April 2021 Issue
Soy Antinutrients
By Carrie Dennett, MPH, RDN, CD
Today’s Dietitian
Vol. 23, No. 4, P. 22
Is the concern about these compounds found in many plant foods much ado about nothing?
The question of whether people should avoid foods that contain antinutrients is a hot topic, with many vigorous opinions available via a quick internet search. While several healthful foods have been thrown under the bus by those in the “avoid antinutrients” camp, soy—a staple food in healthful traditional Asian diets—often takes an extra hit because it contains so many different types of antinutrients.
Antinutrients are naturally occurring compounds found in many plant-based foods—think pulses (beans, peas, and lentils), whole grains, nuts, seeds, and many vegetables. Many bloggers, book authors, and other “experts” claim that people should avoid foods that contain antinutrients for a few reasons, including the fact they can theoretically bind to certain nutrients, reducing the body’s ability to digest and absorb them.
While it’s true that antinutrients can have negative health effects, they also can have positive effects. Is it enough to look only at what antinutrients may do in lab and animal studies, or should the focus be on whether antinutrients are a problem in foods as they’re typically prepared and eaten by humans? Today’s Dietitian takes a closer look.
Phytate
Soybeans contain high concentrations of phytate (also known as phytic acid), one of the most common types of antinutrients—found in seeds, nuts, legumes, and grains.1 Phytate can bind strongly to iron and zinc—and calcium, magnesium, and potassium to a lesser degree—forming insoluble complexes that can’t be absorbed by the intestine, making them less bioavailable.2 Phytate also reduces the bioavailability of vitamins A, B12, D, and E.3 However, phytate has antioxidant and anti-inflammatory properties, and research suggests it may help prevent CVD and some forms of cancer, including colon, liver, lung, breast, skin, and prostate cancers.4 In vitro and in vivo studies have shown that phytic acid reduces proliferation of cancer cells and may contribute to cancer cells reverting to normal cells.5 Since phytate primarily is found in high-fiber foods, this may be one reason why high-fiber diets are associated with lower cancer risk.
Moreover, phytate has been observed to reduce starch digestion, lower the glycemic index of foods, and generally have positive effects on blood glucose levels, although most research to date is on rodents.2,4 It also has shown positive effects on blood cholesterol in rodent studies, possibly due to increased excretion of bile acids and lipids in the feces.4 Another proposed cholesterol-lowering mechanism of phytate is that it inhibits the liver enzymes involved in lipogenesis, the production of triglycerides and fatty acids from glucose.2 Phytate also may prevent kidney stone development by inhibiting formation of calcium salt crystals.1,4
In food preparation, fermentation and sprouting reduces phytate levels,4,6 but cooking at temperatures used in home kitchens doesn’t.7 Some foods are processed with phytases—enzymes found in plants, fungi, animals, and, to a lesser extent humans, that can break down phytate—to reduce phytate in the final product. Humans also degrade 37% to 66% of phytate during digestion in the stomach and small intestine when the diet is rich in plant phytases, but phytases generally are inactivated by cooking and food preparation. The Western-style diet is low in phytases, as plant intake is low and those plants often are cooked, so people eating this style of diet will have low rates of phytate degradation during digestion.4 If clients are eating both cooked and raw plant foods, this should be less of an issue.
Ironically, that relative lack of breakdown may be a good thing. Phytate’s health benefits are more significant for people living in the United States and other developed countries, where rates of cancer, especially colon cancer, are higher, possibly because of higher fat and lower fiber intakes—in other words, because of a Western-style diet. These populations generally don’t suffer from mineral deficiencies because they’re eating a varied and balanced diet.4 In low-income countries, poor mineral absorption from high-phytate diets is a major factor in iron and zinc deficiency.4,7 The bottom line is that there’s no evidence that eating moderate amounts of soy causes deficiencies.
Lectins
Soy contains moderate concentrations of lectins, a group of proteins known as glycoprotein found in most plants, especially beans and whole grains.8 Lectins can interfere with the absorption of minerals, especially calcium, iron, phosphorus, and zinc.9 Much also has been made of the idea that lectins are a threat to gut health, in part because they can bind to epithelial cells lining the intestine, potentially altering gut permeability and passing through the gut into the bloodstream.5 But the only evidence supporting that claim comes from studies that involved feeding either isolated lectins or raw beans to animals.
