Dietary fibre, fiber, or roughage, describes the fibrous parts of plants that pass through the small intestines undigested, as we lack the enzymes needed to break them down. There are two main types of fibre, soluble fibre and insoluble fibre.

Soluble fibre (pectins, alginates, inulin, beta-glucans), as its name implies, dissolves in water to form a gel. Soluble fibre is most important in the stomach and upper intestines, where it mops up fats, cholesterol and sugars to slow the rate at which they pass into your circulation. It also helps to soften stools and prevent constipation.

Insoluble fibre (eg lignin, cellulose, hemicellulose) does not dissolve in water and is most important in the large bowel, where it acts like a sponge to absorb water, bacteria and toxins, and provides bulk to aid bowel emptying.

There is a lot of cross over between the two types of fibre, however, and its solubility doesn’t always predict which foods will provide which health benefits. Once they reach the large bowel, both soluble and insoluble fibre are fermented and broken down by bacterial enzymes, and have beneficial effects on the balance of gut bacteria.

Dietary sources of fibre

Fibre is a form of carbohydrate composed of long chains of sugar molecules joined together. Unlike other carbohydrates, however, fibre resists digestion in the stomach and small intestine, and reaches the large bowel intact.

All plant foods contain both soluble and insoluble fibre, though some sources are richer in one type than another.

Foods that are high in soluble fibre include: rice bran, avocado, barley, flax seeds, chia seeds, citrus fruits, oat bran, oats, beans/legumes, peas, figs, apples, strawberries, tomatoes, apricots, carrots.

The richest dietary source of soluble fibre is chia seeds, which consist of 37% fibre – 80% of which is soluble fibre. That’s why chia seeds dissolve to form a thick gel. A supplement source is inulin, a fructo-oligosaccharide prebiotic fibre that feeds the beneficial probiotic bacteria in your gut.

Foods that are good sources of insoluble fibre include: wheatbran, wholemeal bread, cereals, sweetcorn, corn bread, brown rice, wholemeal pasta, rhubarb, blackberries, cabbage, spinach, kale, lettuce, turnip, Brussels sprouts, cauliflower, peas, lentils and chick peas. A supplement source of insoluble fibre is psyllium husks.

You need both types of fibre for optimum gut health.

How much fibre you need

Until recently, recommendations were to obtain at least 18g fibre per day – significantly higher than the average intake of 12 grams per day. Yet in 2015, the Scientific Advisory Committee on Nutrition increased this recommendation further to a daily intake of 30g per day for everyone over the age of 16. This means most people need to almost double their fibre intake.

In evolutionary terms, even this level of intake is still low – our stone-age ancestors regularly ate over 100g fibre per day from a variety of plant sources.

Benefits of a high fibre diet

A high-fibre diet helps to prevent constipation, diverticular disease, irritable bowel syndrome and may even protect against bowel cancer. The effects are more far-reaching than your gut however, as the beneficial effects of fibre on digestion and bowel bacteria mean that a high fibre diet also helps to decrease blood pressure, reduce inflammation, improve glucose control and cholesterol balance, as well as protecting against obesity, type 2 diabetes, heart attack, stroke and fatty liver disease.

Researchers estimate that, for every 7g increase in fibre that you eat, you have a:

• 9% lower risk of cardiovascular disease
• 7% lower risk of stroke
• 7% lower risk of bowel cancer
• 6% lower risk of type 2 diabetes.

When changing to a high fibre diet, or taking fibre supplements, increase your intake gradually to prevent feelings of bloating and distension which can occur during the first two to three weeks.

It is also important to ensure a good fluid intake, and to eat as many different sources of fibre as possible. New research suggests that bowel bacteria quickly adapt to the types of roughage in your diet. If you mainly eat fibre of one type (eg bran) bowel bacteria will respond within a week or two by increasing their output of enzymes needed to ferment this. The fibre reaching your colon will then be broken down more quickly so some benefits are lost.

Fibre and cholesterol

Fibre binds to bile salts, and to cholesterol and dietary fats to reduce or slow their absorption increase the amount voided. This can have a significant effect on cholesterol levels. Studies show that eating 10 g psyllium seed per day, for at least six weeks, can reduce blood LDL-cholesterol levels by between 5% and 20%. Eating 60 g of oatbran per day (baked into muffins) was found to significantly lower cholesterol levels, while eating pulses regularly lowers total cholesterol levels by at least 7%, with reductions in ‘bad’ LDL-cholesterol and increases in ‘good’ HDL-cholesterol. These benefits are largely attributable to their soluble fibre content.

