Although the pounds will dwindle, so will your metabolic rate and most likely your lean body mass — which in the end is exactly what you don't want.
"If you go on, say, a 900-calorie-a-day diet, you will have a hard time getting the nutrients you need," says Rebecca Mohning, a nutritionist. "Without the daily requirement of protein, you will break down your lean muscle mass."
"Basically, the body will make sure it gets what it needs to function — and if it doesn't get what it needs from food, it will take what it needs from the muscles," says Virginia nutritionist Danielle Omar, who owns Foodconfidence.com. "It's not that smart when you consider that you are in essence eating away at your own muscle mass."
And less lean muscle mass means you burn fewer calories — probably not what you were going for.
You will also lower the body's basal metabolic rate (BMR) — the minimum amount of energy you need to keep the basic functions going (such as liver and brain function and breathing; breaking down food requires about 10 percent of the total BMR).
"It just covers the basal processes of the body. Nothing on top of it — not even walking," says Scott Kahan, a doctor and director of the National Center for Weight and Wellness.
So what determines the metabolic rate and how is it calculated?
The basal metabolic rate is determined by age, gender, height
"Whenever you lose weight, whether intentionally or not, your BMR goes down," Kahan says.
Age also affects BMR. "This is why you sometimes see people in their late 20s surprised at gaining a few pounds when they don't feel like they have changed any of their eating habits," Mohning says.
This is because after the age of 25 — which is the age where we stop growing bone — the metabolic rate goes down by 2 percent or more per decade, she says. So, in order to stay at the same weight without a change to your level of activity, you would have to cut your daily calories by the same amount.
For example, a woman might get away with consuming 1,800 calories a day up to age 25. But in the next decade she would have to cut that intake down to 1,728 calories per day. In the next decade it would be down to 1,658.
Yes, says Kahan, it is complicated and it's difficult to get an accurate reading. For the number to be truly accurate it has to be done in a facility with trained personnel who can measure your body's oxygen intake and carbon dioxide output, he says.
"And it can have unintended consequences," Kahan says.
For example, if you find out that your BMR is low you might feel it's hopeless, he says.
"Knowing your metabolic rate can lead to a very fatalistic view," he says. "If your metabolic rate is 1,200 calories a day, you are going to have a more difficult time losing weight, but not necessarily less opportunity to stay healthy," he says. "We want to emphasize health."
Mohning, though, says that it can work to the contrary. People who blamed their "slow metabolism" their whole lives realize their metabolism actually is no worse than the next person's.
So how about supposedly "metabolism-boosting" foods — are they for real?
"It's nothing to get too excited about," Mohning says. Though some things, such as green tea, have been shown to increase calorie burn, the change is very small. The same goes for spices like cayenne pepper.
The safer, better method to increase your calorie burn is to increase your lean body mass.
"The only real solution is to put on muscle," says Omar, who adds that losing more than one or two pounds a week for most women and more than two or three pounds a week for most men is not advisable.
Building muscle and staying active in general is also a good way to avoid yo-yo dieting, Mohning says.
Typically, people will lose the desired 10 to 20 pounds by crash dieting, she says, but then they gain it back because they have lowered their metabolic rate. And when they gain the weight back, they gain it in fat — not the muscle they initially lost as a result of the crash diet.
So, if you are a perpetual crash-dieter, there's still good news.
Your basal metabolic rate is not permanently damaged no matter how many bad yo-yo diets you have been on in your life, Mohning says. As long as you stay active and find a measured approach to eating, you are good to go, she says.
"I think that could be very encouraging for people to know. No permanent damage is done."
Boston is a fitness trainer and freelance writer.