As a gut microbiologist I have known for a long time that the gut bacteria helping to keep us healthy rely on the food we eat to survive and produce those health benefits. As a general rule in healthy adults, the more diverse the bacterial population the better for health, and this means consuming a more 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 does the bacterial community within our gut contribute to our health? One important example is the synthesis of specific fermentation products (short chain fatty acids, or SCFAs) that allow the human 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 much research has been done determining that the products of a healthy gut microbiota also have health benefits way beyond the gut. The lowering of pH that prevents growth of pathogens, also increases absorption of calcium, improving bone health. Some of the SCFAs can 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 and… the list goes on.
One of the most important recent findings is that the two-way communication pathway between the gut and the brain is influenced by products from microbes. 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 fecal samples. Studies have also shown that supplementing the diets of aging mice with fiber, or directly feeding butyrate, improves their ability to perform tasks and remember things. Taken together these findings indicate that sufficient fiber consumption, alongside other healthy lifestyle choices, may help maintain brain health in the elderly and prevent cognitive decline.
Fiber 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 fiber. Eating plenty of fiber is crucial to maintain microbial activity – and the more different types of fiber, the more different types of microbe will survive (1). Diets high in complex carbohydrates, particularly fiber, have many beneficial effects. As well as increasing concentrations of beneficial bacterial metabolites like butyrate, fiber is also important for decreasing gut transit time, which reduces exposure to potentially toxic dietary components and any byproducts of less desirable microbial metabolism. Diets which are high in protein and low in fiber result in more protein fermentation by bacteria in the colon, releasing harmful compounds (including nitrosamines and heterocyclic amines). During a four-week study, a high protein-low carbohydrate diet significantly decreased cancer-protective metabolites and increased concentrations of hazardous metabolites, suggesting long-term adherence to such diets may increase risk of colonic disease.
Plant-based foods are the most fiber-rich foods – and 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 fiber per day yet according to the NHS (National Health Service in the UK) 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 fiber-rich foods, with a few simple changes, it can be easy to eat more. Simple dietary swaps can help us reach that target 30g/day – choose higher fiber breakfast cereals and whole grain bread/rolls; keep skins on potatoes (for mashing/baking or even making chips); have nuts (unsalted), chopped fresh fruit, raw veg or even higher fiber biscuits, including oatcakes, as snacks. More subtle 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 or past-its-best veg in homemade soups.
More benefits of plant-based foods
As well as high fiber content, plant-based foods contain many other important nutrients including vitamins, antioxidants and phytochemicals (molecules that mop up damaging free radicals in our gut 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 (find a mention in this ISAPP podcast). Fresh, frozen and tinned (without added sugar) products are all equally important fiber sources.
The most widely advocated high fiber diet – with the most research proving its important health benefits – 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 aging” 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 focussed 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 can be important additional sources of soluble fiber and help to maintain a healthy microbiome and improve bone, heart and mental health. Since they also act as natural sweeteners, but are not digested by human enzymes, they can be used as sugar replacements to reduce blood glucose spikes and improve glycemic index. Prebiotics can be useful supplements to a healthy high fiber diet, and can assist the transformation of a low fiber diet into a higher fiber diet.
Some people may not want to eat more fiber are because they feel it causes flatulence (passing wind) and bloating. If you normally have a very low fiber intake, it can help to increase fiber 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 – especially fiber – digestion by gut microbes it can’t completely be avoided.
Take-home message
Hopefully reading this reminds us all that fiber is so much more than an important source of roughage to 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 brain, heart and bone health. Since there are so many different types of fiber-containing foods, relatively simple diet changes are enough to achieve the recommended intake, with potential health benefits all around the body. So next time you think of saying “I don’t like fiber”, rephrase the sentence into “I don’t like that fiber” – and reach for a different fiber-containing food source, one you do like, instead.
References
(1) Louis P, Solvang M, Duncan SH, Walker AW, Mukhopadhya I. Dietary fibre complexity and its influence on functional groups of the human gut microbiota. Proceedings of the Nutrition Society 2021;80(4):386-397.
