Healthy Dieting

Fibre is the part of carbohydrate that resists breakdown by enzymes in the intestine. Most fibre passes straight through to the large bowel ready for excretion in the faeces. Thus you can eat fibre without absorbing the energy from it.

The benefits from eating fibre-containing foods have a lot to do with ‘energy density‘. This term refers to the number of calories in a given weight of food. Many foods naturally high in fibre have a low energy density. In contrast, many fast foods laden with fat and little or no fibre have a high energy density. If you were to eat two meals, both with the same amount of energy, the high-fat meal would be much smaller and could be consumed a lot faster.

A recent review of the effects of energy density on obesity concluded that ‘a low-fat, low-energy dense diet results in a spontaneous fall in energy intake whilst maintaining feelings of satiety’. This means that you can eat less fat and less energy and still avoid the ‘perpetual hunger that often accompanies active periods of food restriction’.

As with most modifications to your diet, moderation is important. While too little fibre can cause constipation, too much, especially in the form of wheat bran, can decrease the absorption of the important minerals iron and zinc. It can also make you a little unpleasant to be around—if you get the drift! A target intake of around 30-40 grams of fibre a day is what you should be aiming for.

Choose low glycaemic index foods

Because of their more complex structure, starches are more slowly digested than sugars. So it’s been thought that these would be more beneficial in dealing with a disease such as diabetes; and for weight loss. But recently it’s been shown that digestion and metabolism is not that simple. Some carbohydrates are not just broken down according to their simple or complex nature. Hence, another measure has been developed to explain this.

The Glycaemic Index (GI) is a method of classifying the glycaemic (i.e. blood sugar) response to carbohydrate-rich foods. Put another way, it’s a measure of how quickly carbohydrate reaches the bloodstream. Foods can be tested and ranked out of 100 according to a measure of their GI, a bit like the satiety index (SI) discussed in Tips 14 and

Foods closer to 100 are more quickly digested, so foods with a lower GI should be preferred.

It doesn’t necessarily follow that all sweet sugary foods will be quickly absorbed and therefore have a high GI. It’s much more complex than this and depends on a number of characteristics of the food consumed. For example:

Glucose (GI = 100) has a higher GI than fructose (GI = 20). Temperate fruits (e.g. apples, pears, oranges) have a higher fructose content than tropical fruits (eg. watermelon, rockmelon, mangoes).

The ratio of two types of starches (amylose:amylopectin) can affect the GI of foods. Amylopectin is more easily digested than amylose. Therefore, foods such as legumes, with a high amylose:amylopectin ratio, have a lower GI than rice, which has a higher amylopectin content.

Processing can increase the GI of a food. Wholemeal bread, where the grains have been ground down and incorporated into the dough, is more quickly digested (and hence has a higher GI) than a wholegrain rye bread, where the grains are still intact.

Soluble fibre can decrease the rate of absorption of glucose and hence decrease GI. Fibre supplements have not been found to have the same effect because the fibre is not packaged into the food.

Large amounts of fat can slow down digestion. While this results in a low GI for foods that contain both carbohydrate and fat (e.g. ice cream), the fat content should take first priority when making food selections.

Low GI foods are scored below 55, intermediate from 55 to 70, and high above 70. You can combine a high and low GI food to produce an intermediate GI meal. The use of low GI foods can be a useful addition to a reduced-fat, high-fibre eating plan.

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