In Depth
Feeding the Diabetic Brain
Ian Simpson studies how the brain gets and uses energy—and how diabetes disrupts that process.
The brain, despite its meager weigh-in at two percent of the body's mass, is our most voracious organ. Our brains consume 60 percent of the sugar coursing through our bloodstreams, a total of about 450 calories each day, a couple candy bars worth of energy. And because the brain can't store energy as fat or glycogen—a storage molecule made of glucose—like other parts of the body can, it needs a continuous supply of fuel. Demand surges when we are faced with complicated mental tasks, like puzzling over a tax form or grasping for the perfect word to complete the perfect sentence.
Transfer/watercolor by Karen Korell
Neuroscientist Ian Simpson has spent the past 10 years understanding what and how our brains eat. The "what" is easy. Basically the brain is picky, like a kid who eats only green peas, and only if they're arranged in neat rows on the tines of a fork. Simpson puts it a different way: "Like every cell in the body, brain cells use glucose for energy, but that's all they'll eat," he says. It's one of the few molecules allowed to pass through the protective barrier that separates the brain from the bloodstream. (The brain does have an "alternative mechanism" for getting energy, Simpson concedes. In the absence of glucose—during a period of starvation or when someone is on the Atkins diet or in the case of a breastfed infant—the body will convert fatty acids to acids known as ketone bodies that can pass into the brain and serve as food.)
Much less is known about how the brain eats, how it regulates supply and demand of energy. "The brain is this little engine that burns through an enormous amount of calories," says Simpson, "but we're still not sure what it does with all that energy." And when something goes wrong—in the case of diabetes, for example—that balance of supply and demand gets particularly complicated.
The hallmark of diabetes is this: The blood is too rich in glucose and yet muscle cells are stressed and starving. The muscle cells can't get enough energy because the diabetic body can't properly provide or use insulin—the hormone that stimulates the uptake of glucose into muscle cells. And when diabetics take insulin for treatment, they often get "too much stimulation and too much uptake," says Simpson, which leaves them with too little glucose in their bloodstreams.
While brain cells don't rely on insulin to get energy, the brain still has to deal with high and low levels of glucose in the blood-stream. Very low levels can put diabetics on the edge of a brain shut-down. This can cause what Simpson describes as "diabetic accidents": passing out behind the wheel of a car, for example. And high levels of glucose in the blood make diabetics susceptible to all sorts of physical and neurological problems, like memory loss and stroke.
Simpson, a researcher at Penn State's College of Medicine, thinks regulating glucose in the diabetic brain all comes down to better understanding how the brain gets and uses its energy—during periods of feast and famine, good health and poor health. He is focusing on the rather complex mechanism by which glucose is transported across that border-control system known as the blood-brain barrier.
Next page: "Simpson himself is diabetic..."