The Humble Soybean

Under the Microscope

The soybean is a humble, yet powerful legume that has been cultivated for centuries. With over 200 soy-related peer-reviewed articles published annually and research spanning 25 years, the soybean is a nutritional powerhouse that can be enjoyed in a variety of ways. Let’s take a closer look and discover all the incredible things it can do.

 

Nutritional Properties of Soybeans

MACRONUTRIENTS OF BOILED SOYBEANS AND SELECTED LEGUMES PER CUP

LEGUMEkCalProtein (g)Fat (g)
Soybeans29631.315.4
Lentils23017.90.8
Pinto beans24515.41.1
Chickpeas26914.54.3
Kidney beans22515.40.9
Black beans22715.20.9
Baked beans251131.3

PROTEIN IN SOYBEANS

The soybean is notable for:

  • Its total protein (higher than in other beans)
  • Its protein quality (higher than that of other plant proteins and similar to animal protein)

PDCAAS (protein digestibility amino acid scores) is the measurement of protein quality. Soy products have a PDCAAS of 0.9 – 1.0 depending on the soy food in question, which is very good. Examples include Soya protein isolate, soya concentrate and some Asian soya foods.

CARBOHYDRATES IN SOYBEANS

  • Low: making it ideal for diabetics
  • Mostly oligosaccharides (predominantly stachyose) – poorly digested – travels to the colon and stimulates the growth of good bacteria such as bifidobacterial. = PREBIOTICS

FATS IN SOYBEANS

  • 10-15% Saturated fatty Acids
  • 19-41% Monounsaturated fatty acids
  • 62% Polyunsatyrated fatty acids (omega-6 = linoleic; and omega 3 = a-linolenic. These occur in a ratio range of 5-13:1, which is very favourable)
  • Soybean oil accounts for 40% of the intake of both essential fatty acids

QHC: 20.5g of soybean oil may reduce the risk of CHD.

MINERALS IN SOYBEANS

  • Iron 84mg/cup cooked soybeans
  • Calcium 175mg/cup cooked soybeans

Note on absorption:

Traditional view New research (improved methodology)
IRON All plant sources of iron are poorly absorbedIron absorption from soy is quite good because it is mostly in the form of ferritin
Phytate in soy inhibits iron absorptionAdaption to the inhibitory affects of phytate on iron absorption
Iron status influences nonheme iron absorption: low iron stores = higher absorption rates
CALCIUMPhytate and oxalate inhibit absorptionFactional calcium absorption from cow’s milk was 0.377 compared with 0.310 and 0.414 from high- and low-phytate soybeans

 

Isoflavones

You may have seen the word “isoflavones”  and wondered what they are. Isoflavones are a type of phytonutrient, which is a naturally occurring compound found in plants. Isoflavones have received a lot of attention for their potential health benefits, including protective effects against cancer and heart disease.  Isoflavones are found in many different plant foods, but soybeans contain uniquely rich amounts.

  • Consumption in Asian vs non-Asian countries is: 30-50mg/day in Japan but less than 3mg/day in the US, Canada and Europe!
  • Occur in soybeans mainly as glycosides but upon ingestion, the sugar is hydrolyzed thereby allowing absorption to occur
  • 1 gram soy protein in soybeans and traditional soy foods is equivalent to +/- 3.5mg of isoflavones. Therefore 1 serving traditional soy food e.g. 100g tofu OR 250ml soymilk, typically provides 25mg isoflavones

CHEMICAL STRUCTURE OF ISOFLAVONES

Isoflavones are similar to the hormone estrogen which allows them to bind to both estrogen receptors (ER) – ERα and ERβ. Each ERs have different tissue distributions and, when activated, have different and sometimes opposite physiological effects.

As such this allows them to exert estrogen-like effects under certain experimental conditions and so are referred to as phytoestrogens. Although research shows that isoflavones bind more weekly than estrogen, circulating levels of isoflavones in response to consuming 2 servings of soy foods = 3 times higher than estrogen

ESTROGEN AND ISOFLAVONES: THE DIFFERENCE

  • Estrogen binds to and transactivates ERα and ERβ equally
  • Isoflavones preferentially bind to and transactivate ERβ

This preference of isoflavones for Erβ is the primary reason that:

  1. isoflavones are capable of having tissue-selective effects
  2. they are classified as selective estrogen receptor modulators (SERMs)

Meaning that: In tissues that have estrogen receptors, SERMs exert estrogen-like effects in some cases. But no effects OR anti-estrogenic effects in others.

