Can you drink coffee before a fasting blood test?

Whether you want to see how high your cholesterol is or whether your diabetes management efforts are working, you should go for a blood test. Usually, you should drink or drink 8-12 hours before taking a blood sample. You can’t have a solid diet, but your body needs hydration and water works best when it comes to maintaining hydration. However, if you like to wake up to the smell of coffee, you might want to drink a few drinks in the morning. However, drinking coffee before a fasting blood test may not be the best idea, especially if you have other ingredients like milk and sugar.

Is it okay to drink coffee before a fasting blood test?

Coffee is one of the widely consumed drinks around the world. According to a study published in Biocemia Medica in 2023, she drinks about 1.6 billion cups of coffee in a day. I don’t think many people, including you, can drink coffee before a fasting blood test. Fasting blood tests are an important part of diagnosing and following many health conditions. For example, if your fasting blood glucose is above 126 milligrams (mg/dl) per deciliter, two separate tests mean that you have diabetes, according to the World Health Organization.

You must have noticed that in fasting, you are usually instructed not to eat or drink anything other than water. Therefore, you may have doubts about drinking coffee before a fasting blood test. A 2020 study published in Current Nutrition Development showed that blood glucose levels were not affected by pre-test black coffee consumption.

Here’s what happens when you drink coffee before a fasting blood test:

1. Fasting blood glucose test

“Black coffee may be allowed, but coffee containing sugar or milk is not recommended,” says Dr. P Venkata Krishnan, an internal medicine specialist. Milk and sugar are carbohydrates that can directly shoot blood sugar levels. “This contributes to high glucose levels and affects the diagnosis and control of diabetes,” experts say.

2. Lipid Profile

Milk or sugar-containing coffee contains lipids (fatty compounds), which increase triglycerides, a common type of fat in your body. “Drinking coffee before a fasting blood test can temporarily increase poor levels of triglycerides and low-density lipoprotein or cholesterol, which can overestimate cardiovascular risk,” experts say.

3. Iron test

Coffee is rich in polyphenols and can disrupt the body’s absorption of iron. “If you take it before an iron test, it can cause a false diagnosis because it can cause very low levels of iron,” experts say. Milk also contains calcium, which can further reduce iron absorption, leading to a false diagnosis of iron deficiency or anemia.

4. Vitamin B12 Test

Black coffee does not directly affect B12 levels, but it can increase digestion rate and affect nutrient absorption. “Coffee containing milk means you’re taking calcium, which inhibits the absorption of vitamin B12 and shows that your readings are low,” says Dr. Krishnan.

Does caffeine affect thyroid stimulating test results?

A thyroid stimulating hormone (TSH) test is required to diagnose and monitor thyroid diseases such as hypothyroidism and hyperthyroidism. These tests determine the amount of TSH, a hormone produced by the pituitary gland that governs thyroid function. “While TSH tests do not require fasting, coffee consumption before testing can affect results,” experts say.

Caffeine in a cup of coffee is known to stimulate the nervous system and can interfere with hormone regulation. “Caffeine can lead to changes in TSH levels by affecting the hypothalamic-pituitary-thyroid (HPT) axis, which governs thyroid activity. This can lead to minor changes in TSH measurements, which can affect diagnosis and treatment decisions,” the expert said.

Coffee before fasting blood test: What should the gap be?

Fasting blood tests require time to abstinence on food and drinks to get proper discoveries. “For all blood tests that require fasting, it is best to stay away from coffee for at least 8-12 hours before the test,” experts say. This prevents caffeine and other substances from affecting the outcome. “If you drink coffee in the morning, keep at least a 12-hour gap before you start fasting,” experts say. If you need to take a test in the morning, it’s better to skip the coffee the night before.

Coffee before fasting blood test: How long does caffeine stay in your system?

Brain and body stimuli lead to short-term increases in arousal and energy. That’s why people love to drink coffee and tea in the morning. “After drinking a drink, caffeine is absorbed into the bloodstream in 15 to 45 minutes,” experts say. It takes 3-6 hours for most people to remove half of the caffeine from the body. “Depending on factors like age, metabolism, liver, and more, it can take up to 10 hours or more to completely remove it from the body.

Food and drinks are excluded for fasting blood tests, except for water. Hydration is important during fasting, as dehydration can make it more difficult to ingest blood. However, drinking coffee before a fasting blood test can affect your results, so you only have regular water.