Nutrition (5 page)
POLYOLS
Some ‘diet’ products contain sugar-free sweeteners such as xylitol, lactitol, sorbitol and maltitol. These polyols are classed as carbohydrates as they are derived from sugars, but despite their sweet taste, they are not processed in the body like sugars and do not promote tooth decay. Sorbitol, lactitol and xylitol do not raise blood glucose levels, but maltitol has the same effect on blood glucose levels as sucrose. In practice, however, the consumption of polyols is limited by their laxative effect. Some people are more sensitive to this than others, so that intakes above 10 g to 20 g per day can cause flatulence, bloating and diarrhoea.
Dietary fibre
Dietary fibre – sometimes referred to as ‘roughage’ – consists of the non-digestible carbohydrates in your diet. While fibre passes through your small intestines unchanged, and provides little in the way of energy or nutrients, it encourages peristalsis and aids the digestion and absorption of other foods.
There are two main types: soluble and insoluble fibre. All plant foods contain both types, but some sources are richer in one type than another. Oats, figs, barley, apples, prunes and kidney beans, for example, are rich in soluble fibre, while wheat, brown rice, rhubarb, leafy vegetables, peas and chickpeas are good sources of insoluble fibre.
Soluble fibre (such as pectins, gums and mucilage) form a gel when mixed with liquid and is particularly important in the stomach and upper intestines. It slows the digestion and absorption of other carbohydrates, helping to blunt the rate at which blood glucose levels rise. It also attracts fats to slow their absorption, and lowers blood fat levels. Once soluble fibre reaches the large bowel, it is fermented by bacterial enzymes to release nutrients and smelly gases. Insoluble fibre (e.g. cellulose) is most important in the large bowel, where it adds bulk, absorbs water, bacteria and toxins and hastens stool excretion. Much of the increased bulk of bowel motions associated with a high-fibre diet is due to increased bacterial multiplication in the gut. For every gram of fibre eaten, bowel motions increase by around 5 g in weight.
Fibre is found mainly in fruit, vegetables and wholegrain products. Our ancestors followed a diet that provided 100 g or more of fibre per day. Ideally, you need a fibre intake of at least 20 g to 30 g daily. When increasing fibre intake, it is also important to drink at least two to three litres of fluids per day as this is absorbed by the fibre and helps to bulk it up for optimum effect.
The fibre content of various fruits and vegetables is shown in
Table 3
.
Table 3
.
Food | Fibre content per 100 g |
Bran | 36 g |
Bran cereal | 13 g to 25 g |
Apricots, dried | 18 g |
Prunes | 13 g |
Figs, dried | 8 g |
Muesli (unsweetened) | 8 g |
Oatmeal | 7 g |
Kidney beans, boiled | 7 g |
Wholemeal bread | 7 g |
Mixed nuts | 6 g |
Peas | 5 g |
Dates, dried | 4 g |
Wholemeal spaghetti, cooked | 4 g |
Brown bread | 4 g |
White bread | 2 g |
Apples, raw | 2 g |
Rhubarb | 2 g |
Pears | 2 g |
Brown rice, boiled | 1 g |
Dietary fats
Fats are the most energy-dense food groups, supplying 9 kcal energy per gram of fat (compared with 4 kcal per gram for protein and carbohydrate). It is therefore an efficient way for your body to bank excess calories for use in future lean times.
Despite their bad name, dietary fats are vital for good nutrition. Like protein, fatty acids play an important structural role in the body, providing building blocks for making cell membranes and nerve sheaths. They are also used to make sex hormones, immune regulators and bile salts and to transport fat-soluble nutrients such as vitamins A, D, E and K. However, eating too much fat is associated with weight gain and can have adverse effects on blood fat levels and your risk of coronary heart disease. Current advice is that dietary fats should provide no more than 30 per cent of daily energy intake. For someone with an average energy intake of 2,000 kcal per day, 30 per cent of energy intake represents around 67 g of fat (1 g fat supplies 9 kcals energy).
