L. iners on the other hand, only produces L-lactic acid. Furthermore, L. iners does not have the ability to produce hydrogen peroxide. It has been suggested that the production of hydrogen peroxide serves as one of the mechanisms by which Lactobacillus species inhibit the colonization of anaerobic bacteria in the vaginal environment. This evidently demonstrates the relative inadequacy of L. iners, despite being the second most abundant species of vaginal lactobacilli, in preserving vaginal health, particularly when contrasted with L. crispatus.

But the science of lactic acid does not stop there. Depending on a pH value, lactic acid (LA) can be in protonated (H⁺) or deprotonated form. A study found that the protonated lactic acid has anti-inflammatory effect on wall cells of cervix and vagina. Both D- and L-lactic acid, when in protonated form, had the ability to suppress inflammation. Therefore, not just isomeric but also ionization status of lactic acid has a significant role to play in biological activity of lactic acid. It was also shown that low vaginal pH (≤3.8) enhances diffusion of lactic acid through vaginal wall cells. Once in the cell, lactic acid gets deprotonated, and cell wall channels pump that proton outside the cell, making the vaginal environment acidic. The relationship between lactic acid and vaginal health is evidently complex, with the influence of lactic acid transcending mere reduction of vaginal pH levels.

Infectivity of HIV virus reduced significantly by the effect of L-lactic acid at pH 3.8, but not with the same concertation of L-lactic acid and Sodium L-lactate, at pH 7.0 (adjusted from Aldunate et al., 2013).   

Hydrogen peroxide and vaginal pH

In a laboratory experiment that closely mimicked vaginal environment, scientists found that hydrogen peroxide did not inactivate BV-causing bacteria (Gardnerella vaginalis, Prevotella bivia, Prevotella corporis, and others). Hydrogen peroxide only expressed activity when high concentrations were applied, which had a toxic effect toward vaginal lactobacilli. Moreover, hydrogen peroxide antimicrobial property was easily inactivated by additional flush of vaginal fluid. In a contrast, in the same experiment, lactic acid was able to demonstrate antimicrobial effect against all BV-causing pathogens. Lactic acid did not negatively influence Lactobacillus bacteria and kept the same effect even with addition of vaginal fluid.

Boric acid and vaginal pH

Boric acid is a weak acid and has pH around 5. It has been used as a home remedy for centuries. Can boric acid and boric acid suppositories help maintain or restore vaginal pH balance?

Though beneficial effect of boric acid on vaginal health has been reported, this effect may not come from the ability of boric acid to restore vaginal pH balance. Rather, it is probably due to antifungal effect of boric acid. In a clinical study on 111 diabetic women who had thrush, boric acid suppositories were especially efficient against Candida glabrata, a member of pathogenic yeasts from Candida family that causes thrush.

Vinegar and vaginal pH

Vinegar is a popular home remedy, often used to self-treat different vaginal infections. Vinegar contains acetic acid, which is a weak acid. Even though it is an acid, and can lower vaginal pH, it does not have the same effect as vaginal ‘native’ organic acids, such as lactic acid. The figure below compares the potential of 3 acids under the same pH conditions to reduce the infectivity of a HIV virus. The significant reduction is achieved by far by lactic acid, compared with acetic acid and hydrochloric acid. This laboratory experiment proves that not all acids are the same.

Inactivation of HIV at pH 3.8 with different acids over time (adjusted from Aldunate et al., 2013).  

Activated charcoal and vaginal pH

There is an interesting study where scientists used activated charcoal to improve vaginal health. The hypothesis underpinning this investigation was that porous activated charcoal would exhibit a reduced binding capacity for beneficial bacteria compared to other bacterial species. The findings of the study demonstrated a significant reduction in malodour, a decrease in discharge, and a reduction in pH, with a minimal decrease in lactobacilli (observed in only one out of 32 subjects). Activated charcoal, a finely powdered black substance, is a widely recognized natural remedy. However, it is not advised for prolonged use due to potential interactions with other substances. Given its black hue and substantial potential for staining, the practicality of its application in the vaginal area may impact its usability.

