Research has begun to demonstrate that the quality of bacteria in one's gut is important for human health. Let me call the quality of bacteria in one's gut "microbiome capital." Anthony Komaroff makes the argument that microbiome capital could plausibly be important early in his Journal of the American Medical Association summary article "The Microbiome and Risk for Obesity and Diabetes":

Beginning at the moment of birth, each human increasingly coexists with microbes. By the time individuals reach adulthood, they are colonized by many more microbial cells than the roughly 13 trillion human cells. More important still, these microbial cells (the microbiota), collectively, have exponentially more genes (the microbiome) than do human cells, around 250 to 800 times more.

Moreover, many genes in the human microbiome generate proteins, including hormones, neurotransmitters, and molecules of inflammation, that can enter the circulation and affect health. In light of this, it is reasonable to question whether the genes of the microbiome might play a greater role in health than do human genes. Recent evidence suggests that the microbiome may affect the probability of many major diseases, including obesity and diabetes.

Current knowledge hints that it might be possible to create a proxy for the quality of bacteria in one's gut—and thus for microbiome capital—from the ratio of bacteria from the phylum Bacteroidetes to bacteria from the phylum Firmicutes. Anthony writes: 

About 90% of gut bacteria are in 1 of 2 phyla: Bacteroidetes and Firmicutes. Firmicutes generate more harvestable energy than Bacteroidetes. Obese humans have relatively more Firmicutes ...

Many experiments in mice, detailed as follows by Anthony, indicate that microbiome capital matters:

• Gut microbiota from obese mice and from lean mice were transplanted into germ-free, lean mice, all of whom had the same daily caloric intake. Over the next 2 weeks, the mice receiving microbiota from obese mice became obese, whereas those receiving microbiota from lean mice remained lean.1

• Gut microbiota from conventionally raised animals were placed in the guts of lean germ-free mice. Without any increase in daily caloric intake, the body fat content of the animals increased by 60% within 14 days, and they developed insulin resistance.2

• Obese mice underwent Roux-en-Y gastric bypass (RYGB) surgery or sham surgery. Mice that underwent RYGB surgery had the expected weight loss and a characteristic change in the gut microbiome, whereas mice that underwent the sham surgery did not. Transfer of bacteria from mice that underwent RYGB surgery to mice that underwent the sham surgery resulted in weight loss, although not as great as seen following RYGB surgery. ...

The last experiment is important because in many ways it has been somewhat mysterious exactly why gastric bypass surgery helps so much in weight loss, given that greater hunger could easily counteract any purely mechanical reduction in food intake at a given sitting.

The comparison to capital accumulation is tightened by the influence of obesity on microbiome capital. Here is Anthony's description of that:

These experiments suggest that the composition of gut microbiota can influence obesity. However, other experiments suggest that obesity can influence the composition of gut microbiota. For example, when obese people diet and lose weight, the proportion of Bacteroidetes increases relative to Firmicutes. Conversely, when obese people resume their previous diets and gain weight, the proportion of Firmicutes increases.

All of this is only tantalizing if a therapeutic result for humans cannot be confirmed. The closest thing so far is this result that Anthony reports:

... only experimental evidence can suggest a causal connection. At least 1 study does. Treatment-naive men with the metabolic syndrome had their gut flora eliminated by polyethylene glycol lavage. Then they were randomized to receive small intestinal infusions (through a gastroduodenal tube) either from lean male donors or from their own feces. In men who received infusions from lean individuals, insulin sensitivity increased. This effect declined over time, and there was considerable individual variability.

That is, getting gut bacteria from someone thin tended to move the recipient away from the insulin-resistance that is so closely related to diabetes, though it only did so for a while. 

This is an important area of research. I hope it proceeds expeditiously. For economists theorizing about obesity and diabetes, it points to changes in microbiome capital as a possible mechanism for phenomena. One big missing piece in the research is that it does not address whether or not high-quality probiotics taken by mouth can help with weight loss and with restoring insulin sensitivity. 

