Soy isoflavones sound like a laboratory term, yet they have quite a colourful history: one was named after the Japanese word for soy, and another was discovered in a European dyeing shrub. In Zielona Góra we make natto from fermented soy, and isoflavones are one of the natural components of that grain. Here we have gathered what is known for certain: where their names come from, how fermentation changes them, and what the scientific literature says about them.
⚠️ Are you taking anticoagulant medication (for example acenocoumarol or warfarin)? Natto is one of the richest natural sources of vitamin K2, which may affect how these medicines work. Consult your diet and supplements with a doctor. See the important legal and medical information.
In brief
- Soy isoflavones are natural polyphenols from the phytoestrogen group, mainly daidzein, genistein and glycitein.
- In soy they occur in a form bound to sugar, and during natto fermentation some of them convert into aglycones, that is free forms.
- The name daidzein comes from the Japanese word for soy, and genistein was discovered in the dyeing shrub dyer’s greenweed.
- In the European Union, soy isoflavones have no authorised health claim (Regulation (EC) 1924/2006), which is why we describe them in terms of structure, origin of names, and what has been measured in research.
What isoflavones are and where their names come from
Isoflavones are natural polyphenols present in soy. They are classified as phytoestrogens because their structure resembles estrogens, hence the very name: the Greek “phyton” means plant. The three most frequently described are daidzein, genistein and glycitein, and their names have interesting origins. Daidzein took its name from “daizu”, the Japanese word for soy. Genistein was first isolated at the end of the 19th century from dyer’s greenweed (Genista tinctoria), a shrub used in Europe to dye fabrics yellow, and it is from its Latin name that the word genistein was formed.
What fermentation does to isoflavones
In raw soy grain, isoflavones are attached to a sugar molecule, that is they occur as glycosides. During natto fermentation, the bacteria Bacillus subtilis natto detach this sugar residue and aglycones are formed. The name itself says what it is about: “aglycone” means in Greek a form “without sugar” (the prefix “a” means absence, “glykys” means sweet). It is still the same family of compounds, only in free form. The literature describes that aglycones differ from the bound forms in the way they are absorbed. This is a description of structure and process, not a declaration of effect.
What the individual studies describe
The aglycone form and fermentation
In studies by Ping et al. (2012) and Xiang et al. (2022) it was described that natto fermentation by various Bacillus subtilis cultures affects the content of isoflavones in aglycone form and the antioxidant potential of the samples as measured in the laboratory. These are results of laboratory analyses concerning the composition and properties of the samples, not clinical studies in humans.
Observational studies in humans
In the Takayama cohort study (Nagata et al., 2017), the relationship between soy and natto intake and mortality in the Japanese population was analysed. This is an observational study, which describes correlations in a specific population, does not prove a causal relationship and does not justify attributing any specific effect to natto. The results should not be generalised directly to other populations.
Why the response can vary
The individual response to isoflavones can vary, partly because of gut microflora. Some of the daidzein is converted by intestinal bacteria into a compound called equol, but only part of the population has the ability to do this. This is one of the reasons why research results on isoflavones are so inconclusive.
Isoflavones in our natto
Isoflavones are a natural component of all soy, so you will find them in all our natto. Fresh natto says most about them, because it is simply soy straight after fermentation, without further processing. We make it in Zielona Góra from certified organic soy.
See our fresh natto in the shop
Safety and precautions
Natto is a food safe for most healthy people. Because isoflavones are phytoestrogens, and natto is rich in vitamin K2, in several situations it is worth being cautious and discussing soy intake with a doctor.
This applies in particular to people taking anticoagulant medicines (for example vitamin K antagonists such as warfarin or acenocoumarol), because of the vitamin K content in natto. Caution is also advisable with hormone-dependent conditions or after experiencing them, and with thyroid disorders, especially when soy intake is high. Natto contains soy, so it is not suitable for people with a soy allergy. Neither a food supplement nor any food product replaces a varied diet and a healthy lifestyle.
Summary
Soy isoflavones are phytoestrogens naturally present in soy, and their names lead from Japanese soy to a European dyeing shrub. During natto fermentation, some of them convert into the aglycone form. The available data comes mainly from laboratory and observational studies, which do not prove a causal relationship with any specific effect in humans. Natto is worth treating as part of a varied diet, not as a medicine.
Frequently asked questions
Where do the names daidzein and genistein come from?
Daidzein took its name from “daizu”, the Japanese word for soy. Genistein was isolated in the 19th century from dyer’s greenweed (Genista tinctoria), a European shrub used for dyeing fabrics, and it is its Latin name that gave it its designation.
What are isoflavone aglycones?
They are free forms of isoflavones, which are formed when the sugar residue attached to them is detached during fermentation. The word “aglycone” means “without sugar” in Greek. The literature describes that they differ from the bound forms in the way they are absorbed.
How many isoflavones does a portion of natto contain?
A portion of natto (about 50 g) provides isoflavones, largely in the aglycone form. The exact amount depends on the raw material and the course of fermentation.
Do soy isoflavones have confirmed effects in the European Union?
No. Soy isoflavones have no authorised health claim in the European Union, which is why a reliable description is limited to structure, origin and what has been measured in research, not to effects. The available data comes mainly from laboratory and observational studies.
Who should be cautious about soy intake?
Caution and consultation with a doctor are worth considering with hormone-dependent conditions, thyroid disorders and when taking anticoagulant medicines. Natto contains soy, so it is not suitable for people with a soy allergy.
Sources
- Xiang A. et al. Determination of 14 Isoflavone Isomers in Natto by UPLC-ESI-MS/MS and Antioxidation and Antiglycation Profiles. Foods. 2022;11(15):2229. Link
- Ping SP. et al. Effect of Isoflavone Aglycone Content and Antioxidation Activity in Natto by Various Cultures of Bacillus subtilis During the Fermentation Period. J Nutr Food Sci. 2012. Link
- Nagata C. et al. Dietary soy and natto intake and cardiovascular disease mortality in Japanese adults: the Takayama study. Am J Clin Nutr. 2017;105(2): 426-431. Link
- Regulation (EC) No 1924/2006 and Regulation (EU) No 432/2012; EU Register of Nutrition and Health Claims.
Legal and medical note. This article is educational and does not replace the advice of a doctor or pharmacist. The substances described here have no authorised health claims in the European Union (Regulation (EC) 1924/2006), and a food supplement is not a medicine and does not replace a varied diet. Full information is on the Legal and medical information page.
