If you have a physically demanding job that keeps you moving all day, you may have greater endurance if you eat small snacks throughout the day instead of having a single large meal at lunchtime. Researchers at the University of Montana in Missoula showed that snacking on carbohydrates may prolong your endurance during a long day of continuous movement. They asked men and women to exercise intermittently for ten hours. Each hour, they performed nine minutes of upper-body weight lifting, 19 minutes of cycling, and 20 minutes of walking on a treadmill, with a one-minute rest between each exercise. This was followed by a 10-minute rest and feeding period. Those who took carbohydrates every hour were able to keep more sugar (glycogen) in their muscles, and were able to exercise longer without feeling tired. Journal reference
When you exercise, your muscles get their energy from muscle sugar and fat stored in muscles, sugar and fat from the bloodstream, and to a lesser degree from protein. When your muscles run out of their stored sugar, they require more oxygen to burn food for energy. This can make your muscles feel tired and be more difficult to coordinate. Eating any source of sugar or carbohydrates during exercise preserves muscle sugar and increases endurance. More
This is very good advice. I always am sure, though, to combine a small amount of protein with the carbohydrates, as I feel that just carbohydrates as a snack make me "crash" (feel light-headed and hungry) fairly quickly.
I weight train three days a week and I've started incorporating some complex carbs into my breakfast routine and making a protein shake with skim milk. I've noticed a big difference in my endurance and it also seems to reduce muscle soreness afterward.