Gut Flora: Their Role in Health and Disease

Probiotics and prebiotics are now widely used by patients-and even present in many common foods. These products have been touted as helpful in a wide range of conditions, from irritable bowel syndrome (IBS) to depression. But what is the science behind these claims? Just what role do gut flora play in health and disease-and what is the evidence for manipulating them to improve health outcomes?

Provocative-and sometimes surprising-answers to these questions were provided by Walter J. Coyle, MD, Program Director of the Division of Gastroenterology at the Scripps Clinic in San Diego, in his April 22nd talk on gut flora. After a quick but informative review of the microbiology of the human gut (more than 500 species of gut flora have been identified, with most of these belonging to either the Firmicutes or Bacteroidetes phylum), he looked at the scientific evidence for influencing several different diseases through manipulation of these flora.

Deploring the fact that “the science is terrible” on probiotics and prebiotics, Dr Coyle limited his claims and conclusions to those that are supported by strong studies, and he stressed repeatedly the need for more “good science” in this area. Despite the paucity of strong evidence, those good studies that are available point in some intriguing directions.

Can your gut flora make you fat? Perhaps the most interesting of the data he presented were those showing a role for gut flora in the obesity epidemic. The microbes in the gut are extremely active metabolically. However, certain species are more active than others, with the result that in some persons, gut flora may produce 100 kcal per day more than are produced in other persons. Over the course of years, these extra calories add up.

A study by Ley and colleagues showed that obese mice and obese people have more Firmicutes and fewer Bacteroidetes than do mice and people of normal weight. Another study-in mice with different gut flora (“obese” flora [ie, more Firmicutes] versus “normal” flora) but the same starting weight-provides an answer to the chicken-or-egg question of whether the altered flora produce obesity or the other way around. This study showed that when fed identical diets, the mice with the “obese” flora gained more weight than the mice with the normal flora. Moreover, the beginnings of the obesity epidemic back in the 1950s correspond  with the introduction into the American diet of high-fructose corn syrup-which is actually a prebiotic, and which causes a ballooning of the number of Firmicutes in the gut. These data suggest the intriguing possibility that the manipulation of patients’ gut flora-through diet, probiotics, or prebiotics-might be a valuable new weapon in the “battle of the bulge.”

The role of gut flora in rosacea. Another intriguing study Dr Coyle presented was one showing that small intestinal bacterial overgrowth (SIBO) is common in patients with rosacea. The study also showed that resolution of SIBO in patients with rosacea (through treatment with rifaximin) produced dramatic improvement in their rosacea: complete resolution in 78% and improvement in another 17%.

Probiotics: which ones are effective for which conditions? Dr Coyle reviewed the evidence for a number of currently available probiotics. He noted that 2 good studies have shown Bifidobacterium infantis 35624 to decrease symptoms in patients with IBS. Several double-blind, placebo-controlled trials have shown Saccharomyces boulardii to significantly reduce the risk of recurrence of Clostridium difficile diarrhea in patients who have had multiple occurrences of the infection (although it was only modestly effective at preventing recurrence in patients with a first episode of C difficile diarrhea). Although most data show only modest effectiveness for yogurts that contain probiotics, one randomized controlled trial demonstrated that feeding a probiotic yogurt to hospitalized patients who were being given antibiotics reduced the incidence of C difficile infection (12% in patients who received the probiotic yogurt versus 34% in those who received placebo).