Before-and-after adverts, showing pictures of people who have lost weight or become fitter, feature in thousands of magazines. But how reliable are they, asks Justin Parkinson.

Wow, what a transformation. Two volunteers go from looking pale and unfit to tanned, toned and dynamic. Before and after photos show the benefits of a change in lifestyle - eating better, exercising more and, in many cases, taking dietary supplements to help the process along.

So how long did it take for the man and woman on the left to turn into those on the right?

Just under two hours.

They volunteered for photos taken as part of BBC Wales's Week In Week Out's investigation into sports supplements. The "regime" consisted of spray-tanning, 15 minutes of light exercise, improved posture and the introduction of more subtle lighting.

Media playback is unsupported on your device Media caption How to create the "Before and After" effect

"I was amazed when I first saw the difference," says Joe, the male volunteer, who also had his chest shaved for the shoot. "We hardly did anything in between. There was hardly any editing of the photos, either. It just goes to show what complete rubbish some of these adverts must be."

Physical self-improvement is a long-established business. During the 1940s, Charles Atlas advertised his bodybuilding courses by describing himself as a the "97lb weakling who became 'the world's most perfectly developed man'". The pieces often featured stories of how skinny young men on beaches had followed his diktats for a short period, returned and successfully confronted bullies who had kicked sand in their faces.

These days, thousands of nutritional supplements are sold with the stated aim of helping people develop their bodies. The industry is worth more than £300m a year in the UK and, with concerns over obesity far higher than during the post-World War Two period, the global weight-loss industry is expected to be worth £220bn by 2017.

The basic formula remains the same. "If you're in charge of advertising diet products, body-building supplements or vitamins for a client you'd pretty much get fired if you didn't come up with at least one campaign featuring a before-and-after shot," says Peter Davies, director of the RMS public relations agency.

Under EU rules, claims about rapid weight loss or before-and-after photographs which state or imply a rate or amount of weight loss are prohibited, according to a Health Supplements Information Service spokesman.

He adds that there is no specific prohibition against "before-and-after" pictures in relation to muscle gain, but using them to make a claim in relation to a product could be viewed as misleading. A protein product can only ever be marketed as providing the materials for muscle gain that is actually achieved through working out.

"One old trick clients used to try was to simply avoid the use of the words 'before' and 'after'," says Davies. "They'd simply print the pictures alongside each other with no text to lead the reader to assume they were 'before-and-after' images." However, the rules have tightened up, he adds, and "anything that misleads the punter will be pulled" by the Advertising Standards Authority.

Could there even be an upside to before-and-after adverts?

"You could instantly see the volunteers' confidence growing after they were shown the 'after' pictures," says the photographer Antti Karppinen, who carried out the shoot for Week In Week Out. "They were surprised how much better they looked. It was a boost to them."

Week In Week Out will be shown on BBC One Wales at 22:40 GMT on Tuesday 3 March.

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