Couples who live together tend to develop similar gut microbiomes over time, and that microbial convergence may help explain why some men pick up extra weight after moving in with a partner, Australian clinical nutritionist Lee Holmes wrote in Daily Mail Australia.
Holmes cited research showing that partnered individuals share more bacterial species than those who live apart and that the longer couples cohabit the more closely their gut microbial profiles resemble one another, a pattern Holmes attributed to shared meals, living environments and close physical contact.
Holmes said men appear especially susceptible to colonisation by their female partners' microbes. Citing findings from human and animal studies, she noted that male guts often begin with lower microbial richness and diversity than female guts, and that sex hormones and immune genetics that diverge after puberty — including the effects of testosterone and estrogen and differences linked to the X chromosome — shape bacterial populations.
Those microbial differences, Holmes wrote, can have metabolic consequences. She described several interacting factors that can contribute to what she called the "relationship belly": the male partner's gut adopting bacteria found in his partner, synchronised eating patterns and portion sizes, hormonal changes associated with settled relationships such as increased oxytocin, and harmonised daily habits including snacking and sleep.
Holmes listed signs that a partner's microbiome is being influenced, including sudden tolerance for foods previously disliked, matching digestive reactions such as bloating after the same meals, parallel weight fluctuations and a tendency to share infections, which she said can reflect immune-system synchronisation.
The piece also addressed household dynamics that can drive microbiome change. Holmes wrote that women often have greater influence over household food choices regardless of who cooks, and that dietary preferences can therefore shift the foods consumed in a home. She said those shifts can steer a partner's gut ecosystem toward the foods and carbohydrates that support the resident microbiome, and that the partner whose microbiome is initially less diverse may experience larger changes over time.
To manage these changes without drawing attention to them, Holmes suggested practical adjustments including serving different portion sizes of the same meals to account for men's typically higher caloric needs, incorporating prebiotic foods such as Jerusalem artichokes and oats, and adding fermented foods or a synbiotic supplement. She recommended zinc-rich foods and cruciferous vegetables to support hormones that influence male metabolism, and noted that regular skin-to-skin contact can facilitate bacterial exchange. Holmes also proposed occasional use of male-targeted probiotics or fermented foods to address individual gut needs.
Holmes argued that shared microbiomes can offer benefits. In the article she said couples whose gut bacteria align may see improved immune function, mood synchronisation and potential long-term advantages from exposure to a more diverse microbial community, which she described as a largely positive aspect of close relationships.
The claims in Holmes' column draw on a growing body of scientific literature that examines the human microbiome and how behavior, cohabitation and biology shape it. Researchers have documented household-level microbial overlap in multiple studies, and scientists continue to investigate how those changes may affect metabolism, immunity and other aspects of health. Holmes framed the microbiome changes as an additional dimension of how partners become biologically intertwined over time.
Holmes' column appears amid broader public interest in the microbiome and in lifestyle factors that affect weight and health. While she promoted specific dietary and behavioral strategies, she also maintained that the health benefits of shared microbes can outweigh modest weight gains. The article presented her perspective and recommendations; readers seeking medical or nutritional guidance should consult qualified health professionals and current peer-reviewed research for personalized advice.