The size and complexity of the human brain is what separates us from lower animals. However, for several hundred million years in the early evolution of life on earth, animals did not have brains. Then, the gut was the most important and complex part of the body. The gut determined when the organism was hungry. It decided what was food and what was not. It sought and found food, digested it, distributed the nutrients and propelled the waste out of the body. The gut developed its own set of chemical messengers to perform these duties, as well as its own communication system -- a nervous system of sorts -- to make certain that every cell in the gut knew what to do and when to do it.

As evolution progressed, the brain developed to control behavior and interactions with the environment. It far surpassed the gut in complexity. Yet even as animals evolved, the gut continued to contribute critical information, not only to the body, but to the increasingly complex brain. There is a long list of chemical messengers in the gut that affect the human brain. In fact, many are found in both gut and brain tissue. Thus includes commonly known substances, such as insulin, as well as lesser known ones, such as cholecystokinin, ghrelin, vasoactive intestinal polypeptide, pancreatic polypeptide, neuropeptide Y and scores of others. Scientists speak of a gut-brain axis -- a complex, ongoing neurochemical interaction between the gut and brain.

An interesting, yet neglected, component of the gut is the collection of bacteria referred to as the intestinal flora. The human intestine is inhabited by over 400 different species of bacteria that number in the hundreds of trillions. We usually associate bacteria with filth and disease. However, among the many varieties of bacteria in the gut are types that are necessary for optimal human health. Our relationship with gut bacteria is symbiotic, that is, necessary and mutually helpful. We give the bacteria a warm, safe place to live, with plenty to eat, and they provide us with a variety of essential housekeeping and maintenance functions. Conversely, studies show that unhealthy populations of bacteria in the gut can contribute to both physical and mental illnesses.

People suffering mental illness, including major depression, schizophrenia and anxiety disorders, tend to have abnormal varieties of bacteria in their intestines. There is evidence that some psychiatric symptoms can be improved by restoring normal intestinal flora populations. There are now at least two formal studies showing that probiotics can improve mood. In one study, published in the British Journal of Nutrition, the addition of probiotics, i.e., healthy bacteria in capsules, for one month improved mood and reduced levels of stress hormones. A similar study in the European Journal of Clinical Nutrition showed that daily addition of probiotic-containing yogurt over three weeks significantly improved mood in normal subjects. A study in the journal Gut Pathogens found that addition of probiotics to sufferers of Chronic Fatigue Syndrome resulted in significant reductions in levels of anxiety. Whether this was a specific effect on chronic fatigue or a general effect on anxiety was unclear.