Don't miss these other posts on diet and health and on fighting obesity:

• Stop Counting Calories; It's the Clock that Counts
• 4 Propositions on Weight Loss
• Forget Calorie Counting; It's the Insulin Index, Stupid
• Obesity Is Always and Everywhere an Insulin Phenomenon
• The Problem with Processed Food
• Letting Go of Sugar
• Prevention is Much Easier Than Cure of Obesity
• Which Is Worse for You: Sugar or Fat?
• Our Delusions about 'Healthy' Snacks—Nuts to That!
• My Giant Salad
• Using the Glycemic Index as a Supplement to the Insulin Index
• How Fasting Can Starve Cancer Cells, While Leaving Normal Cells Unharmed
• Why You Should Worry about Cancer Promotion by Diet as Much as You Worry about Cancer Initiation by Carcinogens
• Good News! Cancer Cells are Metabolically Handicapped
• How Sugar, Too Much Protein, Inflammation and Injury Could Drive Epigenetic Cellular Evolution Toward Cancer
• Meat Is Amazingly Nutritious—But Is It Amazingly Nutritious for Cancer Cells, Too?
• The Keto Food Pyramid
• Sugar as a Slow Poison
• How Sugar Makes People Hangry
• Why a Low-Insulin-Index Diet Isn't Exactly a 'Lowcarb' Diet
• Hints for Healthy Eating from the Nurse's Health Study
• The Case Against Sugar: Stephan Guyenet vs. Gary Taubes
• The Case Against the Case Against Sugar: Seth Yoder vs. Gary Taubes
• Gary Taubes Makes His Case to Nick Gillespie: How Big Sugar and a Misguided Government Wrecked the American Diet
• Against Sugar: The Messenger and the Message
• A Conversation with David Brazel on Obesity Research
• Magic Bullets vs. Multifaceted Interventions for Economic Stimulus, Economic Development and Weight Loss
• Mass In/Mass Out: A Satire of Calories In/Calories Out
• Carola Binder: The Obesity Code and Economists as General Practitioners
• Carola Binder—Why You Should Get More Vitamin D: The Recommended Daily Allowance for Vitamin D Was Underestimated Due to Statistical Illiteracy
• Jason Fung: Dietary Fat is Innocent of the Charges Leveled Against It
• Faye Flam: The Taboo on Dietary Fat is Grounded More in Puritanism than Science
• Diseases of Civilization
• Katherine Ellen Foley—Candy Bar Lows: Scientists Just Found Another Worrying Link Between Sugar and Depression
• Ken Rogoff Against Sugar and Processed Food
• Kearns, Schmidt and Glantz—Sugar Industry and Coronary Heart Disease Research: A Historical Analysis of Internal Industry Documents
• Eating on the Road
• Intense Dark Chocolate: A Review
• In Praise of Avocados
• Salt Is Not the Nutritional Evil It Is Made Out to Be
• Confirmation Bias in the Interpretation of New Evidence on Salt
• Whole Milk Is Healthy; Skim Milk Less So
• Is Milk OK?
• How the Calories In/Calories Out Theory Obscures the Endogeneity of Calories In and Out to Subjective Hunger and Energy
• Putting the Perspective from Jason Fung's "The Obesity Code" into Practice
• 'Forget Calorie Counting. It's the Insulin Index, Stupid' in a Few Tweets
• Julia Belluz and Javier Zarracina: Why You'll Be Disappointed If You Are Exercising to Lose Weight, Explained with 60+ Studies (my retitling of the article this links to)
• Diana Kimball: Listening Creates Possibilities
• On Fighting Obesity
• The Heavy Non-Health Consequences of Heaviness
• Analogies Between Economic Models and the Biology of Obesity
• Debating 'Forget Calorie Counting; It's the Insulin Index, Stupid'
• Podcast: Miles Kimball Explains to Tracy Alloway and Joe Weisenthal Why Losing Weight Is Like Defeating Inflation

Also see the last section of "Five Books That Have Changed My Life" and the podcast "Miles Kimball Explains to Tracy Alloway and Joe Weisenthal Why Losing Weight Is Like Defeating Inflation." If you want to know how I got interested in diet and health and fighting obesity and a little more about my own experience with weight gain and weight loss, see my post "A Barycentric Autobiography."