Soyfoods As a Source of Iron in Diets Devoid of Meat
By Virginia Messina, M.P.H., R.D. and Mark Messina, Ph.D.
Your diet, whether or not it contains meat, is abundant in iron. But how much of that iron actually gets into your bloodstream? That question has interested nutritionists for decades and with good reason.
For one thing, iron deficiency is a significant problem worldwide. And it doesn't occur just in developing countries. Inadequate iron intake is common in the United States, although the incidence of actual deficiency is rather low (1). Since iron is critical to our ability to use oxygen, the effects of iron deficiency are serious.
The actual biological need for iron is pretty small — about 1 to 1-1/2 milligrams a day seems to be enough to support all iron-dependent functions (2). But in order to meet biological needs, people need to consume as much as 10-15 milligrams of iron each day. The reason that the recommended intake is so much higher than the actual requirement is because so little of the iron in foods is actually absorbed by the body.
A general rule of thumb is that we absorb about 10 to 15 percent of the total amount of iron in our diet (2). The amount of iron we absorb depends on many different factors, though, so the amount of iron we actually need to consume becomes a pretty complex issue. For example, people whose diets are low in iron absorb far greater amounts of this nutrient (3). Also, in times of need, iron absorption can increase ten fold (3).
The type of iron found in a food affects its absorption, too. Plant foods contain non-heme iron, which is not well-absorbed. Meat contains both non-heme iron and heme iron; heme iron is very well- absorbed. The whole dietary picture makes a difference, too. Vitamin C enhances the absorption of non-heme iron, although it has no effect on heme iron (4). Vegetarians who rely solely on non-heme iron in their diet can boost the absorption of that iron by including a food high in vitamin C at every meal.
Other dietary components can interfere with iron absorption. Tannins, which are chemicals found in tea, greatly reduce the absorption of iron (5). Frequent consumption of tea in developing countries is probably one reason for rampant iron deficiency in those areas of the world (6).
These factors tend to complicate the iron picture quite a bit. Foods that are rich in iron may not necessarily be the best sources of iron. One food that has been the subject of much study is soybeans.
Soybeans, and the foods made from them, are high in non-heme iron. But traditional nutritional wisdom, which seems to be borne out by most studies, is that iron from soyfoods is poorly absorbed (7). Soy protein also inhibits overall absorption of iron from meat (8). But most iron studies involving soy don't duplicate the eating patterns of vegetarians.
In addition to combining soy with meat, (which of course vegetarians don't eat), these studies primarily use soy protein products, such as soy isolates and soy flour. Vegetarians are more likely to eat tofu and drink soymilk than to consume soy isolates. Even so, the results of these studies are worth noting.
In one USDA study, more than 200 adults and children consumed one or two meals a day that contained either an all-beef product, or a beef product that contained 20 percent soy protein (9). The soy protein was in the form of soy isolate, soy concentrate or soy flour. In this study, the addition of soy did not adversely affect iron status. In fact, there was some indication that iron status actually improved. This may have been, in part, because researchers have found that while soy reduces the absorption of the non-heme iron in meat, it actually increases the absorption of heme iron (4).
In another study, six male subjects consumed a diet that contained soy protein concentrate as the sole source of iron for 82 days (10). Iron levels did go down slightly, but the researchers attributed it primarily to the fact that the students gave so many blood samples during the course of the study. In this study, it was estimated that about 16% of the iron was absorbed. Iron is absorbed better from soybeans than from many other plant foods, such as corn, rice, and other legumes (11, 12), but that 16% figure is probably too high when one considers a wider range of research. One very important study, which used traditional soy foods, found that on average, iron absorption from products such as tofu and miso, ranged from 5 to 10 percent (13).
If one uses an average figure of 7.5% absorption, then one serving of tofu, which contains about 6.5 milligrams of iron, would provide about 1/2 milligram of iron, or about 1/2 the amount needed by an adult male (14). That's impressive, when considering that one serving of ground beef provides 50% more iron, (using an absorption figure of 35%) but also supplies twice as many calories and many times more saturated fat (14).
What Does It Mean?
