What to Do If You Forget to Fast for Your Cholesterol Test

Health Professional

You show up at the laboratory for a fasting cholesterol (lipid) panel. Although the doctor's office advised you to not eat anything after midnight, you forgot. Out of habit, you went ahead and ate breakfast. You didn't even realize your mistake until you arrived at the lab and the technician asked if you'd been fasting.

Was your trip wasted? Should you just reschedule and come back fasting?

Well, if the results of two large studies offer any insight, no: You may have done the smartest thing you could have done, in fact.

First of all, why is it that we nearly always check fasting levels? No study has ever demonstrated that fasting triglycerides are, in any way, superior to non-fasting levels drawn after eating.

There are two principal reasons. One: Triglycerides increase with tremendous variation after a meal, depending on how much fat was in the meal, what kind of fat, how overweight you are, the presence of pre-diabetes or diabetes, how depleted you are in omega-3 fatty acids, and genetic factors. Two: LDL cholesterol is calculated (yes, calculated, not measured) using an equation that assumes that the triglyceride level is a fasting value. Because of the after-eating increase in triglycerides, it can misleadingly increase LDL.

But there's an emerging appreciation that after-eating, or postprandial, triglycerides may shed even more light on risk for cardiovascular disease than fasting values.

Two large studies have demonstrated that, the higher the after-eating level of triglycerides, the greater the risk for cardiovascular disease. The Women's Health Study of over 26,000 females showed that, while fasting triglycerides had little predictive value independent of other risk factors, triglycerides measured at 2-4 hours after eating had the greatest predictive value for future heart attack and other cardiovascular events, with the highest values predicting 6.27-fold greater cardiovascular risk. Interestingly, cardiovascular risk began to increase with after-eating triglycerides of ≥86 mg/dl.

In the Copenhagen City Heart Study, 7587 women and 6394 men were followed for 26 years. For both men and women, cardiovascular risk increased with non-fasting triglycerides of 88 mg/dl or greater, with 2.2-fold increased risk for heart attack for females, 1.6-fold increased risk for males, in the range of 88-176 mg/dl. Graded risk developed with increasing non-fasting triglyceride levels.

Why would triglyceride levels after eating provide insight into risk for heart disease?

Triglycerides are really an easily measured index of the variety of particles that flood the bloodstream after eating. Ingested fats, in particular, are packaged into large particles, called chylomicrons, within minutes after consuming a fat-containing food. Chylomicrons are degraded in the bloodstream to chylomicron remnants, which, in turn, are metabolized to another particle called VLDL. Chylomicrons, chylomicron remnants, and VLDL are all composed mostly of triglycerides. Measuring triglycerides thereby provides an indirect method of assessing the triglyceride-containing particles that develop after a meal.

Recent studies have demonstrated triglyceride-rich particles, especially chylomicron remnants and VLDL, trigger the sequence of steps that lead to atherosclerosis in the coronary and other arteries and, eventually, heart attack and stroke.

Here's where it gets a bit tricky: While fats are the component of food that are packaged into triglyceride-containing particles, other factors beyond fat are the dominant determinants of postprandial responses, including the severity of increased triglycerides and triglyceride-containing particles. In particular, being overweight (especially visceral obesity, the sort in the abdomen), physical inactivity, omega-3 fatty acid deficiency, and carbohydrate intake are the long-term factors that cause exaggerated postprandial patterns, including increased triglycerides.

So, if you find yourself at the lab with a belly full of breakfast, you might consider having the blood drawn, then discussing the importance of the triglyceride value you now have with your doctor. It might provide information superior to the test originally ordered. (Of course, always discuss the advisability of doing so with your doctor.)