Quick Answer. According to Quest Diagnostics, a lipid or glucose tolerance test requires eight hours of fasting. Health care providers may have other recommendations, depending on the type of test. Fasting means consuming no food or beverages, except water, for several hours, explains Quest Diagnostics. Fasting is often required before blood tests because nutrients in food and drink can absorb in the bloodstream, affecting the test results. Not fasting before such tests may yield inaccurate results.