This article summarizes some of the same things I've been posting here. From NY Times:
We may think of ourselves as just human, but we’re really a mass of microorganisms housed in a human shell. Every person alive is host to about 100 trillion bacteria cells. They outnumber human cells 10 to one and account for 99.9 percent of the unique genes in the body.
Our collection of microbiota, known as the microbiome, is the human equivalent of an environmental ecosystem. Although the bacteria together weigh a mere three pounds, their composition determines much about how the body functions and, alas, sometimes malfunctions. Like ecosystems the world over, the human microbiome is losing its diversity, to the potential detriment of the health of those it inhabits.
Dr. Martin J. Blaser, a specialist in infectious diseases at the New York University School of Medicine and the director of the Human Microbiome Program, has studied the role of bacteria in disease for more than three decades. In his new book, “Missing Microbes,"Dr. Blaser links the declining variety within the microbiome to our increased susceptibility to serious, often chronic conditions, from allergies and celiac disease to Type 1 diabetes and obesity. He and others primarily blame antibiotics for the connection.
The damaging effect of antibiotics on microbial diversity starts early, Dr. Blaser said. The average American child is given nearly three courses of antibiotics in the first two years of life, and eight more during the next eight years. Even a short course of antibiotics like the widely prescribed Z-pack (azithromycin, taken for five days), can result in long-term shifts in the body’s microbial environment.
But antibiotics are not the only way the balance within us can be disrupted. Cesarean deliveries, which have soared in recent decades, encourage the growth of microbes from the mother’s skin, instead of from the birth canal, in the baby’s gut, Dr. Blaser said in an interview.
This change in microbiota can reshape an infant’s metabolism and immune system. A recent review of 15 studies involving 163,796 births found that, compared with babies delivered vaginally, those born by cesarean section were 26 percent more likely to be overweight and 22 percent more likely to be obese as adults.
The placenta has a microbiome of its own, researchers have discovered, which may also contribute to the infant’s gut health and help mitigate the microbial losses caused by cesarean sections.
Further evidence of a link to obesity comes from farm animals. About three-fourths of the antibiotics sold in the United States are used in livestock. These antibiotics change the animals’ microbiota, hastening their growth. When mice are given the same antibiotics used on livestock, the metabolism of their liver changes, stimulating an increase in body fat, Dr. Blaser said.
Even more serious is the increasing number of serious disorders now linked to a distortion in the microbial balance in the human gut. They include several that are becoming more common in developed countries: gastrointestinal ailments like Crohn’s disease, ulcerative colitis and celiac disease; cardiovascular disease; nonalcoholic fatty liver disease; digestive disorders like chronic reflux; autoimmune diseases like multiple sclerosis and rheumatoid arthritis; and asthma and allergies.