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Tofu: Myths, Facts, and Folklore


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Photo Credit: soy beans, originally uploaded by ozdigital.

Tofu has been crowned accolades like none other. In fact, in 1999, the FDA recommended consumption of 25g of soy protein per day, along with a diet low in cholesterol and saturated fats, to reduce heart disease. On the other hand, it has also been under intense scrutiny and at times considered a health risk. Separating the fact from the hoopla is no easy task, but it is important to understand potential risks and benefits of soy.


The protein content of soybeans is 36-56%. Soybeans are considered a complete protein in that the contain most of the essential amino acids. It is the only plant protein with the largest concentration of isoflavones. Isoflavones are a class of phytoestrogens. Phytoestrogens are a group of non-steroidal plants with estrogen-like activity. The amount of isoflavones varies depending on the type of soybean, geographical area of growth, harvest years of the soybeans, and methods of processing. Isolated soy proteins, tofu and soymilk provide 0.1-2 mg of isoflavones per gram of soy protein. Alcohol extracted products, such as soy protein concentrate, are stripped of much of their isoflavone content and provide almost insignificant amounts of isoflavones. Usually less then 0.3mg per gram of soy protein. Soy products that contain most of the bean, mature soybeans, roasted soybeans, soy flour, textures soy protein contain the largest quantity of isoflavones: 0.1-5mg per gram of soy protein. Whole soybeans have the highest concentration of isoflavones, which decreases with increasing processing of soy. Soymilk, tofu and soy food products contain only 2% of the original isoflavone content of soybeans.

The predominant isoflavones in soy are genistin and daidsin. After ingestion, they are converted to their bioactive forms: daidzein and genistein. The estrogenic activity of these 2 isoflavones is 100-1000 times less then that of estrogen found in the body. Their affinity for the estrogen receptor is 1000-10,000 times less than that of naturally occuring estrogen found in the body. However, in an individual consuming 50-80mg of soy protein per day, their concentration in the plasma can be 100 times higher than the estrogen found in the body.

Health Benefits: Lowering Cholesterol

What are some of the health benefits of soy? The benefits are wide and varied. Soy has been shown to decrease bad cholesterol: LDL, triglycerides, liver cholesterol, and liver triglycerides. Animal studies have shown that soy proteins decrease intestinal cholesterol absorption and increase bile acid secretion. The cholesterol lowering effects are inversely related to the baseline level of cholesterol. In essence, higher baseline cholesterol levels show a more dramatic decline than lower baseline cholesterol levels.

Health Benefits: Combatting Atherosclerosis

Soy decelerates the progression of atherosclerosis secondary to antioxidant, antiproliferative and antimigratory effects on smooth muscle cells. The normal vascular reactivity of blood vessels is maintained. Genistein inhibits an enzyme, tyrosine kinase, associated with plaque development. In 2 separate studies, Soy protein isolate with isoflavones was shown to decrease diastolic blood pressure ( the bottom number on BP readings) in women but not in men.

Health Benefits: Fighting Cancer

In cancer, soy has been shown to decrease the risk for colon cancer by 50% secondary to the antioxidant effects of phytoestrogens, such as genistein. In human prostate cancer cells, genistein and its precursor have been shown to inhibit cell growth at high concentrations. Soy in the diet of rats has shown to prevent prostatitis. Interestingly, estrogens have also been shown to be associated with BPH(benign prostatic hypertrophy) and prostate cancer.

Health Benefits: Combatting Obesity

Soy has been studied as a means of combating obesity. Intake of soy suppresses food intake, increase satiety and or increases energy expenditure resulting in decreased body fat gain and weight loss. Soy protein has been shown to decrease insulin level in plasma by inhibiting release of insulin from the pancreas or by increasing lipolysis (fat breakdown) by the liver. Soy with isoflavones (100mg per day) has a favorable effect on bone mineral density.

