Hypothesis

Lactonifactor spp. act as a keystone bacterium that enables daidzein‑to‑equol conversion by producing lactate and acetate that feed butyrate‑generating taxa such as Eubacterium coprostanoligenes and Coprococcus. In young adults with high microbial diversity, Lactonifactor abundance supports a cooperative network that channels carbon flux toward equol synthesis. With aging, Lactonifactor declines, disrupting this cross‑feeding and shifting the community toward pro‑inflammatory taxa, thereby lowering equol‑producer prevalence despite adequate daidzein intake.

Testable predictions

1. Cross‑sectional correlation – In a cohort of adults aged 20‑90, fecal Lactonifactor relative abundance will be positively associated with equol‑producer status (binary) and with urinary equol/daidzein ratio, independent of total daidzein intake.
2. Age interaction – The slope of the Lactonifactor–equol relationship will weaken after age 65, reflecting a threshold where Lactonifactor loss outweighs substrate availability.
3. Prebiotic intervention – Administration of a galactooligosaccharide (GOS) supplement known to stimulate Lactonifactor growth will increase its abundance by ≥1.5‑log units within 4 weeks and convert ≥30% of baseline non‑producers to equol producers in participants >65 y.
4. Metabolite flux – In vitro co‑cultures of Lactonifactor strain with E. coprostanoligenes will show elevated lactate consumption and increased equol yields compared with monocultures, demonstrating a cross‑feeding mechanism.

Novel mechanistic reasoning

Unlike Slackia isoflavoniconvertens, which directly reduces daidzein, Lactonifactor lacks the isoflavone‑reductase gene cluster but encodes high‑affinity lactate dehydrogenase and acetate kinase pathways. We propose that Lactonifactor’s primary role is to scavenge excess lactate produced by primary fermenters (e.g., Bifidobacterium spp.) and to release acetate and formate as by‑products. These metabolites lower colonic pH and serve as substrates for butyrate‑producing genera that, in turn, maintain redox conditions favorable for the NADH‑dependent isoflavone reductase activity of Slackia. Thus, equol production emerges from a metabolic chain: primary fermenters → Lactonifactor (lactate sink) → SCFA producers (butyrate) → Slackia (equol synthase). Age‑related depletion of Lactonifactor breaks this chain, causing lactate accumulation, pH rise, and inhibitory effects on Slackia activity, even if Slackia remains detectable.

Falsifiability

If longitudinal sampling shows no change in Lactonifactor abundance preceding shifts in equol status, or if GOS supplementation fails to raise Lactonifactor levels or equol conversion, the hypothesis would be refuted. Similarly, if co‑culture experiments reveal no lactate‑dependent enhancement of equol production, the proposed cross‑feeding mechanism would be invalid.

References

[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC12918213/ [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC12515389/