Fibre is the indigestible carbohydrate that is found in whole grains, vegetables, fruits, nuts, seeds, lentils and beans. Humans do not have the enzymes to break down fibre, but the beneficial bacteria that are living in our guts do. They feast on the fibre and during the process of digesting it, they produce many different metabolic by-products that we can benefit from. For example, some of the different bacterial species in our guts can produce B-vitamins, serotonin (the molecule that makes us feel happy) and vitamin K.
Fibre absorbs water in the gut, making the food bulkier and easier to pass through the body. This decreases the amount of time that food waste spends in the body and reduces the risk of infection. A high fibre intake is associated with low incidence of bowel diseases such as bowel cancer, colitis and diverticulitis. A person on a low-fibre diet can have a gut transit time as high as 72 hours, whereas a person eating a high-fibre diet can reduce this time to 24 hours, helping any carcinogens or toxins to be quickly removed from the body. It follows that eating sufficient fibre makes you less likely to suffer from constipation.
The British Nutrition Foundation recommends that adults have 30g of fibre a day. To give you an idea of what this looks like a jacket potato with the skin on has 4.7g, a bowl of Bran Flakes has 7.4g, 2 slices of wholemeal bread contains 5.2g, 1 portion of boiled broccoli has 2.2g and an apple has 1g.
After a large fibre intake, some people experience gas and bloating. One way to avoid this is to gradually increase the amount of fibre in the diet so your gut can adapt and get used to it over time. To have the best effect, an increase in fibre should be accompanied by an increase in fluid intake.
Foods containing fibre also slow down the absorption of sugar from the food, helping to keep you feeling fuller for longer and maintaining energy levels. For example, if you compare eating one apple with drinking the juice of that one apple, the whole apple contains fibre and keeps you feeling fuller for longer than the juice (which contains no fibre) might. A high fibre diet might be helpful, therefore, for controlling appetite, balancing blood sugar levels and weight management. It can also help reduce cholesterol.
Fibre comes in several different forms: pectins, mucilages, resistant starches, prebiotics, gums, soluble and insoluble fibres. Some bacteria in our guts are very selective and will only eat certain types of fibre and others are not fussy and will eat any fibre. By eating a wide variety of different fibres we feed a diverse bacterial population, which is beneficial for our health. The best way to do this is to eat lots of different plant-based foods weekly. Choose from whole grains, beans, legumes (chickpeas, lentils, split peas), nuts, seeds, vegetables and fruit.
Here are some simple ways to increase the fibre content of your diet:
Swap white bread, pasta and rice for wholegrain varieties
Don’t drink your fruit (i.e. fruit juice) – eat it!
Eat five servings a day of vegetables – a variety of different colours is best
Don’t overcook your vegetables – they should still have some “bite”
To summarise: Fibre cannot be digested by the human body, but is beneficial for our digestive, metabolic and cardiovascular health. Fibre is found in plant-based foods.