I've seen it in almost everyone I work with at some point or another. They have no energy; they can't lift as much as they normally do; they can't keep moving; they feel like crap for no good reason. Depending on the time of day, I ask the appropriate question: "did you eat breakfast / lunch / dinner today?" They look thoughtful and say "no... is that why I feel like this?"
The opposite problem happens too. Someone comes in for a workout right after a big meal and feels nauseated from warmup to cooldown.
There is a right way to fuel your body for exercise, so that you're feeling energized throughout, don't run out of steam and don't feel nauseated and overfull. Lots of people who are trying to lose weight figure they will exercise without eating beforehand, thinking that this will force their bodies to burn fat. In actuality, the body will react by slowing the metabolism, burning fewer calories overall both during and after exercise. It also ensures that the exerciser will be unable to work out with as much intensity as their normal potential would allow; making exercise less effective.
This does not mean it's okay to overdo it on junk food before hitting the treadmill! Putting in more calories than you will expend in exercise will result in gaining weight, not losing it. And while sugary foods might give you a rush of energy to begin with, the quick blood sugar crash makes for the same problem you would have had if you hadn't eaten at all half an hour into the workout.
The best way to prepare the body for exercise is to eat a healthy meal around 2-3 hours beforehand, or if it has been longer since the last meal, a smaller snack about half an hour to an hour beforehand. Whatever you eat should have a balance of simple and complex carbohydrates and protein, which will provide lasting energy throughout your workout. Some of my favorite pre-workout snacks are below:
*1/2 peanut butter & honey sandwich on whole wheat bread
*"cottage doubles" cottage cheese & fruit
*1/2 whole wheat bagel w/ 1 tbsp cream cheese
*1/4 cup of trail mix with nuts & dried fruit
*1 apple sliced with peanut butter
Be creative, or just stick with things you like. Just don't try to workout with no fuel!
Also make sure to drink 16-24 oz of water before exercise, and have more available during and after exercise. This will keep your body properly hydrated, energized and able to keep going. Give it a try and see how much better you feel!