Extreme diets, including one that bans solid food, have been branded "rubbish" by a top dietitian. Nikki Hart said quick-fix weight loss set people up for a dieting nightmare, particularly the heavily advertised Lemon Detox Diet which allows only liquid.

"They are rubbish because they involve cutting out certain food groups, especially carbohydrates," she said.

"You'll have bloating when you start eating again and it will affect your self-confidence, so you'll want to diet again. This is how women get trapped into yo-yo dieting."

Victoria Beckham and Beyonce are among celebrities to sing the praises of diets that promise weight loss of up to 6kg in 10 days.

But Hart says rapid weight loss can make you "severely dehydrated", starve your body and force you to break down lean muscle mass for energy.

With many Kiwis thinking about getting in shape for summer, the Herald on Sunday tested four popular seven-day detox and weight-loss programmes - the Lemon Detox Diet, Celebrity Slim, the cabbage soup diet and the Quick Cleanse 7-Day Detox.

The most extreme weight loss was 4kg in a week on the Lemon Detox Diet, followed by a 3.6kg loss on cabbage soup, 2.4kg using Celebrity Slim and 1.5kg on the Quick Cleanse detox.

Hart, presenter of TV series Eat Yourself Whole, says all four are too extreme for the body to handle.

The heavily advertised Lemon Detox Diet is dangerous, she said.

"To lose 500g of body fat costs your body around 3500 kilocalories, so if you've lost 4kg you've basically started to eat your own muscle mass and your body is in starvation mode."

The diet only allows a drink consisting of squeezed lemons, Madal Bal natural tree syrup and cayenne pepper for seven to 10 days.

You also must drink a litre of sea salt water every morning, a detox tea in the evenings, and fresh orange juice and peppermint tea if necessary.

The cabbage soup diet involves eating as much of the soup recipe as possible, with a different type of food each day.

Celebrity Slim, costing $79.99 for seven days, gives dieters two protein meal replacement shakes each day. You are allowed one other meal, not including carbohydrates, and some fruits.

The Quick Cleanse detox, available for seven or 15 days, cuts out coffee, alcohol, dairy, sugars and meat and requires detoxers to take various organ cleansing pills each day.

Beyonce reportedly lost 9kg on the Lemon Detox Diet and was quoted in a magazine saying: "I lived on water, cayenne pepper and maple syrup for 14 days. It was tough; everyone was eating and I was dying."

Victoria Beckham has been linked to fad diets such as seaweed shakes and edamame beans.

A 2006 Consumer New Zealand survey into detox diets concluded many were not worth doing because scientific evidence of the benefits was "thin on the ground" and some could reinforce "bizarre eating habits". But we continue to buy often expensive diet products. In America the dieting industry reportedly pulls in about US$100 billion ($132 billion) a year.

Nutritionist and co-owner of Auckland's CrossFit New Zealand gym Darren Ellis said such diets prey on people who are desperate to lose weight.

Ellis said all were essentially the same "reduced-energy, low-carb" diets in different packaging.

"They're just reinvented to look like the latest thing. They end up making people feel bad about how they're going to look in a bikini," he said.

"When you lose weight that fast your body rebels and gets a signal that its going into starvation, so your metabolism reduces to actually conserve body fat and use up muscle," he said.

Endree Saade, managing director of Lemon Detox Diet Australia, said sales had grown by 75 per cent since September last year. He said the breakdown of muscle mass is true of a regular fast, but the lemon drink provides you with potassium, zinc, calcium and magnesium, making it more of a liquid food.

The detox is also aimed at cleansing the body.

Jordan Jamieson of Celebrity Slim said their "lifestyle" product would ideally help customers lose up to 10kg over eight weeks.

"It's not a fad shake diet like others are, it's complete longer-term meal-replacement programme which can also help balance blood sugar levels and help customers lose however much weight they wish," he said. The manufacturers of the Quick Cleanse Detox did not respond to a request for comment.

Eat like a child

Not convinced by the diets we tested? Dietitian Nikki Hart's tips for the best ways to lose weight for summer are to exercise regularly and eat the types of food you would feed to a child up to age 5.

Even cutting down on caffeine and alcohol can make a world of difference to your energy, concentration levels - and waistline.

"Children before school age have a high fibre diet with minimally processed foods, lots of fruit and veges, low-fat milk and wholemeal bread and they have oodles of energy," said Hart. "Adults should take a leaf out of that book. There is no quick fix. There's too many of these diets out there. They aren't healthy."