Working out on an empty stomach, such as running first thing in the morning before breakfast, can cause you to feel sick to your stomach during the exercise. However, it's not a good idea to eat immediately before you exercise either, because your digestion can be compromised as your blood flow is redirected to your muscles. This can also cause an upset stomach. The best idea is to find the balance so you're working out after eating, but not immediately after.
Exercise on Empty Stomach
Although exercising on an empty stomach can help you burn more fat because there are no carbohydrates immediately available in your digestive system, the difference in the amounts of fat burned is minimal, according to Iowa State University. Most people who exercise before eating choose to do it in the mornings before breakfast, when the body has been fasting for more than eight hours.
Working out on an empty stomach can cause your body to pull protein from your muscles rather than fat from your body, says the Australian Sports Commission. Although you might burn slightly more fat than if you exercise after eating, your body won't pull all its necessary energy from the fat. When you take protein from the muscles, you can't build your muscles effectively. Also, as your body tries to draw energy from your digestive system, your blood sugar can lower and lead to nausea.
Nausea is a common symptom of low blood sugar, which can occur when your body is seeking energy but none is readily available. You might also feel dizzy or faint, which can be dangerous while working out. You can fall or drop weights, injuring yourself or someone else. If you start to feel faint, dizzy or nauseous, stop working out and sit down immediately.
Eating just before you work out isn't the best solution, as your body pushes oxygenated blood to your digestive system to help it process food. If your muscles compete with your digestive system during exercise, your food won't digest properly and you can begin to feel sick. Instead, eat something one to two hours before your workout for the best results. Mix some carbs with protein, such as cereal with low-fat milk or cheese with whole-wheat crackers. If you must eat immediately before a workout, use something that has balanced protein and fat such as a spoonful of peanut butter or a handful of almonds.