Why the fuss about fibre?
Nutritionists often say “you are what you eat”…you are also what you feed your gut microbes. Your gut bacteria rely on the food you eat to survive and produce many health benefits. In healthy adults, the more diverse the bacterial population the better for health, and this means consuming a diverse diet. Just as different humans prefer different things to eat, so different bacterial species have their own ‘dietary’ preferences. These diverse bacteria interact to produce the many metabolic products (molecules) that keep us healthy.
How exactly do they contribute to our health? One important example is the synthesis of specific fermentation products (short chain fatty acids, or SCFAs) that allow the cells lining the gut to grow and reproduce, maintaining an essential healthy cell turnover. Some of these same fermentation products also suppress inflammation and inhibit the growth of cancerous cells. And the very existence of these fermentation products, which are weak acids, lowers the pH in the gut (or intestine) helping to prevent the growth of bacteria that may make us sick, like some food-borne pathogens.
Going beyond the gut
In recent years research has shown that the products of a healthy gut microbiota also have health benefits beyond the gut. The lowering of pH that prevents growth of pathogens, also increases absorption of calcium, improving bone health. Some of the SCFAs interact with specific receptor molecules in the gut, telling us that we are full and to stop eating, thus having an impact on weight gain. Bacterial products are also now known to improve mood, help strengthen the immune system, improve cardiovascular health etc.
One of the most important recent findings is that the two-way communication nerve pathway between the gut and the brain is influenced by products from microbes, and it mainly (80%) flows from the gut to the brain, not the other way around. Clinical studies have shown the benefits of specific probiotics reducing stress in young people sitting exams. For older populations, microbial production of the SCFA butyrate may be important in maintaining brain function and preventing cognitive decline. There is a growing body of evidence that people living with Alzheimer’s disease tend to have lower amounts of both butyrate-producing bacteria and butyrate itself in faecal samples. Studies have also shown that supplementing the diets of ageing mice with fibre, or directly feeding butyrate, improves their ability to perform tasks and remember things. Taken together these findings indicate that sufficient fibre consumption, alongside other healthy lifestyle choices, may help maintain brain health in the elderly and prevent cognitive decline.
Fibre as the key to unlocking gut benefits
For many of the bacteria providing this myriad of health benefits, their preferred energy sources are foods containing fibre. Eating plenty of fibre is crucial to maintain microbial activity – and the more different types of fibre, the more different types of microbe will survive. Diets high in complex carbohydrates, particularly fibre, have many beneficial effects. As well as increasing concentrations of beneficial bacterial metabolites like butyrate, fibre is also important for decreasing gut transit time, which reduces exposure to toxins and any by-products of undesirable microbial metabolism. Diets which are high in protein and low in fibre result in more protein fermentation by bacteria in the colon, releasing harmful compounds (including nitrosamines and heterocyclic amines). Studies show that a high protein-low carbohydrate diet significantly decreased cancer-protective metabolites and increased concentrations of harmful metabolites, suggesting long-term adherence to such diets may increase risk of colonic disease.
Plant-based foods are the most fibre-rich. These include legumes (lentils, beans, chickpeas); cereals and whole grains (oats, barley, brown rice); fruits; vegetables; nuts and seeds. The target for a healthy adult is to consume 30g fibre per day, yet according to the NHS most people only manage 20g/day. For teenagers the target is 25g/day, but most only consume about 16g.
With so many different options for fibre-rich foods, with a few simple changes, it’s easy to eat more. Simple dietary swaps can help us reach that target 30g/day – choose whole oats and whole grain bread; keep skins on potatoes and carrots; snack on nuts (unsalted), chopped fresh fruit, raw vegetables or even higher fibre biscuits, including oatcakes. Tricks to increase family intake include mixing whole wheat and white pasta, mixing brown and white rice, adding beans, pulses or extra vegetables to stews and curries, and using any leftover veg in homemade soups. My favourite is blending cooked red lentils into vegetable soups - this not only adds fibre but also protein.
More benefits of plant-based foods
As well as fibre, plant-based foods contain many other important nutrients including vitamins, antioxidants and phytochemicals (molecules that mop up damaging free radicals preventing cell damage). These micronutrients are also important for health and are released following fibre degradation and fermentation. Easy to follow guidance includes ‘eat a rainbow’ or setting oneself a challenge to consume 30 different plant-based foods a week (this would include herbs and spices). Scientists are investigating how the diversity of different foods contributes to the diversity within the gut microbiota – and therefore their metabolites.
The most well-researched high fibre diet is the Mediterranean diet. In addition to a higher fish consumption, the Mediterranean diet is generally higher in fruit, vegetables, cereals and pulses and lower in refined foods than a standard Western diet, providing lifelong health benefits. It is no coincidence that there are more people in the “healthy ageing” category in countries where this type of diet is prevalent. The DASH (Dietary Approaches to Stop Hypertension) diet designed to lower high blood pressure is also focused on increasing consumption of fruit, vegetables, and cereals while limiting salt, sugar and saturated fat intake. The MIND (Mediterranean-DASH Intervention for Neurodegenerative Delay) diet is basically a merging of the Mediterranean and DASH diets, focusing on improving brain health, while also benefiting heart health).
Prebiotics are helpful additional sources of soluble fibre, which not only improve microbiome, bone, heart and mental health, but also reduce blood glucose spikes and improve glycaemic index. Prebiotic supplements can assist the transformation from a low into a higher fibre diet.
Some people may not want to eat more fibre because they feel it causes flatulence and bloating. If you normally have a very low fibre intake, try increasing your fibre consumption slowly, allowing your body (and gut bacteria) to adjust to the change in your diet. However, since gas production is a natural part of food (fibre) digestion by gut microbes it can’t completely be avoided.
Conclusion
Fibre is so much more than roughage helping our digestive systems and improving gut transit. It is a key food source for our gut microbes, which during degradation and fermentation release important bacterial metabolites that circulate around the body improving our overall health, especially brain, heart and bone. With many different types of fibre-rich foods, diet changes are easy to achieve the recommended intake. For every 10g/day increase in fibre, it has been shown to achieve a 10% reduction in all-cause mortality (up to 45%) ie it will help you live healthier for longer.