Composition variability in soy-derived dietary supplements designated for menopausal symptom prevention

Abstract

An increasing number of soy isoflavone supplements are commercialized and many investigations are conducted to define their real impact on human health. The presence of other compounds (soyasaponins, phytosterols, polyunsaturated fatty acids…) is rarely considered when discussing the bioactivity of an isoflavone-enriched product. Moreover, the process used can modify the phytochemical content and composition of the final product. This report evaluated the variability in content and composition of isoflavones as well as soyasaponins, proteins, fatty acids and α-galactooligosaccharides of 25 soy based dietary supplements. For isoflavones and soyasaponins, analyzed by high performance liquid chromatography coupled with an ultraviolet detector (HPLC/UV), the ‘intra product’variability was investigated by analyzing two different lots of five of these 25 dietary supplements. Proteins were determined through the quantification of total nitrogen by an elementar analyzer, fatty acids by gaz chromatography, and α-galactooligosaccharides were analyzed using a refractive index detector.These components showed a high variability: the total isoflavone contents, expressed as aglycone equivalents, varied from 4.4 mg/g (16.7 lmol/g) to 95.3 mg/g (365.6 lmol/g), and the isoflavones/ soyasaponins ratio varied from 0.9 (more saponins than isoflavones) to 12.9. In the same way, the protein contents ranged from 0.4 to 42.9%, and the lipid contents from 1.6 to 20%. A high variability was also observed in the profiles of these metabolites. All these differences allowed us to distinguish two main classes of dietary supplements; the whole seed based products, with genistein occurring as the major isoflavone, and the soy germ based products, with a low genistein but high glycitein content. Soy germ and whole seed based products displayed also very contrasted profiles for the other components. An additional variability, more related to the process used, was detected when the conjugation profiles were taken into account, as both heat and pH can selectively affect isoflavone and soyasaponin conjugation. Analyzing different lots revealed the importance of raw material and process cumulated variabilities: total isoflavones displayed large decreases (30%) or increases (17%), and soyasaponin contents varied from 80% decrease to 30% increase. The characterization of the complex phytochemical mixture of a natural dietary supplement needs to be clarified since some active compounds may be responsible for additive, synergistic or antagonist effects only attributed to isoflavones. At least, given its influence on the end product composition, the information about the raw material origin (germ or whole seed) is of great interest.