Nutrition profile of soy

Soy is a high-quality protein. It is one of only two known plant foods (the other is amaranth seed) to contain all the essential amino acids, similar to those found in meat.

Some soy products are sources of calcium and iron, such as Chinese tofu or tempeh (made with a calcium coagulant) and calcium-fortified soy drinks. The soybean is:
• high in fibre
• high in protein
• low in saturated fat
• cholesterol free
• lactose free
• a good source of omega-3 fatty acids
• a source of antioxidants
• high in phytoestrogens.

Soybeans and phytoestrogens

Soybeans contain hormone-like substances called phytoestrogens (‘phyto’ means plant), which are naturally occurring chemicals found in plants. Given the right conditions, these compounds copy the action of the female hormone oestrogen, but are much less powerful – about 1,000 times less.

There are many types of phytoestrogens, one example being isoflavones. Soybeans are the most common source of isoflavones in food with rich sources found in soy nuts and tempeh. Isoflavones are very strong antioxidants and can mimic the effects of oestrogen, but their full physiological effects are unknown.

Health benefits of soy foods

Research suggests that soy foods and food containing soy have a range of health benefits.

Soybeans and menopausal hot flushes

Due to their phytoestrogen content, it has been thought that soy can reduce menopausal symptoms, such as hot flushes. This is due to the observation that Asian women tend to have soy-rich diets and typically have fewer hot flushes during menopause than their Western counterparts (who tend to have meat-rich diets). However, recent studies have found little effect on a soy-rich diet or supplementation.

It is thought that a soy-rich diet helps reduce menopausal symptoms such as hot flushes, because the phytoestrogens act like a mild form of hormone replacement therapy (HRT). Measured reductions in the rate of hot flushes associated with soy consumption vary from 1.9 per cent to 45 per cent.

More research is needed, but soybeans seem to offer promise in helping some women to manage menopausal hot flushes. If phytoestrogens do work, studies suggest that you need at least two to three serves of soy products daily. This would mean either:
• 500 ml of soy milk per day
• 100 g of tofu per day
• Four to five slices of soy linseed bread per day (depending on the brand).
Soybeans and coronary heart disease

Oestrogen may protect women against coronary heart disease (CHD) during their reproductive years, but rates of CHD increase remarkably after menopause. Soybeans have been shown to lower total cholesterol and LDL cholesterol levels, both known risk factors for CHD.

A meta-analysis (an analysis of multiple studies on a topic) of 41 clinical trials found that 20 g to 61 g of soy protein can significantly reduce total blood cholesterol levels, LDL (bad) cholesterol levels and triglycerides. The results also showed that soy protein supplementation slightly increased HDL (good) cholesterol levels. This amount of soy protein is found in two to three serves of soy products.

It is not known whether the phytoestrogens or the soy proteins (or both, working in combination) or the other characteristics of soy (including high-fibre content and low saturated-fat content) are responsible for these health benefits. However, studies have shown that eating soy protein without isoflavones results in only small cholesterol reductions and isoflavone supplements alone have minimal cholesterol lowering effects.

The cholesterol-lowering benefits of eating soy foods may be improved if the total diet is high in carbohydrate. This seems to help with the breakdown of the isoflavones.

In 1999, the United States Food and Drug Administration acknowledged the heart health benefits of including at least 25 g soy protein daily in a diet low in saturated fat and cholesterol. This equates to approximately four servings of soy daily.