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Anti-Nutrients Play Key Role In Plant Health and Survival 

Mark Messina, Ph. D.Many plant foods contain multiple compounds historically classified as anti-nutrients, which are defined as food components that diminish or inhibit the utilization of specific nutrients. There are also inherent “toxicants” in plants that when consumed in large amounts may give rise to adverse effects.1 These compounds play myriad roles in the health and survival of plants.

It is difficult to envision a plant food that doesn’t contain at least one such component. For example, cruciferous vegetables contain glucosinolates, which can inhibit thyroid function.2 Potatoes and tomatoes contain solanine, a glycoalkaloid that works by interacting with mitochondrial membranes.3 These types of compounds are ubiquitous throughout the plant kingdom; although they are not limited to plants. Raw eggs contain avidin, which inhibits biotin absorption.4

All of the foods mentioned above can be safely consumed because conventional food processing greatly reduces the concentrations of anti-nutrients and/or because harmful effects only occur after consuming excessive amounts . This situation is fortunate because many of the anti-nutrients in plants are under study for their potential health benefits. For example, the glucosinates in cruciferous vegetables are widely touted for their chemopreventive properties.5

The soybean is a good example of a food known for containing several anti-nutrients — some with potential health benefits and some without. Knowledge of the physiological properties of these soybean components, and how they are affected by processing, is necessary for understanding the effects of soyfood consumption on health.

Oxalate

Renal Stones

Oxalate is considered an anti-nutrient because it inhibits calcium absorption6 and can increase the risk of developing kidney stones.7 More than one million cases of kidney stones are diagnosed annually in the United States.8 In most industrialized countries, approximately 80% of the kidney stones are composed of calcium salts and usually occur as calcium oxalate.9 Oxalate is found primarily, but not exclusively, in plants. Soybeans are considered to be a food containing a moderate amount of oxalate although among legumes, soybeans have an oxalate content closer to the top than to the bottom. For example, soybeans contain 56 mg oxalate per 100 g (wet weight) whereas small white beans and garbanzo beans contain 78 and 9 mg per 100 g, respectively.10 Soybeans are considerably lower in oxalate than the prototypical high-oxalate foods like spinach (1145 mg/100 g) and chocolate (155-485 mg/100 g dry matter).11 Cooking can greatly affect the oxalate content of foods; for example, boiling vegetables causes a much greater oxalate loss than steaming, whereas baking potatoes12 or roasting peanuts13 has no effect on oxalate content.

Low oxalate diets may be recommended, for individuals with a history of kidney stones and/or are hyper-absorbers of oxalate. Such individuals are generally advised to avoid foods that contain more than 10 mg oxalate per serving. Research published in 2005 found that soymilk and nearly all of the tofu tested were well below this limit; although recent data provide higher values for soymilk.14 In contrast to tofu, edamame and soynuts were above the limit. Unfortunately, because of methodological difficulties, many of the older values for the oxalate content of foods may be inaccurate. Some differences are expected because of the effects of processing and because plant variety influences oxalate content. For example, one study found soybean varieties ranged as much as fourfold in oxalate content.15

In addition to the absolute amount of oxalate in a food, the proportion that is soluble, as opposed to the insoluble form, may matter because the former appears to be better absorbed.17 Furthermore, there are many factors besides oxalate intake that affect kidney stone formation. A recent article on lifestyle factors for reducing risk of developing kidney stones recommended increasing water intake, decreasing animal protein intake, limiting sodium intake and increasing the consumption of fruits and vegetables.17 Several of these risk factors may account for why a preliminary British survey found that vegetarians were only half as likely to develop kidney stones as non-vegetarians.18

Recent data also indicate that adhering to the DASH diet is very protective against renal stones.19 Interestingly, in an analysis from the Nurses’ Health Study II, which included 96,245 female participants over an 8 year follow up period, phytate intake was associated with a 63% reduced risk of stone formation.20 Phytate is found in high amounts in soyfoods and other beans as well as in whole grains. Previous assumptions were that phytate would increase, not decrease risk. However, at least in vitro experiments, phytate is a strong inhibitor of calcium oxalate crystal formation.21

There are multiple factors in individual foods that potentially impact risk of developing renal stones. Several soyfoods have a sufficiently low oxalate content to fit within the restrictions of individuals on low-oxalate diets. These diets also contain other factors (e.g., plant protein, phytate) that may decrease risk.

Calcium Absorption

Calcium absorption from high-oxalate foods is generally quite poor, which is why spinach and rhubarb are not considered to be good sources of this mineral despite being very high in calcium.6 The ability of oxalate to bind calcium is why high-calcium foods appear to decrease risk of kidney stones – because calcium binds with oxalate in the gut which prevents absorption.

Oxalate in soybeans is primarily in the form of calcium oxalate crystals.22,23 Since calcium oxalate is considered insoluble, and soybeans are also high in phytate (another compound that inhibits calcium absorption) one would expect calcium bioavailability from soybeans to be quite poor.24 However, abundant evidence indicates this is not the case.

