Soyfoods contain compounds that inhibit the absorption of minerals (including iron, calcium and zinc) and, because of that, can cause mineral deficiencies or imbalances.
|Soyfoods do not cause mineral deficiencies or imbalances and recent evidence indicates mineral absorption from soy is much better than previously thought.|
For a food to be considered a good source of a nutrient requires not only that it contain ample amounts of that nutrient, but that the nutrient is well absorbed. There are factors in food that both inhibit and enhance nutrient absorption and this is particularly true in the case of minerals. There is perhaps no better example of the importance of absorption than in the case of spinach and calcium. Spinach is especially high in calcium; in fact, one cup of cooked spinach, which is only 41 calories, contains 245 milligrams, almost as much calcium as one cup of cow’s milk. Spinach is also high in iron as one cup provides more than 6 mg (the RDA is 8 and 18 milligrams for men and women aged 19 to 50). However, spinach is not considered a good source of either iron or calcium because the absorption of these minerals is so poor .
The fact that iron is poorly absorbed from spinach isn’t surprising because this is the case for most plant foods [2-4]. There is an important caveat however. Absorption is typically assessed in response to the consumption of a single meal. However, acute studies tend to exaggerate the effects of factors that both enhance and inhibit mineral absorption in comparison to the impact of these factors over the long term ; This is likely because the body has physiological mechanisms to compensate for the differences in mineral absorption and perhaps also because in a mixed diet the effects of enhancers and inhibitors of absorption tend to balance each other out.
No food is a good source of all nutrients nor does it have to be to be viewed as nutritious. However, because soyfoods are frequently used in place of animal foods, it is important to have some understanding of whether they are good sources of three nutrients in particular – zinc, iron, and calcium. Red meat is a good source of zinc and iron and as noted, dairy foods of calcium.
Soybeans are high in a natural plant chemical phytate . Phytate binds minerals such as iron, zinc, and calcium and as a result, limits their absorption. This doesn’t mean however that foods containing phytate should be avoided. In fact, whole grains contain lots of phytate but the nutrition community recommends the intake of whole grains because the overwhelming preponderance of evidence indicates that they are nutritionally beneficial [7, 8]. Interestingly, although phytate does inhibit mineral absorption, there is interest in phytate as a potential anti-carcinogen .
In regard to calcium and soy, the critical question is whether calcium is well absorbed from calcium fortified soymilk (nearly all soymilk in the United States is calcium fortified) because people who drink soymilk likely do it in place of cow’s milk, and the latter is a major source of calcium in the US diet. The evidence unequivocally shows that despite containing phytate and oxalate, calcium absorption from soymilk is essentially equivalent to calcium absorption from cow’s milk . Calcium absorption from calcium-set tofu is also quite good .
Soyfoods such as tofu and soy burgers may end up partially replacing meat in the diet. Thus, it is important to understand how such substitutions affect mineral absorption and status. This having been said, whether soyfoods are good sources of iron and zinc is really an issue primarily important for vegetarians because it takes relatively little meat to meet the dietary requirements for these minerals. Soybeans are quite high in iron but as previously noted, as is the case for most plant foods, iron absorption from soy has traditionally been considered to be poor. However, recent clinical research indicates that this view may be wrong. It now appears that the methodology used in older studies for assessing iron absorption from soy may have greatly underestimated iron absorption [12, 13]. Further, because of the specific form in which the iron in soybeans exists, it appears that iron is extremely well absorbed from soyfoods . More research is needed before definitive conclusions can be drawn but so far the results are quite encouraging.
Zinc absorption from soy is only about 25% lower than from meat [15-18]. However, the issue isn’t absorption as much as it is content – soybeans are not especially rich in this mineral. In fact, zinc is often considered a problem nutrient for vegetarians. Plant foods such as beans, nuts, and oats contain about 1 to 2 milligrams per serving (the RDA is for adult men and women is 11 and 8 milligrams, respectively). Fortunately, many foods are fortified with zinc, especially breakfast cereals.
