VIII. 2. Corticosteroids

Corticosteroids quiet inflammation powerfully, yet they also thin the gut flora and open the door to fungal overgrowth — fiber and care help buffer the cost.

Corticosteroids and the Gut – Powerful Medicine, Complex Consequences

Corticosteroids suppress inflammation effectively – but the gut pays a price [220].

The gut microbiota effects of corticosteroids were largely overlooked in clinical medicine for decades, obscured by the legitimate therapeutic urgency of the conditions for which they are prescribed. A systematic review by Huang and colleagues published in Frontiers in Microbiology in 2019 brought this gap into focus by analyzing microbiota studies in patients receiving systemic corticosteroids across multiple clinical contexts – inflammatory bowel disease (Inflammatory Bowel Disease: Crohn's disease and ulcerative colitis), rheumatoid arthritis, organ transplantation, and asthma. [220] The consistent finding across populations and steroid types was a reduction in microbial diversity, with relative loss of Faecalibacterium prausnitzii and other butyrate-producing Firmicutes, and relative enrichment of fungi, particularly Candida species. The fungal enrichment is mechanistically expected: corticosteroids reduce mucosal immune surveillance, and Candida, normally present at low abundance, takes advantage of the reduced immune pressure. In patients receiving high-dose or prolonged steroids, clinically significant fungal overgrowth occasionally becomes a secondary complication. [144] An important nuance identified in the review was the distinction between systemic and locally acting corticosteroids. Inhaled steroids in asthma patients showed oral and oropharyngeal microbiota effects – particularly oral Candida enrichment – but smaller gut microbiota effects than systemic preparations. Topical skin preparations showed minimal gut effects. The dose–route–effect relationship is clinically relevant for choosing the minimum effective steroid exposure when options exist. [24] The microbiota-protective strategy during steroid courses involves the same elements relevant to antibiotic protection: high dietary fiber, avoidance of co-administered broad-spectrum antibiotics when possible, and consideration of targeted probiotic co-administration when courses are long or doses are high. The evidence for specific interventions during steroid courses is less developed than for antibiotic courses, but the mechanistic rationale is shared.

Corticosteroids are a class of steroid hormones derived from cholesterol, produced naturally by the adrenal cortex (primarily cortisol) and used clinically in synthetic forms (prednisone, prednisolone, dexamethasone, budesonide, methylprednisolone, and others). Their primary therapeutic value lies in potent anti-inflammatory and immunosuppressive effects, making them first-line or rescue treatments for a broad range of conditions including inflammatory bowel disease (IBD (inflammatory bowel disease: Crohn's disease and ulcerative colitis)), autoimmune disorders, severe allergic reactions, asthma, and post-transplant rejection prevention [24].

The relationship between corticosteroids and the gut microbiota is complex and bidirectional. On one hand, in conditions like active Crohn's disease or ulcerative colitis, corticosteroids reduce the inflammatory environment that itself disrupts microbiota composition. On the other hand, corticosteroids directly alter the intestinal epithelial barrier, mucosal immune function, and microbial community structure through mechanisms independent of the underlying disease.

Systemic corticosteroids increase intestinal permeability^[G]. They suppress tight junction protein expression, reduce mucus secretion, and impair the integrity of the intestinal epithelial layer. This creates conditions where luminal bacteria gain easier access to the mucosal surface, paradoxically increasing bacterial translocation risk even as immune responses are suppressed.

In vitro studies suggest that glucocorticoids may alter bacterial gene expression, membrane integrity, and biofilm formation in some species, though direct antimicrobial effects on the gut microbiota in clinical settings remain incompletely characterised. In vivo, systemic corticosteroid use is associated with characteristic shifts in microbial community composition, generally toward reduced diversity and enrichment of opportunistic organisms.

The immunosuppressive effects of corticosteroids reduce the mucosal immune surveillance that normally contains and shapes the gut microbiota. Secretory IgA production is suppressed, mucosal macrophage and dendritic cell function is altered, and the regulatory T-cell (immune cells that suppress inflammation and promote tolerance) environment of the gut is disrupted. These immune changes modify the selective pressures on microbial communities in ways that favour dysbiotic shifts.

Fungal overgrowth, particularly Candida species, is a recognised complication of corticosteroid therapy. Suppression of mucosal immunity and bacterial community disruption create ecological openings that Candida and other opportunistic fungi can exploit. Oral, oesophageal, and intestinal candidiasis are clinical risks in patients on prolonged systemic corticosteroids.

Locally acting corticosteroids (budesonide, beclomethasone) have reduced systemic absorption and a more targeted anti-inflammatory effect in the gut. Their microbiota impact is generally less pronounced than systemic agents, though not negligible, particularly with prolonged use.

Duration and dose are the primary determinants of microbiota impact. Short courses of corticosteroids for acute conditions produce more limited and partially reversible microbiota changes. Long-term maintenance therapy, particularly at systemic doses, produces more substantial and sometimes persistent dysbiosis.

Managing Microbiota Health During Corticosteroid Therapy

In clinical microbiota care, corticosteroid use is flagged as a significant modifying factor that requires active dietary and lifestyle countermeasures. The goal is not to avoid clinically necessary corticosteroid therapy but to mitigate its microbiota consequences while it is used.

