DALLAS — In the months after Doug Robinson started driving a truck, he noticed his clothes were increasingly more snug-fitting. He was already overweight but soon realized that spending up to 11 hours behind the wheel, frequently eating fast food, and not exercising was a poor combination.

When his employer, US Xpress, took part in a weight-loss challenge sponsored by the Truckload Carriers Association, the 321-pound, 6-foot-1-inch Robinson signed up.