It is time to look at another common rumor regarding soy foods; the dangers posed by phytates. Phytate is the name for the salt form of phytic acid, a phosphorus containing molecule in many plant tissues.
The Weston A. Price Foundation writes, “High levels of phytic acid in soy reduce assimilation of calcium, magnesium, copper, iron and zinc. Phytic acid in soy is not neutralized by ordinary preparation methods such as soaking, sprouting and long, slow cooking. High phytate diets have caused growth problems in children.”
The “natural health” website Mercola writes, “Soy contains phytates. Phytates (phytic acid) bind to metal ions, preventing the absorption of certain minerals, including calcium, magnesium, iron, and zinc -- all of which are co-factors for optimal biochemistry in your body. This is particularly problematic for vegetarians, because eating meat reduces the mineral-blocking effects of these phytates (so it is helpful—if you do eat soy—to also eat meat).”
Soy Online Service adds, “Soybeans contain very high levels of phytate and their[sic] are numerous reports of reduced bioavailablity[sic] of various metals from foods containing soy; this has particular significance for vegetarians and infants fed soy-formulas.”
We already learned from last year's post on The Dangers of Phytoestrogens that, “Comprehensive literature reviews and clinical studies of infants fed SBIFs[soy based infant formulas] have resolved questions or raise no clinical concerns with respect to nutritional adequacy, sexual development, neurobehavioral development, immune development, or thyroid disease. SBIFs provide complete nutrition that adequately supports normal infant growth and development.” Interestingly however, there is still quite a bit of truth to these concerns over phytate. Soy does contain phytate and phytate does reduce our absorption of several minerals.
There is one redeeming aspect to all of this however. Soy is not a particularly exceptional source of phytate. The 2001 book Food Phytates brought together a lot of information about phytates and published it in a very convenient fashion. Part of this book involved gathering data on quantity of phytate in various foods and arranging that into a convenient table. Here are a few bits of data from that table:
Source - %Phytate by mass
Dolique Beans – 5.92-9.15%
Brazil Nut – 1.97-6.34%
Almond – 1.35-3.22%
Tofu – 1.46-2.90%
Linseed – 2.15-2.78%
Pinto Beans – 0.61-2.38%
Soybeans – 1.00-2.22%
Peanuts – 1.05-1.76%
Kidney Beans – 0.89-1.57%
Tempe – 0.67-1.08%
Soy Milk – 0.05-0.11%
Do keep in mind that you probably consume much more soy milk by mass than you would soybeans or almonds. The book also mentions that, “Dry cereals account for 69.5% of the total global crop seeds/grains/fruit each year but synthesized 77.3% of the total PA [Phytic Acid]. Legumes account for 7.6% of the annual global production of crop seeds/grains/fruits and 13.0% of the total PA.” I encourage you to look through the entire table for yourself, which is available via Google Books.
The American Dietetic Association in their position paper regarding vegan and vegetarian diets brings up phytic acid or phytate several times, regarding calcium, zinc, and iron.
One recent study attempted to measure the absorption of calcium and zinc in Nigerian children with and without rickets. In the study the authors, “sought to examine 1) the effect of a typical Nigerian meal on the absorption of zinc and calcium 2), the effect of meal dephytinization on calcium and zinc absorption, and 3) whether the relationships between mineral absorption, meal consumption, and dephytinization were different in children with and without rickets.” Dephytinization is the word for removing phytate from a substance. The study authors went about this by giving the study participants a bowl of porridge along with a cup of orange juice fortified with both calcium and zinc to be consumed half way through. They repeated this process with both regular and phytate reduced porridges. While the study was largely about rickets, those findings had no detectable effect on the results. “Calcium absorption did not differ significantly between children with and without rickets for any permutation of meal.” Interestingly, they also note that the phytic acid had no significant impact on calcium absorption either. “Calcium absorption with fermented[phytate reduced] porridge (50.7 ± 19.1%) did not differ from unfermented porridge (50.1 ± 17.3%; P = 0.94).” The study also found rickets to have no detectable effect on zinc absorption. It did however find that dephytinization had a significant impact. “Enzymatic dephytinization increased zinc absorption during the second absorption study (55.5 ± 18.0% vs. 32.2 ± 14.8%; P < 0.001). Dephytinization resulted in a mean relative increase in zinc absorption of 101 ± 88%.” This study suggests that phytic acid has at most a modest impact on calcium absorption, but may affect the absorption of zinc fairly significantly.
The effect of phytic acid on iron absorption has been much more thoroughly studied. One study attempted to model iron absorption based off a number of factors and was able to achieve an r^2 value of .987 (usually interpreted as 98.7% of the variation in iron absorption rates could be accounted for by their model). The two main terms in their model were phytic acid content and ascorbic acid content (more commonly known as vitamin C). Their model found that the ratio of iron that was absorbed from an ordinary wheat roll increased linearly with the amount of ascorbic acid, decreased with the logarithm of phytic acid content. This means that as vitamin c increases the rate of iron absorption continues to increase at a similar rate, while as phytic acid increases it has a diminishing impact on the amount it inhibits absorption. Overall however, if you look at absorption from the vegetarian meals they studied the absorption of iron in those meals was still much lower than absorption from meat-containing meals. Perhaps this is why the American Dietetic Association suggests, “because of lower bioavailability of iron from a vegetarian diet, the recommended iron intakes for vegetarians are 1.8 times those of nonvegetarians.”
So while soy certainly isn't a guilty culprit, phytic acid absolutely has a negative impact on our absorption of minerals as vegetarians. What, however, is the net impact of all this on overall health? Perhaps a paper titled Health Effects of Vegan Diets in the American Journal of Clinical Nutrition can help answer this for us.
“[T]he risk of iron deficiency anemia are similar for vegans compared with omnivores and other vegetarians. Vegans often consume large amounts of vitamin C–rich foods that markedly improve the absorption of the nonheme iron.”
“Phytates, a common component of grains, seeds, and legumes, binds zinc and thereby decreases its bioavailability. However, a sensitive marker to measure zinc status in humans has not been well established, and the effects of marginal zinc intakes are poorly understood. Although vegans have lower zinc intake than omnivores, they do not differ from the nonvegetarians in functional immunocompetence as assessed by natural killer cell cytotoxic activity. It appears that there may be facilitators of zinc absorption and compensatory mechanisms to help vegetarians adapt to a lower intake of zinc.”
“More recent studies with postmenopausal Asian women showed spine or hip BMD was significantly lower in long-term vegans. Those Asian women, who were vegetarian for religious reasons, had low intakes of protein and calcium. […] The higher risk of bone fracture seen in vegans appears to be a consequence of a lower mean calcium intake. No difference was observed between the fracture rates of the vegans who consumed >525 mg calcium/d and the omnivore fracture rates.”
Overall it seems that our lower rates of absorption of zinc and iron are accounted for by higher intakes of those minerals. Getting enough calcium can be a concern for some vegans, but for those who do they seem to utilize it just fine. If there is one thing to take away from all this, I think the American Dietetic Association summarizes it best. “[A]ppropriately planned vegetarian diets, including total vegetarian or vegan diets, are healthful, nutritionally adequate, and may provide health benefits in the prevention and treatment of certain diseases. Well-planned vegetarian diets are appropriate for individuals during all stages of the life cycle, including pregnancy, lactation, infancy, childhood, and adolescence, and for athletes.”
Plan your diet well, eat a healthy variety of foods, and no, eating soy is not going to kill you.