Prebiotics are substrates that are selectively utilised by host microorganisms in ways that confer health benefits. In practical terms, they are dietary fibres and polyphenols that are not digested in the small intestine and that selectively promote the growth of specific beneficial bacteria, particularly Bifidobacterium and Lactobacillus species, in the large intestine. Unlike probiotics, which introduce specific bacterial strains, prebiotics nourish the resident beneficial bacteria already present in the gut.
The Mental Health Mechanism
A randomised double-blind trial from the University of Oxford found that healthy adults who consumed a prebiotic supplement of fructooligosaccharides and galactooligosaccharides for three weeks showed significantly reduced attentional vigilance to negative stimuli and reduced anxiety-related morning cortisol compared to placebo. These are measurable effects on the psychological and physiological dimensions of stress and anxiety from dietary fibre alone.
Best Dietary Prebiotic Sources
- Garlic and onions: fructooligosaccharides that selectively feed Bifidobacterium; use raw in salad dressings and as a cooking base in all savoury dishes
- Jerusalem artichokes: one of the most concentrated dietary sources of inulin at 14 to 19 grams per 100 grams; introduce gradually to avoid excessive fermentation symptoms
- Asparagus: contains inulin and other prebiotic fibres; lightly steam and eat regularly during the spring season
- Leeks: related to garlic and onions, containing similar prebiotic fructooligosaccharides
- Slightly unripe bananas: contain resistant starch and FOS; riper bananas have less prebiotic content
- Chicory root: the most concentrated commercial source of inulin; used in chicory coffee and as a partial coffee substitute