The basic building blocks of fat are called free fatty acids (carboxylic acids). These contain a chain of carbon atoms that are linked to each other through single or double chemical bonds.
A fatty acid whose carbon atoms are joined using only single bonds is described as saturated, as all its available bonds are fully saturated with additional hydrogen atoms. A fatty acid that contains one or more double bonds is known as an unsaturated fat; of these, a fatty acid with a single double chemical bond is described as monounsaturated, while one with two or more double bonds is referred to as polyunsaturated.
A fatty acid whose carbon atoms are joined using only single bonds is described as saturated, as all its available bonds are fully saturated with additional hydrogen atoms. A fatty acid that contains one or more double bonds is known as an unsaturated fat; of these, a fatty acid with a single double chemical bond is described as monounsaturated, while one with two or more double bonds is referred to as polyunsaturated.
The number and position of the double bonds within a fatty acid determines whether it acts as a solid or liquid at room temperature. In general, saturated fats tend to be solid at room temperature, while monounsaturated and polyunsaturated fats tend to be oils. The position of the double bonds also determines how the fatty acid is metabolized in your body.
Triglycerides
The fats in your diet, and in your body stores, are mainly in the form of triglycerides. These E-shaped molecules contain three fatty-acid chains linked to a glycerol backbone. The three fatty-acid chains in a triglyceride molecule do not have to be the same. One could be a saturated fatty acid, one a monounsaturated fatty acid and one a polyunsaturated fatty acid. Most fats contain a blend of these three different types of fatty acids. Almost half (47 per cent) of the fatty acids found in beef fat are monounsaturated, for example, even though this animal fat is classed as saturated.
In general, animal-based foods contain a higher percentage of saturated fat than vegetable foods. The exact breakdown will vary depending on the country, and how the animals or plants from which the fats are obtained were farmed. For example, grass-fed cattle tend to have a healthier fatty-acid composition, with more essential fatty acids, omega-3 polyunsaturated fats and antioxidants than grain-fed cattle. The fat from grass-fed beef may also have a more yellow appearance due to the higher content of beneficial carotenoid pigments. This is especially true of Guernsey Channel Island cows, whose fat (and the butter and cream made from their milk) is a lovely rich colour. Why? Because these cattle are genetically less efficient at converting the yellow carotenoid pigment, betacarotene, on to vitamin A. Despite these considerations, typical values for the types of fatty acid obtained from a variety of sources are shown in
Table 4
. Those with the least saturated fat and the most monounsaturated fat are usually considered the most healthy, which is why nutritionists encourage you to use more olive, nut and rapeseed oils during cooking and in salad dressings, rather than corn and safflower oils.
Table 4
. Those with the least saturated fat and the most monounsaturated fat are usually considered the most healthy, which is why nutritionists encourage you to use more olive, nut and rapeseed oils during cooking and in salad dressings, rather than corn and safflower oils.
