The discovery of prebiotics, a fibre-rich food group first defined in 1995, has greatly enhanced our understanding of gut health.
The food promotes the diversity and function of good bacteria in our gut, a key indicator of health. But recent research suggests the gut-friendly foods may have other benefits, including helping us sleep.
Expanding our notions of the gut microbiome, a new study was published in March 2020 in Scientific Reports, found that rats that grazed on a diet enriched with prebiotics, had longer periods of restorative non-rapid-eye-movement (NREM) sleep compared to those that didn't.
A further goal was to test an earlier hypothesis by University of Colorado researchers that prebiotics might modulate changes in the gut caused by stress which could in turn impact sleep.
The rats were further divided into those put into a stressful environment and those left alone in their cage. The researchers found lower levels of stress hormone metabolites in the stools of rats on the test diet.
Dr Suzanne Mahady, a gastroenterologist and clinical epidemiologist at Monash University (a world leader in gut disorders), says the key finding was that a diet rich in prebiotics was associated with reduced stress hormones in the stool, even in the context of increased stress.
"While the findings are interesting, they cannot be readily translated to humans without more research."
A phenomenon dubbed the 'brain-gut axis', suggests the brain and gut may have a bi-directional relationship.
Dr Mahady says, "While the findings are interesting, they cannot be readily translated to humans without more research.
"It's a nice line of research and one to watch. However, there's still years' worth of work to fully understand the links and also understand how we would use this approach in humans."
For example, the study only tested four prebiotic substances: galactooligosaccharide (found in legumes and some nuts); polydextrose (PDX) a food additive; lactoferrin, present in breast milk; and milk fat globular protein.
Dr Mahady says it's important to take the findings in context to the broader approach to getting good sleep, including ensuring exercise, healthy relationships and stress reduction.
In the meantime, there's no harm adding more of these healthy, fibrous foods into your diet.
Types of dietary fibres so far found to have a prebiotic action include fructans (found in onion, garlic, leeks and wheat) and galactooligosaccharides.
Nutritionist and dietician Kara Landau, media representative nutrition advisor to the Global Prebiotic Association and founder of Uplift Food, says prebiotics have many positive effects, including "reducing inflammation, helping cells be more responsive to insulin, improving digestion and more".
However, various surveys have found many people don't know what prebiotics are, or confuse them with probiotics.
Probiotic foods, like probiotic supplements, are rich sources of beneficial bacteria. They include fermented foods like yoghurt, sauerkraut, tempeh, miso, kombucha and pickles.
Prebiotics, on the other hand, are nutritional ingredients, including types of fibre, that pass undigested through our bodies. Probiotics selectively utilise and thrive on them. More recently, resistant starches (those that the body can’t digest) and some polyphenols (compounds found in plant foods), have also been found to have prebiotic action, Landau says.
"Some of the richest sources of prebiotics are not commonly consumed," Landau reveals. These include chicory root, dandelion greens, Jerusalem artichokes, green (unripe) bananas, legumes, lentils and raw oats.
More commonly eaten foods that contain prebiotics include asparagus, onions, garlic, cashews, pistachios, and cooked and cooled grains and potatoes.
The process of cooling grains prior to consumption turns regular carbohydrates into prebiotic resistant starches, she explains.
To increase your intake, Landau suggests including more plant-based ingredients in your diet, especially those rich in prebiotics.
"In addition, cooling any grains that you consume prior to consumption makes a significant difference," she says.
Landau is unsurprised by the sleep/probiotic link. "Although this study was only performed on animals, I believe the findings appear in line with other areas of research and prebiotics and inflammation.
"It's a good starting point for further investigation."