For children with type 1 diabetes, blood sugar levels that are either too low or too high may disrupt their ability to concentrate and think, a small study shows.

Blood-sugar lows are known to temporarily cloud mental functioning in people with diabetes, but the effects of high blood sugar on cognition have been less clear. In addition, compared with adults, little is known about how blood-sugar fluctuations affect children's thinking and concentration.

In the new study of 61 kids with type 1 diabetes, researchers found that both sugar highs and sugar lows impeded the children's average performance on tests of math and reaction speed.

In fact, very high blood sugar levels had the same impact as very low levels, according to the researchers, led by Dr. Linda Gonder-Frederick, of the University of Virginia Health Sciences Center in Charlottesville.

They report the findings in the journal Diabetes Care.

For the study, the researchers gave the children, 6 to 11years of age, personal digital assistants (PDAs) programmed with two cognitive tests that they completed before measuring their blood sugar each day. Each child took the tests several times per day over four to six weeks.

In general, the study found, the children were slower to complete the tests when their blood sugar was either too low or too high. The effects were not universal, however. The researchers found that individual children varied widely in how blood-sugar extremes affected their mental performance.

They call for more research into why some children are more vulnerable than others.

Also unclear is why high blood sugar impairs mental performance. It's possible, the researchers say, that elevated sugar disrupts blood vessel functioning in the brain, or interferes with the normal activity of brain chemicals like serotonin.