Slimming clubs should tell fat members they could die from cancer to encourage them to lose weight, an expert has said.
Steve Miller, a weight loss mentor, said dieters get stuck in a routine of losing 1lb (0.4kg) or less each week.
But if team leaders were harsher and banned ‘excuses’ it could actually motivate members to achieve long-term results, he said.
Being overweight can cause cancer as well as other life-limiting conditions, such as heart disease and type 2 diabetes.
Other industry experts agree clubs such as Slimming World need a makeover, but said problems lie with a lack of emotional support for dieters.
Psychologists say members can leave heavier than when they started because clubs don’t help them recognise why they overeat in the first place.
Mr Miller told MailOnline: ‘I think there needs to be some kind of new invigoration in content in a slimming club. It’s just sitting moaning, whinging, and clapping for half a pound.
‘It’s not about being horrible or cruel to people, it’s about being cruel to be kind. You have to be honest about the reality of fat. Fat people often die.
‘These leaders should be saying you have lost half a pound, but you’re currently 22st (139.7kg) and 5ft1 and if you don’t lose weight, you might die of cancer.
‘Does that mean they have to tell themselves to get a grip? Absolutely. Less moaning, more action, and then you can celebrate.
‘They need a makeover to be braver and bolder.’
Being overweight or obese is the second biggest preventable cause of cancer in the UK, second only to smoking.
More than one in 20 cancer cases are caused by being fat, figures show. Weight loss can cut the risk drastically.
And carrying too much weight also significantly raises the risk of heart disease, high blood pressure, type 2 diabetes and breathing difficulties.
Clubs like Slimming World and Weight Watchers have been around since the 1960s to help people drop the pounds. Thousands have found success with the clubs which continue to soar in popularity as an affordable way to change habits.
The British Dietitian Association praise the group meetings for an easy way for members to share ideas and recipes with each other – but admit they may not appeal to everyone.
The clubs’ plans centre on changing eating habits such as eating more vegetables, filling up on low-calorie foods and cutting back on sweet treats.
But the ‘rules’ have come under controversy in the past – many independent experts have slammed the lack of education on nutrition provided.
Sophie Bertrand, a registered associate nutritionist on Harley Street, said the rules – such as point counting, low-fat and weekly weigh-ins – can cause an unhealthy relationship with food.
She said: ‘Slimming clubs usually encourage people to manage food intake with some kind of point system.
‘As a result [this]likely means you will be required to categorise foods as “good” or “bad”, depending on the number of “points” or “syns” they are worth.
‘Creating “rules” around food is a dangerous game and it encourages an unhealthy relationship with food.’
In response, Slimming World ‘strongly refuted’ the claims that their plan encourages ‘unhealthy behaviours’, and that their methods are based on scientific research.
Ms Bertrand warned that generally, constant dieting can lead to further health problems – studies have shown dieting can slow metabolism and create feelings of guilt and anxiety around food.
Diets can also lead to weight regain because the body and mind naturally fight against a change in diet, perceiving it to be a stressor.
Ms Betrand said: ‘There is a body of research highlighting various consequences of constant dieting. You can’t diet forever.’
The comments come ahead of an inevitable surge of sign-ups to diet clubs in the new year, with women and men across the country hoping to shift their Christmas bulge.
Although slimming club newbies will expect some success, one expert warned there may be disappointed.
Joanna Konstantopoulou, a psychologist and founder of the Harley Street-based Health Psychology Clinic, said positive results aren’t usually sustained.
She told MailOnline: ‘For most people, the results are short-lived as people revert back to their old behaviours.
‘When people start dieting, they have willpower and motivation for success, but when the diet fails, they begin to blame themselves for this failure.’
Ms Konstantopoulou said for long-term change, there needs to be a psychological element involved.
She said: ‘People need to understand what contributed to their weight gain in the first place.
‘Perhaps they were comfort eating or stress eating. Once the root of the problem is identified, that’s when real, long-lasting changes can be made.
‘Having a healthy relationship with food is so important when it comes to overall wellbeing and slimming clubs may further damage ones relationship with food.’
Body mass index (BMI) is what the NHS use to measure if a person is too overweight for their height.
A BMI of 25 to 29.9 is overweight, 30 to 39.9 is obese, and 40 is severely obese. BMI can be calculated here.