Thinking blurs when blood sugar strays.

Once assumed to pose mainly long-term health risks, hyperglycemia--or too much sugar in the blood--also can slow mental capacity in the short term, according to a new study. Other work shows low concentrations of sugar in the blood, leaving a person weak and groggy, can impair driving.

Researchers recruited 105 people with type I, or juvenile-onset, diabetes to follow a daily routine that included monitoring their blood sugar and self-assessing how clearly they were thinking. For a month, participants recorded their physical condition on a hand-held computer. They documented such high-blood sugar symptoms as headache, mouth dryness, and need to urinate.

Every day, each person took a brief arithmetic test to assess mental quickness. In periodic meetings with researchers, participants also were asked to list in 60 seconds as many words as possible beginning with a given letter.

At month's end, researchers combed the results for correlations between test performance and blood sugar concentrations.

The data harbored a surprise. While test scores at times of low concentrations of blood sugar were predictably worse than normal, high levels seemed to impair mental capacity, too, says psychologist Daniel J. Cox of the University of Virginia Health Systems in Charlottesville, who presented the data.

The range considered normal is 80 to 240 milligrams of glucose per deciliter (mg/dl) of blood. In the study, a reading of 270 mg/dl or slightly higher corresponded with a 10 percent decline on the mental tests. That plunged to 25 percent when the reading topped 300 mg/dl. When their concentrations of blood sugar were high, patients took longer to perform their self-tests than when their glucose was normal, he says. This is the first large-scale study of adults that clearly shows cognitive impairment from hyperglycemia, Cox says.

Meanwhile, results from a separate study by Cox and his colleagues bolster the belief that low blood sugar hampers driving ability. The researchers compared driving records of 1,000 diabetic people with those of their spouses. People with type II, or adult-onset, diabetes were no more likely to get into accidents, but those with type I diabetes were involved in roughly twice as many mishaps as their spouses were, Cox reported.

Cox recommends that doctors pay attention to the driving habits of their diabetic patients. Only half the type I diabetes patients in this study had ever talked to their physicians about driving. One way of reducing driving risks, Cox says, is for people with diabetes to spread their insulin intake over the day to preempt sudden dips in blood sugar.