Considering some of the outsized negative claims about lectins, there’s not much research on their effects on human health. What’s clear is that lectins are almost entirely destroyed by heat, with moist heat being the most effective—for example, there’s evidence that cookies made with soy flour still retain some lectin activity, although not at a level thought to pose a health risk.8,10 Boiling beans for 15 minutes, simmering for two hours, or pressure cooking for 45 minutes has been demonstrated to lower lectin activity below detectable levels. It’s also worth noting, especially to clients, that the benefits of a plant-based diet, which naturally contains many sources of lectins, is well established.
Protease (Enzyme) Inhibitors
Soy and other legumes contain high levels of protease inhibitors that protect the growing plants against insect damage but also bind to the digestive enzymes chymotrypsin and trypsin in animals and humans. This can interfere with digestion of protein and trigger the pancreas to produce excessive amounts of digestive enzymes, leading to an enlarged pancreas in animal research,3,8,10 much of which used raw legumes or isolated protease inhibitors. Research is scant on human subjects. In one human study, direct infusion of purified Bowman-Birk inhibitor (BBI)—one common type of protease inhibitor—significantly increased production of trypsin and chymotrypsin in the pancreas, but, in real life, humans don’t consume isolated enzyme inhibitors. Kunitz-type inhibitors in soy appear to be completely inactivated by human gastric juice, while BBI remains active.8
It’s important to note that humans also rarely eat soy or other legumes without previous heat treatment, and cooking and soaking to a lesser extent reduces levels of these enzyme inhibitors.6,8 Unlike with lectins, dry heat—such as roasting and toasting—appears to be more effective than moist heat in deactivating protease inhibitors.5 In recent years, new techniques have been developed to reduce protease inhibitor concentration and activity in soymilk.11
BBI has anti-inflammatory, antimicrobial, and immune-modulating properties, and purified BBI has been used as a cancer-preventive agent because of its ability to block proliferation of cancer cells. Soybean BBI has been shown to have the ability to suppress expression of various oncogenes—genes that have the potential to cause cancer. It’s also been used to help treat inflammatory bowel disease and other inflammatory and autoimmune diseases.11 Soy BBI also has been used to prevent replication of HIV.12 Kunitz-type inhibitor from soy also may help prevent cancer cell invasion and metastasis.13
Oxalates
Oxalates are present in clinically meaningful amounts in about 75% of all flowering plants, including soy.3 A 2005 study found a wide range of oxalate levels in soyfoods, from 2 to 58 mg per serving. According to criteria established by researchers, soyfoods that have undergone more processing, such as tofu, soymilk, and soy sauce, tend to be low oxalate, while more minimally processed soyfoods, such as soy flour, textured vegetable (soy) protein, soy nuts, edamame, and soy nut butter, are inclined to be high oxalate.1
Oxalates are a major component in kidney stone formation, and foods containing more than 10 mg of oxalate per serving are considered high-oxalate foods. However, while frequently consuming much soy or other higher-oxalate foods can lead to the formation of kidney stones in susceptible people, simply reducing dietary oxalates isn’t a healthful prevention strategy.14 Oxalates also bind to calcium and other minerals in the small intestine, reducing absorption and potentially leading to deficiencies.3
Taylor Wolfram, MS, RDN, LDN, a Chicago-based vegan dietitian, says she isn’t concerned about oxalates in soy—she simply makes sure her vegan clients know that oxalates significantly reduce calcium absorption from greens, so they shouldn’t rely on spinach and Swiss chard for calcium, even though those greens do have a high calcium content. Instead, she suggests they include a wide array of leafy greens and cruciferous vegetables in their diets.
