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Soyfoods and stomach cancer

 

Dietary factors are recognised as playing an important role in the aetiology of stomach cancer: a high intake of nitrates and related compounds and salted foods increase the risk and a high intake of fruit and vegetables decrease the risk. It has been suggested that soyfoods may also be associated with stomach cancer. There are two main categories of soyfoods: non-fermented (e.g. soymilk, tofu and soy beans) and fermented (e.g. soy paste). This meta-analysis examines the relationship between fermented and non-fermented soyfoods and risk of stomach cancer.

Message

The associations found, i.e. fermented soyfoods increase risk of stomach cancer and non-fermented soyfoods decrease risk of stomach cancer, may be due to consumption of other foods in the diet, such as salt and fruit and vegetables. Nevertheless, practical advice remains largely independent of further research: eat less salt, more fruit and vegetables and replace some animal protein with vegetable protein.

Reference

A H Wu et al. A meta-analysis of soyfoods and risk of stomach cancer: the problem of potential confounders. Cancer Epidemiology, Biomarkers and Prevention 2000 9: 1051-1058.

Search

The literature was searched using MEDLINE and reference lists of English language publications between 1966 and 1999, conducted in Asia or among Asians.

Intake of fermented soyfoods was investigated in 15 studies (five cohort and ten case-control). In 14 studies, stomach cancer cases ranged from 44 to 668; the exception had 5,247 cases. The soyfood investigated in the majority of studies was miso (others being fermented soybean and soybean paste). A high intake was compared with a low intake (e.g. daily versus less than once a month).

Intake of non-fermented soyfoods was investigated in 11 studies (2 cohort and 9 case-control). Cases of stomach cancer ranged from 93 to 819. The soyfoods investigated were tofu, bean curd and soybean products comparing a high with low intake.

In the majority of studies only age and gender were adjusted for in the analyses.

Results

When all the studies were included in the analysis examining fermented soyfoods, the results were the reverse of what was expected. Compared with a low intake of fermented soyfoods, a high intake was associated with a decreased risk of stomach cancer. When the largest study (which found a reduced risk) was removed from the analysis a high intake of fermented soyfoods was then associated with an increased risk of stomach cancer.

A high intake of non-fermented soyfoods was associated with a decreased risk of stomach cancer (relative risk 0.72, 95% confidence interval 0.63 to 0.82) compared with a low intake.

However, fermented and non-fermented soyfood consumption may be associated with a high consumption of salt and fruit and vegetables respectively. Further analysis revealed that, within the same studies, the risk of stomach cancer associated with fermented soyfoods and a high salt intake followed a similar pattern. Similarly, the risk of stomach cancer associated with non-fermented soyfoods and a high fruit and vegetable intake followed a similar pattern.

Comment

Given the limited information on each study, there is no apparent explanation for the largest study finding the unexpected association between fermented soyfoods and stomach cancer risk. If we accept the results of the re-analysis, they suggest that a high intake of fermented soyfoods increases the risk of stomach cancer, whereas a high intake of non-fermented soyfoods lowers the risk. However, looking at the data more closely showed that consumption of fermented and non-fermented soyfoods may be correlated with high salt and fruit and vegetable intake respectively. Salt and fruit and vegetable intake were not adjusted for in these studies and so the apparent effects of soy on stomach cancer risk may be due to these associations. Until studies consider these other dietary factors, the role of soyfoods in the aetiology of stomach cancer remains uncertain.

However, dietary advice in relation to lowering stomach cancer risk is not wholly dependent upon the outcomes of future studies. The findings from this meta-analysis is consistent with current guidelines, i.e. decrease salt intake (which would include fermented soyfoods such as soy paste or miso, which have a high salt content); increase fruit and vegetable consumption; and try replacing some animal protein with vegetable protein (which would include non-fermented soyfoods such as tofu).