Research has long suggested a link between our diet and our mental health. The gut microbiome — the collective genome of trillions of bacteria that live in the intestinal tract that are created largely by what we eat and drink — appears to influence our mood and mind-set.
But human studies large enough to pinpoint what bacteria matter, if they matter at all, have been missing.
That’s slowly changing. The largest analysis of depression and the gut microbiome to date, published in December, found several types of bacteria notably increased or decreased in people with symptoms of depression.
“This study provides some real-life evidence that you are what you eat,” says study author Andre Uitterlinden, who researches genetics at Erasmus Medical Centre in Rotterdam, Netherlands.
Or to be exact, how you feel is closely related to what you consume.
The gastrointestinal system has been featured in brain research for centuries. In the early 1800s, John Abernethy, a popular London physician, held that “gastric derangement” was the root of all mental disorders.
And gastrointestinal symptoms often are reported in people with psychiatric illness. Weight and appetite changes are common among people with depression, from adolescence to older age. Anxiety has been tied to a heightened risk of nausea, heartburn, diarrhea and constipation. The link between food and mood is there even when we reach for macaroni and cheese to comfort us during a stressful time.
Interest in the gut-brain axis has had a resurgence in the past 20 years. A host of studies has pointed to a connection between the microbiota living in our intestinal tract, and our minds, including our memory, mood and cognitive skills.
Such research has spawned an industry of probiotics, prebiotics and fermented everything. Scientific names like bacteroidetes and lactobacillus, two of the most common bacteria found in healthy humans, have become household terms.
The health trend has gotten a bit ahead of the evidence. Most of the studies linking depression and the gut, for example, have been in animals and studies involving human participants have been small.
Still, the evidence thus far shows a link between the two. In one noteworthy study, entitled “Transferring the Blues,” bacteria-free rats given fecal samples from humans diagnosed with major depression became anxious and disinterested in pleasurable activities. Their metabolism of tryptophan, a chemical connected to depression, changed. But the mechanics behind the microbe-mood pathway — and which bacteria matter — has been harder to uncover.
Bacteria that predict depressive symptoms
This new study moves that needle, largely because of its size. The investigators, led by Najaf Amin, who researches population health at Oxford University, analyzed data from the Rotterdam Study, a decades-long effort to understand the health of the local population.
Amin and her colleagues focused specifically on a phase of this study that included fecal sample collection from more than 1,000 individuals. These participants
