Obesity is linked to metabolic syndrome, a cluster of conditions that include hypertension, high blood sugar, excess body fat around the waist, and abnormal triglyceride or cholesterol levels.

As obesity rates continue to soar, understanding how these conditions work together and what can be done to prevent them is more pressing than ever.

Metabolic syndrome is now considered to be a chronic inflammatory disease, involving altered relationships between gut bacteria and the gut.

Western society has experienced a huge shift in eating habits in recent decades; there is now a much greater emphasis on processed foods, which, notably, lack fiber. This has had an impact on gut bacteria and, according to some, could help explain the increased prevalence of metabolic syndrome.

A diet lacking fiber alters the composition of gut bacteria, lowering numbers overall and changing the ratios of species. Also, low-fiber diets increase bacteria's ability to encroach upon the gut's epithelial cells; this provokes an inflammatory response.

Reduced fiber, altered gut

A paper, published recently in the journal Cell Host and Microbe, explores the relationship between obesity, gut bacteria, inflammation, and fiber intake in new detail.

Earlier studies have shown that supplements of a fermentable fiber — inulin — reduce fat buildup and the symptoms of metabolic syndrome. However, consuming enough inulin comes with negative consequences, such as flatulence and bloating. For this reason, understanding exactly how fiber imparts its health benefits is important. Once the mechanism is described, it might be possible to find more efficient, less windy ways to achieve the same goal.

A team of scientists — led by Dr. Andrew Gewirtz, professor at the Institute for Biomedical Sciences at Georgia State — recently set out to develop a clearer picture.

Earlier studies have concluded that short-chain fatty acids (SCFAs) may play a role in reducing inflammation and improving metabolism; this is thought to be mediated by the activation of free fatty acid receptor GPR43. Dr. Gerwirtz and his team wanted to test this theory.

Fiber's role in the gut and beyond

To answer these questions, the researchers fed mice with one of two diets, both of which were high in fat and known to induce obesity in rodents:

• Low fiber content (5 percent cellulose as a source of fiber)
• High fiber (either inulin or insoluble cellulose)

As expected, after a 4-week period, the mice fed the diet enriched with inulin showed reduced obesity and a reduction in the size of fat cells; the inulin-fed mice also had lower cholesterol and lower incidence of abnormal blood sugar levels (dysglycemia).

Mice fed cellulose, however, only showed slight reductions in obesity and dysglycemia.

The positive effects seen in the inulin-fed mice were due to a number of factors: gut bacteria levels were restored, there was an increase in the production of intestinal epithelial cells, and expression of the protein interleukin-22 (IL-22) was restored.

"This study revealed the specific mechanism used to restore gut health and suppress obesity and metabolic syndrome is the induction of IL-22 expression. These results contribute to the understanding of the mechanisms that underlie diet-induced obesity and offer insight into how fermentable fibers might promote better health."

Dr. Andrew Gewirtz

IL-22 appears to prevent inflammation by stopping gut bacteria from invading epithelial cells. The authors hypothesize that IL-22 stops bacterial encroachment by increasing the rate at which new epithelial cells are produced, and by upping production of antibacterial proteins.

Interestingly, the researchers found that neither inhibiting SCFA production nor removing GPR43 had any effect on metabolic syndrome. These findings were unexpected and go against earlier research.

As Western society struggles under the weight of an obesity epidemic, any study that gives new insight into obesity and metabolic disorders is important.

Researchers are gradually delving deeper into the relationships between gut bacteria, diet, and inflammation, and the picture is growing ever clearer.

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