Saponins
Soy and other legumes are rich in saponins, a class of bioactive compounds found in many plant species that protects plants against pathogens, pests, and foraging animals due to antimicrobial, antifungal, antiparasitic, insecticidal, and antifeedant properties. In general, saponins have many properties that can have positive or negative effects in humans and various animals.15
Popular claims on the internet state that these saponins can block nutrient absorption, “damage” cholesterol, disrupt endocrine function, kill cells, and increase intestinal permeability. While saponins have been shown to increase intestinal permeability in vitro and in animal research, which could theoretically contribute to autoimmune disease,3 most effects touted as negative can be considered positive. In fact, saponins in animal and in vitro studies also have shown that saponins have antioxidant, immune-modulating, anticarcinogenic, anti-inflammatory, and antimicrobial properties.16
Notably, saponins may have cardiovascular benefits due to their cholesterol-lowering effects through a variety of mechanisms.2 Many saponins form insoluble complexes with cholesterol in the gut, suppressing intestinal reabsorption of cholesterol. They’re similarly able to reduce reabsorption of bile acids. This has been demonstrated in rodent models. Animal research also has demonstrated that soy saponins have antithrombotic effects by reducing blood platelets and fibrinogen levels.2,17
Furthermore, saponins have been shown to modulate expression of genes related to the enzymes and transporters involved in carbohydrate metabolism, which can help normalize blood glucose and insulin levels. They’re capable of improving insulin resistance through alteration of other mechanisms involved in carbohydrate metabolism.2 And saponins may play a role in soy’s protective effects against cancer.3 In vitro studies have found that soy saponins suppress cancer cell growth and inhibit survival, especially colon cancer cells.18,19 In lab research, saponins have been shown to downregulate genes related to the formation of fat cells from stem cells (adipogenesis), reduce production of pancreatic lipase, and inhibit intestinal fat absorption. This could have benefits for body composition.2
While saponins can interfere with nutrient absorption, as with other antinutrients, the degree of interference will depend on the individual person’s metabolism, what other foods are in the meal, and how the antinutrient-containing food is prepared. Studies of the effects of various cooking and food processing methods on saponin levels have produced inconsistent results, with some showing a significant decrease, and others almost no decrease.16
Bottom Line for RDs
Soy and other foods containing antinutrients are considered traditional foods that are staples in the diets of some of the healthiest populations around the world. The demonization of specific foods or food components because they theoretically could cause problems—even if highly unlikely based on how we eat them—or because they do cause problems for a small percentage of people is a troubling trend.
Given all the information—and misinformation—online about antinutrients, how should dietitians approach the topic with clients who ask about it or believe they should avoid certain foods because they contain antinutrients?
Other than oxalates, Wolfram says she doesn’t express much caution around antinutrients. However, if a client specifically asks, she says she tends to say something short and sweet, such as, “There are some naturally occurring compounds in plants that might interfere with the absorption of some nutrients, but that effect is usually small and insignificant, and in many cases, cooking often neutralizes it. The nutritional benefits of eating an abundance, variety, and balance of plant foods far outweigh any concerns about antinutrients.” By balance, Wolfram says she means making sure the client is getting enough legumes, grains, nuts, and seeds in addition to fruits and veggies. Then, if someone wants to know more, or asks about a specific antinutrient, she’ll give more specific information.
Regarding the effects of cooking, fermenting, and processing on degrading antinutrients, Wolfram says she doesn’t advise people to worry about this. “Soy is just one component of many in a varied, balanced eating pattern, even for plant-based eaters who eat more soy than meat-eaters,” she says. “And many other plant foods deliver the nutrients of concern here.”
Much of the confusion about antinutrients may come from simply not seeing the forest for the trees. In other words, some individuals only focus on the fact that, yes, some foods have antinutrients and that antinutrients could have some unwanted effects. They don’t consider that the ways in which people prepare and consume those foods may help degrade the antinutrients, or the fact that antinutrients, and the foods containing them, also have health benefits.
“Unfortunately, this narrow view and understanding of nutrition is so rampant in wellness conversations by folks who aren’t RDs, and those who don’t know how to critically evaluate research,” Wolfram says. “We have to spend a lot of our time as RDs explaining why folks don’t need to worry so much.”
She says dietitians can sound boring when reminding people to eat enough of all the food groups, take any necessary dietary supplements, and practice intuitive eating, but, for the generally healthy adult, that’s what good nutrition is about. “I see ‘antinutrients’ as a trendy topic that ‘wellness’ folks use to stay relevant and garner fear and attention that keeps their audiences attentive. Oxalates and phytates aren’t new—they’re just a little tidbit the wellness community has picked up and decided to co-opt, especially those pushing a meat-heavy or meat-only diet. Kind of reminds me of food trends. Never saw cauliflower becoming trendy. What will it be next?”
