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Our good friend Marlene (not her real name), recently announced that she and all the other women in her office have committed to lose ten pounds by March, aiming to shed a pound a week. This is not the first time she's made such a pact but one of many, and each time that unwanted ten pounds has briefly departed but then boomeranged back usually bringing one or two extra pounds along with it. The persistence and willingness of her and her friends to try and try again is impressive. Unfortunately, it is doomed to fail once more because it flaunts their bodies' built-in wisdom. Here's why.
Marlene is fairly average, and according to annual government surveys, the average American women takes in 1860 calories a day; but let's round that up to 2000. A pound of fat equals 3500 calories, so losing a pound a week will mean cutting her intake by 500 calories a day. It may not seem like much, but it's a reduction to just three quarters of what she usually eats. Marlene doesn't want to talk about how much she weighs, but let's say its 150 pounds. Based on her age and weight, we can estimate that about 50 pounds of that 150 is fat. So, to meet her goal of losing ten pounds of fat, she would have to lose 20 percent of her stored fat, a pretty significant portion.
Let's now consider this situation from the viewpoint of the part of Marlene's brain that controls her weight, her hypothalamus. This is in the ancient core of the brain that isn't directly connected to the higher, more recently evolved brain centers, so her hypothalamus doesn't "know" that Marlene wants to lose weight. This is important because Marlene's hypothalamus has a target for her weight called her set point, and it makes constant adjustments to keep her weight about the same from week to week. It constantly checks on how much fat Marlene has and regulates her appetite so that she eats just enough to keep her weight at the set point.
Marlene's set point is determined by the genes she inherited from her parents, her experience in her mother's womb, and her having had two children. Certain foods may also raise the set point and, as we are learning, so can dieting. Marlene's hypothalamus has determined that she needs to have 50 pounds of fat, and it can tell exactly how much she has from hormones produced by her fat cells.
Now Marlene has been chugging along following her hypothalamus's careful guidance on how much she should eat when, all of a sudden, her intake of calories drops by one quarter! Her hypothalamus is telling her to eat, but she isn't responding, and she is starting to lose some of her stored fat. Her hypothalmus can only conclude that Marlene no longer has enough food available to keep her where she should be, so it takes a series of physiological steps to protect her fat. The result of these reponses is that cutting 500 calories is no longer enough to lose a pound a week, so Marlene cuts even more. But this makes her hypothalamus even more determined to protect her fat. And so it gets harder and harder for Marlene to lose, and after a while she will decide she has lost enough and will stop her diet even if she didn't drop the full ten pounds.
Now, the gaurdian of her set point, her hypothalamus will push her to eat more than 2000 calories a day so that she can make up for the fat she has lost. Without being aware of it, she will regain her weight even though she feels that she is eating the same way she was before her diet. But her hypothalamus will continue to encourage her to gain weight even after she has regained the weight she lost, and studies show that these effects may continue for a year or more. As a result Marlene will probably finish her diet cycle a pound or two heavier than where she began it.
Marlene is just one of 45 million overweight American women who try to lose weight by dieting each year, and on average they have gained 20 pounds in the past 10 years; so their efforts to lose weight by dieting have not worked. Pound-a-week dieting not only fails to lower weight set points; it actually raises them.
So why do our brains do this? One reason is that a woman needs unusually large amounts of fat-about six times more than other animals-to provide essential fats that are needed for the growth and development of her children. So her brain is determined to protect this fat. The other reason is that our Stone-Age ancestors often had times when food was plentiful and times when it was scarce, so having a cushion of stored fat was essential if they were to survive the bad times. When we diet, our brains think that the bad times are back, so it pushes us to eat more even after we have regained the weight we lost, because it isn't sure now how long the good times will last. The more often we diet, the more we convince our brain that we live in a place where the food supply is unreliable and the more it wants us to eat when the eating is good and the higher our setpoints go.
Frequent dieting is one likely reason why the setpoints of American women have risen by an average of twenty pounds over the past forty years, but changes in the American diet have probably played an even more important role; so what you eat can also affect your setpoint. There is more about these issues in our book, Why Women Need Fat.
Written by Will Lassek and Steve Gaulin
