Dr Karine Nohr looks at the evidence that soy products can benefit post-menopausal women
I play in a women’s tennis doubles league and we are mostly middle aged, fit and well. Not infrequently, I get asked for some advice about medical problems.
‘So should I be drinking soya milk, then?’ was a recent typical example. ‘Should we give the kids soy milk?’ , ‘are veggie burgers healthy?’, ‘does it matter what form the soy comes in?’ and ‘Is soy actually useful for hot sweats?’ were others. So what of this soy debate?
For reducing hot flushes, the evidence is contradictory. Soy contains isoflavones, which distinguishes them from other legumes, and they have a similar structure to oestrogen and can have oestrogenic effects.
In premenopausal women, soy has anti-oestrogenic effects, displacing oestrogen from receptors. In adolescent Asian females, soy intake is inversely proportional to breast cancer rate and this benefit persists even when Asian women immigrate to Western cultures. This protective effect is more pronounced pre-menopause than post-menopause. In post-menopausal women with low oestrogen levels, soy can have weak oestrogenic effects.
One of the soy isoflavones is daidzen. Daidzen is converted to equol in the gut and 30% of western populations are equol converters. A high proportion of the south east Asian population are equol converters, and it may be for this reason (unproven) that this population have a high soy diet and a low breast cancer rate.
Soy binds to the same receptor as tamoxifen. At present, there is insufficient data to evaluate the effect on soy on breast and other cancers. Gut flora can influence isoflavone availablity and it may be this reason that there are discrepant trial results in different populations.
Overall, a high soy diet seems to be associated with a modest reduction in the frequency and severity of hot flushes, but this is not demonstrated in all trials.
As regards the effect of soy on CVD, there is good evidence that soy reduces lipid levels. This is because soy is a good source of saponins and phytosterols, which can decrease cholesterol absorption. There is a modest reduction of total cholesterol and LDL-cholesterol but only an insignificant decrease in triglycerides and increase in HDL-cholesterol. These changes may be more significant in hyperlidiaemic patients. There is no evidence that a high soy diet reduces the frequency of cardiovascular events in a western population.
As mentioned in a previous blog, the frequency of prostate cancer is reduced in an Asian diet. This also seems to apply to endometrial cancer and thyroid cancer. But the jury is out as to whether this is dietary, environmental or genetic.
All dry beans are important sources of complex carbohydrates. They are a good source of both insoluble fibre, maintaining good gut health, and soluble fibre, helping to keep down cholesterol levels. In type 2 diabetics, there is some good evidence that a soy-containing diet lowers glucose, HbA1c and improves glucose tolerance.
What kind of soy is ‘healthy’ soy? Processed soy protein is a by-product of the production of soy oil, which is a huge multi-million dollar industry. This brings up geopolitical issues, as huge tracts of land have been turned over to its production, resulting in the destruction of, for example, massive areas of Brazilian rain forest, also to the demise of indigenous populations.
Unprocessed unfermented soy, such as edamame, soy milk, tofu and tempeh, are thought to be a far more healthy source than isolated soy products, as they will have a higher proportion of isoflavones and not just be a source of protein. Additionally, phytic acid is neutralised in the process of fermentation. The prescence of phytic acid in fermented soy can be problematic, as it binds/chelates with minerals like calcium, magnesium, copper, iron, and especially zinc in the GI tract. Additionally, MSG is formed during soy food processing and additional amounts are added to many soy foods.
Exposure of children to soy formula in infancy does not appear to cause health or reproductive problems later in life . However, soy milk that’s not designed for infants should not be used as a substitute for infant formula, as it can lead to nutrient deficiencies.
So, in conclusion, unprocessed unfermented soy seems to be a useful component of a healthy diet. This seems to be even more true for adolescent girls. Although there is evidence for cardiovascular benefit, it is conflicting and supplements are not recommended. Beware of a high daily intake (>100g) of isolated soy products.
Dr Karine Nohr is a GP in Sheffield
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