SELECTIVE ESTROGEN RECEPTOR MODULATORS

In the pharmaceutical industry – the most widely used are:

  • Tamoxifen
  • Raloxifene

Isoflavones have many health benefits, Yet controversies exist: concerns regarding their estrogen-like properties may exert untoward effects in some individuals e.g. postmenopausal women.

However the EFSA concluded that in postmenopausal women, isoflavones DO NOT adversely affect the 3 organs that were investigated: breast , thyroid, uterus. Recently, the North American Menopause Society also concluded that isoflavones do not increase risk of breast or endometrial cancer.

Isoflavones should not be equated with estrogen:

ISOFLAVONESESTROGEN
Do not stimulate the vaginal maturation index or increase C-reactive proteinDOES stimulate the vaginal maturation index, and increases C-reactive protein
May exert potentially-relevant hormone-independent physiological effectsEstrogen is a hormone – so can cause hormone-dependent physiological effects

Not only should isoflavones NOT be equated with estrogen but soy foods should not be equated with isoflavones. The soybean, like all foods, is a collection of many biological molecules.

 

The Effects of Soy Protein on Cardiovascular Disease Risk Factors:

FACTS:

  • Elevated LDL-cholesterol is a well-established risk factor for CHD
  • LDL-cholesterol lowering is still the primary strategy to prevent CHD
  • Clinical research (since 1967!!) demonstrates the hypocholesterolemia effects of soy protein
  • This fact was formally recognized by the FDA in 1999, and by 10 other countries since then
  • FDA recommendations: 25g /day soy protein – easily met through a combination of soy foods
  • In addition to lowering LDL-cholesterol, soy protein also modestly lowers triglyceride levels and raises HDL-cholesterol levels

 

Blood Pressure:

  • Clinical data shows that soy protein is hypotensive
  • The public health benefits of even the modest proposed hypotensive benefits of soy protein are relevant as reducing systolic BP by just 2-5mmHg may reduce stroke by 6-14% and CHD by 5-9%

While there are some excellent results from clinical trials, the mechanism by which soy protein exerts its hypotensive has not yet been identified.

 

Breast Cancer:

FACTS FROM THE RESEARCH:

  • The protective effects afforded by isoflavones may be similar to the observed protective effect of early pregnancy against breast cancer.

The positions of the American Cancer society and the American Institute for Cancer Research are that:

  • Soy foods can be safely consumed by women with breast cancer
  • And that post-diagnosis soy intake improves the prognosis of breast cancer patients

The World cancer Research Fund International states that:

  • There is a possible link between consuming soy foods and improved breast cancer prognosis

 

Mental Health:

Limited research, but interesting preliminary results show that:

  • Isoflavones may function as antidepressants
  • Over a 3 month period 100mg/day isoflavones reduced depressive symptoms in clinically depressed postmenopausal women to a similar extent as Zoloft (50mg/day) and Prozac (10mg/day)
  • In addition, the combination of Zoloft and isoflavones resulted in greater reduction in symptoms than the other 3 individual treatments

 

To Summarise:

  • Soy protein is higher in quality than other legume proteins
  • Soybeans are a good source of both essential fatty acids
  • Soy protein directly lowers LDL-cholesterol
  • Soy protein may also modestly lower blood pressure
  • Replacement of commonly consumed sources of protein in Western diets by soy foods may also lead to a favorable change in the fatty acid content of the diet
  • The most distinctive aspect of the soybean is its isoflavone content – which have many health benefits
  • Isoflavones can definitely alleviate hot flushes and improve arterial health in postmenopausal women
  • Concerns that the estrogen-like properties of isoflavones produce untoward effects in some subpopulations, such as postmenopausal women, are NOT supported by the clinical and epidemiologic research
  • Evidence indicates that soy foods can be consumed by all individuals except those who are allergic to soy protein (uncommon)
  • When adding soy to the diet it is important to consider the overall nutritional quality of a particular soy food since many Western soy foods include a variety of non-soy ingredients.
  • US FDA recommendations for LDL-Chol reduction: 25g / day soy protein
  • Populations and clinical studies involving adults suggest benefits are associated with approx. 2 – 4 servings per day.
  • Ideally soy foods are incorporated into the diet by displacing less healthy foods and as part of an overall healthy diet designed to lower risk of chronic disease

The versatility of this little bean is amazing – from reducing cholesterol to fighting cancer, soybeans really are something special. If you haven’t already, make sure to add them to your diet today.

 

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