John Livesey PhD, Scientific Officer, Department of Endocrinology, Christchurch Hospital, New Zealand

Soy products have attracted much attention in recent years as possible beneficial additions to the traditional Western diet. The hope is that increased soy consumption will reduce the incidence of Western afflictions such as heart disease and cancer and so increase longevity. In addition, for vegetarians and vegans, soy products are a relatively cheap and convenient source of high quality protein.

There is much circumstantial evidence in the medical literature for beneficial effects of soy on human health. In relatively short-term human experimental studies consumption of soy products (as compared to animal products) is found to:

Alter blood lipid1 and homocysteine2 concentrations in ways likely to reduce the risk of heart disease.
Lower blood pressure in patients with hypertension3.
Alter bone metabolism in a way that may reduce the risk of osteoporosis 4.
Improve memory5.
Decrease menopausal symptoms6.
Further, observational and epidemiological studies of humans suggest that soy consumption may:

Reduce the risk of breast cancer7,8.
Reduce the risk of prostate cancer9.
Reduce the need for hysterectomy10.
Increase bone mineral density11.
Reduce the risks of lung cancer12 and stomach cancer13 in Asian populations where these diseases are relatively common.
Assist in body weight control14.
Reduce the risk of thyroid cancer15.
Despite this apparently weighty evidence for health benefits from soy (only a small fraction of studies with similar conclusions have been referred to), not everyone is convinced.

Two years ago an epidemiological study of elderly Japanese-Americans in Hawaii found a correlation between eating tofu in middle age and poor cognitive function, enlargement of the ventricles and low brain weight in old age16. It is not clear though what to make of these findings. They have yet to be confirmed by any other study and they appear to run counter to the finding of lower rates of dementia in Okinawa (and Japan generally) than are found the United States17. Soy consumption is considerably higher in Okinawa than in the US. Further, two recent short-term studies suggest that soy or soy products may improve memory5,18. It has been suggested that the high aluminium content of Hawaiian tofu may be responsible for the deterioration of mental function seen there. Another possibilty is that the tofu consumers may have been relatively deficient in vitamin B12 compared to the non-tofu consumers, who were presumably eating meat. B12 deficiency can result in neurological problems.

New Zealand researchers have questioned the safety of soy for infant feeding19,20, citing largely theoretical concerns about possible anti-estrogen and anti-thyroid effects from the isoflavones (phytoestrogens) that are found in much higher levels in soybeans than in common Western foods. However a recent study of 248 young adults who were feed soy formula as infants found no statistically significant differences between them and 563 young adults feed cow milk formula as infants21. With the primary aim of studying effects that might be expected from any estrogen-related activity of the soy formula, the subjects were questioned about pubertal maturation, menstrual and reproductive history, height, weight, current health and education. On this basis thirty primary comparisons were made between the two groups and using the strictly correct statistical interpretation (Bonferroni adjustment for multiple comparisons), no significant differences between the soy-fed and the cow milk-fed groups were found.

If the statistical criteria are relaxed and each of the primary comparisons considered in isolation, the only significant differences found were that women in the soy formula group had a longer duration of menstrual bleeding (by 0.4 days) and greater discomfort during menstruation21.

If unplanned comparisons suggested by the data are also examined, the relaxed statistical criteria find a further statistically significant difference between the two groups21. A greater proportion of the soy-fed group (17%) were regular users of asthma or allergy drugs than were the cow milk-fed group (10%). If however irregular users of these drugs were included with regular users, there was no significant difference between the groups for the proportions of subjects using asthma and allergy drugs. No significant difference in the incidence of thyroid disease was found for the two groups22. Thus it would appear on current evidence that feeding infants soy formula is not associated with a substantial risk to health in early adult life, though more research is needed23, particularly with much larger numbers of (human) subjects.

This lack of obvious effect of soy isoflavones on infant development may be because humans do not absorb the isoflavones intact and only their glucuronide and sulfate metabolites are detectable in the blood24.

Articles have also appeared in more popular media casting doubt (usually quite vigorously) on the merits of consuming soy (see references 25 and 26 for examples). Concerns in these articles centre around soy’s high content compared to other plant foods of phytoestrogens and anti-metabolites such as anti-trypsin, phytic acid, hemagglutinins (lectins) and antithyroid agents. It is also claimed that soy increases the body’s requirement for vitamins B12 and D, and that toxic lysinoalanine, nitrosamines and monosodium glutamate are formed during the processing of soy.

The possible adverse effects of these substances in the human diet are largely speculative. An alternative view is that some of these anti-metabolites may have desirable anti-cancer properties. We just don’t know yet.

At least some of these anti-soy claims appear to be weak since anti-trypsin and lectins are inactivated by heat (ie cooking)27, and infants fed soy based-formula maintain similar vitamin D levels28 and growth rates29 to those fed cow milk-based formula.

It was reported from a retrospective study in 1990 that children with autoimmune thyroid disease (a rare condition) were more likely than their unaffected siblings to have been fed soy based-formula30. This finding though still awaits confirmation by other studies and the relationship found could be an example of reverse causation. That is to say, the babies may have been given soy-formula for some reason related to a predisposition to an autoimmune condition.

In 1991 a Japanese group reported that in an uncontrolled trial consumption of 30g of soybeans per day by adults resulted in half their subjects developing goiters31. However subsequent better designed studies in the USA using soy proteins failed to find such an effect or indeed any other clinically significant hormonal effects32,33. Indeed, the claimed effect of soybeans would predict a high incidence of goiter in high soy consuming areas such as Japan and Okinawa, but this appears not to be the case. There is also evidence that soy consumption may be protective against thyroid cancer15.

It is difficult though to completely discount concerns about soy since appropriate large-scale epidemiological studies have not yet been done. What is needed are studies comparing disease and death rates for large numbers of high soy and low soy consumers living similar lifestyles. More definitive answers should be available in a few years because Professor Gary Fraser of Loma Linda University, California, has recently been funded to study the effects of soy consumption among 125,000 Seventh Day Adventists.

Allergy to soy though is well documented. For example, 10% of infants allergic to cow milk were also allergic to soy formula34, but soy allergy appears to be much less common than allergy to cow milk35.

The best evidence presently available on the long term effects of soy on human health is firstly the study of young adults fed soy as infants21 mentioned above and secondly the observation that in countries where soy is widely consumed people tend to have better longevity than most. The Japanese, regular soy consumers, lead the world in longevity36. In particular, in Okinawa, where for older persons the soy consumption of about 80g per day37, mainly as tofu, exceeds that in the rest of Japan, the average longevity (81.2 years) also exceeds that of the rest of Japan36,38.

In conclusion, whether or not soy consumption really does benefit your health overall remains to be determined by good epidemiological studies. On the other hand, if it is harmful, the harm would seem not to be very great in view of the excellent longevity found in high soy consuming areas of the world.

Similar conclusions are reached in articles by Greg Caton and Dr John McDougall.

REFERENCES

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