It is the time of year when people's minds often turn to slimming. And with or without New Year's resolutions and the excesses of Christmas, increasing numbers of people do need to lose to weight. Currently 62 per cent of adult women in the United Kingdom are overweight or obese, with the proportion forecast to increase yet further. Of all the Government's health targets, the one aiming to reduce the number of obese women from 21 to 8 per cent and of men from 18 to 6 per cent by 2005 will is unlikely to be met. Yet at any one time, more than a third of women are "dieting" and the slimming industry is enormous.

The key to successful weight loss lies in negative energy balance achieved by a combination of reduced calorie intake and increased physical activity. Authoritative guidelines, used by most health professionals in advising their overweight patients, stress the need for healthy, low calorie meals, individual assessment, realistic weight goals, gradual weight loss, psychological support and long-term planning to ensure maintenance of weight lost. Such guidelines identify only limited roles for meal replacements and very low calorie diets (VLCDs), and none at all for non-prescription slimming products of the type that are said to restrict appetite, enhance thermogenesis or reduce fat absorption.

Yet many people who are overweight, or think they are overweight, seek help from sources other than health professionals. Increasingly, this includes the internet, and advice on slimming features prominently on health-related web sites. Interestingly, in the three weeks before Christmas 2001, mainly as a result of subscribing to a number of nutrition newsletters, I received on average one e-mail each day advertising an internet slimming programme.

So what will a potential slimmer find on the internet and to what extent will the information conform to the principles of healthy weight loss normally recommended by health professionals? Entering the search string "weight loss diets" into the search engine "Yahoo" currently generates over half a million links, while "Excite" generates over a million. I would not pretend to have looked at more than a minuscule proportion of these, but an impression gained is that for every 50 sites based in the United States, there is at most one or two in Britain. A small number are supported by government agencies (eg, the Food Standards Agency in the UK), a few by professional organisations (eg, the American Dietetic Organisation, the American Association for the Study of Obesity) and a few by reputable self-help organisations (eg, the British Heart Foundation, the American Heart Association). These tend to offer free advice on healthy eating and exercise, with examples of foods and appropriate meals.

However, most sites are from individuals or companies offering various weight loss programmes or products which can vary in price between around £10 and £100 for a short (eg, a month's) course. The main types of products or programmes offered include:

• Advice on losing weight and balanced diets (these are mainly sites supported by government or professional organisations)

• Meal replacements, very low calories diets, drinks, energy bars and biscuits

• Slimming plans (eg, six to 12 weeks of daily calorie controlled menus)

• Herbs and other supplements that are purported to reduce cellulite, reduce appetite, increase thermogenesis or reduce fat absorption

• Workout routines

• Vitamins, minerals and other supplements to increase nutrient intake for the slimmer

• Books offering advice on slimming, exercise and cooking, including recipes

• Surgery

Serious concerns have frequently been raised about the quality and safety of health advice offered on the internet, and this certainly applies to advice on slimming. In 1992, the US National Institutes of Health technology conference published a list of recommendations for people considering weight loss programmes, which stated that "in evaluating a weight loss method or programme, one should not be distracted by anecdotal 'success' stories, or by advertising claims".

The NIH also said that information that should be obtained about the programme includes:

• The percentage of all participants who complete it

• The percentage of those completing the programme who achieve various degrees of weight loss

• The proportion of that weight loss that is maintained at one, three and five years

• The number of participants who experienced negative medical effects as well as their kind and severity

It further added that it was not aware at the time of any programmes (on the internet or anywhere else) which could fulfil these criteria. During my limited search on the net, I did not find any either, although not all sites are poor by any means (see above).

The main issues are safety and efficacy, not to mention the extravagant claims that are made about products. Such claims promise things like:

Rapid weight loss Rapid weight loss is promised on a large proportion of sites. Claims like "you can lose 12 pounds a week" while using the product or following the programme are obviously false. Apart from initial rapid weight reduction due to loss of glycogen and water, it would be impossible to lose 12 pounds a week. If an individual reduced energy intake by 2,000kcals/day (an excessive reduction even if starting on a very low calorie diet), this would be equivalent to a loss of around three pounds a week. Names of products (eg, Reductamin, Thermothin) often allude to weight loss, and there is frequent use of meaningless terms such as "fat digester" and "metabolic activator".

Long-term or permanent weight loss Claims are made like "you've tried every diet under the sun and put weight back on. This is the diet where you will discover the secret of permanent weight loss." Such claims are targeted at those who have lost weight but have never been able to keep it off — ie, most dieters. Although weight is relatively easy to lose, long-term weight maintenance is extremely difficult to achieve. There is no evidence to suggest that any special supplement can help the individual to achieve this more successfully than lifestyle changes.

No diet or exercise required Many internet sites claim that weight can be lost without reducing calorie intake or increasing physical activity. "Lose weight while you sleep; no dieting, no strenuous exercise," says one site, flying completely in the face of the first law of thermodynamics.

Consumer testimonials Testimonials are on almost every site. "Three months ago I weighed 256 pounds. Now I'm down to 132 pounds. I didn't have to worry about what I ate. It's a miracle." Many such testimonials claim a degree of weight loss that is impossible.

Before and after photographs "Before" photographs often show the individual in a poor posture, with untidy hair, a glum expression and unfashionable clothes. "After" photographs on the other hand usually show the successful dieter with a smile on her face, a new hairstyle and fashionable or skimpy clothes, standing with her shoulders back and stomach tucked in.

Scientifically proven or endorsed by doctors or nutritionists Sites claiming endorsement by doctors or nutritionists do not tend to provide details of the research studies (eg, how many subjects, length of study, where it was conducted and where published). Professionals endorsing the products may not have seen trial evidence, may not have reviewed it properly if they have, and may have a financial interest in the product.

Safety Some programmes make claims like "no dangerous pills, not a prescription weight loss drug, not an amphetamine". However, products containing herbal and other ingredients have not been tested in long-term trials, so there is no guarantee that they are safe. Nor have they been proven to be efficacious.

So are there any internet sites offering advice which conforms to the principles of healthy lifestyle changes? I found some sites (this list is not intended to be exhaustive) which, on the whole, provide sound nutritional advice for potential dieters (see Panel). These are all based in the UK, not that this matters greatly, although when it comes to advice on recipes, eating out and so on, British people may find them more culturally relevant than US sites.

Conclusion

Information on weight loss on the internet ranges from the sound to the ridiculous. Pharmacists should advise their customers to view sites with a degree of scepticism. Not only do some of them make extravagant claims, but they also increase the risk of adding to what is an already serious health problem by diverting individuals away from weight loss methods that are safe and evidence-based.

Good internet sites are to be welcomed because the problem of obesity is growing and this medium has the potential to reach a wide audience.