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Ask a Dietitian

"Diana, just a quick heads up to let you know we are still using your cookbook and the guys will often be heard saying what would Diana say about this or that....really good feed back... I made your potato salad and the oriental coleslaw on Sat. for a family luncheon and had rave reviews so thanks again."

Maeghan Henke
BC Hydro

The Soy Story

June 27th, 2006


Soy foods have become very popular in recent years due to their health benefits. However, recently there have been many claims in the media against the benefit of soy in the diet, including allegations that it raises the risk of cancer, osteoporosis, nutrient deficiencies, thyroid problems, and reproductive difficulties. First, it is important when reading information to always check the source. Health professionals make it their job to look at all the current research and the total evidence rather then just a few studies. Second, be wary of conclusions drawn from animal studies (as foods can react differently in humans and animals) and epidemiological studies that do not always show cause and effect. The following is research into the claims against soy and what the verdict is on if we should be consuming soy in our diets.

Soy foods and the Thyroid
The thyroid is a small gland that is located at the base of the neck and is responsible for producing hormones that influence the balance in the body (homeostasis). Goitrogens are compounds found in many foods such as millet, broccoli and soy, which potentially can interfere with thyroid function if iodine intake is low. Iodine is an element required in small amounts for healthy growth and development and is important for thyroid function. Iodine is mostly found in milk products, seafood and iodized salt. For example, about 2-3 servings a week of seafood will give you enough iodine for the week. Vegans who do not eat animal products may need to incorporate sea vegetables (such as dried hijiki, kelp or kombu) into their diets and season lightly with iodized salt to ensure that they are getting enough. Make sure that you look at the label when choosing the type of salt. For example, regular sea salt is a poor source of iodine.
Bottom line: There has not been any evidence to date that soy products cause thyroid problems in healthy, well nourished people who consume adequate iodine.

Soy foods and Nutrient Absorption
There has also been the claim that soy causes nutrient malabsorption due to their phytate content. Many other foods also contain phytates including whole grains, legumes, nuts and seeds. Phytates are plant compounds that have the potential to inhibit the absorption of iron, zinc and calcium. However, calcium is well absorbed from fortified soy products. In fact, some studies have found soy isoflavones to promote bone health. While soy products have been found to inhibit iron absorption this effect can be partially overcome by consuming a food providing vitamin C (eg. oranges) with your iron foods. While zinc is poorly absorbed, it is not a reason to avoid soy products. There are many other sources of zinc including meat, dairy, nuts, seeds, whole grains and fortified foods. By consuming some of these iron, zinc and calcium rich sources at different times to your soy foods, one can also reduce the problem of malabsorption.

Soy and Breast Cancer
Many concerns have been raised regarding soy foods and the risk of breast cancer. This is because of the isoflavone content that soy is naturally rich in. Isoflavones are a class of phytoestrogens that are plant-derived compounds with estrogenic activity that have been hypothesized to either increase cancer cell growth or reduce it. To date, there is no clear evidence that phytoestrogen intake in amounts consumed daily through soy products influences the risk of developing cancer. In fact, a review of epidemiological studies (the study of cultures) indicated a small association between soy intake and a reduced risk of breast cancer. Additionally, there is some evidence to suggest that intakes of soy early in life may help to decrease the risk of breast cancer later in life. However, currently high-dose isoflavone supplementation is not recommended.

Soy Foods and Osteoporosis
Soy isoflavones have been found to reduce bone resorption and bone formation and thus can play a role in alleviating osteoporosis. Soy is also a rich source of calcium and thus may play a role in diminishing bone loss. Research has also found that soy food consumption may reduce the risk of fracture in postmenopausal women, especially those in the early years following menopause.

Soy Foods and Heart Disease
There is some discrepancy as to how much of an impact soy foods have on reducing cholesterol levels or preventing heart disease. When soy protein is used to displace dairy protein or animal meats (that are higher in saturated fat), soy appears to have a positive lowering effect on reducing LDL-cholesterol (bad cholesterol) by a few percentage points (~3%). There were no significant effects on HDL-cholesterol, triglycerides or blood pressure. Dietary soy and soy protein isolates also have antioxidative properties and appear to be cardio-protective. In addition, cholesterol lowering foods such as soy, almonds, oats, plant sterol enriched margarines and barley appear to work better in combination rather then in isolation, suggesting a synergistic benefit. Again, the use of isoflavone supplementation is not recommended.

Soy and Reproductive Health
Scientists first became concerned about the potential link between isoflavone consumption and reproductive health when female sheep grazing on isoflavone rich clover in Australia began having breeding problems. However, the consumption of clover was extremely high (higher then the typical consumption of Asians). In addition, different species vary in their responses to particular compounds and sheep are especially sensitive to isoflavones which would influence the outcome. There is currently no evidence of reproductive problems in those that eat soy products from human studies, despite the weak estrogen like effects that soy isoflavones possess.

Bottom line: Considering the evidence, soy foods appear to be perfectly safe if consumed in reasonable amounts as part of a healthy diet. Approximately 2-3 servings (eg. 1 cup of milk is a serving) a day of soy is considered a reasonable amount. Those who have had estrogen-positive breast cancer may want to be more restrictive of their soy intake. However, there is not enough evidence that soy has harmful effects to suggest the need to avoid soy altogether. Remember that with any foods, variety and balance is key. Too much of anything is never good!