Fasting before lipid test usually unnecessary

Q. Do I need to fast before my cholesterol test?

A. For most people, the answer is no. Here's the backstory: For decades, we've told people not to eat or drink (except for water and medications) for eight to 12 hours before a cholesterol test. This was based on the thought that a measurement done after fasting would provide a more accurate assessment of all the lipids (fats) in the blood, including LDL cholesterol and triglycerides. However, studies show that the differences between fasting and non-fasting values for cholesterol are usually negligible.

An analysis using data from a clinical trial of 4,100 people compared the results from lipid tests done with and without fasting. The LDL (bad) cholesterol values were only about 4 milligrams per deciliter (mg/dL) higher when people weren't fasting. Because the cholesterol guidelines focus on LDL, whether you are fasting won't matter for determining whether your LDL is at goal. So if people come to my clinic and they've had breakfast or lunch, we go ahead and take a blood sample.

But triglycerides are another story. After you eat, your body immediately converts any excess calories from food into triglycerides, which are then stored in fat cells. In addition, any saturated fat you eat is broken down into smaller fatty particles called chylomicrons, which enter the bloodstream and further contribute to a temporary rise in triglycerides. Some people have high triglyceride levels, which is more common among those who are overweight or have diabetes or prediabetes (and some people have inherited a tendency for this problem). For these people, not fasting can often lead to very high triglyceride levels. So anyone who has ever had a high triglyceride level should always fast before a lipid test. This will provide a more accurate assessment of their triglyceride value, which is then used to inform treatment recommendations.

If a person's triglycerides are higher than 150 mg/dl on a fasting lipid test, that is linked to a higher risk of heart attack or related cardiovascular issues. Treatment starts with diet and exercise but may include medications, although the latter are usually reserved for people who have heart disease or are at high risk for it, including those with diabetes.

When I see a new patient, I look back at their earlier lipid test results. If most of their triglycerides are within the normal range (below 150 mg/dL), I tell them they don't need to fast before their blood test. But if some of their values are high, I ask them to fast. Because fasting can be challenging if you can't schedule a morning blood test, ask your doctor if not fasting is an option for you.

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