CHICAGO — When Jay Smith fell off his jogging routine, he gained 25 pounds and saw his average blood sugar level climb into dangerous territory.

So Smith, a 48-year-old Type 1 diabetic, did what many people do when they need to get in shape: He began a diet and workout regimen. But while exercise benefits everyone, diabetics in particular can see dramatic improvements in their health.

After completing the 12-week Weight Achievement and Intensive Treatment (Why WAIT) fitness program at the Boston-based Joslin Diabetes Center, Smith lost the 25 pounds and saw his hemoglobin A1C level, a measurement that reflects a person’s average blood sugar level over several months, drop from 8 percent to 5.8 percent (7 percent or less is a common treatment target for diabetics). He also has been able to cut his insulin intake in half and reduce his cholesterol and blood pressure medications.

“Diabetics should consider exercise as a prescription; it is cheap and extremely effective,” said Dr. Osama Hamdy, medical director of the Obesity Clinical Program at Joslin, a research and clinical care center affiliated with Harvard Medical School. Hamdy also co-wrote “The Diabetes Breakthrough,” an upcoming book about the Why WAIT program, slated for March publication.

The weight loss that exercise helps hasten is a major driver of health improvements, particularly for Type 2 diabetics, who often struggle with obesity and constitute about 90 percent of the country’s 23.6 million diabetics. Hamdy’s research on the Why WAIT program found that when obese Type 2 diabetics lost 7 percent of their body weight, their insulin sensitivity improved by 57 percent, equivalent to taking two medications. Eighty-two percent of the participants were able to reduce their medication by 50 to 60 percent.

His results mirrored an earlier major study of prediabetics, who are at greater risk of developing Type 2 diabetes because their blood glucose levels are higher than normal. That study found that doing 30 minutes of physical activity five days a week reduced the five-year risk of getting diabetes by 58 percent. That was more effective than taking the common antidiabetic drug metformin, which reduced diabetes incidence by 31 percent, according to the study, published in 2002 in the New England Journal of Medicine. The Department of Health and Human Services estimates that a quarter of U.S. adults 20 years or older, about 57 million people, had prediabetes in 2007.