Dietary fibre is an essential part of a healthy diet and is important for many functions in the body such as regulating bowels, increasing satiety (fullness), and regulating blood cholesterol, glucose, and insulin levels.

While the benefits to physical health are clear and consistent, a growing body of evidence now suggests that dietary fibres are also important to mental health. Diets high in fibre from fruits, vegetables, legumes, and whole grains (e.g. the Mediterranean diet) have demonstrated potential to improve the risk for mood disorders including depression and anxiety, which are the most common mental conditions across the globe, as well as treat them when they occur.

What are dietary fibres?

Dietary fibres are a diverse group of molecules that are usually part of plant cell walls and that also include storage molecules such as resistant starch. Owing to the diversity of dietary fibres, they are classified based on their molecular structure, composition, solubility, and fermentability. Humans lack fibre-digesting enzymes and the fibrous part of the food thus escapes digestion in the stomach and small intestine and enters the colon. Here, fibre becomes available for fermentation by gut microbes, which use these fibres to create molecules such as short-chain fatty acids. Many of these molecules have anti-inflammatory properties, as well as many other actions in the body.

Fermentable fibres, particularly those with prebiotic properties such as fructans and galacto-oligosaccharides, are capable of influencing biological pathways that are relevant to depression and anxiety through the microbiota-gut-brain axis. People experiencing psychiatric disorders often have an altered gut microbiota composition, with increased proportion of microbes that can trigger inflammation, potentially affecting the body and brain. Therefore, modifying the gut microbiota through fibre intake may be a potential strategy for those experiencing common mental disorders.

Prebiotic fibres positively affect the gut microbiota by increasing the abundance of beneficial bacteria such as Bifidobacterium and Lactobacillus. These bacteria can also produce chemicals and proteins that interact with the brain and can influence depression and anxiety. Gut bacteria play an important role in metabolising tryptophan, which affects the amount of serotonin in the brain. More fermentable types of fibre may therefore be specifically useful for influencing depression and anxiety.

Our research

Taking into consideration the potential of dietary fibres to influence mood disorders, we conducted a comprehensive systematic literature review and meta-analysis that examined fibre intake in relation to depression and anxiety. We examined both observational studies and randomised controlled trials (RCTs), which are the gold-standard in research trials.

We reported on 18 observational studies, which revealed a significant moderate inverse association between total dietary fibre intake and depressive and anxiety outcomes (i.e. greater fibre intake was linked with lower depressive and anxiety symptoms). Despite the positive findings, the generalisability of our findings remain limited as most of the studies were done on healthy people and had small effect sizes. Observational studies by their nature do not allow us to determine cause and effect; for example, people eating more fibre may also have other lifestyle behaviours that may affect anxiety and depressive symptoms, such as being more physically active. This is why RCTs are important to tease out cause and effect.

All of the 10 RCTs used single fibre supplementation, particularly fermentable or prebiotic fibres at varying amounts, rather than diet. Our analysis showed that fibre supplementation did not improve either depressive or anxiety outcomes compared to placebo. However, when we performed sub-group analysis to examine whether our findings differed based on fibre supplement type, we found some evidence that galacto- oligosaccharides, a type of prebiotic fibre, led to improved anxiety outcomes compared to placebo.

Although our findings did not demonstrate overall beneficial effects of fibre supplementation in improving depressive or anxiety outcomes, the potential role of fibres on mood disorders cannot be ruled out entirely. This is because the RCTs included in the meta-analysis had limitations in study design, such as small participant sizes and not including participants with elevated depressive or anxiety symptoms.

It is likely that changing the whole diet will be more beneficial than single-fibre supplementation. This is because whole-diet modification means  increased diversity of fibre in the diet, with different fermentability profiles, with a broader range of effects on the gut microbiota and its metabolites. Given that the many thousands of different bioactive compounds in plant foods – such as polyphenols – are also important ‘food’ for gut bacteria, this approach is likely to have a stronger influence on depressive or anxiety outcomes.

How much fibre should I eat?

In Australia, the recommended amount of fibre is 25g for women and 30g for men per day. Eating in line with the Australian Dietary Guidelines will provide you will enough fibre each day. Have a look at our fibre counter to get an idea of how much you can find in fibre-rich foods.

Tips to eat more fibre:

  • Eat plenty of plant foods – fruits, vegetables, wholegrains, legumes, nuts and seeds
  • Choose whole, dried or canned fruit instead of juice
  • Leave skins on fruit and vegetables (even on potatoes, carrots, pumpkins, and apples) as a lot of the fibre and other nutrients are in the skin
  • Choose wholemeal or wholegrain breads, rices, and pastas
  • Have a high fibre breakfast – for example unprocessed oats, bran, muesli, wholemeal or sourdough toast
  • Swap white flour for wholemeal when cooking or baking
  • Include high fibre snacks – for example high fibre crackers, nuts, fruit, vegetable sticks and hummus
  • Add high fibre vegetables and legumes to salads, such as corn, potato or sweet potato
  • Swap meat for legumes (i.e. chickpeas, beans and lentils) in soups, casseroles, salads and pasta sauces

Prebiotic fibres

Specific prebiotic fibres may be especially important for promoting the growth of beneficial bacteria in our gut. Prebiotic fibres can be found in:

  • Some types of vegetables such as artichokes, asparagus, beetroot, celery, fennel, garlic, green peas, leek, onion, savoy cabbage, shallots, snow peas, spring onion, and sweetcorn.
  • Some types of fruits such as apples, dried dates, figs, grapefruit, nectarines, pomegranate, watermelon, and white peaches.
  • Most legumes such as baked beans, chickpeas, lentils, red kidney beans, and soybeans (edamame).
  • Most wholegrains such as barley, bran, buckwheat, oats, rye, and spelt.
  • Some types of nuts & seeds such as almonds, cashews, hazelnuts, linseeds, and pistachios.

Important things to consider as you increase the amount of fibre in your diet:

Fibre needs to absorb fluid in your gut, so it is important that you’re drinking enough water. Most people need about 2-3 litres of fluid per day. Without enough water, your stools may be too hard and become difficult to pass. Also, your gut can take some time to adapt to a higher fibre diet, so increase your fibre slowly to avoid temporary gut symptoms like bloating and gas.

Overall, our research has found a small but positive link between higher fibre intake and lower levels of anxiety and depressive symptoms. We found that fibre supplementation may not have a huge effect on anxiety or depressive symptoms, but fuelling yourself with fibre-rich foods is a great way to support your body and mind. If you want to learn more about prebiotic fibres and their effect on mental health, watch the Food & Mood Academy’s on demand webinar on the topic.