Going With the Gut

It's been with us for the eons that humans have walked the earth. But only in the past decade have scientists recognized that a vast array of bacteria, collectively known as the microbiome, inhabits our gut and serves as an important organ of the body.

Billions of those bacteria, called probiotics, do good: They stimulate the development and function of the immune system, combat the growth of harmful micro-organisms, aid in the process of digestion, even promote healthy body weight. But probably the most unlikely discovery has been that, in ways still under study, the gut plays an important role in mental health. "You can't have a healthy brain without a healthy gut," says psychiatrist Ted Dinan of Ireland's University College Cork.

Maintaining the microbiome in fighting fettle—feeding the beneficial bacteria and keeping them metabolically active—is a massive task that relies on the human diet. Enter prebiotics, nondigestible carbohydrates, otherwise known as fiber. All prebiotics are fiber, but not all fiber is prebiotic—only those complex carbohydrates that resist digestive enzymes but lend themselves to fermentation by bacteria in the large intestine.

In humans, the best-known prebiotics are inulin, fructooligosaccharides (FOS), and galactooligosaccharides (GOS), all variants of sugar substances. And all three, researchers recently discovered, reduce levels of the stress hormone cortisol and influence emotional processing in the brain. They activate a channel of biochemical signaling between the gastrointestinal tract and the nervous system. Prebiotics also increase levels of BDNF, or brain-derived neurotrophic factor, which promotes mental flexibility by stimulating the growth of new neurons.

In a study reported in European Neuropsychopharmacology, Dinan and colleagues fed mice a combination of FOS and GOS for 10 weeks and subjected them to tests that induce behavioral states similar to anxiety and depression in humans. The two prebiotics, he found, functionally "looked like an antidepressant," keeping rats from developing the equivalent of behavioral despair. The prebiotic combo also reduced levels of corticosterone—rodent cortisol—and elevated levels of BDNF in the hippocampus, the brain area crucial for learning and memory.
"We're seeing the first indication that changes in the microbiota by eating these two prebiotics may have a significant impact on stress responses," says Dinan.

It may not be necessary to consume specific prebiotics to promote health and mental health. It may be more important to increase consumption of all fiber-rich foods—something Americans have a particularly hard time doing.

Although prebiotics are distinguished by their susceptibility to fermentation by gut microflora, all dietary fiber undergoes some degree of fermentation, says biologist Jens Walter of the University of Alberta. He believes that a diet rich in many kinds of fiber would go a long way to restoring the microbial biodiversity that sustained humans until the advent of the Western diet, which brought on many of the chronic diseases common today.

"We're starving our microbiota by eating a refined diet," Walter says. The Recommended Dietary Allowance for fiber intake is 25 to 35 grams a day, depending on gender and age. Nutritional surveys show that most people don't get even half that amount. Yet Walter believes that unless we consume double the RDA, we're putting ourselves at risk of disease.

Between 50 and 100 grams per day, he says, is closer to the diet of our ancestors. In a study reported in Nature last year, African Americans who ate a traditional South African diet, with 55 grams of fiber a day, showed reduced markers of colon cancer within just two weeks.

Raw onion, garlic, leeks, lentils, Jerusalem artichokes, asparagus, and bananas are among the foods richest in natural fiber and prebiotics. The good news, says Walter, is that getting prebiotics in supplement form is just as beneficial.

Prebiotics in the Body

1. Fuel Growth and Activity of Probiotics
2. Prevent Obesity
3. Lower Cardiovascular Risk
4. Enhance Mineral Absorption
5. Prevent Colon Cancer
6. Normalize Elimination
7. Reduce Inflammation

Fast Facts

• All prebiotics resist digestion in the small intestine and reach the colon, where they undergo fermentation by gut bacteria.
• Foods rich in prebiotics have been consumed since prehistoric times.
• People eating non-Westernized diets tend to have significantly healthier gut flora than those in Europe and North America.
• A key factor in the rise of obesity may be the shift away from the diet under which the human microbiome evolved.
• The prebiotic inulin is found naturally in many plants—and in breast milk.
• There's some evidence that a deficiency of beneficial bacteria in the gut may be passed on to future generations.
• Increasing fiber intake is best done gradually to avoid bloating.
• Even current dietary recommendations to consume 25 to 35 grams of fiber a day pale next to the 50 to 150 grams our ancestors likely consumed every day.