The building blocks of carbohydrates and fibre are sugars. Carbohydrates are classified according to the size of the chains of sugar molecules they contain; fibre is classified according to the type of sugar it contains.
The stored energy supply of green plants consists of carbohydrates. Plants make carbohydrates using carbon dioxide, water and energy from sunlight. Carbohydrates consist of combinations of carbon, hydrogen and oxygen atoms. They are categorized as simple and complex according to their size.
The simplest carbohydrates are simple sugars or mono-saccharides, which contain few atoms. The most important simple sugar is glucose (found in fruit and vegetables). Simple sugars are digested and absorbed quickly (although some simple sugars take longer as they need to be converted to glucose before use).
Simple sugars can be joined together to make more complex double sugars or disaccharides, such as sucrose (found in table sugar, honey, fruit and vegetables) and lactose (found in milk).
Chains of simple sugars join together to form complex carbohydrates or polysaccharides. The main complex carbohydrates are starch and fibre. Starches are digested and broken down slowly to provide glucose as energy for the body; fibre contains less digestible carbohydrates. Cereals, vegetables, fruits, pulses (beans, peas and lentils), nuts and seeds all contain both starch and fibre.
Dietary fibre refers to all the edible and largely indigestible parts of plant foods. Whole grains, fruit, vegetables and pulses all contain dietary fibre. Fibre is the structural part of plants – the connective tissue that supports the cells and the outer coating or skin. There is no fibre in animal products such as meat, cheese or eggs. There are two types of fibre – soluble and insoluble.
This consists of non-cellulose carbohydrates such as pectins (the substances that hold plant tissues together).
This consists of cellulose (chains of glucose units) and lignin (a non-carbohydrate material found, for example, in the structure of wood).
SOURCES OF CARBOHYDRATES AND FIBRE
Carbohydrates are readily accessible from a wide range of plant sources. Wholegrain cereals are a particularly important source of unrefined complex carbohydrates. Other plant sources include fruit, vegetables, pulses, nuts and seeds. These foods should be favoured over processed carbohydrates and simple sugars (sweets, cakes, honey, jam and sucrose). Most plant foods also provide both insoluble and soluble fibre but usually contain more of one type than the other.
The range of cereals
Cereals are one of the most important sources of complex carbohydrates, and this importance is reflected in the enormous range of staple carbohydrate foods around the world, from rice, bread and pasta to maize and millet. Wholegrain cereals are particularly nutritious since they contain B vitamins, fibre and protein. They are also particularly valuable for vegetarians.
Originally cultivated in central America, maize or corn is the basis of a wide range of foods around the world including cornflour, cornmeal, breakfast cereals and popcorn. Coarsely ground cornmeal is used to make dishes such as polenta in Italy, tortillas in Latin America, and mealie in Africa. Maize is also high in protein.
A favourite in Scandinavian countries, rye is a nutritious grain used to make a moist, heavy bread, as well as crispbreads. The nutritional value of rye is similar to that of wheat.
For thousands of years, rice has been the staple food for about half the world’s population. In southeast Asia, China and Japan, noodles made from rice flour are an important part of the diet. Brown rice is a good source of B vitamins.
Probably the earliest cultivated grain, barley is a staple food in the Middle East. In the West, pearl barley is added to soups and stews. Although the hull and bran have been removed, pearl barley still contains fibre and is easily digested.
Although classed as a whole grain, buckwheat is a tiny triangular seed. It is a staple food in Russia, where buckwheat flour is used to make hearty blini pancakes. It has a high fibre and protein content.
This grain-like product is actually the seed of a plant native to South America which was cultivated by the Incas. Quinoa is high in protein, and, like amaranth, its protein is more complete than that in other grains; it is also high in iron. Quinoa contains no gluten, so is useful for those with gluten allergy.
The seed of a plant originally cultivated in Mexico by the Aztecs, amaranth has a nutty flavour and can be used whole or ground into flour. It is higher in protein than most cereals, and its protein is more complete than that of other grains.
This is a staple food in Asia and North Africa, where it is made into flat breads. An all-round nutritious grain, millet generally has more and better protein than wheat, rice or corn.
The seed of life for one-third of the world’s population, wheat is used in many forms: wheat flour is used to make bread and pasta; milled hard wheat (known as semolina) is used in the production of pasta and couscous (a very small pasta) and in baking; and bulgur wheat consists of cracked wholewheat grains. Wheatgerm is often added to baked goods such as bread and muffins. Wheat is a useful source of protein, and the germ is a good source of the B vitamin, folate, and vitamin E. Oats Common throughout northern Europe and North America, oats are traditionally used in breakfast porridge and in baking. Oatmeal is oat grain that has been hulled, steamed and rolled flat. Soluble fibre in oats may lower blood cholesterol levels.
