Vitamin D Meets Your Microbiome

Vitamin D, often referred to as the "sunshine vitamin," is synthesized in the skin when UVB rays convert 7-dehydrocholesterol into previtamin D3, which then becomes vitamin D3. This process is affected by factors such as season, latitude, skin pigmentation, sunscreen use, and age. Once produced, vitamin D is metabolized in the liver to form 25-hydroxyvitamin D, the primary circulating form used to assess vitamin D status. It is further converted in the kidneys to 1,25-dihydroxyvitamin D, the biologically active form that regulates calcium and phosphate metabolism and supports numerous cellular functions throughout the body (Wacker et al., 2013). 

The gut is home to a large portion of the body’s immune defenses, including both the innate immune system, which involves local immune cells that reside in the gut lining, and the systemic immune system, which includes cells like dendritic cells that help regulate broader immune responses (Wiertsema et al., 2021). Nutrients found in seasonal produce—such as fiber, antioxidants, and vitamins like vitamin D—can influence the activity of these immune cells. Vitamin D, in particular, interacts with dendritic cells to help maintain immunity.

In an observational study of individuals with inflammatory bowel disease (IBD), researchers examined the relationship between sun exposure habits and vitamin D status. A majority of participants (69%) had deficient or insufficient serum 25-hydroxyvitamin D (25(OH)D) levels, a concern given the role of vitamin D in immune regulation and inflammation. Additionally, 67% reported low sun exposure based on questionnaire data. A modest but statistically significant correlation was observed between total sun exposure scores and serum 25(OH)D concentrations, especially during the summer months. However, sun protection behaviors—such as sunscreen use—did not significantly influence vitamin D levels. Multivariate analysis revealed that active IBD symptoms were the only independent factor associated with low sun exposure (Olmedo-Martín et al 2019). It's important to consider lifestyle and disease activity when evaluating vitamin D status in patients with IBD.

Conclusions can be drawn regarding seasonal fluctuations in vitamin D levels and gut microbiota composition in inflammatory bowel disease (IBD). A study with a cohort of patients found a link between reduced sunlight exposure and microbial shifts. In a cohort of 74 individuals with Crohn’s disease (CD) and ulcerative colitis (UC), serum 25-hydroxyvitamin D levels were significantly lower during winter and spring, corresponding to reduced sun exposure in central European regions. These lower vitamin D levels were associated with altered gut microbiota profiles, including increased abundance of potentially pro-inflammatory bacterial groups such as Actinobacteria and Proteobacteria. In contrast, during the summer and autumn months—when vitamin D levels were higher—there was a reduction in inflammation-associated genera like Fusobacterium, Eggerthella, and Helicobacter, alongside increases in potentially beneficial taxa including Clostridium and Pediococcus (Soltys et al., 2020). Seasonal changes in vitamin D status may influence gut microbial composition in IBD, potentially impacting disease activity.

In vitamin D-deficient individuals, including those with and without ulcerative colitis (UC), an 8-week course of vitamin D supplementation significantly raised serum 25(OH)D levels. Among patients with active UC, this was associated with reduced intestinal inflammation, as shown by lower faecal calprotectin and improved systemic markers. While overall microbial diversity did not change, an increase in Enterobacteriaceae was observed in UC patients (Garg et al, 2018). This shift may reflect an immune-related change in microbial composition during reduced inflammation, though the clinical relevance of this specific increase remains unclear and warrants further investigation.

In a study of otherwise healthy vitamin D-deficient women, vitamin D supplementation significantly improved gut microbial diversity and altered microbiota composition. Increases in the Bacteroidetes to Firmicutes ratio and higher levels of beneficial genera such as Akkermansia and Bifidobacterium were observed. Distinct shifts in enterotypes, particularly in the dominant genera Bacteroides and Prevotella, were noted after supplementation. Greater microbial changes occurred in individuals who responded well to supplementation, while non-responders showed a decrease in Bacteroides acidifaciens (Singh et al., 2020). Vitamin D can positively modulate the gut microbiome and that individual microbial signatures may influence supplementation response.

To observe the effects of vitamin D intake, a study with healthy young adults explored the relationship between vitamin D levels, gut microbiota, and inflammation. Higher vitamin D intake was associated with distinct microbiota profiles, including greater abundance of Prevotella and lower levels of Haemophilus and Veillonella. Lower circulating vitamin D was linked to higher lipopolysaccharide levels, suggesting increased gut permeability. Inverse correlations were found between 25(OH)D and inflammatory markers (PCR, E-selectin) as well as certain gut bacteria, including Coprococcus and Bifidobacterium. However, these associations weakened after adjusting for inflammation, indicating that inflammatory status may mediate the relationship between vitamin D and gut microbiota (Luthold et al., 2017).

Supplementing vitamin-D may help support a healthier gut by increasing microbial diversity and boosting beneficial bacteria like Akkermansia and Bifidobacterium. Individual gut microbiome differences may influence how well you respond, so paying attention to your gut health could help you get more from vitamin D. Monitoring your gut microbiome could help provide insights into vitamin status (Singh et al., 2020).