XII. 3. Hormonal Changes (Pregnancy, Menopause)

At the hormonal turning points of pregnancy and menopause the gut microbiota shifts too, and through the estrobolome it feeds back on circulating estrogen.

How Gut Microbes Influence and Respond to Endocrine Shifts

Pregnancy and menopause induce profound hormonal shifts that reshape the gut microbiota, impacting metabolism, immunity, and overall health [24] [144].

The gut microbiota undergoes systematic changes across reproductive life stages in women that were characterized in detail by Koren and colleagues in a landmark paper published in Cell in 2012. Longitudinal stool samples from pregnant women showed that the gut microbiota changed substantially between the first and third trimesters: microbial diversity decreased, Proteobacteria and Actinobacteria increased, and the microbiota shifted toward a configuration that, when transplanted into germ-free^[G] mice, produced metabolic phenotypes – increased adiposity, insulin resistance, and higher inflammatory tone – resembling those seen in non-pregnant hosts with metabolic syndrome. [298] The interpretation was not that third-trimester microbiota is pathological, but that it serves the energetic demands of late pregnancy: the metabolic phenotype that would be disease-promoting in a non-pregnant host is adaptively calibrated in late pregnancy to maximize maternal energy extraction from food and support fetal growth. The hormonal drivers of these changes – progesterone, estrogen, and placental lactogens – act through hormone response elements in gut epithelial cells and through systemic effects on immune regulation that alter the intestinal environment. [24] In menopause, estrogen decline produces changes in the gut microbiota that include reduced Lactobacillaceae and altered Bacteroidetes/Firmicutes ratios in observational studies. Hormone replacement therapy has been shown to partially normalize postmenopausal microbiota composition toward premenopausal patterns in small trials. The "estrobolome" – the collection of gut microbial enzymes capable of deconjugating estrogen metabolites – links gut microbiota directly to circulating estrogen levels, creating a bidirectional regulatory relationship between the microbiota and the hormonal environment. [39] For clinical management, the implication is that hormonal life-stage transitions are also microbiota transitions, and that dietary and lifestyle support for microbiota stability during pregnancy and perimenopause may modulate the secondary hormonal and metabolic effects of these transitions.

The influence of pregnancy on the gut microbiota was systematically characterized by Koren and colleagues in a study published in Cell in 2012, which followed 91 pregnant women from the first to the third trimester and found dramatic shifts in gut microbiota composition across pregnancy – comparable in magnitude to the differences seen between healthy individuals and those with metabolic disease. By the third trimester, pregnancy-associated microbiota were enriched in Proteobacteria and Actinobacteria and showed reduced alpha-diversity^[G]. When these third-trimester microbiota were transferred to germ-free mice, the mice developed increased adiposity and insulin resistance – raising the question of whether third-trimester microbiota changes, while inflammatory in profile, serve adaptive functions for maternal energy storage and fetal provisioning. [298] The hormonal drivers of pregnancy microbiota shifts include elevated progesterone (which slows gut motility, prolonging microbial fermentation time and allowing community shifts), elevated estrogen (which directly modulates microbial gene expression through estrogen receptor signaling), and the immunological adaptation of pregnancy (which shifts immune tone toward tolerogenic phenotypes that also affect microbial colonization patterns). [24] The menopause transition produces the complementary picture: the decline in estrogen is associated with gut microbiota shifts toward lower Lactobacillus abundance, reduced microbial diversity, and a microbiota profile that increasingly resembles that of males – consistent with the known estrogen-modulating effects on gut microbiota composition. Postmenopausal women show higher relative Bacteroidetes and lower Firmicutes than premenopausal women matched for diet and BMI. The gut microbiota shift at menopause has been proposed as a contributing mechanism to the postmenopausal increase in cardiovascular disease risk, metabolic disease, and inflammatory conditions. [39] [39]

Pregnancy and menopause mark significant endocrine transitions in a woman’s life, and the gut microbiota participates in these processes in ways that are increasingly recognized by research. The intestine and its microbial community continuously interact with host hormones, immune signals, and metabolic pathways, making them part of a broader physiological dialogue rather than passive bystanders [144].

During pregnancy, multiple studies document changes in the composition and function of the gut microbiota as gestation progresses, often including increases in members of the Pseudomonadota (formerly Proteobacteria) and Actinomycetota (formerly Actinobacteria) phyla and variation in diversity across trimesters. These patterns appear correlated with systemic adaptations in energy metabolism and immune regulation, although the precise causal pathways remain a subject of further research. Environmental factors such as diet, antibiotics, and body weight also shape the microbiota during pregnancy and intersect with endocrine signals.