Dietary fat | % saturated | % polyunsaturated | % monounsaturated |
Macadamia nut oil | 14% | 5% | 81% |
Hazelnut oil | 8% | 14% | 78% |
Olive oil | 15% | 10% | 75% |
Almond oil | 8% | 18% | 74% |
Rapeseed oil | 7% | 32% | 61% |
Walnut oil | 10% | 70% | 20% |
Flaxseed oil | 9% | 72% | 19% |
Safflower oil | 10% | 76% | 14% |
Sunflower oil | 12% | 22% | 66% |
Corn oil | 13% | 58% | 29% |
Beef fat (dripping) | 50% | 3% | 47% |
Pork fat (lard) | 43% | 10% | 47% |
Butter fat | 68% | 4% | 28% |
Saturated fats
Saturated fats have gained a reputation as ‘bad’ fats, as consuming an excess of these has been associated with an increased risk of coronary heart disease. Not all studies show this, however, and, although it is commonly believed that saturated fats are converted into cholesterol in the liver, this is not absolutely true. Only saturated fatty acids with chain lengths of 12, 14 or 16 carbon atoms have an effect on your blood cholesterol levels. Saturated fats with other chain lengths are not converted into cholesterol in the liver at all, and therefore have a neutral effect on your cholesterol levels. Overall, a third of dietary saturated fats – including stearic acid (18 carbon atoms) found in milk fat, cocoa butter and meat fat – have no cholesterol-raising activity. This does not mean that a high saturated-fat intake is harmless. Like all types of fat, it has a high calorie content and an excess is linked with obesity. And, if you have a high cholesterol level, you may have inherited genes that mean you process saturated fat less efficiently than other people. Ideally, saturated fats should supply no more than 7 to ten 10 cent of your energy intake, which, for most people, means cutting back. Replace them with more beneficial monounsaturated fatty acids (found in olive, rapeseed, macadamia and avocado or walnut oils, as shown in
Table 4
), or omega-3 polyunsaturated fats (found in fish, flaxseed and walnuts oils).
Table 4
), or omega-3 polyunsaturated fats (found in fish, flaxseed and walnuts oils).
Monounsaturated fats
Monounsaturated fats, such as oleic acid, consist of chains of carbon atoms in which there is only one double (unsaturated) bond. They are metabolized in such a way that they lower blood levels of harmful low-density-lipoprotein (LDL) cholesterol but have no effect on beneficial high-density-lipoprotein (HDL) cholesterol. As such, a diet high in monounsaturates may help to reduce your risk of atherosclerosis, high blood pressure, coronary heart disease and stroke. This is thought to explain some of the benefits of the so-called Mediterranean diet. Ideally, monounsaturated fats should supply around 12 per cent of your energy intake. For most people, this means eating more monounsaturates.
Polyunsaturated fats
Polyunsaturated fatty acids (PUFAs) have a molecular structure whose carbon chains contain two or more double bonds. There are two main types of polyunsaturated fat in the diet: if the
first
double bond involves the third carbon atom, it is classed as an omega-3 fatty acid; if the
first
double bond involves the sixth carbon atom, it is classed as an omega-6 fatty acid.
first
double bond involves the third carbon atom, it is classed as an omega-3 fatty acid; if the
first
double bond involves the sixth carbon atom, it is classed as an omega-6 fatty acid.
PEROXIDES
Having so many double bonds makes polyunsaturated fats highly reactive and susceptible to chemical changes known as oxidation. This can produce toxic substances (lipid peroxides) that are believed to trigger hardening and furring-up of artery walls if there are insufficient antioxidants to prevent oxidation. Factors which encourage the formation of these toxic lipid peroxides include eating excessive amounts of polyunsaturated fats (PUFAs), overheating PUFA oils so they smoke while cooking and reheating cooking oils.
Because of the position of their first double bond, your body handles omega-3 and omega-6 oils in different ways. Omega-6 fatty acids are converted into substances (series 2 prostaglandins, series 4 leukotrienes) that tend to have an inflammatory action and increase blood stickiness, which can lead to unwanted blood clots. In contrast, omega-3 fatty acids act as building blocks to make substances (series 3 prostaglandins, series 5 leukotrienes) that are anti-inflammatory and have a blood-thinning action.
The omega-6 fatty acid, gamma-linolenic acid (GLA) is one of the few omega-6s that can reduce inflammation if intake is sufficiently high. It is found in evening primrose and starflower oils, which are popular supplements for people with dry, itchy skin such as in eczema (an inflammatory condition).
The omega-6 fatty acid, gamma-linolenic acid (GLA) is one of the few omega-6s that can reduce inflammation if intake is sufficiently high. It is found in evening primrose and starflower oils, which are popular supplements for people with dry, itchy skin such as in eczema (an inflammatory condition).
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