If you are trying to keep up a healthy, active lifestyle you will want to eliminate junk food from your diet. If you eat it once in a while, as a "treat," you probably won't suffer any consequences. If junk food is your regular choice, however, or if you eat it before your workout, you might end up paying the price.
Lack of Energy
Junk food often contains high amounts of sugar, which can directly affect your energy levels. When you eat something full of simple sugars -- like a donut or a piece of candy — it immediately raises your blood sugar. You will get a quick burst of energy, but an equally quick crash not long afterward. How long the burst of energy lasts depends on your current weight, what you eat and how much. However, if your workout is an hour or two hours long, junk food is not likely to provide you with enough energy.
Junk food is heavy on your stomach and could cause indigestion, heartburn and other gastrointestinal issues. If you eat junk food before you exercise, you might end up feeling sick, especially if you go for a workout that requires a lot of jumping, twisting or moving around. For example, you might not feel too bad on the stationary bike, but try running on the treadmill or doing yoga while you're nauseous and you might end up feeling very sick.
Lack of Definition
Junk food also tends to be high in fats as well as sugars, both of which can lead to weight gain. If you're trying to gain muscle definition, adding fat to your body can make it impossible. Fat accumulates under the skin, on top of the muscle, so even if you exercise daily, you won't see muscle tone until you get rid of the fat. Also, muscle growth requires protein, not fats or sugars.
Junk food can lead to a number of health conditions that can affect your ability to exercise. High blood pressure, for example, is a common consequence of eating excess sodium and fats, according to MedlinePlus. Sodium and fats are commonly found in junk food. A high-intake of junk food could also lead to other health problems, including diabetes and obesity. Any medical condition you have will affect the type and amount of exercise you can perform.