nutr 60:333-40.
- Watanabe S, Terashima K, Sato Y, et al. 2000. Effects of isoflavone supplement on healthy women. Biofactors 12:233-41.
- Lu LJW, Anderson KE, Grady JJ, et al. 1996. Effects of soya consumption for one month on steroid hormones in premenopausal women: implications for breast cancer risk reduction. Cancer Epidemiol Biomarkers Prev 5:63-70.
- Martini MC, Dancisak BB, Haggans CJ, et al. 1999. Effects of soy intake on sex hormone metabolism in premenopausal women. Nutr Cancer 34:133-9.
- Duncan AM, Merz BE, Xu MX, et al. 1999. Soy isoflavones exert modest hormonal effects in premenopausal women. J Clin Endocrinol Metab 84:192-7.
- Nagata C, Takatsuka N, Inaba S, et al. 1998. Effect of soymilk consumption on serum estrogen concentrations in premenopausal Japanese women. J Natl Cancer Inst 90:1830-5.
- White LR, Petrovitch H, Ross GW, et al. 2000. Brain aging and midlife tofu consumption. J Am Coll Nutr 19:242-55.
- 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.
- Duffy R, Jarrett N, Fluck E, et al. 2002. Dietary soy improves memory in humans. J Nutr 132:587S.
- Kritz-Silverstein D, von Muhlen D, Barrett-Connor E. 2002. The soy and postmenopausal health in aging (SOPHIA) study: overview and baseline cognitive function. J Nutr 132:586S.
- Mitchell JH, Cawood E, Kinniburgh D, et al. 2001. Effect of a phytoestrogen food supplement on reproductive health in normal males. Clin Sci 100:613-8.
- Horn-Ross PL, Hoggatt KJ, Lee M. 2002. Phytoestrogens and thyroid cancer risk in women. J Nutr 132:574S-5S.
- FAO/WHO. Protein Quality Evaluation Report of Joint FAO/WHO Expert Consultation. Rome: Food and Agriculture Organization of the United Nations, 1991.
- Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Washington, DC: National Academies Press; 2002.
- 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 71:1166-2011.
- Hallberg L, Hulthen L. Prediction of dietary iron absorption: an algorithm for calculating absorption and bioavailability of dietary iron. 2000. Am J Clin Nutr 71:1147-2011.
- Sandstrom B. 2001. Micronutrient interactions: effects on absorption and bioavailability. Br J Nutr 85 Suppl 2:S181-S185.
- Murray-Kolb LE, Welch R, Theil EC, Beard JL. 2003. Women with low iron stores absorb iron from soybeans. Am J Clin Nutr 77:180-4.
- Hexane in veggie burgers: little science behind the claims. (Examiner.com; 04/26/10)
- Safety of Soyfoods (Academy of Nutrition and Dietetics)
- Soy and Health Update: Evaluation of the Clinical and Epidemiologic Literature By Mark Messina, PhD, 2016
Click here to see the milk alternative tables, updated in 2013.