Ulcerative colitis: a randomized controlled trial highlights the positive effects of the Mediterranean diet

The symptoms of ulcerative colitis, i.e., bloody diarrhea, abdominal pain, cramps, tenesmus and fatigue, have a major impact on the quality of life of patients. Altered gut microbiota may lie at the root of the problem. However, this same microbiota is influenced by diet. That is why this randomized controlled trial was set up by researchers and clinicians at the University of British Columbia, Canada. They assessed the effectiveness of the Mediterranean diet in treating symptoms, inflammation and gut microbiota. The adults recruited (65% women, median age 47) either adopted a Mediterranean diet for 12 weeks, with advice from a dietitian (15 patients), or continued with their usual diet (13 patients, control group).

Preventing ulcerative colitis relapses 

At the end of the 12-week study, the Mediterranean diet was well tolerated and reduced the frequency of attacks. Although all patients (except one experiencing a benign episode) were in remission at the start, slight activity was observed in one-third of the patients in the Mediterranean group while almost half of the patients in the control group suffered from a mild-moderate attack. The Mediterranean diet also helped to reduce fecal calprotectin levels which predict impending relapses and measure intestinal inflammation: 20% of patients on this diet had levels above 100 μg/g compared with 75% of those in the control group. 

Improved gut microbiota

Microbiota analysis also showed the benefits of the Mediterranean diet, with an increased presence of protective bacteria, notably Firmicutes (Ruminococcus spp., Flavonifractor spp., Clostridium M, Blautia A, and Lactococcus spp.), as well as a decrease in potentially pathogenic bacteria such as Veillonella dispar, Veillonella obetsuensis, Prevotella copri, Streptococcus australis, and biofilm-forming species. The researchers also observed a significant increase in fecal secretory immunoglobulin A (sIgA) after 12 weeks on the Mediterranean diet. This sIgA plays an essential role in maintaining mucosal homeostasis as it binds to pathogenic bacteria and prevents them from gaining access to the intestinal epithelium. Fecal secretory immunoglobulin A could thus explain the negative association between opportunistic pathobionts and the Mediterranean diet.

More short-chain fatty acids

Lastly, the Mediterranean diet coincided with an increase in the production of short-chain fatty acids (SCFAs), known for their immunomodulatory properties and role in promoting intestinal homeostasis, with observed higher levels of total SCFAs as well as butyric, acetic and valeric acids.

As such, the well-tolerated Mediterranean diet appears to be a reasonable and healthy dietary model that can be an option for ulcerative colitis patients in remission to prevent relapses, in addition to their standard medical treatment.