If you’re on the glucose roller coaster, it’s time to seek level ground. A new study presents compelling evidence that when it comes to cardiovascular health, blood glucose that swings wildly up and down is bad news.

Hyperglycemia, or high blood glucose, is known to be a ticket to diabetic complications. One reason, experts believe, is that hyperglycemia triggers production of free radicals, highly reactive chemicals that can damage cellular molecules, in a process called oxidative stress. To keep an eye on hyperglycemia, doctors check the A1C, an estimate of a person’s average blood glucose over the past three months. However, since it is an average, the A1C doesn’t reveal the pattern of high and low oscillation. A person with dramatic fluctuations could have the same A1C as someone whose blood glucose remains relatively constant. But are their risks the same?

Not according to results from the new study, published online in Feb. 2008 in the journal Diabetes, which indicate that people with oscillating glucose levels are more prone to endothelial dysfunction—free radical damage, or oxidative stress, to the endothelium, the layer of cells that lines the arteries—compared with people who have more stable blood glucose levels. “Endothelial dysfunction has been proved to be an independent predictor of cardiovascular disease in type 1 and type 2 diabetes,” says lead author Antonio Ceriello, MD, professor at Warwick Medical School in the United Kingdom.

In the study, 27 people with type 2 diabetes and 22 without diabetes submitted to 24 hours of strict drug-induced blood glucose and insulin regulation. One group of people had their blood glucose manipulated to oscillate between normal (90 mg/dl) and high blood glucose (270 mg/dl). The second group maintained a stable, though high (270 mg/dl) blood glucose. The third group’s blood glucose was set to an intermediate high level (180 mg/dl) —its average blood glucose over the 24 hours was equal to that of the oscillating group.

The researchers found that the oscillating group had more endothelial dysfunction (indicating abnormal function of the blood vessels) than either of the two stable blood glucose groups. The oscillators also showed more signs of impact from free radicals. So even though the second group with persistently high blood glucose was more hyperglycemic than the oscillating group, the people with consistently high blood glucose still showed better blood vessel health.

It is unclear from this study how blood glucose oscillation could cause greater damage than hyperglycemia alone. Ceriello thinks it might have to do with how cells respond to and protect themselves from oxidative stress. “When the glucose is constantly high, you recognize that something is wrong. So you prepare the cell to react,” says Ceriello. “When it goes up and down, [the cells] don’t have time to realize it’s really dangerous. They need time to generate the antioxidant defenses.”

Whatever the reason, this study demonstrates that erratic blood glucose levels may potentially contribute to cardiovascular disease. And that seems reason enough to avoid the rollercoaster.