Losing weight rapidly, only to pile it all back on again - yo-yo dieting - is a trap many fall into; a survey in 2014 found 60 per cent of yo-yo dieters will try up to 20 diets in their lifetime.
And you don't have to be losing and gaining huge amounts of weight to be a yo-yo dieter (it can be as little as between 7lb and 1st), but the weight gain and loss occurs over weeks and months, rather than years.
This way of dieting has been blamed for a range of health issues including hormone imbalances and osteoporosis.
A study presented last week at the American Heart Association's Scientific Sessions found it could also harm the heart - and the risk increases the more often you try and fail to keep weight off.
"This is particularly worrying since we know many yo-yo dieters have been on multiple failed diets, each time regaining more weight than they lose," says Rebecca McManamon, of the British Dietetic Association.
Here the experts explain why yo-yo diets are bad news...
YOU'LL PUT WEIGHT ON EVEN FASTER
Studies show the body strives to maintain its weight at a "set point".
"The body will protect itself against weight loss during a period of calorie restriction, which is probably an evolved protection mechanism", says Rebecca McManamon.
It also means people don't suddenly gain weight if they overeat for short periods.
"However, in protracted periods of starvation, weight loss does occur and your body gets used to functioning at a lower energy level," she explains.
"Unfortunately, this resets the metabolism so when you start eating normally again, you will put on weight more quickly."
(Intermittent fasting, as in the popular 5:2 diet, when people fast for two days and eat normally for five, is thought to be too short a period to kick start 'starvation mode'.)
Muscle loss is another problem. We need muscle to burn calories.
"When the intake of calories drops below a certain level, the body will start to digest your muscle cells as well as remaining body fat to make energy," says Kathryn Freeland, a personal trainer who runs Absolute Fitness in London.
"Less muscle mass can slow down metabolism even further, compounding the problem of rapid fat gain when you start to eat normally again."
HARMS THE HEART MORE THAN OBESITY
In the latest study, presented last week, researchers from the Memorial Hospital of Rhode Island analysed data from 158,000 women over 50.
Over 11 years, the women of normal weight who admitted to yo-yo dieting more than four times, were three-and-a-half times as likely to die from a heart attack than women whose weight stayed stable, even if they were obese.
Dr Somail Rasla, lead author of the study, says gaining weight as part of yo-yo dieting increases heart rate, blood pressure, cholesterol and blood sugar levels, which do not fall back to normal when weight is lost again.
If these cycles keep repeating, these health problems worsen over time, putting strain on the heart.
Dr Mike Knapton, associate medical director at the British Heart Foundation, says the findings seem to back up previous studies linking yo-yo dieting to cardiovascular disease.
Yo-yo dieting increases inflammatory markers in the blood which are associated with lots of diseases, including arthritis.
YOU WILL END UP LOOKING OLDER
Yo-yo dieting has one obvious effect - constantly losing and regaining weight can leave permanent stretch marks and sagging and drooping skin.
"This depends on how much weight you gain and lose, how often and also your age and the elasticity of the skin, but it is a common issue," says Dr Nick Lowe, a consultant dermatologist in Harley Street, London.
Older women with less elastic skin will be more prone to sagging. Quick weight gain followed by rapid weight loss puts more strain on skin than gradual weight change.
Restrictive diets can also affect the plumpness of the skin, as a lack of protein in the diet can affect the production of collagen, which gives skin its firmness.
STOPS FERTILITY THERAPY WORKING
Famine followed by feast can disrupt the release of hormones from the pituitary gland which drives the menstrual cycle, stimulating the ovaries to produce eggs.
If a woman rapidly loses weight this process shuts down because the body goes into starvation mode and concentrates its resources on vital functions, which don't include reproduction, says Gillian Lockwood, a consultant obstetrician, gynaecologist and fertility expert based in the Midlands.
Unfortunately, according to Dr Lockwood, many overweight women who want IVF on the NHS are told that they have to lose weight in order to qualify for the free treatment, and this can encourage crash dieting.
'If a woman is 6st overweight, it will take her a year or more to lose that safely, but many women and their partners feel a sense of urgency.
"Not surprisingly, they may dramatically cut their calorie intake, sometimes down to 400 calories a day."
And women who rapidly lose weight to reach the necessary BMI have very poor outcomes when it comes to getting pregnant, as the IVF is unlikely to work.
This applies only to crash dieters.
BAD NEWS FOR YOUR BONES
Yo-yo dieting does seem to affect bone density, although the jury is still out as to whether yo-yo dieting or periods of restrictive dieting is the problem.
Our bone is constantly being broken down, absorbed into the body and replaced.
A recent U.S. study found that women whose weight fluctuated had increased bone resorption when their weight fell, which was not replaced when they regained the pounds.
Feasting periods do not make up for bone loss in the fasting times.
GOOD DIET IS VITAL FOR HEALTHY HAIR
Trichologist Anabel Kingsley says yo-yo dieting is a common cause of poor hair conditions and hair loss: "One of the first things we always ask is about people's diet, which is fundamental to healthy hair.
"Yo-yo dieters are stressing their hair."
She says that, at any one time, 90 per cent of head hair is in a growth phase and just 10 per cent is in the resting phase, when it sheds.
"Restricted calorie intake pushes more hairs into the shedding phase because the hair follicles have fewer nutrients to sustain them."
Rapid weight gain also disrupts normal hormone levels and stresses the hair follicles.
GUM DISEASE AND LOSS OF TEETH
"People on restrictive diets are not getting enough key minerals and vitamins such as calcium to keep their teeth and gums healthy," says Mervyn Druian, of London Cosmetic Dentistry.
"Yo-yo dieters are more prone to gum disease, which can lead to tooth loss."
Those who have dramatically cut back on eating also produce less saliva, which is triggered by the chewing action, which helps protect the teeth from acid attack.
On the weight gain cycle, people who eat a high carbohydrate diet are at risk from tooth decay.
"Bacteria in the mouth thrive in these conditions and release acid in larger quantities," adds Dr Druian.
IT MAY EVEN BE LINKED TO CANCER
Studies have suggested there may be a correlation between yo-yo dieting and some cancers.
One theory is that it may lead to a reduction in the number and effectiveness of immune cells, which are the body's initial line of defence against tumour cells.
A 2004 study showed a reduction in these cells the more times post-menopausal women lost weight.