Eating shortly before a blood test may skew the results, so it's likely that your doctor will ask you to fast for a brief period to ensure an accurate result.

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Fasting for Medical Reasons

Although you may not be aware of it, chances are you're familiar with medical fasting. Abstaining from food is frequently a requirement for contemporary medical procedures such as cholesterol screenings, blood sugar checks and major surgeries.

Fasting in medicine serves a very specific purpose. Eating before a blood test, for example, may interfere with the test results until your body can completely digest the food you consume. Because cholesterol and other blood tests check the level of fat in your blood, the food you eat and the resulting fat it adds to your blood stream can produce misleading test results.

Fasting before you go in for blood work gives the doctor a clearer picture of any indicators that may be present in your blood. If your fat (lipid) levels are high in a fasting state, then you may be at risk for high cholesterol and all of the health problems it can cause: heart disease, stroke, heart attack, blocked arteries and more.

Fasting is also a common practice before major surgery involving anesthesia. When patients who have eaten recently go under anesthesia, there is the potential that they may vomit and inhale their stomach contents, which can be deadly [source: Cox] Thus, any patient who's due for surgery in the morning is usually instructed not to eat anything the night before. However, this practice may become outdated soon, as researchers are finding that regurgitation during surgery is quite rare. Furthermore, fasting is often associated with adverse effects, such as hunger and dehydration, and can also cause headaches, dizziness and nausea in some people.

When a person stops eating for more than a few hours, the body looks to stored forms of energy to function normally. If you don't eat protein, your body starts to break down your muscle (which is protein) in order to get the necessary amino acids (the building blocks of protein). A long-term fast can damage your heart, which is a muscle, as well as your liver and kidneys, which can decrease in size, if you don't consume enough protein. Insufficient protein will also impair the functions of these organs.

Some medical procedures that typically require fasting include:

  • Blood check
  • Cholesterol testing
  • Glucose testing
  • Surgery with full anesthesia
  • Diabetes screening