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Judith J. Wurtman, PhD Headshot

Cravings: Why Are Premenstrual Women Desperate for Chocolate?

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I nearly fell off the treadmill the other day when I heard on TV that snapping a rubber wristband against the wrist would stop the need to eat cookies.

"I would like to see a premenstrual woman zooming in on a piece of chocolate, suddenly stop, snap the wristband and then settle for a celery stick," I muttered to myself as I resumed my running.

A craving for carbohydrates is generated by the brain, not the taste buds, and is as natural as feeling thirst when the body needs water. Thus snapping a rubber band against the wrist to stop cravings is about as effective as hitting your head against the wall when you are thirsty and trying not to drink.

The thirst and the cravings are signals from our bodies to do something. Thirst is a demand that we drink to increase our blood volume. Carbohydrate craving is a demand that we eat something sweet or starchy because the brain needs to make serotonin. However, unlike thirst, the craving for carbohydrates is often accompanied by deterioration in mood. Studies we carried out at MIT many years ago found that when people had an urge to eat carbohydrates, they were usually stressed, irritable, angry, depressed, cranky, distracted, tired or impatient, or all of the above. These moods of course, reflect a change in serotonin activity or levels.

Serotonin is made after any non-fruit carbohydrate is eaten and digested. Insulin is released and the pattern of amino acids in blood shifts to allow tryptophan to enter the brain. Tryptophan is converted to serotonin very quickly and serotonin levels increase in the brain. This discovery was made in the early 1970s.

I agree with the prohibition against eating carbohydrates that are greasy or high in fat like fried batter, chips, doughnuts or other sweet or starchy fatty foods. The brain doesn't want the fat; it wants you to eat carbohydrates. Whether you choose to eat the carbohydrate in the form of a healthy whole-grain food such as multigrain Cheerios or a high-fat, sugary food like a candy bar is another matter. All that the brain requires is that you consume about 30 grams of carbohydrate and -- this is important -- that the carbohydrate food is low in protein. Protein prevents tryptophan from getting into the brain. (Fructose, the sugar in fruit and in many sugary drinks, is the one carbohydrate that does not lead to serotonin production.)

Carbohydrates that contains large amounts of fat, such as cookies and ice cream, are digested more slowly than fat-free carbohydrates, so it takes the brain a longer time to make serotonin.
Just as we lose our thirst after drinking water, we lose our cravings after serotonin is made. And as an added benefit, those unpleasant moods we experienced along with our cravings are replaced by a decrease in stress and an increase in calmness, energy, focus and patience. Of course, if people are prevented from eating carbohydrates by wristbands, the cravings don't go away. Instead, they will seem to take on the form of an addiction. We all know that when we are terribly thirsty, all we can think about is how and when we can drink. When the brain needs to make serotonin, the cravings can become just as insistent.

And who has the worst, most insistent carbohydrate craving? Women with PMS. The saying, "I could kill for chocolate," is, one hopes, not based on reality but there are plenty of anecdotes of women braving hurricanes and blizzards to get their carbohydrates when they have PMS.

We studied the food choices of women with severe PMS who stayed in the MIT Clinical Research Center at the beginning and end of their menstrual cycle. These normal weight women increased their calorie intake by more than 1,100 calories daily when they were premenstrual -- and the calories came entirely from carbohydrates.

When we discovered that inadequate serotonin activity was behind their food cravings and their premenstrual moods, we tried treating these symptoms by giving the women a fat-free, protein-free carbohydrate beverage twice daily. We reasoned that if serotonin could be increased with a small amount of carbohydrate, then perhaps these women could feel better without resorting to drugs or herbal supplements. The carbohydrate intervention worked extremely well; not only did the women feel significantly better, they also were able to control their appetite.

When the brain wants carbohydrates to increase serotonin, neither wristbands nor will power is going to prevent someone from doing what the brain wants. Of course, the carbohydrate craver should be encouraged to choose healthy carbohydrates and remember that only 30 grams, not 30 cookies, have to be consumed.

The payoff is worth it. A lot of carbohydrate cravers are going to be much happier after their cravings are satisfied -- and their wrists won't be sore.

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