Stress

How Gut Bacteria Change the Stress Response

Several randomised controlled trials have tested this directly in people, which is unusual for a topic this soft-sounding. Stress is not only a feeling. It is a measurable sequence in which the brain signals the adrenal glands to release cortisol, a hormone that raises alertness and blood sugar. Gut bacteria appear able to turn the volume on that sequence up or down. The trials that have tested it are small and their results are mixed. What makes them worth reading is that some of them changed a hormone in the blood, not just how people rated their mood.

The fibre that lowered morning cortisol

Researchers gave healthy volunteers galacto-oligosaccharide for three weeks. Galacto-oligosaccharide is a fibre the body cannot digest, so it travels intact to the large intestine and is broken down by bacteria there instead. It occurs naturally in beans, lentils and chickpeas, and it is one of the main fibres in human milk.

Two things changed. The cortisol surge participants produce shortly after waking was reduced, which is a standard measure of how reactive the stress system is. In a computer task measuring attention to emotional words, they also attended less to negative material.

The effect was specific to that fibre rather than to fibre in general. That is what you would expect if particular organisms, rather than fibre itself, are doing the work.

Source: Prebiotic intake reduces the waking cortisol response and alters emotional bias in healthy volunteers

Adding bacteria directly, and what it did

Feeding existing bacteria is one approach. Introducing new ones is another, and it has also been tested.

Healthy volunteers took a combination of two probiotic strains for 30 days. Probiotics are live bacteria taken deliberately with the aim of changing the gut community. The strains were Lactobacillus helveticus R0052 and Bifidobacterium longum R0175.

Compared with placebo, the probiotic group reported less psychological distress. Their urinary free cortisol fell, which meant the change was not only in how they described feeling.

Introducing bacteria this way only helps if they are then fed. Think of it as stocking a pond. Adding fish achieves nothing if there is no food in the water, and the same applies to bacteria arriving in a gut with little fibre reaching them. They fail to establish and the effect fades.

Source: Beneficial psychological effects of a probiotic formulation (Lactobacillus helveticus R0052 and Bifidobacterium longum R0175) in healthy human volunteers

Food against supplements, tested head to head

That raises a practical question. If both feeding bacteria and adding them can work, which does more?

One trial compared them directly. Adults were randomly assigned to a diet high in prebiotic foods, to probiotic supplements, or to both together.

The high-prebiotic diet came out ahead on the trial's main measure of mood. It also improved perceived stress scores and sleep by the end of the study.

The scale of the testing is worth knowing. A review gathered 46 randomised controlled trials covering 3,516 participants. Across those, the effect on cortisol was most consistent in healthy people who were not taking other medication.

Results do vary between trials, and not every study has reached a measurable change in stress markers. Where an effect has been found, food has performed at least as well as supplements.

Source: Effect of Probiotics Supplementation on Cortisol Levels: A Systematic Review and Meta-Analysis

Source: Effects of a high-prebiotic diet versus probiotic supplements versus synbiotics on adult mental health: The Gut Feelings randomised controlled trial

Source: Prebiotic Consumption Alters Microbiota but Not Biological Markers of Stress and Inflammation or Mental Health Symptoms in Healthy Adults

What this means for you

Stress that's persistent, disrupting your sleep, work, or relationships is worth taking to a doctor or therapist. In addition, the trials above found galacto-oligosaccharide, the fibre concentrated in beans, lentils and chickpeas, lowered the waking cortisol response within three weeks, and food matched supplements when the two were compared directly. A varied, plant-rich diet supports the same gut bacteria this research ties to a calmer cortisol response. That's the same idea behind GutLab, everyday gut health starts with regularly feeding the microbiome a broad range of plant-rich ingredients.