There are a lot of weight loss plans out there, promising dramatic results in a short space of time. And no wonder - given that the third most searched "how to" term on Google is "how to lose weight", it's inevitable that there are people and companies out there looking to capitalise by selling the latest quick fix.

Of course, losing weight fast sounds great, but there's a big catch. While seeing a difference in your body shape early on can be motivating and satisfying, nutritionists and dietitians warn that losing weight quickly is mostly unsustainable and doesn't promote a long-term healthy relationship with food.

Though crash or "fad" diets that lack nutrients are unlikely to have a huge impact on overall physical health because they are short term, their impact on mental well being can be long lasting.

"My biggest concern with losing weight rapidly is the affect it has on someone's mental wellbeing," says Dr Frankie Phillips, registered dietitian and nutritionist and spokesperson for the British Dietetic Association.

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"Crash diets often leave people feeling demoralised when they start to regain weight so quickly."

A review published by Yale University's Department of Psychology acknowledged the correlation between yo-yo dieting (or weight cycling) and mental health issues. "Weight cycling appears linked to increased psychopathology, lower life satisfaction, more disturbed eating in general and perhaps increased risk for binge eating," it says.

While the British Dietetics Association (BDA) promotes safe and sustainable weight loss of between 0.5 to 2lbs a week, dieters on certain weight loss programmes - such as the juicing diet, the GM diet and the New Atkins diet - report extreme weight loss of up to 15lbs in two weeks.

So what principles do these fast weight loss diets have in common, how do they work, and should you give them a go?

INTRODUCE A CALORIE DEFICIT

Perhaps the most crucial body factor in any weight loss plan is to be in a calorie deficit. That is, simply, to eat fewer calories than you require for you current body weight.

Eat fewer than this every day and it's likely that you'll drop weight. However, you need to do this in a safe range - a 500 calorie deficit on your daily needs is recommended by the BDA.

You can use a calorie counter to find out your daily intake needs to be in a calorie deficit. It's both realistic and not bad for your health.

If you knocked much more off, you'd likely not have a very good time on your diet, and you also would be far more likely to break the rules.

A very low calorie diet (VLCD) where dieters eat just 800 calories a day is only suitable for obese adults with a BMI of 30 or over and should be clinically supervised.

When following a calorie controlled diet, it's a good idea to record calories and keep an eye on your daily target. Using an app like MyFitnessPal to count calories in your first 3-4 weeks can help you stay on track.

INCREASE ACTIVITY

The more you exercise, the more energy demands you'll be placing on your body. If you're already in deficit of, or even slightly above, your calorie needs, the likelihood is you'll lose weight if you exercise intensively for three or four times a week.

Different types of exercise put different strains on the body, and it really depends on how heavy you are already as to how much you'll burn via exercise.

The simple rule is, the more intense your form of exercise, the more likely you are to burn calories faster - thus the more likely you are to get into calorie deficit.

RECONSIDER CARBOHYDRATES

Many restrictive diets result in fast weight loss because of the lack of carbohydrates. Why does this work? "It's all about water weight," says Dietitian and BDA Spokesperson Chloe Miles.

"When you eat carbohydrates, your body stores it as glycogen in the liver and muscle. Your muscles store approximately 500g and your liver approximately 100g and it's thought that every gram of glycogen in the human muscle is bound to 3g of water," she says.

So, cut out carbohydrates and you also deplete this store of water, thus losing weight. This is probably why some low carb dieters report 4-5lbs weight loss in just two weeks. The problem, again, is that the water - and hence weight - is regained as soon as you reintroduce carbs to your diet.

SLEEP

Getting your eight hours shut eye is just as important as your diet and exercise regime when it comes to weight loss.

In a study by the University of Leeds, 1,615 adults reported how long they slept and kept records of food intake. Indicators of overall metabolic health such as blood pressure, blood cholesterol, blood sugar, and thyroid function were monitored, as well as weight and waist circumference recorded.

Those who slept for six hours or less a night had waists that were on average 1.1 inches (3cm) larger than those who slept for nine hours.