Fats in the blood are called lipids. All the fats you eat are changed into cholesterol or another group of lipids called triglycerides.
If you have too much of some kinds of cholesterol in your blood, this puts you at higher risk of heart diseases. The only way to diagnose high cholesterol is to have a blood test.
Doctors usually recommend that you don’t eat anything for several hours before the test, so that all your food has had time to be digested and won’t affect the results. This is because after a meal the level of fats in your blood is higher than normal.
This type of test is called a fasting lipid (or lipoprotein) profile. You can sometimes arrange to have it done first thing in the morning, before you have breakfast. But this can often be inconvenient and seem like a hassle, both for the people having the test and their doctors. It can also mean that tests and appointments have to be rearranged if, for example, you forget and eat something in the hours before a test.
Researchers have started to look at whether this recommendation is strictly necessary, and if it’s possible to have a fasting lipid profile without avoiding food in the hours leading up to the test.
In this study, researchers looked at the blood samples given by people who had lipid profile tests during a six-month period in 2011 in Canada. They divided the tests according to how long the person who gave the blood sample had fasted for, ranging from fasting for one hour before the test, to avoiding food for 16 hours. They then looked to see whether the length of time people had fasted for made any difference to their lipid levels and the results of their tests.
A total of 209,180 people were included in the study. Regardless of how long people had fasted before having the test, there was little difference in the average levels of total cholesterol and a type of cholesterol called high-density lipoprotein (HDL) cholesterol.
The average level of a type of cholesterol called low-density lipoprotein (LDL) cholesterol varied a little more. There was around a 10 percent difference in LDL cholesterol levels between people who fasted for different lengths of time. There was also a 20 percent difference in average triglyceride levels between people who fasted for different lengths of time.
How reliable is the research?
These results were drawn from a large study. This makes the results more reliable, as there are enough samples to be confident that any patterns seen are not just due to chance.
The researchers had to rely on people estimating how long they avoided food before their blood test, and we can’t be sure how accurate this information is.
What does this mean for me?
This study suggests that you may not need to avoid food for long periods of time before having a blood test for high cholesterol. It challenges a long-held belief and the current practice among doctors. If there’s a particular reason you would find it difficult to fast before your lipid profile test, you can discuss this with your GP or practice nurse for advice.
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