The newspaper article by a freelance journalist kindly submitted here proved to be interesting to read all the way through, and prompted me to read all the comments here posted before I saw the thread, and also to look up the researchers mentioned in the article and the author of the article. There are interesting ideas here, but this is very preliminary research, based on early animal studies mentioned with human studies with very small n and a short time frame. ("The researchers gave 12 healthy women fermented milk containing a probiotic supplement made up of five different strains of bacteria thought to have a positive effect on the intestines. The women drank the milk two times a day over the course of four weeks. Another group of 11 women drank milk without probiotics.")

It's quite a bit too early to tell if altering your personal dietary habits will have any predictable long-term effects on your mood (the article title says "may" after all) and especially what alterations in intestinal microorganisms have what interaction with what other health outcomes.

So far, "modern life" appears to be good both for reducing mortality and reducing morbidity, as life expectancy at age 40, at age 60, and at even higher ages is still rising throughout the developed countries of the world.[1] A variety of incremental changes over the last century have allowed us all to enjoy longer, healthier lives than our ancestors a hundred years ago, as reported in an article in a series on Slate, "Why Are You Not Dead Yet? Life expectancy doubled in past 150 years. Here’s why."[2] If deeper understanding of the health effects of intestinal microorganisms leads to even more incremental improvement in disease prevention and treatment, and increase in healthy lifespan, that would be great. So I heartily support research on these topics. (I remember that the father of Freakonomics co-author Steven Levitt spent his career at my alma mater university studying intestinal gas with federal grants--I'm not making this up.) But we can't be sure yet what the recommended practice will be for how we eat or whether or not we should have particular microorganisms put into our intestines.

Meanwhile we can be happy that ongoing medical research and perhaps other factors we don't fully understand seem to be driving steady increases in healthy lifespan. Girls born since 2000 in the developed world are more likely than not to reach the age of 100, with boys likely to enjoy lifespans almost as long. The article "The Biodemography of Human Ageing"[3] by James Vaupel, originally published in the journal Nature in 2010, is a good current reference on the subject. Vaupel is one of the leading scholars on the demography of aging and how to adjust for time trends in life expectancy. His striking finding is "Humans are living longer than ever before. In fact, newborn children in high-income countries can expect to live to more than 100 years. Starting in the mid-1800s, human longevity has increased dramatically and life expectancy is increasing by an average of six hours a day."[4]

[1] http://www.nature.com/scientificamerican/journal/v307/n3/box...

[2] http://www.slate.com/articles/health_and_science/science_of_...

[3] http://www.demographic-challenge.com/files/downloads/2eb51e2...

[4] http://www.prb.org/Journalists/Webcasts/2010/humanlongevity....