A recent review article in the Journal of Allergy and Clinical Immunology addressed the role of gut microbiota in health and illness.
The author pointed out that the GI tract serves two main functions: 1. Digestion and absorption of foods and nutrients; 2. Immune function. He also pointed out that these don’t operate independently, but rather, are fully intertwined.
The GI tract is the home for the majority of our immune system cells and proteins. This is so because the GI tract is home to billions of microbes that require immune surveillance. Disruptions in these microbes can impact both digestion and immune function.
One example is celiac disease. It is an inherited condition caused by autoimmunity directed against gluten. New research is finding that despite the inherited tendency, many individuals won’t develop the disease if their gut bacteria are normal.
On the other hand, the more disrupted the gut flora, the more likely that the immune system will cause the inflammation that leads to the disease. Sadly, once the disease starts, it leads to greater alteration in the gut flora, which in turn leads to more inflammation – a bad synergism.
Another example is obesity and metabolic syndrome (insulin resistance and high lipids). Two broad observations are relevant: 1. Children who receive multiple courses of antibiotics are more likely to become obese than children who don’t (antibiotics alter gut flora.) 2. Societies whose cows, beef, chickens and pigs receive antibiotics with their food are also more likely to become obese than those who don’t.
In a similar vein, gastric bypass surgery is more effective at both weight loss and improvement in metabolic syndrome than is lap-band surgery. The former leads to a positive improvement in gut flora not seen with lap-banding.
Even more interesting is the fact that in gastric bypass, patients the metabolic syndrome improves even before there is any noticeable weight loss. The bad gut bacteria breakdown fats into more easy-to-absorb particles, hence greater weight gain and higher cholesterol levels.
Finally, in mice experiments: Transfer of healthy gut bacteria from lean mice to obese mice leads to weight reduction in the latter without reduction in caloric content.