Our scientist Toby Murcott puts them under the microscope

COFFEE SLENDER

What is it? An apparently ordinary coffee containing a proprietary extract of green coffee called Svetol, which is supposed to alter the body's sugar-processing metabolism.

Any evidence? There is one published clinical trial of 50 overweight patients. Those taking 400mg of the extract a day lost on average 5.7 per cent of their body weight, while those on the placebo lost 2.8 per cent. However, this was a small study and Coffee Slender warrants further investigation.

Cost £19.99 for 60 tablets. Stockist coffeeslender.co.uk , 0845 0946284

Science rating 1/5

LIPOBIND

What is it? The package says that it is a “clinically proven fat binder”. The principle is reasonably well established. Many fruits and vegetables contain fibre that binds fats in the stomach, preventing them from being absorbed.

Any evidence? LIPObind contains fibre from Opuntia ficus indica, the prickly pear, which has been shown to bind fats in the test tube. However, LIPObind's claims to be “clinically proven” appear to be based on one tiny 1998 study of ten people. No conclusions can be drawn until more research is completed.

Cost £24.95 for 60 tablets. Stockist boots.com ; 08450 708090

Science rating 0.5/5

TESCO SLIM AID

What is it? A herbal remedy containing fucus (a seaweed), dandelion and boldo (a South American herb).

Any evidence? Fucus has high levels of iodine, an essential ingredient of the hormone thyroxin, produced in the thyroid glands. Thyroxin plays an important role in regulating metabolism. Roughly speaking, increase the level of thyroxin and metabolism goes up, burning energy faster. If you are lacking in iodine, your thyroid can't produce enough thyroxin and so your metabolism can be sluggish. The idea is that taking fucus will stimulate the thyroid, increasing the metabolism, so burning calories faster. However, most people have sufficient iodine in their diets and taking more will not increase metabolism. I could find only one study that looked at the weight-loss powers of fucus. It compares a mixture of fucus and another seaweed called spiruline with a placebo. In a test of 39 obese patients with high blood pressure, those taking the fucus-spiruline mix did no better than the placebo group.

For information on dandelion and boldo, see below.

Cost £4.99 for 100 tablets. Stockist tesco.com ; 0845 600411

Science rating 0.5/5

BOOTS ALTERNATIVE DIET AID

What is it? A herbal pill, with dandelion root powder, boldo extract and fucus.

Any evidence? Dandelion has some diuretic properties and a diuretic will produce very short-term weight loss, but that's because you pee more. Boldo extract appears to have some laxative effect, which has short-term weight-loss implications.

Cost £3.79 for 60 tablets. Stockist boots.com; 08450 708090

Science rating 0.5/5

4.3.2.1 SHAPE UP DAY AND NIGHT

What is it? Herbal pills that contain plant extracts such as green tea, barley and fennel.

Any evidence? The only ingredient that appears to have been analysed is green tea. One recent trial suggests that it might help weight loss. Sixty obese Thais took either green tea or a placebo while eating the same carefully controlled diet. After 12 weeks those on green tea did have an increased metabolism and lost some weight compared with those on a placebo. But this does not mean that 4.3.2.1 will have the same effect. A 2006 study of 100 patients found that a herbal remedy containing some of the same ingredients did have a weight-loss effect. But the formulation was different, so it's impossible to say whether the effect would be the same.

Cost £9.95 for 30 tablets. Stockist active-health.co.uk ; 01253 316965

Science rating 0.5/5

SLIMTHRU

What is it? An emulsion of palm and oat oil, a sort of artificial double cream, if you like. It is supposed to make you feel fuller for longer.

Any evidence? A fatty meal will make you feel full more quickly, but the problem is that fats are high in calories. This attempts to provide the fat without too many calories but there's no evidence that it helps weight loss.

Cost £12.49 for 12 tablets. Stockist hollandandbarrett.com ; 0870 6066605

Science rating 0/5

DIETRIM

What is it? A vitamin and mineral supplement.

Any evidence? If you have a poor diet, you might need some supplements, but you need to have a very poor diet. Weight loss is about reducing the number of calories you eat and it is perfectly possible to have a balanced, but smaller, diet. There is no no scientific evidence to suggest that vitamin supplements will make you slimmer.

Cost £12.95 for 60 tablets, Stockist boots.com; 08450 708090

Science rating 0/5

HOODIA GORDONII

What is it? Extract from an African cactus-like plant. The San people of the Kalahari desert use it to cope with hunger on long hunting trips.

Any evidence? It contains a chemical, called P57, which suppresses appetite. It is thought to work by tricking the brain into thinking you are full, and, to date, no harmful side-effects have been found. A company called Phytopharm is developing P57 into a slimming product that will be available in a couple of years.

Cost £39.99 for 20 tablets. Stockist hollandandbarrett.com; 0870 6066605

Science rating 3/5

ADIOS

What is it? A mixture of butternut, fucus, boldo and dandelion root.

Any evidence? Fucus, boldo and dandelion root have not been shown to encourage weight loss (see above). I've not been able to find any research on butternut.

Cost £6.35 for 100 tablets. Stockist tesco.com; 0845 600411

Science rating 0.5/5

Toby Murcott is a science writer and broadcaster