— Carrie Dennett, MPH, RDN, CD, is the nutrition columnist for The Seattle Times, owner of Nutrition by Carrie, and author of Healthy for Your Life: A Holistic Guide to Optimal Wellness.
References
1. Al-Wahsh IA, Horner HT, Palmer RG, Reddy MB, Massey LK. Oxalate and phytate of soy foods. J Agric Food Chem. 2005;53(14):5670-5674.
2. Ribeiro PVM, Andrade PA, Hermsdorff HHM, et al. Dietary non-nutrients in the prevention of non-communicable diseases: potentially related mechanisms. Nutrition. 2019;66:22-28.
3. D'Adamo CR, Sahin A. Soy foods and supplementation: a review of commonly perceived health benefits and risks. Altern Ther Health Med. 2014;20(Suppl 1):39-51.
4. Schlemmer U, Frølich W, Prieto RM, Grases F. Phytate in foods and significance for humans: food sources, intake, processing, bioavailability, protective role and analysis. Mol Nutr Food Res. 2009;53(Suppl 2):S330-375.
5. Mohan VR, Tresina PS, Daffodil ED. Antinutritional factors in legume seeds: characteristics and determination. In: Caballero B, Finglas PM, Toldrá F, eds. Encyclopedia of Food and Health. Academic Press; 2016:211-220.
6. Abu-Salem FM, Mohamed RK, Gibriel AY, Rasmy NMH. Levels of some antinutritional factors in tempeh produced from some legumes and jojobas seeds. Int Sch Sci Res Innov. 2014;8(3):296-301.
7. Gibson RS, Raboy V, King JC. Implications of phytate in plant-based foods for iron and zinc bioavailability, setting dietary requirements, and formulating programs and policies. Nutr Reviews. 2018;76(11):793-804.
8. Lajolo FM, Genovese MI. Nutritional significance of lectins and enzyme inhibitors from legumes. J Agric Food Chem. 2002;50(22):6592-6598.
9. Vasconcelos IM, Oliveira JTA. Antinutritional properties of plant lectins. Toxicon. 2004;44(4):385-403.
10. Friedman M, Brandon DL. Nutritional and health benefits of soy proteins. J Agric Food Chem. 2001;49(3):1069-1086.
11. Gitlin-Domagalska A, Maciejewska A, Dębowski D. Bowman-Birk inhibitors: insights into family of multifunctional proteins and peptides with potential therapeutical applications. Pharmaceuticals (Basel). 2020;13(12):421.
12. Ma TC, Zhou RH, Wang X, et al. Soybean-derived Bowman-Birk inhibitor (BBI) inhibits HIV replication in macrophages. Sci Rep. 2016;6:34752.
13. Kobayashi H. Prevention of cancer and inflammation by soybean protease inhibitors. Front Biosci (Elite Ed). 2013;5:966-973.
14. 6 easy ways to prevent kidney stones. National Kidney Foundation website. https://www.kidney.org/atoz/content/kidneystones_prevent. Reviewed September 25, 2020.
15. Moses T, Papadopoulou KK, Osbourn A. Metabolic and functional diversity of saponins, biosynthetic intermediates and semi-synthetic derivatives. Crit Rev Biochem Mol Biol. 2014;49(6):439-462.
16. Singh B, Singh JP, Singh N, Kaur A. Saponins in pulses and their health promoting activities: a review. Food Chem. 2017;233:540-549.
17. Imai S. Soybean and processed soy foods ingredients, and their role in cardiometabolic risk prevention. Recent Pat Food Nutr Agric. 2015;7(2):75-82.
18. Li YH, Niu YB, Sun Y, et al. Role of phytochemicals in colorectal cancer prevention. World J Gastroenterol. 2015;21(31):9262-9272.
19. Tsai CY, Chen YH, Chien YW, Huang WH, Lin SH. Effect of soy saponin on the growth of human colon cancer cells. World J Gastroenterol. 2010;16(27):3371-3376.