Fainting or "blacking out" while exercising can be dangerous, but the underlying cause is not always a serious condition, according to the National Institutes of Health Medline Plus. Malnutrition, overheating, dehydration and stress can all lead to blacking out, and exercise can hasten the process. In some cases, however, a serious underlying disorder such as heart disease can be the cause. If you black out during exercise despite taking preventive measures such as adequate hydration, consult a doctor.
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Blacking out, medically referred to as "syncope," results from a lack of adequate blood supply to the brain, regardless of the underlying cause. Before blacking out, you may feel light-headed or nauseous, or your heart may flutter or start to beat more rapidly. In most cases, blacking out is only temporary and you will regain consciousness immediately. You may experience only a momentary loss of posture, or you may actually fall to the ground.
Blacking out while exercising can be due to a variety of causes that restrict blood flow to your brain. Dehydration -- not having enough fluids in your body -- can lower your blood pressure and lead to blacking out, particularly if you are sweating a lot. If you have not had enough to eat, your blood sugar will be low, which also affects the blood flow to your brain. Exercising in an overly heated environment can cause a blackout. If you black out while exercising despite being properly cooled, nourished and hydrated, an underlying medical disorder could be the cause. The most common medical problems associated with blacking out while exercising are related to the heart. Some medications, particularly blood thinners used by some heart patients, can lead to blacking out when you exert yourself.
If your blackout was minor and you experienced only a momentary loss of consciousness, stop exercising and rest. Drink some water or water enhanced with electrolytes to replenish vital fluids. Eat a small snack that is high in carbohydrates or sugar, such as a some bread or orange juice, to recover from low blood sugar. If you are in an overly heated environment, find a cooler, preferably air-conditioned, environment in which to rest and cool off. If you have fallen as a result of blacking out, check for injury and seek medical attention if necessary or if in doubt.
Covering your basic needs before exercising can help you avoid black-out spells. Start hydrating the day before vigorous exercise, and drink one to three cups of water before you exercise. Drink plenty of fluids during and after exercise, particularly if you are sweating a lot. Eating a smaller meal with carbohydrates, lean protein and healthy fat at least two to four hours before exercise can help you avoid low blood sugar levels. If you skipped a meal, consume a smaller snack, such as peanut butter and crackers, 30 minutes before exercising. Blacking out during exercise can be dangerous, so if it happens more than once, consult a doctor. Discuss any medications you are taking with your doctor, particularly if you are taking heart medications or any type of blood thinner.