Your gut microbiome has a passport

Your gut microbiome has a passport, according to a new study of 1,976 people from five countries. It was published in Nature Communications, and it compared the USA, Mexico, the UK, Spain and Japan.

And by the end, you'll see why there's no single ideal microbiome for everyone.

Let's start in Japan. Out of 3,375 microbial units, only three were found almost exclusively in Japan. They were strains of Bacillus subtilis. And their link with natto, the fermented soybeans, was very strong.

But it's not just which microbes live in each country. It's their genes, too.

In Mexico, Bifidobacterium longum has largely lost two genes, galC and galD, tied to galactose, the sugar in milk. Fewer than 10% of the Mexican genomes carried them, while in the other countries it was over 70%.

In the US, the diet had about 30% more fat per calorie than the Japanese diet. And the bacteria followed the same pattern: their genes for building fatty acids were more abundant, while in Japan they were less abundant. In other words, more fat on the plate goes with more fat-handling tools in the microbes.

In the UK, dietary folate was among the highest, and the strains showed the biggest drop in folate-making genes. The authors' likely explanation: when folate comes from food, the microbes don't need to make it.

Spain was the opposite. Dietary folate was the lowest, and the strains carried more genes to make it. On top of that, Spain had the highest healthy-eating score, with more legumes and fish, and less salt and sugar.

Spain and Mexico had the most similar microbiomes. Even so, a model trained to read diet quality from the microbiome in one country lost accuracy in another.

So how much does the country matter? It explained 28% of the differences in the microbiome, while body mass index explained just 0.3%. And almost half of the species studied showed genetic differences by country, mostly fiber-fermenting bacteria like Prevotella.

In other words, your microbiome is largely local. It's shaped by geography, by culture, and by what's grown and cooked where you live. That's why one ideal microbiome for everyone doesn't exist.

And the samples? They came from used toilet paper.

Now, the limits. 75% of the participants were women, and diet was recorded with questionnaires. So this is a correlation. It doesn't prove that diet alone causes these differences.