Spotlight on dietary interventions

Dietary interventions were in the spotlight this month, with new dietary guidance for patients with inflammatory bowel disease and two studies finding that the Mediterranean diet's effects on the gut microbiome may have clinical benefits.

First, the International Organization for the Study of Inflammatory Bowel Disease convened a working group to formulate nutritional recommendations for physicians, dietitians, and patients based on the best available evidence. The recommendations focus on dietary patterns to control and prevent relapse of inflammatory bowel disease and were published online on Feb. 14 by Clinical Gastroenterology and Hepatology.

The authors selected the most important food groups, dietary components, and food additives to address for patient dietary guidance and reviewed the published literature for each. Recommendations were provided separately for Crohn's disease and ulcerative colitis and were chosen from four categories (prudent to increase consumption, prudent to decrease or avoid consumption, safe to consume, or insufficient evidence to make a claim). Consensus was achieved for all food types except pasteurized dairy consumption.

Recommendations include the following:

• It is unnecessary to restrict moderate consumption of unprocessed red meat, lean chicken breast, and eggs in patients with Crohn's disease (high level of evidence); however, in those with ulcerative colitis, it is prudent to reduce intake of red and processed meat (low level of evidence).
• It is prudent to recommend moderate to high consumption of fruits and vegetables in Crohn's disease (low level of evidence). In ulcerative colitis, there is insufficient evidence to recommend any specific change or restriction in intake of fruit and vegetables (very low level of evidence).
• It is prudent to reduce exposure to saturated fats (low level of evidence) in patients with Crohn's disease. In those with ulcerative colitis, it is prudent to reduce consumption of myristic acid from palm oil, coconut oil, and dairy fats (low level of evidence) and to increase consumption of omega-3 fatty acids from fish (low level of evidence), but not from supplements (high level of evidence). For patients with either condition, it is prudent to avoid trans fat (very low level of evidence).
• There is insufficient evidence to recommend restriction of wheat and gluten, changes in low-level alcohol consumption, or any specific change in intake of complex carbohydrates, refined sugars, or fructose in patients with either Crohn's disease or ulcerative colitis (low level of evidence for all).
• For patients with either condition who have persistent symptoms despite resolution of inflammation (and absence of strictures in Crohn's disease), it may be prudent to use a diet low in fermentable oligo-, di-, and monosaccharides and polyols (low level of evidence).
• For patients with either Crohn's disease or ulcerative colitis, it may be prudent to limit intake of maltodextrin-containing foods and artificial sweeteners and processed foods that contain carrageenan, carboxymethylcellulose, polysorbate-80, titanium dioxide, and sulfites (very low level of evidence for all).

Next, two studies, both published in Gut, found that a specific dietary intervention, the Mediterranean diet, alters the gut microbiome and may have positive effects in older people and those with cardiometabolic risk factors.

For the first study, published on Feb. 17, researchers profiled the gut microbiota in 612 nonfrail or pre-frail older adults from the U.K., France, Netherlands, Italy, and Poland before and after administration of a 12-month Mediterranean diet intervention tailored to elderly adults and assessed the diet's impact on frailty. Adherence to the diet was associated with specific changes in the microbiome that were positively associated with several markers of lower frailty and improved cognitive function and were negatively associated with inflammatory markers, including C-reactive protein and interleukin-17. The associations were independent of patient factors, such as age and body mass index.

The study authors noted that there was a high level of microbiome variability across individuals in the five countries. They added that maintaining a long-term Mediterranean diet may be impractically expensive or logistically impossible in many countries and that it may not be realistic for some older patients with problems like dysphagia.

The second study, published on Feb. 19, also explored the effects of a Mediterranean diet intervention on the gut microbiome. Researchers recruited 82 healthy overweight and obese people with a habitually low intake of fruit and vegetables and a sedentary lifestyle to participate in a randomized controlled trial of the diet. Forty-three participants consumed a Mediterranean diet tailored to their habitual energy intakes, and 39 maintained their regular diets. During the study period, researchers monitored their dietary adherence, metabolic parameters, gut microbiome, and systemic metabolome.

Participants in the Mediterranean diet group significantly increased their daily intake of dietary fiber by twofold and their dietary vegetable:animal protein ratio by 2.5-fold compared with the control group (P<0.001). After four weeks, those in the intervention group had significant reductions in total plasma cholesterol level and high-density lipoprotein cholesterol level compared to those in the control group. The decrease in cholesterol level was proportional to adherence to the Mediterranean diet. The intervention was also associated with microbiome changes, including increased gut microbial gene richness in individuals with reduced inflammation, a rise in the fiber-degrading Faecalibacterium prausnitzii, and a decrease of the potentially proinflammatory Ruminococcus gnavus.

“Taken together, our results indicate that [a Mediterranean diet] may remodel the intestinal microbiome towards a state that promotes metabolic and cardiovascular health,” the study authors wrote.