The maternal microbiota influences early microbial exposures for the infant. While the newborn’s gut microbiota is established through a combination of maternal vaginal, skin, environmental, and breast milk microbes, there is evidence that gut-associated bacteria contribute to this initial colonization and that variation in these early communities can have lasting consequences for immune development and metabolic programming.

In the menopausal transition, when estrogen and progesterone production from the ovaries declines, research suggests there are corresponding shifts in gut microbial composition and diversity. Large population studies indicate that postmenopausal women tend to exhibit lower microbial diversity and altered community structures, with patterns that in some studies resemble those seen in men. These associations are likely mediated by hormonal influences on gut physiology, immune function, and bile acid signaling, although the exact mechanisms are still being defined.

A critical concept in this context is the estrobolome, the subset of gut microbial genes and taxa capable of producing enzymes such as β-glucuronidase that act on estrogen conjugates in the enterohepatic circulation. By modifying how estrogen metabolites are processed in the gut, the estrobolome has the potential to influence systemic levels of bioactive hormones, a mechanism that is plausible and supported by enzymology studies, but whose clinical implications for menopausal symptoms and disease risk continue to be investigated.

The relationship between hormonal changes and the microbiota is bidirectional. Shifts in sex hormones alter gut transit, mucosal immunity, and bile acid pools, all of which can shape microbial communities. In turn, microbial metabolism can influence host hormone availability and immune signaling. This feedback loop does not operate in isolation; factors such as diet, lifestyle, and genetics modulate both microbial and endocrine responses.

Supporting microbial resilience across these life stages — through balanced nutrition, physical activity, and judicious medication use — may contribute to more stable metabolic and inflammatory profiles, although it is important to recognize that microbial changes are part of broader physiological shifts. Ongoing research aims to clarify how these interactions operate at the molecular level and how they might be leveraged for health optimization during pregnancy, menopause, and aging.

How to Support Microbiota During Hormonal Shifts

Support during pregnancy and menopause is best built on consistent dietary patterns that provide natural fibers, resistant starches, and a broad spectrum of plant compounds rather than single supplements.

Fermented foods such as yogurt, kefir, or traditionally prepared vegetables can be viewed as cultural sources of microbial signals, helping maintain SCFA-related functions without replacing balanced nutrition.

Regular, moderate physical activity – often described as Zone 2 movement – contributes to intestinal motility and metabolic flexibility, factors that indirectly shape microbial communities.

Medication use deserves careful attention; antibiotics and long-term acid suppression may influence the estrobolome and gut ecology, making periodic therapeutic review an important part of care.

Diets dominated by ultra-processed products tend to narrow microbial diversity, whereas meals based on minimally processed ingredients create a more permissive environment for beneficial taxa.

Polyphenol-rich foods, including berries, green tea, and colorful vegetables, act as microbial substrates and signaling molecules, supporting anti-inflammatory metabolic pathways.

Probiotics may have a role in selected situations, yet their effects are strain- and context-dependent and should complement, not replace, lifestyle foundations.

Psychological stress modifies gut function through neuroendocrine routes; practices that stabilize the stress response can therefore have secondary microbial benefits.

Sleep regularity helps synchronize host circadian rhythms with microbial activity, highlighting the importance of temporal structure in gut physiology.

Collaboration with healthcare professionals allows dietary and lifestyle measures to be adapted to the individual hormonal phase and clinical background, recognizing that responses differ widely.

Microbiota Effects

• Pregnancy is associated with functional and compositional microbiota shifts that support fetal growth; however, when these changes are accompanied by obesity, high-glycemic diets, or antibiotic exposure, they may correlate with a higher risk of gestational diabetes rather than acting as a sole cause [298] [298].
• During menopause, many women show reduced abundance of SCFA-producing taxa (e.g., Faecalibacterium, Roseburia) and lower overall diversity, changes that may contribute to a pro-inflammatory milieu but interact with age, adiposity, and lifestyle factors [39] [24].
• The estrobolome—microbes with β-glucuronidase activity—participates in enterohepatic estrogen recycling; its altered activity can influence circulating estrogen fractions, yet direct clinical effects on bone or mood remain associative rather than proven.
• Hormonal fluctuations can modify mucosal immunity and tight-junction regulation, creating variable intestinal permeability; this is a spectrum of mechanisms rather than a single entity termed “leaky gut.”
• Decreased SCFA availability may weaken epithelial energy supply and regulatory T-cell signaling, affecting the gut–immune axis, but symptom severity depends on host genetics and diet.
• Evidence linking Veillonella expansion to exercise derives mainly from athletic cohorts; in pregnancy and menopause its relevance is plausible but not established as a therapeutic target.
• Microbial metabolism of dietary phytoestrogens (lignans → enterolignans; isoflavones → equol) shows marked inter-individual variability, determined by the presence of specific bacterial converters.
• The gut–brain axis^[G] is influenced by microbial metabolites (SCFAs, tryptophan^[G] derivatives), yet the clinical value of so-called psychobiotics during hormonal transitions is modest and strain-specific.
• Effects on mineral absorption involve multiple systems; while microbiota may influence calcium and magnesium handling, osteoporosis risk is primarily driven by endocrine and mechanical factors.
• Data on probiotics improving vaginal or urogenital microbiota in menopause are heterogeneous; benefits appear formulation-dependent and should not be generalized to all products.
• Fungal (mycobiome) and viral (virome) communities also shift with hormonal states, but human evidence is limited and currently insufficient for targeted clinical manipulation.

