You know what it's like: You're high energy one minute, slumped over the next. Why? Blood sugar. Lisa Davis gets to the bottom of her daily ups and downs.
Half the time, I bear a strong resemblance to a shrew (the animal version, not the Shakespearean kind): I'm small, fast moving, and cute from a distance—but I'll chew right through you if you get between me and a meal. The rest of the time, my disposition is closer to that of a corpse. You could stand on me to eat your own lunch and I wouldn't notice.

There are a lot of people who, like me, are perfectly fine except that every now and then they get shaky, headachy, maybe just a little bit frantic or even cranky because they're starving, damn it... people who are lovely, even-keeled souls unless they crash, and then they're not on the floor, they're well below it.

A decade or so ago, there was a popular diagnosis for this kind of behavior. According to a number of books and self-help groups, hypoglycemia (a fancy term for abnormally low blood sugar) was to blame for premeal jitters and postmeal stupor, not to mention forgetfulness, depression, yelling at your spouse, and cuddling up with sweets. It was the diagnosis your doctor refused to make, advocates claimed, even though the syndrome was rampant.

Except that it wasn't. Careful studies proved that, aside from diabetics, very few people have blood sugar that gets seriously out of whack. Good thing, too, because persistent hypoglycemia is a signal that something is very wrong. "The diseases that cause hypoglycemia are serious, with major health implications," says Mayo Clinic endocrinologist F. John Service, MD, a longtime expert in the field. Which doesn't necessarily mean that your mood and energy swings are unrelated to blood sugar. If the hormones that regulate it are just a little overeager, you may be prone to a host of unpleasant symptoms that have nothing to do with being ill or abnormal, and everything to do with the way you eat.

Forget the sensory pleasures of digging into a garlicky bowl of pasta or licking a dripping chocolate ice cream cone. All your body really wants of a culinary creation is to mash it, dissolve it into a slurry, and break the carbs down into simple sugar that can float in your blood to each and every cell. Blood sugar, aka glucose, provides the energy that keeps your brain buzzing and your heart ticking, and all your other muscles and organs functioning nicely. So it's no wonder there's a complicated hormonal system to nip and nudge at blood sugar to keep it in normal range. Insulin ushers glucose into needy cells or into the liver for storage; glucagon and other hormones prompt the liver to release stored glucose if blood levels get low. These hormones keep your blood sugar where it should be even if you miss meals or work out strenuously.

But you can mess with the system by overdoing foods that are high in simple carbohydrates—obvious sugary items like candy, cake, nondiet soda, and syrupy waffles, as well as white bread, bagels, and orange juice. Inhale a big load of pure simple carbs at one sitting (a box of fudge, a big stack of pancakes), and your blood sugar will shoot up. To deal with the jump, insulin will shoot up, too, according to Roland G. Hiss, MD, a hematologist who has been an investigator at the Michigan Diabetes Research and Training Center for more than 20 years. And the higher the sugar intake, the more likely it is that your body will produce excess insulin for the job.

An overenthusiastic insulin response sweeps too much sugar from the blood, dropping it below your normal set point. That triggers a release of other chemicals bent on raising blood sugar, among them adrenaline, one of the stress hormones that produces the fight-or-flight response in a panic situation. All you did was pour a little too much syrup on your pancakes a couple of hours ago, and here you are with the sirens blaring, red lights flashing, and then come the jitters, racing heartbeat, cold sweats, that I-could-kill-someone feeling.

"The body hates low blood sugar," Hiss says, "and that's because of the brain. It must have a constant blood sugar level to function—and if it doesn't, all hell breaks loose." Once the body senses a blood sugar dive and goes into panic mode, the repercussions can last a half hour or longer. It won't matter if you throw food into your mouth by the fistful; you're going to have to wait out the aftershocks until the level of stress hormones falls.

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