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Most of the side effects of isoflavones are associated with long-term use of supplements and not from dietary sources such as soy products. But epidemiological (the branch of medicine dealing with the incidence and control of disease) data have shown that there is a link between long term soy consumption and Kawasaki disease (KD), and that soy isoflavones are involved in the development of the disease.
Another study discovered that exposure to soy-based infant formula resulted in negative effects on the long-term development of infants.
Breast cancer cell growth has been linked with long-term exposure to genistein, resulting in what is called “soy protein isolate-induced tumors and advanced growth phenotypes.”
Animal studies have shown that evidence of the isoflavone genistein may have adverse effects on the developing female reproductive tract.
When ingested on a short-term basis (up to six months in duration) soy is considered possibly safe. Common side effects may include:
- • GI upset
- • Constipation, bloating and nausea
- • Allergic reactions (involving rash, itching, and in severe instances, anaphylaxis)
- • Loss of appetite
Swelling of the ankles and abdominal tenderness have been noted in high doses of isoflavones of four to eight milligrams per kilograms (mg/kg).
Long-term use of soy extract supplements may result in abnormal tissue growth in the uterus.
Precautions (Contraindications)
There is not enough clinical research data to support the safe use of soy products when pregnant or breastfeeding, particularly at higher doses.
There have been some preliminary research findings that link infant soy formula with delayed development in infants, but according to Drugs.com, “The National Toxicology Program (U.S. Department of Health and Human Services) has concluded that there is minimal concern for developmental effects in infants fed soy infant formula.” Long-term use of soy formula should always be discussed with the healthcare provider.
Children should not eat soy in large amounts. It’s uncertain whether soy is safe for kids at high doses.
Those with asthma or hay fever may have a higher risk of an allergic reaction to soy.
Those with breast cancer should discuss the use of isoflavone supplements with their oncologist or healthcare provider. The research data are mixed, and it’s possible that soy may act like estrogen and promote cancer cell growth in certain types of breast cancer.
Children with cystic fibrosis should avoid taking isoflavones because they may interfere with the way these kids process protein.
The use of isoflavones in people with diabetes should be discussed with the healthcare provider prior to use because soy products may lower blood sugar, interfering with diabetes medication.
Hypothyroidism may be worsened when using soy isoflavones.
Those with kidney stones should avoid taking soy isoflavones as they may increase a chemical, called oxalates, that contributes to kidney stones. Also, those with kidney conditions should avoid the use of phytoestrogens such as soy isoflavones, which could be toxic at high doses for those with kidney failure.
Drug Interactions
Isoflavones may adversely interact with some medications including:
- Monoamine oxidase inhibitors (MAOIs): A type of antidepressant which interacts adversely with fermented soy products
- Antibiotics may reduce the effectiveness of soy products by negatively impacting the natural gut flora needed to effectively process isoflavones.
- Estrogens such as Premarin, estradiol, and other HRT for menopause should not be taken with isoflavones because the isoflavones may decrease the effect of estrogen.
- Nolvadex (tamoxifen) is a drug used in the treatment of the type of cancer influenced by estrogen and should not be taken with isoflavones.
- Coumadin (warfarin) soy products may lower the effectiveness of Coumadin. Red clover may have blood-thinning properties and should not be taken with Coumadin.
Isoflavones may adversely impact the speed in which the liver metabolizes certain medications. These drugs include:
- • Tolbutamide (hypoglycemic agent)
- • Glipizide (hypoglycemic agent)
- • Phenytoin (anticonvulsant)
- • Flurbiprofen (anti-inflammatory agent)
- • Warfarin (anticoagulant)
Anyone taking prescription or over the counter medications should consult with the healthcare provider before taking isoflavones or any other herbal or nutritional supplement. *