Health risks: Goiter and Gout

Some of the risks associated with soy consumption involve goiter, gout, and effects on breast tissue. Soy has been linked with goiter. In those that are iodine deficient, namely the elderly and infants, there is an increased antithyroid effect of soy. Iodine supplementation is protective and iodine reverses infant goiter associated with soy. Tofu ingestion increases the amount of uric acid concentration, uric acid clearance by the kidney, and urinary excretion of uric acid. The increased plasma concentration of uric acid is small. In patient’s with gout, with a uric acid clearance greater than 0.6 L/min, there was no significant rise in plasma, urinary, and clearance of uric acid. In fact, tofu may be a preferable source of protein especially in patients with gout with a uric acid clearance greater than 6.0L/min.

Health Risks: Breast cancer and hormonal effects

In Japan, the incidence of breast cancer is 1/3 that of Western countries. Part of this lower incidence is speculated to be secondary to the diet and the relatively high intake of soy (phytoestrogens). Approximately 70% of breast cancers are estrogen sensitive. Estrogen can promote tumor growth in these estrogen sensitive tumors. Breast cancer incidence increases with age, despite the fact that post-menopausal women are no longer producing estrogen from their ovaries. This may because other tissues in the body (adipose, breast, brain, muscle, skin and bone) are able to synthesize estrogen from circulating androgens and steroid precursors. Phytoestrogens are able to inhibit the enzymes which convert circulating precursors into active estrogen. At low doses, phytoestrogens stimulate tumor cells, but at higher doses phytoestrogens have been shown to inhibit tumor cells, in vitro and in vivo. Human studies suggest no consistent effect of phytoestrogens on normal breast tissue, but a possible proliferative effect on breast cancer tissue. Soy may exert a weak estrogen effect on breast tissue in premenopausal women. Soy also has many anti-estrogen effects on estrogen dependent cancers. Soy has been shown to increase menstrual cycle length, which over a lifetime results in 2 fewer years of menstruation (which is protective against estrogen dependant breast cancer). Soy causes a decrease in urinary estrogens and a decrease in genotoxic estrogen metabolism. Soy is both protective against breast cancer and promoting of breast cancer. Isoflavones, have no effect on the endometrium (lining of the uterus). In men, there is no adverse effect of soy isoflavones on sperm quality. For menopausal symptoms, soy shown non-significant relief of symptoms. Soy may have a modest effect on hot flashes, but studies are not conclusive.

Health risks: Cognition and Soy Allergies

Soy consumption was linked with decreased cognitive function. Two other short-term studies suggest that soy may improve cognitive function and memory. Soy is less allergenic (allergy producing) than cow’s milk. A meta-analysis of 17 different studies looking at allergen reactivity showed soy allergies occur in 3-4% of infants and children compared ot cow’s milk allergy(25%).

In traditional chinese medicine, tofu is considered Yin depleting. Yin energy is quiet, potentiated, inward and nourishing.

Admittedly, research on soy risks and benefits is conflicting and confusing. One reason may be that the dietary preparation of soy proteins used in clinical studies is widely varied. Preparation methods should be standardized so that results can be appropriately compared. With out standardization of experimental methods, results will continue to be conflicting. One must apply sound judgement and assess their individual medical status to determine their personal risks of consuming soy, or for that matter any other food item. The preparation, production, processing, storage, farming, and harvesting of all consumable goods is replete with risks, benefits, and side effects.

References:

  1. Adv Exp Med Biol.
  2. J Nutr.
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  8. Am J Manag Care
  9. Environ Health Perspect.
  10. CMAJ
  11. Am J Clin Nutr.
  12. J Clin Endrocrinol Metab.
  13. Acta Biomed.
  14. Int J Med Sci.
  15. Ann Intern Med.
  16. Endocr Relat Cancer
  17. J Am Coll Nutr.
  18. J Nutr
  19. FDA

1 Comment »

  name wrote @ June 14th, 2009 at 1:44 pm

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Very interesting sites.,

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