High blood sugar levels (search) can hamper thinking, new research shows. The pattern was seen in a study of people with type 1 and type 2 diabetes (search).

Almost 200 adults with type 1 diabetes and 36 with type 2 diabetes participated. They used handheld computers to test their mental skills. Then, the participants used blood glucose monitors to check their blood sugar levels; they entered the results into the handheld computer devices.

The tests were quick mental quizzes. One test required participants to think of as many words as possible that began with the letter “A,” tapping the computer’s “enter” key each time they thought of one.

Subtraction was also tested. Participants did the math mentally, without paper, pencil, or calculators. A third test gauged reaction times.

The point of the tests was to see if blood sugar levels were in sync with test performance, for better or worse. The answer was yes, at least, for some people.

More than half of the group (55 percent) didn’t think as clearly as normal when their blood sugar was too high. They responded more slowly or made more verbal and subtraction errors when their blood sugar levels were high.

These skills are important for daily tasks such as balancing checkbooks, calculating insulin dosing, and school and work performance.

Not everyone was affected. “These effects were highly individualized,” say the researchers, who included Daniel Cox, PhD, of the University of Virginia Health System.

High blood sugar’s flip side, low blood sugar, has also been linked to slower thinking. That means top-quality thinking might require optimal blood sugar levels.

Keep that in mind the next time you need maximum brain power. For instance, students facing final exams might not want to rely on sugary foods and refined carbs for fuel.

“Excessive carbohydrate loading ... may be counterproductive, if such actions lead to hyperglycemia [high blood pressure],” say the researchers.

Fuzzy thinking could also be an important clue for people with diabetes, signaling them to check and adjust their blood sugar as needed. The study appears in the January issue of Diabetes Care.

By Miranda Hitti, reviewed by Brunilda Nazario, MD

SOURCES: Cox, D. Diabetes Care, January 2005; vol 28: pp 71-77. News release, American Diabetes Association.