Strong social ties have long been linked to better health and longer life. People who are married or living with a partner tend to live longer, experience fewer chronic illnesses, and report better mental wellbeing than those who are socially isolated.

Until recently, these benefits have been explained mainly through behaviour and psychology, such as emotional support, shared routines, and reduced stress.

Recent research published in Nature Scientific Reports now suggests there may also be a biological link. A large study of older adults in the United States indicates that close personal relationships, particularly high‑quality marriages, are associated with differences in the gut microbiome. This is the vast community of bacteria that lives in the digestive system and plays an important role in health.

The findings add a new layer to our understanding of how social life and physical health are connected, especially in later life.

What we already know about the gut microbiome

The gut microbiome consists of trillions of bacteria, viruses, and fungi that live mainly in the intestines. These microorganisms help break down food, produce vitamins, regulate the immune system, and influence metabolism.

Growing evidence links the gut microbiome to conditions such as obesity, type 2 diabetes, inflammatory bowel disease, and heart disease.

One key feature of a healthy microbiome is diversity. A more diverse microbial community, meaning a wider range of different bacteria, is generally associated with better health and greater resilience to disease. Lower diversity has been linked to chronic inflammation and metabolic problems.

The microbiome develops rapidly in early childhood and becomes broadly stable by the age of three to five. However, it can still change throughout life in response to factors such as diet, medication, illness, and lifestyle. Less is known about how adult social relationships might influence it.

A unique human study linking relationships and microbes

The new research draws on data from the Wisconsin Longitudinal Study. This long‑running project has followed thousands of people who graduated from high school in Wisconsin in 1957, along with many of their siblings and spouses, for nearly six decades.

For this analysis, researchers examined stool samples from 408 adults aged between 58 and 91. Using genetic sequencing, they identified the types of bacteria present in each person’s gut. These data were then linked to detailed survey information on health, diet, living arrangements, and social relationships collected over many years.

Because the study included married couples, siblings, and unrelated individuals, it allowed researchers to compare the effects of adult relationships with those of shared genetics and early‑life environments.

What the research found

The study reported three main findings.

First, people who lived with a spouse or partner had a more diverse gut microbiome than those who lived alone. This included higher measures of both microbial diversity and richness, which reflect how many different bacterial types are present and how evenly they are distributed.

Second, spouses had gut microbiomes that were more similar to each other than those of siblings or unrelated individuals. This similarity increased with the length of the marriage, suggesting that long‑term cohabitation plays a role.

Third, relationship quality mattered. Married couples who described their relationship as “very close” showed greater microbial similarity and higher diversity than those who said they were only “somewhat close”. Couples in less close relationships did not differ significantly from people living alone.

Among people who did not live with a partner, those who reported more frequent social contact with friends and relatives also tended to have more diverse gut microbiomes.

In plain terms, the research suggests that close, supportive relationships are linked to a richer and more shared community of gut bacteria.

How social relationships might influence gut bacteria

The study was not designed to pinpoint exact mechanisms, but several plausible explanations exist.

People who live closely together tend to share daily environments, routines, and behaviours. This includes eating together, touching shared surfaces, and having physical contact. All of these can allow bacteria to pass from one person to another.

Previous research has shown that people who live together share not only gut bacteria but also skin and oral microbes. Some evidence suggests that bacteria from the mouth can influence the gut, providing another possible route of transfer.

The researchers also found that many of the bacteria shared between close partners were anaerobic, meaning they do not survive well in oxygen. This suggests that direct person‑to‑person contact, rather than exposure through the wider environment, may be important.

Emotional closeness may increase the frequency of these interactions. Couples who feel closer are likely to spend more time together and engage in more physical affection, even in later life, which could increase opportunities for microbial sharing.

How strong is the evidence

This was a well‑designed observational study in humans, using detailed social and health data alongside microbiome analysis. The researchers adjusted for many known factors that influence gut bacteria, including age, sex, diet, antibiotic use, diabetes, and heart disease.

However, important limitations remain.

The study cannot prove cause and effect. It is not possible to say whether close relationships lead to changes in the microbiome, or whether people with certain microbiomes are more likely to form close relationships. The microbiome was measured at a single point in time, so changes over years or decades could not be tracked.

The participants were mostly white Americans from one US state, which limits how widely the findings can be applied. Diet was measured in general terms rather than in fine detail, and unmeasured lifestyle factors may still play a role.

For these reasons, the findings should be seen as suggestive rather than conclusive.

What this means for the public

The research does not change current medical advice. It does not mean that people should seek relationships for the sake of their gut bacteria, or that living alone is harmful in itself.

What it does suggest is that social connection may be part of the broader environment that shapes health, alongside diet, exercise, and healthcare. For older adults in particular, maintaining meaningful social ties may support health in ways that extend beyond mental wellbeing.

The findings may also help explain why social isolation has been linked to poorer health outcomes in many studies. Changes in the gut microbiome could be one of several biological pathways involved.

Implications for treatment and prevention

There are no treatments or interventions based on this research at present. Gut‑targeted therapies, such as probiotics or dietary changes, are already an active area of research, but this study does not support any specific approach.

From a public health perspective, the findings reinforce the importance of addressing loneliness and social isolation, especially in ageing populations.

Policies that support social engagement may have benefits that extend beyond psychological health, although more evidence is needed to confirm biological effects.

What remains unknown

Many questions remain unanswered. Researchers do not yet know which specific social behaviours matter most, how much contact is needed to influence the microbiome, or whether these microbial changes translate into measurable health benefits over time.

Future studies will need to follow people longitudinally, include more diverse populations, and examine how changes in relationships relate to changes in gut bacteria and health outcomes.

Understanding these links more clearly could help bridge social science and biology, offering a more complete picture of how human lives shape human health.

A wider perspective

This research adds to a growing body of evidence that health is shaped not only by genes and lifestyle choices, but also by social experience.

The gut microbiome appears to be sensitive to the human environment in which it exists, including close personal relationships.

While much remains to be learned, the study highlights an often overlooked idea: that social connection is not just emotionally meaningful, but may also leave a biological imprint.

In the context of ageing societies and rising concerns about loneliness, this perspective offers a valuable reminder of how closely social life and physical health are intertwined and this factor alone should not be underestimated.