Can you prevent infections with pH balancing suppositories?

Disbalance in vaginal pH is closely related to disbalance in vaginal microbiome and presence of pathogenic organisms.

A study conducted on 494 healthy and asymptomatic Caucasian women found that the disbalance in vaginal microbiome (dysbiosis) is directly related to high vaginal pH and malodorous discharge.

Another clinical study found a significantly decreased lactic acid concentration among patients with bacterial vaginosis (BV), vulvovaginal candidiasis (thrush) and Chlamydia (STI) when compared with healthy women.

 

Log concentration of lactate in vaginal samples of healthy (HC), Chlamydia trachomatis (CT), vulvo-vaginal candidiasis (VVC) and bacterial vaginosis (BV) positive women. Stars indicate significant differences compared with healthy women (adjusted from Ceccarani et al., 2019).  

It is clear that lactobacilli thrive in a low pH milieu, while the growth of potentially pathogenic bacteria is limited in this environment. Based on this fact, as well as clinical connection between disbalanced vaginal pH and microbiome, it has been suggested that acidifying agents would be able to restore a normal vaginal microbiome and prevent future infections. But is this a reality?

It was reported that lactic acid has antimicrobial activity beyond acidity alone. In laboratory environment, lactic acid can disrupt the integrity of bacterial cell membranes (as it can penetrate cell membranes) and even interfere with the ability of pathogens to adhere to host cells. Fascinatedly, in laboratory experiment, vaginal mucus that contained high levels of L. crispatus and D-Lactic acid was able to trap and inactivate HIV-1 virus. But does the same effect happen in real vaginas?

The recent systematic review was looking into the effect of intravaginal lactic acid-containing products and their effect on the composition of vaginal microbiome. Total of 57 articles were reviewed and 7 different products evaluated (which varied in lactic acid concentration and pH). There were various reports, including those where lactic acid was found to be equivalent to the effect of an antimicrobial, inferior to the effect of an antimicrobial, superior in beneficial effect when compared to placebo and equivalent to the effect of placebo. Lactic acid itself did not significantly impact the composition of vaginal microbiome. Therefore, more of high-quality scientific evidence is needed to support the use of lactic acid intravaginally.

In conclusion, many experts in vaginal health concur that simply reducing the pH in the vagina doesn’t guarantee sustained vaginal health. As shown on the example of lactic acid, the relationship between pH and activity of specific organic acids is complex. Only lowering vaginal pH or applying just any acidifying agent is not good enough to ensure vaginal health. As the matter of fact, Candida fungus, responsible for causing vaginal thrush, can quite comfortable live in acidic environment. A clinical study involving 21 healthy and 21 women with thrush did not find any significant difference in their vaginal pH (most of the thrush infected women were asymptomatic).

It is crucial to keep in mind that the beneficial influence of acidic vaginal pH in inhibiting the overgrowth of potential pathogenic microorganisms is primarily attributed to the predominance of specific Lactobacillus species and their production of lactic acid, hydrogen peroxide and bacteriocins. This underscores the critical role of specific Lactobacillus species in maintaining a healthy vaginal microbiome. Having said that, there are a few other non-lactobacilli species capable of producing lactic acid (Atopobium, Megasphaera and Leptotrichia). In the absence of lactobacilli, these organisms can produce lactic acid and give a modest contribution toward acidic vaginal pH.

What can you do to maintain healthy vaginal pH?

Numerous factors in a woman’s everyday routine can lead to an imbalance of the normal vaginal pH. These include engaging in unprotected sexual intercourse, the use of antibiotics, practices like vaginal douching, and fluctuations in the menstrual cycle.

The concept of a healthy vaginal pH should not be considered in isolation from the existence of a healthy vaginal microbiome. They are interconnected aspects of vaginal health.

As vagina is a self-cleaning organ, it does not require special washing, besides normal bathing with clean water. If you really need to apply a vaginal wash, avoid using soaps and washes which are not pH balanced. Avoid douching and products that contain fragrances and artificial ingredients. The most importantly, support your vaginal microbiome with good quality and scientifically designed vaginal probiotic.

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