Gil Clasby lost half her original body weight on the diet
With an annual turnover of £18m, diet company LighterLife claims to have enabled 60,000 people to lose three stones in three months.
The liquid-based programme, aimed at people who are three or more stones overweight, involves dieters consuming just 530 calories a day for 12 weeks.
But Inside Out in the East has heard from some dieters who have experienced disrupted periods, hair loss and water poisoning.
LighterLife say very low calorie diets help people to lose weight quickly and safely.
Concerns about her health led Lizzie Whitehead from Norfolk to tell her LighterLife counsellor that she was giving up the diet.
Ms Whitehead said: "I explained to her that it was due to how the diet had messed up my menstrual cycle and caused me to feel so ill.
"She basically said 'well that's just one of the side effects, feeling ill. You've just got to carry on, you'll get past it'.
"No-one warned me about any side effects at all. She just said to drink plenty of water. No health warnings were given to me at all."
The experience was shared by another dieter Sarah Morley.
She said: "When I came off LighterLife, within about two weeks my hair was falling out in clumps... Nobody had warned me.
"When I mentioned it to my counsellor she said 'isn't the fact that you've lost weight enough? Surely a bit of hair loss is ok?'"
When BBC undercover researchers asked two separate LighterLife counsellors, picked at random, what side effects might be experienced they were told the worst they could expect was a headache.
Inside Out asked Bar Hewlett, a founder and director of LighterLife, to explain why the counsellors did not warn about side effects such as hair loss and interrupted periods.
She said: "If a counsellor is asked about the side effects of weight loss there would be a different answer than if they were asked about the side effects of a very low calorie diet."
She added that these things are made clear in LighterLife literature.
Another former LighterLife dieter Paul Kendrick was admitted to hospital with water poisoning, having drunk too much in a bid to relieve constipation caused by the diet.
He asked LighterLife to warn people against drinking too much water but, counsellors told BBC researchers that it was "not possible" to drink too much water.
LighterLife said they give guidance on drinking water in their literature.
Gil Clasby from Cambridge lost weight successfully.
She said: "I weighed 19 stones in April 2007 and by January 2008 I had lost 9.5 stones - so I actually lost half my bodyweight.
"I liked the counsellor, the way she talked, the facts she gave out.
"I worried about cost but the weight loss was going to be a weight loss that came off quite quickly, quite dramatic compared to anything I'd done before and I had so much weight to lose to be a healthy BMI, that I thought this is probably the only thing that's going to work for me, getting the weight off quickly".
The National Institute for Clinical Excellence (NICE) guidelines state that very low calorie diets should only be used for a maximum of 12 weeks.
Any diet of less than 600 calories should only be used under medical supervision. One counsellor was seen to be undertaking this herself with no reference to a medical professional.
Another client was seen by a "locum" with only two days training - again in breach of the company's own protocol.
Ms Hewlett told Inside Out: "If we find that a counsellor is not abiding by our protocol we will take steps to make sure that that behaviour changes.
"All our counsellors end up with a nationally accredited qualification.
"At the end of about their first four months they start practising in the field and they're heavily supervised and at the end of that supervision time - about two years - they are awarded, by an external learning board, a diploma."
Inside Out filmed the diet being offered to someone with an eating disorder.
The company stands by its policy of allowing people with eating disorders, including compulsive eating, onto the programme.
Ms Hewlett told Inside Out: "The National Eating Disorders Association actually sends people to us who are obese.
"They believe that the programme we have and the services we offer are absolutely suitable for people with eating disorders."
The Eating Disorders Association (now called B-eat), told the BBC: "We don't make referrals to LighterLife or any other diet.
"Anyone with an eating disorder should talk to their GP and be referred for specialist medical help."