Fibre and weight loss

Fibre is filling while providing few calories, so eating more fibre-rich foods is a great way to curb appetite and support weight loss. Fibre also slows the absorption of dietary sugars to reduce the blood glucose swings that can trigger hunger and food cravings. A recent study showed that taking fibre supplements can reduce energy intake and contribute to weight loss by reducing the frequency of eating and the total amount of food consumed. Over 12 weeks, those taking fibre supplements lost 2.5 cm in waist circumference and 1.4kg weight loss without making any other dietary changes.

Fibre and menopause

A high fibre-diet improves oestrogen balance and women who follow a high fibre diet long-term appear to tolerate the drop in oestrogen levels seen around the time of the menopause better than those following a low fibre diet.

Fibre and constipation

Inulin supplements have been shown to reduce functional constipation in elderly people. The inulin dose was initially 20g per day, and after 8 days was slowly increased to 40g per day. Inulin was more effective than the prescribed laxative, lactulose, and increased bowel movements from between one and two per week to as many as eight or nine per week in 70% of cases which, as a GP, I have to say is impressive. Stools were soft, with no diarrhoea, only mild-to-moderate flatulence and no discomfort. Fibre also reduces the risk of diverticular disease and haemorrhoids.

Fibre and IBS

A high-fibre diet helps to prevent constipation (by increasing bowel transit time) and diarrhoea (by absorbing excess fluids) and is helpful for around one-third of people with irritable bowel syndrome (IBS). A quarter of people with IBS find that changing to a high fibre diet initially makes their bloating and distension worse. This effect usually disappears after 2 or 3 weeks so it’s worth trying to persevere, and to build up your fibre intake slowly so your bowel has time to get used to it.

How to follow a high fibre diet

Follow a high fibre diet by eating more unrefined complex carbohydrates such as wholemeal bread, cereals, nuts, grains, root vegetables and fruits.

It’s easy to add extra fibre to meals, to increase their filling effect and help you eat less overall. Some ideas include:

• Eat more plant-based foods – serve larger portions of veg and cut back on meat
• In many plants, the skin contains the most fibre so keeping the skin on apples and potatoes, for example, will increase your fibre intake.
• Adding cooked pearl barley or legumes to home-made soups and stews
• Replacing half the meat in a chili con carne or bolognese sauce with grated vegetables such as carrots, turnip, courgettes/zucchini, aubergine/eggplant, onions and chopped leaks
• Replacing a third of the mince in meatballs with uncooked oats
• Replacing one-third of the flour in baked puddings or fruit crumble toppings with quick oats
• Have dried apricots, dried bananas, or a handful of mixed nuts and seeds as a snack

Foods containing 3g or more fibre per 100g are considered high fibre choices – for example (cooked white pasta and rice don’t quite make the cut but are still useful sources):

A typical day in a high fibre diet might look like:

Breakfast:	Home-made Bircher Muesli OR Porridge topped with dried apricots and a handful of almonds OR Wholemeal toast with shredded marmalade
Snack:	Fresh or dried fruit eg apricots
Lunch:	Courgette & Spelt Grain bowl OR a Mediterranean bean salad, large mixed salad and walnut bread OR a jacket potato with sweetcorn and coleslaw OR falafel, humus and salad in a wholewheat wrap.
Snack:	Handful of pistachio nuts
Dinner:	Sweet potato curry, served with green beans and brown rice; OR Mushroom and walnut lasagne, served with broccoli and sweet potato chunks; Fresh Fruit Salad.

 More recipes are available on the Alpro website and from Waitrose .

Fibre supplements

Fibre supplements include inulin, wheat bran, psyllium (also known as ispaghula), sterculia and linseed. The effectiveness of psyllium derives from its mucilage content, which swells to between and 8 and 14 times its original volume when mixed with water.

It is important to drink plenty of water when eating a high fibre diet. It is also important to increase fibre intake slowly, so your intestines have time to adapt to increased amounts.

When increasing fibre intake, I also recommend adding in a probiotic supplement to ensure a healthy balance of bacteria to ferment fibre in the lower bowel.

Image credits: Pixabay; Waitrose/Alpro