Bioavailability of Soy Isoflavones in Infants

Researchers in Hawaii wanted to determine the bioavailability of isoflavones from soy in infants by indirect or direct exposure. A small study was conducted that measured isoflavone excretion in urine and plasma in 18 mother/infant couples. The results were published in the August issue of American Journal of Clinical Nutrition.

The researchers looked at the soy isoflavone content in the urine of mothers after consuming a known amount of soy milk for 2-4 days. Then urine excretion was measured in those infants who were exclusively breast fed as well as infants who were given tofu as part of a meal. Excretion and therefore bioavailability were compared between both mother and child and breastfed and tofu infants.

According to the results, isoflavones are more bioavailable in children than adults. Isoflavone content in urine of mothers after consuming soy milk ranged from 18-135 where in infants who consumed tofu had a mean urinary excretion level of 229. The level of isoflavones in the urine of exclusively breastfed infants ranged from 29-111, where as the level of isoflavones in breastmilk was only 5-19.

After adjusting for weight and intake breastfed infants had the lowest bioavailability and infants who ate tofu had the highest bioavailability as measured via isoflavone excretion.

Now that more is known about isoflavone metabolism in mother/infant couples and children who consume soy products, information and recommendations about consumption of soy products and the relative effects on children can be developed.

Because isoflavones from soy (and other plants) are phytoestrogens there is a potential for interaction with normal hormone metabolism and development in children. At this point little is known and healthcare providers do not know how much soy is too little or too much or what the long term implications could be for children who consume high amounts of soy either via the breastmilk or by their own abilities. Look for more information to develop in this area.