Patient Guidance

• Try to build your meals around natural fiber sources like vegetables, legumes, oats, and whole grains rather than relying on supplements.
• Include a small portion of fermented food (yogurt, kefir, sauerkraut) several times a week if you tolerate them comfortably.
• Aim for moderate, regular movement such as brisk walking, cycling, or swimming on most days, at a pace where you can still talk.
• Avoid taking antibiotics unless clearly necessary, and discuss timing and alternatives with your doctor during sensitive hormonal periods.
• Choose everyday foods with polyphenols and healthy fats—berries, olive oil, fish, nuts—more often than processed snacks.
• Use simple stress-relief habits like slow breathing, short walks, or gentle stretching to keep the gut–brain axis more stable.
• Protect your sleep rhythm by keeping similar bedtimes and wake times, aiming for about 7–8 hours when possible.
• Consider probiotics only after professional advice; benefits are individual and strain-specific, not universal.
• Limit ultra-processed foods and sweetened drinks that tend to narrow microbial diversity.
• Keep an eye on bone and metabolic health with regular check-ups while supporting them through food and movement rather than pills alone.

References

[24] Sonnenburg JL, Bäckhed F. Diet–microbiota interactions as moderators of human metabolism. Nature. 2016. Link

Review of mechanisms linking the gut microbiota to obesity and type 2 diabetes drawing on translational animal models and human studies. The microbiota emerges as a mediator of dietary impact on host metabolic status, with growing efforts to establish causal relationships in people and develop therapeutic interventions including personalised nutrition.

[39] Koh A, De Vadder F, Kovatcheva-Datchary P, Bäckhed F. From Dietary Fiber to Host Physiology: Short-Chain Fatty Acids as Key Bacterial Metabolites. Cell. 2016. Link

Mechanistic review of short-chain fatty acids (SCFAs) — a major class of bacterial metabolites derived from dietary fibre fermentation — as central mediators between diet, gut microbiota, and host physiology. SCFAs activate G-protein-coupled receptors, inhibit histone deacetylases, and serve as energy substrates, thereby influencing metabolic, immune, and epigenetic processes. The authors synthesize evidence implicating SCFA-mediated signalling in both health maintenance and disease pathogenesis. Provides the conceptual backbone for fibre-based and microbiome-directed therapeutic strategies.

[144] Cani PD, Amar J, Iglesias MA et al. Metabolic endotoxemia initiates obesity and insulin resistance. Diabetes. 2007. Link

Cani and colleagues' 2007 Diabetes paper introduced the concept of 'metabolic endotoxemia' as a microbiota-driven trigger of obesity and insulin resistance. In mice, they show that a high-fat diet increases intestinal permeability and circulating lipopolysaccharide (LPS) levels, which activate TLR4-CD14 signalling and induce low-grade inflammation in adipose tissue, liver and muscle. Chronic subcutaneous LPS infusion in mice was sufficient to reproduce diet-induced obesity, insulin resistance and hepatic steatosis. CD14-knockout mice were protected. The paper established a mechanistic axis linking gut microbiota, barrier function and metabolic disease that has shaped subsequent obesity-microbiome research.

[298] Koren O, Goodrich JK, Cullender TC et al. Host remodeling of the gut microbiome and metabolic changes during pregnancy. Cell. 2012. Link

This study characterized faecal bacteria in 91 pregnant women of varying prepregnancy BMIs and gestational diabetes status and their infants. Mother-infant microbiota similarity increased with child age; infant microbiota was unaffected by maternal health. Maternal gut microbiota changed dramatically from first (T1) to third (T3) trimesters, with increased between-mother diversity, an overall increase in Proteobacteria and Actinobacteria, reduced richness, and the strongest inflammation and energy-loss signals in T3. Microbiome gene repertoires remained constant. The findings document a programmed third-trimester dysbiosis resembling metabolic syndrome.