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

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

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.