In 1991, Heaney et al.,25 found that the calcium from low-phytate soybeans was better absorbed than high-phytate beans, demonstrating an effect of phytate; but even the absorption of the former was nearly equal to the absorption of calcium from milk. Not surprisingly therefore, research shows that calcium absorption from soybeans is better than from other beans.26 More importantly, calcium absorption from calcium-set tofu 27 and calcium-fortified soymilk 28,29 is equal to the absorption of calcium from cow’s milk. However, bioavailability from calcium-fortified products, such as soymilk, depends to some extent on the type of supplemental calcium used.27 Research has shown that when calcium carbonate was used to fortify soymilk, absorption was similar to that seen with cow’s milk.28 In contrast, calcium absorption from soymilk fortified with tricalcium phosphate is about 25 percent lower than that from cow’s milk.30 Most soymilk sold in the United States is fortified with calcium carbonate.

In summary, despite being high in oxalate and phytate, calcium-fortified soyfoods are excellent sources of bioavailable calcium.

Oligosaccharides

Many of us began our lives exposed to a heavy dose of oligosaccharides (carbohydrates that are composed of several monosaccharide residues joined through glycosidic linkages) because breast milk (but not cow’s milk) contains as few as 23 and as many as 130 different oligosaccharides.31 These non-digestible carbohydrates provide a number of benefits to the developing infant.32 Foods, and especially legumes, also contain oligosaccharides.

The oligosaccharide content of beans is roughly 25-50 mg/g dry weight, but soybeans (60 mg/g of defatted meal) have a higher oligosaccharide content (primarily raffinose and stachyose) than most beans.33,34 Because there is no α-galactosidase in the human intestinal mucosa to cleave the α-(1-6) galactose linkage present in galactoside-containing oligosaccharides, they pass into the large intestine where the bacteria there metabolize them forming large amounts of carbon dioxide, hydrogen and sometimes methane.

Almost 50 years ago, diets containing beans were first shown to markedly increase flatulence.35,36 In 1970, it was found that the oligosaccharides were responsible for gas production.37,38 Gastric discomfort and flatulence are one of the most common reasons why people limit their consumption of beans.39–41

Commercial products that contain α-galactosidase are available so that individuals can eat beans without discomfort. Additionally, it is possible to remove substantial amounts of oligosaccharides (up to 60%) and to markedly reduce flatulence, by removing the water in which beans are boiled one or more times.42 Flatulence can also be circumvented by consuming a host of soy products with greatly reduced oligosaccharide content. For example, there are sprouted soybeans and fermented soyfoods such as miso and tempeh in which the sugars have already undergone bacterial metabolism.43–45 However, the extent to which fermentation per se (as opposed to leaching during the hydration, washing and cooking of the beans) is actually responsible for the reduction in oligosaccharide content is unclear.46 In addition to these soyfoods, tofu and isolated soy protein are also very low in oligosaccharides because they have largely been removed during processing.47,48

While eliminating or reducing the oligosaccharide content of soyfoods may have some obvious advantages, doing so may be disadvantageous from a nutritional perspective because of the prebiotic properties of oligosaccharides. A prebiotic is defined as “a non-digestible food ingredient that beneficially affects the host selectively by stimulating the growth and/or activity of one or a limited number of bacteria in the colon.49 Because of their growth promoting effect on bifidobacteria, soybean oligosaccharides have been hypothesized to promote the health of the colon, increase longevity and decrease colon cancer risk.50–54 Bifidobacteria compete with less desirable bacteria found in the colon such as Clostridium perfringens.

Soybean oligosaccharides increase the bifidobacateria population 2 to 10-fold in human subjects.53 Soybean oligosaccharides are commercially available in Japan and China 55 and two decades ago, one group of researchers in Japan suggested that soybean oligosaccharides be used as a substitute for common table sugar.56 Furthermore, the Ministry of Wealth, Labor and Welfare in Japan once recommended that Japanese citizens take 10 ml of soy oligosaccharide daily to boost their immunity and improve overall health.

In more recent studies, it was found that in broiler chicks, soy oligosaccharides increased visible microbial populations attached on caecal walls and increased the population of a group of lactic acid bacteria in the caecal contents, leading the authors of this research to conclude that soy oligosaccharides “show promise for use as a product which may promote competitive exclusion of potential pathogens” and therefore may be “a suitable substitute for dietary antibiotics”.57 Also, in male Sprague-Dawley rats, soy oligosaccharides were found to reduce plasma LDL-cholesterol and triglycerides and to increase HDL-cholesterol. In addition, there was a decrease in plasma thromboxane and an increase in nitric oxide concentrations. As a result, the authors suggested soy oligosaccharides might help prevent atherosclerosis.58

In summary, the oligosaccharides in beans produce gastric discomfort which can cause some people to shy away from eating these foods. However, oligosaccharides may have a number of health benefits. Individuals still wishing to avoid these carbohydrates are able to consume foods made from soybeans because many soyfoods have greatly reduced levels of oligosaccharides. In the future, it may become more common for soyfoods to be made from soybean varieties with a low oligosaccharide content.59,60

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About the Author

Mark Messina, PhD, is the co-owner of Nutrition Matters, Inc., a nutrition consulting company, and is an adjunct professor at Loma Linda University. His research focuses on the health effects of soyfoods and soybean components. He is chairman of The Soy Connection Editorial Board and executive director of the Soy Nutrition Institute.

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