Nutritionists are in agreement that iron from meat is better absorbed than iron from plants. But they also agree that the bottom line — how much iron an individual actually absorbs — is dependent on a lot more than whether or not a person eats meat. Studies show that vegetarians are, in fact, no more likely to be iron deficient than meat-eaters, although iron stores may be somewhat lower (15, 16).
It may be that vegetarians eat copious amounts of iron-rich foods, so that even a low absorption rate won't keep them from getting enough iron. Vegetarians may also consume more vitamin C-rich foods with their meals. But remember that iron deficiency is common among all population groups. People need to pay particular attention to getting enough of this nutrient no matter what type of diet they eat. At the same time, it is worth noting that recent studies show that too much iron may be as great a public health problem as too little (17, 18). High levels of iron in the body have been linked to increased risk of both heart disease and cancer. Plant-based diets may have the real advantage in that they provide adequate iron, but may have built in mechanisms that protect us from iron overload.
References
1. Dallman PR, Yip R, Johnson C. Prevalence and causes of anemia in the United States, 1976-1980. Am J Clin Nutr 39:437-445, 1984.
2. National Research Council. Recommended Dietary Allowances. National Academy Press, Washington DC, 1989.
3. Cook JD. Adaptation in iron metabolism. Am J Clin Nutr 51:301-308, 1990.
4. Lynch SR, Dassenko SA, Morck TA, Beard JL, Cook JD. Soy protein products and heme iron absorption in humans. Am J Clin Nutr 41:13-20, 1985.
5. Gillooly M, Bothwell TH, Torrance JD, Macphail AP, Derman DP, Bezwoda WR, Mills W, Chariton RW. The effects of organic acids, phytates and polyphenols on the absorption of iron from vegetables. Br J Nutr 49:331-342, 1983.
6. Disler PB, Lynch SR, Charlton RW, Torrance JD, Bothwell TH, Walker RB, Mayet F. The effect of tea on iron absorption. Gut 16:193-200, 1975.
7. Morck TA, Lynch SR, Cook JD. Reduction of the soy-induced inhibition of nonheme iron absorption. Am J Clin Nutr 36:219-228, 1982.
8. Bodwell CE. Effects of soy protein on iron and zinc. Cereal Foods World 28:342-348, 1983.
9. Bodwell CE, Miles CW, Morris E, Prather ES, Mertz W, Canary JJ. Long-term consumption of beef extended with soy protein by men, women, and children: II. Effects on iron status. Plant Fds Hum Nutr 37:361-376, 1987.
10. Istfan N, Murray E, Janghorbani M, Evan WJ, Young VR. The nutritional value of a soy protein concentrate (stapro-3200) for long-term protein nutritional maintenance in young men. J Nutr 113:2524-2534, 1983.
11. Layrisse M, Cook JD, Martinez C, Roche M, Kuhn IN, Walker RB, Finch CA. Food iron absorption: a comparison of vegetable and animal foods. Blood 33:430-443, 1969.
12. Lynch SR, Beard JL, Dassenko SA, Cook JD. Iron absorption from legumes in humans. Am J Clin Nutr 40:42-47, 1984.
13. Macfarlane BJ, van der Riet WB, Bothwell TH, Baynes RD, Siegenberg D, Schmidt U, Tal A, Taylor JRN, Mayet F. Effect of traditional oriental soy products on iron absorption. Am J Clin Nutr 51:873-880, 1990.
14. Pennington JAT, Church HN. Food values of portions commonly used. Harper and Row, 1989.
15. Anderson BM, Gibson RS, Sabry JH. The iron and zinc status of long-term vegetarian women. Am J Clin Nutr 34:1042-1048, 1981.
16. Worthington-Roberts BS, Breskin MW, Monsen ER. Iron status of premenopausal women in a university community and its relationship to habitual dietary sources of protein. Am J Clin Nutr 47:275-279, 1988.
17. Weinberg ED. Roles of iron in neoplasia. Biol Trace Element Res 34:123-140, 1992.
18. Salonen JT, Kyyssonen K, Korpela H, Tuomilehto J, Seppanen R, Salonen R. High stored iron levels are associated with excess risk of myocardial infarction in eastern Finnish men. Circulation (in press)
Virginia and Mark Messina are freelance writers from Maryland. They devote a lot of their time to researching the role of soy foods in a healthy diet.
