What bliss it would be if there was a pill which would allow you to eat cream cakes and still lose weight. Of course, manufacturers have been peddling such impossible dreams for years with a variety of weight-loss formulas from plant fibres to caffeine-enriched gum.
A new product on the market is called Fat Magnets. The manufacturers, Fat Magnets International, say that these tablets (which cost about pounds 20 for 60 capsules) "attract fat like iron filings to a magnet" and prevent it being absorbed. Its claims to be a "breakthrough" in weight control, however, are being treated with caution by nutritionists.
The "magic" ingredient in the tablets, and brands like them, is chitosan, a fine powder which is ground out of the shells of prawns, lobsters and crabs. Chitosan is supposed to work by "grabbing" fat lipids in the stomach before they can be absorbed.
The manufacturers describe chitosan as an organically inert substance whose molecules have a positive ionic charge. Ingested fat, when broken down into fatty acid molecules, has a negative charge. The theory is that opposites attract and the fats bind with the chitosan and convert it to a form which passes through the digestive system without being absorbed.
Would-be thinnies are advised to take two to four 500mg tablets around 15 minutes before a meal to ensure the chitosan is in your stomach before the fat can be digested. (This is fine if you are sitting at home but slightly more embarrassing at Christmas parties, gulping down capsules with the mineral water.) The manufacturers claim that taking four Fat Magnet capsules can tie up eight grammes of fat per meal, reducing the fat content of a piece of fried chicken, for example, from 20 grammes to 12.
While chitosan has been well documented as a food additive and used as a binding agent in winemaking and bread, it has only recently been exploited for weight control. Fat Magnets, which have been available in the US for about a year, recently gained the Food and Drugs Administration licence to be sold as a food supplement for its high-fibre content - and not as a medicine - which means health-giving claims cannot be made about it.
Trials so far on chitosan have been small scale, but some look promising. A double-blind trial carried out on 20 semi-obese people in Helsinki over a four-week period found that people who regularly supplemented their meals with chitosan lost a total of 6.7kg in weight as opposed to 2.5kg in the control group. However, nutritionists in Britain remain sceptical. They say there is not yet enough evidence to know whether Fat Magnets work. Dr Susan Jebb, head of obesity research for the Medical Research Council at Cambridge University, called for more extensive research before chitosan's effectiveness can be judged.
"There is no data," she said. "No one has done any large trials, and this leaves scientists in a very difficult position as there is no evidence as to whether it works. We don't have any evidence about its metabolic properties."
If it does work, there is another problem. Chitosan cannot distinguish between "good" unsaturated, vitamin-rich fats found in foods such as olive oil, nuts and avocados and so-called "bad" saturated fats found in chocolate, butter and crisps. Christine Williams, professor of nutrition at Reading University, is concerned that if Fat Magnets do absorb enough fat to help in weight loss, users could also put themselves at risk of losing beneficial fat-soluble vitamins K, A, D and E. This could have disastrous effects, she said.
"Fat-soluble vitamins are antioxidants which are extremely important for the body," Professor Williams said. "They can help protect against cardiovascular conditions and cancers. We need to maintain a good intake of fat-soluble vitamins and good absorption of them."
She also said that there could be unpleasant side-effects if the fat bound to chitosan was not absorbed but excreted. "If fat goes down to the lower intestine and is fermented by the gut microflora, it could leadto bloating and wind," she said. "It is also harmful - and unpleasant - if a lot of fat goes into the large bowel and the faeces."
In response, Fat Magnets International suggest that anyone embarking on long-term use of the tablets should take nutritional supplements; they also say there have been no reports of side-effects.
Dr Jebb also suggested that Fat Magnets may work because of the psychological boost experienced by many people when they start a new type of diet. "Take something like a food-combining diet, where you avoid mixing starch and protein," she said. "There is no evidence that this has any particular metabolic benefits. It works because people end up eating fewer calories.
"People always want to find a way lose weight easily. It is a placebo effect. You think `it's going to work for me'. If people believe it, they make changes in their eating patterns for a time."
One of the UK's leading nutritional scientists, Professor Tom Saunders, from King's College, London, and co-author of You Don't Have To Diet (Bantam Press, pounds 5.99) is sceptical of all diet products.
"The diet industry has been described as a capitalists' dream - products that don't work but still stimulate the market," he said. "There is a problem with obesity in this country and it is because people are physically inactive. There is no evidence that chitosan works as a slimming aid. It is giving people a false hope when what is really needed is a lifestyle change."
However wonderful it would seem to dieters that a pill could solve their weight problem, Professor Saunders and Dr Jebb are adamant that there is only one way to sustain weight loss, and that is picking up your trainers and getting down to the gym after the office party.
"People will only lose weight when their energy intake is less than their energy expenditure," said Dr Jebb. "That really is the only case when a diet product will work. And the most effective way to lose weight is by increasing physical activity.
"There is not a magical way to achieve loss"n