Finally, what matters most is the effect of eating soyfoods on mineral status. In this regard, long term study studies show that when soy partially replaces meat in the diet neither iron nor zinc status is adversely affected. Furthermore, a newly completely 10-week study conducted at Iowa State University found that when premenopausal women consumed 2 to 3 servings of soyfoods daily in place of 2 to 3 servings of similar foods made from animal products zinc and iron status were unaffected (see reference for a description of study) . This study is especially noteworthy because the subjects consumed soyfoods as they typically would in real life. Instead of drinking cow’s milk they drank soymilk, and instead of eating chili with meat, they ate chili made with textured soy protein.
In summary, incorporating soyfoods into the diet has not been shown to adversely affect mineral status in people who consume them. Calcium is well absorbed from soy and new information suggests this is also true for iron. No food is a source of all nutrients nor does it have to be to be considered a healthful addition to the diet. It is best not to rely upon any single food too much as a source of a particular nutrient.
- Weaver CM, Heaney RP, Nickel KP, Packard PI: Calcium bioavailability from high oxalate vegetables: Chinese vegetables, sweet potatoes and rhubarb. J Food Sci 63, 524-5 (1997).
- Hunt JR: Bioavailability of iron, zinc, and other trace minerals from vegetarian diets. Am J Clin Nutr 78, 633S-9S (2003).
- genannt Bonsmann SS, Walczyk T, Renggli S, Hurrell RF: Oxalic acid does not influence nonhaem iron absorption in humans: a comparison of kale and spinach meals. Eur J Clin Nutr 62, 336-41 (2008).
- Sandberg AS: Bioavailability of minerals in legumes. Br J Nutr 88 Suppl 3, S281-5 (2002).
- Hunt JR: Moving toward a plant-based diet: are iron and zinc at risk? Nutr Rev 60, 127-34 (2002).
- Reichwald K, Hatzack F: Application of a modified Haug and Lantzsch method for the rapid and accurate photometrical phytate determination in soybean, wheat, and maize meals. J Agric Food Chem 56, 2888-91 (2008).
- Slavin J: Why whole grains are protective: biological mechanisms. Proc Nutr Soc 62, 129-34 (2003).
- Slavin JL, Jacobs D, Marquart L, Wiemer K: The role of whole grains in disease prevention. J Am Diet Assoc 101, 780-5 (2001).
- Vucenik I, Passaniti A, Vitolo MI, Tantivejkul K, Eggleton P, Shamsuddin AM: Anti-angiogenic activity of inositol hexaphosphate (IP6). Carcinogenesis 25, 2115-23 (2004).
- Zhao Y, Martin BR, Weaver CM: Calcium bioavailability of calcium carbonate fortified soymilk is equivalent to cow’s milk in young women. J Nutr 135, 2379-82 (2005).
- Weaver CM, Heaney RP, Connor L, Martin BR, Smith DL, Nielsen E: Bioavailability of calcium from tofu vs. milk in premenopausal women. J Food Sci 68, 3144-7 (2002).
- Murray-Kolb LE, Welch R, Theil EC, Beard JL: Women with low iron stores absorb iron from soybeans. Am J Clin Nutr 77, 180-4 (2003).
- Lonnerdal B, Bryant A, Liu X, Theil EC: Iron absorption from soybean ferritin in nonanemic women. Am J Clin Nutr 83, 103-7 (2006).
- Lonnerdal B: Soybean ferritin: implications for iron status of vegetarians. Am J Clin Nutr 89, 1680S-5S (2009).
- Sandstrom B, Andersson H, Kivisto B, Sandberg AS: Apparent small intestinal absorption of nitrogen and minerals from soy and meat-protein-based diets. A study on human ileostomy subjects. J Nutr 116, 2209-18 (1986).
- Sandstrom B, Kivisto B, Cederblad A: Absorption of zinc from soy protein meals in humans. J Nutr 117, 321-7 (1987).
- Greger JL, Abernathy RP, Bennett OA: Zinc and nitrogen balance in adolescent females fed varying levels of zinc and soy protein. Am J Clin Nutr 31, 112-6 (1978).
- Sandstrom B, Cederblad A: Zinc absorption from composite meals. II. Influence of the main protein source. Am J Clin Nutr 33, 1778-83 (1980).
- Messina M, Watanabe S, Setchell KD: Report on the 8th International Symposium on the Role of Soy in Health Promotion and Chronic Disease Prevention and Treatment. J Nutr 139, 796S-802S (2009).