How Your Social Life Affects Your Gut

Unless you’ve been living under a rock for the last decade, you know by now that the gut plays a surprisingly large role in physical and mental health. Not just by breaking food down into component nutrients that sustain mind and body but also by conscripting resident bacteria—said to number about 100 trillion—to act as auxiliary organs, deploying their own metabolic processes to influence our every function.

Studies over the past decade or so have shown that microbes produce substances that maintain a strong intestinal wall, preventing the leakage of toxins and bacteria that, once at large, set off inflammation, thought to be a cause of conditions ranging from irritable bowel syndrome to neurodegenerative disorders. They also produce neurotransmitters—serotonin, dopamine, and GABA—that communicate directly with the brain.

It’s just the early stages of understanding the magnitude of the gut’s role in health. For example, there’s evidence that some of the substances (namely, short-chain fatty acids, or SCFAs) produced by certain groups of bacteria boost the efficacy of cancer immunotherapy. A diet that promotes the abundance of such bugs—largely plant-based—may in the future become a standard part of cancer care.

But SCFAs have a much larger portfolio. The product of bacterial fermentation of fiber in the lower intestine, they also influence brain function, regulating appetite, energy balance, and mood states. A dearth of SCFAs is implicated in depression and anxiety.

There are hundreds of bacterial species in the gut. Bacterial diversity is a good thing, a general contributor to and marker of good health. But exactly what does a healthy gut look like? Studies indicate that gut composition varies by many factors.

The Bugs in Your Nabe

One of the most important, it turns out, is who you currently live with. Researchers studying large numbers of individuals of traceable ancestry in different populations have come to the same conclusion: Diet and genetic heritage are known to affect the microbiome, but social relationships shape it most—especially sustained close marital partnerships.

The Dutch Microbiome Project, which characterized gut microbes in 8,208 individuals belonging to three generations, finds that the heritability of gut microbes is at most 1.9 percent. Reporting recently in the journal Nature, the research team observed much greater similarity among the microbiomes of genetically unrelated individuals sharing a household than among relatives who do not share households, including identical twins whose living situations diverge in adulthood.

Only a small proportion of organisms have high heritability, among them Bifidobacterium longum, a multifunctional microbe that colonizes the intestinal tract early in infancy to digest the sugars in breast milk and bolster immunity. The first few years of life are crucial for microbiome development, and childhood is indeed linked to the adult microbiome—but it’s the childhood living environment that is most associated with the adult microbiome.

Growing up in a rural environment brings increases in various bacteria—among them Bifidobacterium species—linked to general health and decreases in bugs that are opportunistically pathogenic. Growing up with pets also has a positive effect on the microbiome, while early exposure to air pollution, notably car exhaust, and passive exposure to cigarette smoke have durably negative effects.

The makeup of the microbiome is highly individualistic, although two large classes of organisms—Bacteriodetes and Firmicutes—make up more than 90 percent of the total. Their ratio serves as a general indicator of health.

But shifts in the balance—dysbiosis—can negatively affect health: Metabolic conditions and cardiovascular diseases, among other disorders are linked to increases in the proportion of Firmicutes, while inflammatory bowel disease is linked to a low proportion of Firmicutes. All around, researchers say, environment dominates over heritability in shaping the microbiome, and it does so throughout life.

The Impact of Intimacy

Despite large individual variation in adult microbiome composition, those who live closely together come to have similar microbes within. The Dutch researchers found that nearly 50 percent of microbial types were significantly affected by cohabitation. In general, the microbiomes of all types of cohabitors (including parents and children and siblings) were more similar than those of participants living separately no matter their relatedness.

But married couples who report having a close, loving relationship have the most similarity, say Israeli researchers, also in Nature. Some of the microbiome similarity, of course, comes from shared diet and shared environmental exposures. But evidence of microbial sharing, especially of rare species, suggests mutual colonization through human interactions. And that puts a spotlight on acts of intimacy—including kissing (the salivary microbiome influences the gut microbiome).

Not only are the microbiomes of cohabiting spouses more similar to each other; they also are more similarly diverse. There is a greater richness of species than in unmarried, noncohabiting individuals.

Cohabitation is not just a matter of social enrichment; there’s biological enrichment as well. Cohabitation brings an invasion of new microbial species to the less diverse microbiome of those who have previously lived alone.

Scientists have known for decades that social relationships, especially marriage, powerfully affect health. In fact, social isolation compromises health at least as much as smoking, bringing significant increases in all-cause mortality.

The decades of research documenting the benefits of marriage on well-being suggest that in addition to the positive effect of psychosocial factors, the gut microbiome may be linking human relationships and health.

Despite the commanding influence of a close, cohabiting relationship, the microbiome can be diversified by diet, especially one rich in plant fiber. It can be influenced by behavior, such as farming and gardening and exercise. It’s modifiable by environment—green space and pollutants around you.

It can also be altered by direct addition of specific species of bacteria, delivered in consumable probiotics or, in that ultimate feat of reverse engineering, by fecal transplant. There are probiotic formulations now marketed for digestive health, others aimed at immune health, and still others for boosting mood. Now that it’s clear that social relations play the largest role in the makeup of the microbiome, can probiotics for singles be far behind?

Bug Bites

• Eating breads, legumes, fish, and nuts is linked to a lower abundance of gut bacteria that cause disease and those that cause inflammation.
• Consumption of nuts, oily fish, fruits, vegetables, cereals, and red wine promotes bacteria that ferment fiber to short-chain fatty acids.
• Polyphenol-rich red wine is also linked to gut microbial diversity.
• Fast foods, processed meats, soft drinks, and sugar promote bacteria that increase the energy harvesting from food, a pathway to obesity.
• Processed foods are linked to an increase in gut bacteria that feed on the mucosal lining of the gut, eroding the protective gut barrier and promoting local and systemic inflammation.
• Air pollutants negatively affect the microbiome, and evidence suggests they increase the risk of gastrointestinal disease by promoting dysbiosis.