Other plant sources
Vegetables (including root vegetables, such as carrots, parsnips, swede, turnips and beetroot, and tubers, such as potatoes and sweet potatoes), pulses (beans, peas and lentils), fruit, nuts and seeds are all good sources of complex carbohydrates and fibre. Tubers and root vegetables store their food in their tuber or root and are higher in carbohydrates than other vegetables.
Fibre is supplied by the outer coating (skin) or plant wall material of vegetables, fruit, grains and seeds. Plant sources of insoluble fibre include fruit and vegetable skins, dried beans and broccoli; plant sources of soluble fibre include fruit, leafy vegetables, cooked pulses (beans, peas and lentils) and vegetables such as carrots and squash. These foods are also rich in other important nutrients, such as proteins, vitamins and minerals. Vegetables also contain protective substances called phyto-chemicals, which are believed to have a beneficial effect on the immune system. Some of the fibre in vegetables is altered by cooking, so they are best eaten raw or lightly cooked – this also keeps vitamin losses to a minimum.
Nuts and seeds are another source of carbohydrates and fibre. They also provide essential fatty acids, proteins, vitamins and minerals.
INCORPORATING CARBOHYDRATES AND FIBRE IN THE DIET
Carbohydrates should provide about 50 per cent of our total energy intake. By favouring unrefined complex carbohydrates and, where appropriate, eating the skins of fruits and vegetables, carbohydrate and fibre intake can be increased simultaneously.
The international consensus is that complex carbohydrate foods should provide about half the daily calorie intake and be eaten regularly. Intake of refined carbohydrates, found in highly processed foods such as white bread, sugar-coated breakfast cereals, cakes, biscuits and table sugar, should be reduced. Instead, favour wholegrain, minimally processed foods. The following suggestions will help you increase your carbohydrate intake:
• Eat a variety of tasty breads that do not require spreads, such as rye bread and wholemeal pitta bread.
• Eat more baked and boiled potatoes.
• Eat more brown rice – experiment with risotto and paella dishes.
• Use wholemeal flour instead of white flour for baking.
• Eat more beans, peas and lentils – pulses are the key ingredient of many dishes and they provide useful amounts of protein and other essential nutrients.
Nutritionists recommend a daily intake of around 30g (1oz) of fibre. This can be achieved during the course of a day by eating, for example, a bowl of muesli, two slices of wholemeal bread, one serving of salad, two apples, two servings of vegetables, a large potato with skin and a handful of nuts and seeds, which can be sprinkled over breakfast cereals or salads. A quick way to determine whether you are getting enough fibre is to take note of the colour of your stools: if they are dark and difficult to pass, you need more fibre; if they are pale tan, soft and easy to pass, you are getting enough fibre.
When you decide to include more fibre in your diet you will need to make a very gradual shift, otherwise you may over-stimulate your digestive system and suffer from uncomfortable flatulence and bloating. Broccoli, peas, cabbage and pulses are good fibre providers but also notorious gas producers; this is because humans do not produce amyloglycosidase, the enzyme that is needed to digest them. Introduce these foods slowly in small portions over a period of several days or weeks.
Many people turn to fibre supplements or drinks to increase their fibre intake. People with constipation, for example, may rely on sprinkling generous helpings of bran over meals. Unfortunately, neat bran and bran supplements are concentrated forms of fibre, and can cause side effects, such as bloating, stomach cramps, diarrhoea and flatulence. Bran powder and fibre tablets can also interfere with the absorption of key minerals, such as iron and calcium. Instead of using fibre supplements, take your fibre from a variety of fresh foods.
It is also important to remember that fibre needs to absorb plenty of liquid if it is going to exert its beneficial effect. If you are consuming fibre in a concentrated form, but failing to drink enough liquid (at least 2.5 litres/4-1/2 pints a day), the fibre can end up forming a plug inside the gut and slowing down the elimination of waste products. The following suggestions can help you to increase your fibre intake. If you have diabetes, check with your doctor before making any changes to your diet.
• Eat wholegrain or bran cereal or wholemeal bread or toast.
• Eat plenty of unrefined foods, such as wholemeal bread and pasta, and brown rice.
• Eat whole fruit instead of drinking juice – a whole orange contains about six times as much fibre as a glass of juice.
• Scrub and eat the skins of fruit and vegetables instead of peeling them.
• Eat fruit with edible seeds, such as figs.
• Eat the stems of vegetables such as broccoli and asparagus – add them to casseroles.
• Add pulses to soups, stews and salads.
• Add grated vegetables, such as carrot, to casseroles and sauces.
Snack on dried fruit, nuts and seeds.