We are pleased to introduce a new series here on The Biome Blog called Ask the Expert with Chief Science Officer Elizabeth DuPriest, Ph.D. Have a question about vaginal biome science? Submit yours here and your answer may be featured in an upcoming newsletter.
Q: Why is lactic acid an ingredient in some vaginal products?
A: Having an acidic vaginal pH (3.5 to 4.5) maintains the health of the vaginal epithelium, prevents bacterial vaginosis, and reduces risk of infection by pathogenic bacteria and viruses like chlamydia and HIV.
Lactic acid is the main contributor to a low vaginal pH in a healthy vaginal microbiome: lactic acid concentrations correlate with vaginal pH with an r2 of between 0.91 and 0.97 (O’Hanlon 2013 and O’Hanlon 2019). Lactic acid is produced as a byproduct of glycogen metabolism by Lactobacillus and related healthy microbes, and also by some unhealthy microbes, though to a lesser degree.
The more lactobacilli you have, the more lactic acid you have and the lower the vaginal pH in general. In addition, different strains and species of Lactobacillus have varying tolerance for acid; the exact type of bacteria a woman has is the primary determinant of her vaginal pH.
Lactic acid inactivates HIV both by its lowering of vaginal pH and through more direct actions (Aldunate 2013), whereas other acidic ingredients (like citric acid) will lower pH but without the direct action against pathogens. Moreover, the ability of lactobacilli to produce lactic acid seems to be limited by environmental pH…when they sense the pH is low enough, they stop producing lactic acid.
Based on this, it is believed that artificially lowering vaginal pH using a different acid may actually prevent lactobacilli from producing this natural pathogen-fighting lactic acid. Products designed for use in the vagina should have lactic acid in concentrations that match vaginal fluid, so as not to disrupt the vaginal microbiome. In addition, avoiding other acids like citric or ascorbic acid is important to allow Lactobacillus to function their best. Products containing lactic acid can even help to correct the lactic acid and pH levels in vaginal fluid of women who don’t have enough lactobacilli naturally. With time, this can help adjust the overall vaginal microbiome and health of vaginal tissues.
Aldunate M, Tyssen D, Johnson A, Zakir T, Sonza S, Moench T, Cone R, Tachedjian G. Vaginal concentrations of lactic acid potently inactivate HIV. Journal of Antimicrobial Chemotherapy. 2013;68(9):2015-25. https://doi.org/10.1093/jac/dkt156.
O’Hanlon DE, Moench TA , Cone RA. Vaginal pH and microbicidal lactic acid when lactobacilli dominate the microbiota. PLOS ONE. 2019;19(1):13. https://doi.org/10.1186/s12866-019-1388-8.
O’Hanlon DE, Cone RA, Moench TA. Vaginal pH measured in vivo: lactobacilli determine pH and lactic acid concentration. MBC Microbiology. 2019;19(1):13. https://doi.org/10.1186/s12866-019-1388-8.