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Isoflavones Have Little Impact on Metabolism


Intervention study conducted at the Max Rubner-Institut

Many women use soy extracts containing isoflavones, freely available as nutritional supplements, to alleviate typical menopausal symptoms. During menopause, the body’s own oestrogen levels decrease, which causes changes in the body’s make-up and lipid metabolism, and puts women at a higher risk of cardiovascular disease. Whether soy isoflavones, which are phytoestrogens, can have a positive impact on lipid metabolism and body fat levels is a matter of controversial debate. Therefore, the Max Rubner-Institut launched a 12-week placebo-controlled intervention study involving 170 healthy menopausal women to investigate this question at its own research facility. In this study, which was financed by the German Research Foundation (DFG), the subjects ingested a daily dose of soy extract, the composition and soy isoflavone content of which had been characterised in detail in advance. The study examined the effect on blood lipids such as total cholesterol, LDL cholesterol, HDL cholesterol and triglycerides as well as on the expression of LDLR and CD36 receptors, which are relevant for lipid metabolism and help regulate blood lipids. Using a modern process called the DEXA method, it also analysed the body composition and distribution of body fat prior to and after the study. Furthermore, the scientists captured possible effects on the thyroid by setting important thyroid parameters. Analysing the test subjects’ blood and urine samples, they were able to track the metabolisation of the ingested isoflavones in the body. However, taking soy isoflavone supplements showed no significant effect on blood lipids and expression of LDLR and CD36 receptors; only LDL cholesterol levels in the blood increased slightly. The women’s body weight and body fat levels had no impact on the parameters set.

The way isoflavones are metabolised in the intestines (producing the metabolite equol), which is often associated with the effect of isoflavones, was not relevant. Taking isoflavones had no impact on the women’s body fat levels. Thyroid effects were not observed, either.

In conclusion, this controlled study did not confirm the often-cited positive impact on lipid metabolism or body fat levels in healthy postmenopausal women; but it did not detect any negative effects, e.g. on the thyroid, either.


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