The humble soybean is the key ingredient in soy milk, soy sauce and all sorts of tofu (doufu), but it is also found in around 60% of all processed foods (including burgers, sausages, yogurt and ice cream) under pseudonyms such as hydrolysed vegetable protein and soy protein concentrate.

The health benefits of soy are widely acknowledged. It is the only plant-based food that contains all the amino acids necessary to make a complete protein. This makes it a great alternative to animal protein. Soy is also low in saturated fat and studies have shown that 25g of soy protein each day can reduce cholesterol levels, thus reducing risk of coronary heart disease. In fact products in the UK and US containing more than 6.25g of soy protein can now use this as a health claim. Many of the health effects of soy are thought to be due to constituent natural chemicals called isoflavones which are a type of plant oestrogen. These bio-active compounds are not as potent as human oestrogen hormone, but may be able to mimic or block the action of oestrogen in the body.

Accordingly, soy products are thought to be able to act as a form of natural Hormone Replacement Therapy (HRT) in menopausal women, and may be useful in relieving symptoms such as hot flushes. Practically, this means including around 3 portions of soy food per day (glass of soy milk, pot of soya yogurt or 100g tofu). These oestrogen-like properties may also assist in lowering the risk of developing osteoporosis, and it has been observed that in countries consuming more soy products, people have a high bone mineral density. Soy consumption is also thought to be protective against endometrial cancer, hence the lower rates observed in Japan and China.

However, in the last few years, a shadow has been cast over the health benefits of the soybean. There is speculation that soy phyto-oestrogens may suppress thyroid function, and worsen hypothyroidism in those with a pre-existing condition. Experiments in animals have shown that very high doses of soy can block thyroid action and lead to a goitres (swelling of the thyroid gland seen with iodine deficiency). Currently health bodies say there is insufficient evidence to advise those with thyroid problems against consuming soy at recommended levels, but they have acknowledged that more studies are required.

Phyto-oestrogenic effects are also a concern for infants, and therefore in countries such as the UK, it is advised that soy milk is not routinely given to babies under 12 months. This is due to fear that the daily amount of phyto-oestrogens delivered when soy milk is the sole source of nutrition could affect the development of a baby's reproductive system at this delicate stage.

Meanwhile research into the role of soy in breast cancer is still highly conflicting. Some studies show no connection, some show decreased rates in rats consuming soy, whilst other human and animal studies suggest that the soy's oestrogen-like effects can increase breast cancer risk.

This nutrition-related column is written by Nina Lenton, a qualified dietitian living in Beijing and working at Bayley and Jackson Medical Center. Contact her at nina.lenton@bjhealthcare.com.

(China Daily 09/12/2007 page14)