I didn't know the answer until I read this study in December's issue of Medicine & Science in Sports & Exercise, which is the official journal of the American College of Sports Medicine. Nine professors and who knows how many members of the Look Ahead Research group somehow got together to write, "Fitness, Fatness, and Cardiovascular Risk Factors in Type 2 Diabetes: Look AHEAD study."
One of the study's authors, Steven Blair of the University of South Carolina, was kind enough to send me the full-text of the article. You can read the abstract online.
This huge study included 5,145 people with type 2 diabetes, all of whom were either overweight or obese. The average age of the people in the study was 59. Not surprisingly, the heaviest people in the study were also the least fit.
The researchers used both the body mass index (BMI) and waist size to determine how fat the people in the study were. Then, the researchers graded the cardiovascular fitness -- not muscle strength -- of the participants after they worked out on treadmills.
Now, after carefully studying the report, I understand why I didn't know whether it was better to be fit or to be thin. The answer, it turns out, depends on which risk factors we consider.
Being fit means that you will have a lower A1C and a couple of other less widely known risk factors (the ankle-brachial index, which shows the severity of peripheral arterial disease or PAD, and the Framingham risk score, which measures your risk of having a heart attack in the next 10 years) than when you have a lower BMI.
On the other hand, a lower BMI was associated with a lower systolic blood pressure.
The conclusion seems inescapable to me. We have to be both fit and unfat if we are to avoid the most common severe complications of diabetes.