单项选择题

Once upon a time, staying a healthy weight was easy. To lose weight you simply had to practise the reverse of home economics-spend more than you earned. Unfortunately for many, but perhaps not surprisingly, it turns out that people are rather more complicated than bank accounts.
To stay a healthy weight, you need a hormone called leptin to work properly. It sends "I’m full" messages from the fat cells up to the brain, where they go, among other places, to the same pleasure centers that respond to drugs like cocaine. Obese people produce plenty of leptin, but the brain doesn’t seem to respond to it properly. Last year, researchers at the Oregon Research Institute scanned the brains of overweight people and found their reward circuits were underactive. They were eating more to try to get the enjoyment they were missing.
There’s a lot of evidence for the fact that most, if not all, of us have a set point around which our weight can vary by about seven to nine kilos, but anything beyond that is a real struggle. Making changes is hard, particularly if your body is working against you. So why not ditch the traditional approaches and try some new methods, based on the latest research, that work with your body rather than against it.
Several years ago researchers at the National Institute on Aging in Baltimore reported that when they gave rats very little food one day and allowed them to eat plenty the next, they showed virtually all the benefits of a permanent calorie restriction diet. The same goes for humans, according to Dr. James Johnson.
How does it work Besides forcing the body to burn fat, it may also trigger hormonal changes. Most people say that the diet takes a bit of getting used to, but is not as grinding as trying to cut back on an everyday basis.
Older dieters may remember something called brown fat. Unlike the undesirable white stuff, this was a dieter’s dream. Instead of storing excess energy as fat, brown-fat tissue burned it off to keep you warm-at least in mice. Brown fat fell out of favor because researchers couldn’t find much in humans but now, thanks to the New England journal of Medicine, it’s back in fashion. The idea is to expose people to cold temperatures. They then make more brown fat and their weight drops. The author’s purpose in writing is to ______.