Some diet aids allow you to eat all you want and still lose weight

It's a nice idea and the magic most of us would like! There are even some shreds of truth here. Some foods produce a greater sense of fullness for the same amount of energy (kilojoules) than other foods. For example, we know that the same amount of sliced apple produces a greater feeling of fullness than apple puree or juice. Potatoes are low in energy (only 3.5 kilojoules per gram), and very filling. It's difficult to overconsume energy on boiled potatoes, provided fat is not added. Fat is high energy value (37 kilojoules per gram or 9 kilocalories), but often contains fat-soluble flavours which people like, and it is used to increase the palatability of other refined foods like pastries, biscuits and icecream.

These findings have been used to develop diet aids. In particular, dietary fibre isolated from various plants has been used to decrease appetite. Very few scientific studies have been carried out on this idea, and those rat have been done have found a slightly positive effect or no effect. There is another point to remember: some components of dietary fibre are extensively broken down by large intestine bacteria. This process can reduce short-chain fatty acids which are used as fuel energy) by the body. Such dietary fibre has an energy value of about 13 kilojoules per gram (3 kilocalories per gram) compared with digestible carbohydrate like sugar or starch (which can be digested and therefore absorbed in the small intestine) at 17 kilojoules per gram (4 kilocalories per gram). Of course, the amounts of dietary fibre used in this way may be relatively small, say 5 ,10 grams, amounting to 60 to 120 kilojoules, but it is not zero.

There is a risk that some dietary fibre supplements taken in tablet form may take up water and swell after swallowing, blocking off the upper gut (oesophagus). Such forms of dietary fibre have now been banned in Australia.