Dietary fiber intake is prioritised during corticosteroid therapy. Maintaining fermentable substrate availability supports residual commensal bacterial activity and SCFA production, partially counteracting the dysbiotic drift caused by corticosteroids. Tolerated fiber-rich foods are encouraged throughout the treatment period.

Fermented foods and probiotic-containing products are considered as complementary supports. While corticosteroids suppress immune responses that normally mediate probiotic-host interactions, maintaining live microbial exposure through diet supports microbial community diversity to the extent the immune environment allows.

Probiotic supplementation during corticosteroid therapy requires clinical judgement. In immunocompromised patients on high-dose systemic steroids, live organism supplementation carries a theoretical risk of opportunistic infection that must be weighed against the microbiota support benefit. In patients on low-to-moderate doses or locally acting agents, probiotic use is generally considered safe and potentially beneficial.

Antifungal prophylaxis is considered in patients on prolonged high-dose systemic corticosteroids, particularly those with prior candidiasis history, diabetes, or concurrent antibiotic use. Oral antifungal agents or reducing dietary sugar and refined carbohydrate intake to limit fermentable substrate for Candida species is recommended; formal antifungal prophylaxis with pharmacological agents is guided by clinical risk assessment.

Tapering corticosteroid doses as rapidly as clinically appropriate reduces cumulative microbiota exposure. Abrupt cessation is avoided for physiological reasons, but the shortest effective course at the lowest effective dose is the standard approach for microbiota protection alongside other clinical considerations.

Microbiota monitoring during prolonged corticosteroid therapy can guide targeted interventions. Stool microbiota analysis at baseline and after treatment initiation allows identification of dysbiotic patterns early enough for dietary or supplemental correction.

Sleep, stress management, and physical activity remain important microbiota supports during corticosteroid therapy. Corticosteroids can disrupt sleep architecture and increase psychological stress through mood effects; maintaining sleep hygiene and stress management practices counteracts these secondary microbiota impacts.

After corticosteroid course completion, microbiota recovery is supported through dietary diversification, increased prebiotic fiber, and structured probiotic or fermented food protocols. Recovery timelines vary by duration and dose of prior corticosteroid exposure.

Microbiota Effects

• Systemic corticosteroid use consistently reduces gut microbial diversity, with particular losses among anaerobic butyrate producers including Faecalibacterium prausnitzii, Roseburia, and Lachnospiraceae members.
• Corticosteroids increase intestinal permeability by suppressing tight junction protein expression and reducing mucus production, creating conditions for increased bacterial translocation despite concurrent immunosuppression.
• Fungal populations, particularly Candida species, expand during corticosteroid therapy due to reduced mucosal immunity and bacterial community disruption, contributing to fungal-bacterial dysbiosis.
• Corticosteroid-associated microbiota shifts include enrichment of Pseudomonadota (formerly Proteobacteria), Enterobacteriaceae, and other opportunistic taxa that thrive under reduced immune surveillance conditions.
• Secretory IgA suppression by corticosteroids reduces immunological containment of luminal bacteria, altering the selective pressure that normally shapes commensal dominance and pathobiont containment.
• Locally acting corticosteroids (budesonide) produce less pronounced systemic microbiota disruption than equivalent anti-inflammatory doses of systemic agents, though luminal effects on mucosal immunity and barrier function persist.
• Microbiota recovery after corticosteroid cessation is partial and time-dependent; some species show rapid rebound while others require months of targeted dietary support for restoration.

Patient Guidance

• Maintain dietary fiber intake throughout corticosteroid therapy to support residual commensal bacterial activity.
• Include fermented foods daily where tolerated to maintain live microbial exposure.
• Discuss probiotic supplementation with your clinician – safety considerations apply at high immunosuppressive doses.
• Reduce sugar and refined carbohydrates to limit fungal overgrowth risk during prolonged courses.
• Monitor for signs of oral or intestinal candidiasis: white patches, increased digestive symptoms, unusual discharge.
• Use the shortest effective corticosteroid course at the lowest effective dose.
• After completing corticosteroids, actively rebuild dietary diversity and consider structured probiotic support.
• Maintain sleep hygiene and stress management practices to counteract secondary microbiota impacts.
• Report new or worsening gut symptoms during corticosteroid therapy to your clinical team.
• Consider stool microbiota monitoring during prolonged therapy if clinically accessible.

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.

[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.

[220] Huang EY, Inoue T, Leone VA et al. Using corticosteroids to reshape the gut microbiota: implications for inflammatory bowel diseases. Inflamm Bowel Dis. 2015. Link

This study delineated dexamethasone-induced changes in the gut microbiota and host mucin regulation in adult male C57Bl/6, germ-free, Muc2± and Muc2-/- mice over a 4-week treatment. Faecal microbiota were profiled by 16S rRNA T-RFLP and amplicon sequencing; intestinal mucosa was analysed for mucin gene expression. Glucocorticoid exposure reshaped microbiota composition and impaired mucin regulation, with downstream effects on colonic inflammation. The findings indicate that pharmacological glucocorticoids modulate gut homeostasis partly through microbiota-mucus interactions.