have to progress to the full- blown disease. sb. jacqueline lonier is with the naomi berrie center at new york presbysterian. >> the most effective way to prevent the progression of prediabetes to diabetes is through diet and exercise-- regular activity, watching carbohydrate intake. >> reporter: 52-year-old paul healy was diagnosed with prediabetes in 2010. then diabetes. since then, he has changed his diet and lost 32 pounds. >> part of what happened when i was diagnosed is that i was really in denial about it. so it took me a while to really get my blood sugars under control and learn how to say no. >> reporter: the c.d.c. estimates 90% of people with prediabetes don't know they have it, but a simple blood test can quickly make the diagnosis. that's crucial because more than half the time, diet and exercise can prevent prediabetes from progressing to diabetes. >> pelley: what are the symptoms of prediabetes?