Coffee before a fasting blood test: Know how it can affect results

Whether you want to check how high your cholesterol is or if your diabetes management efforts are working, you will need to go for blood tests. You usually need to stop eating or drinking beverages 8-12 hours before a blood sample is taken. While you can’t have solid foods, your body will need hydration and water works best when it comes to staying hydrated. But if you love waking up to the smell of coffee, you may want to take a few sips in the morning. However, drinking coffee before a fasting blood test may not be the best idea, especially if it has other ingredients like milk and sugar.

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

Coffee is one beverage that is widely consumed across the globe. People have around 1.6 billion cups of coffee in one day, according to research published in Biochemia Medica in 2023. No wonder many people, including you, may wonder if you can drink coffee before a fasting blood test. Fasting blood tests are an essential part of the diagnosis and follow-up of numerous health conditions. For instance, if your fasting blood glucose is 126 milligrams per deciliter (mg/dL) or higher on two separate tests, it means you have diabetes, as per the World Health Organization.

You must have noticed that in fasting, you are usually instructed not to eat or drink anything except water. So, you may have doubts about drinking coffee before a fasting blood test. During a 2020 study, published in Current Developments In Nutrition, the level of blood glucose was not influenced by black coffee consumption before the test.

Here’s what can happen if you drink coffee before a fasting blood test:

1. Fasting blood glucose test

“Black coffee might be permitted, but coffee with sugar or milk is not advisable,” says internal medicine expert Dr P Venkata Krishnan. Milk and sugar are carbohydrates, which can directly shoot up your blood sugar levels. “This can contribute to high glucose values, impacting the diagnosis and control of diabetes,” says the expert.

2. Lipid profile

Coffee with milk or sugar includes lipids (fatty compounds), and sugar increases triglycerides, a common type of fat in your body. “Drinking coffee before a fasting blood test may temporarily raise triglyceride and low-density lipoprotein or bad cholesterol levels. This might result in overestimation of cardiovascular risk,” says the expert.

3. Iron test

Coffee is rich in polyphenols, which can disrupt absorption of iron by your body. “If taken prior to an iron test, it can cause really low levels of iron, which might result in a false diagnosis,” says the expert. Also, milk contains calcium that can reduce iron absorption even further, leading to a false diagnosis of iron deficiency or anemia.

4. Vitamin B12 test

Black coffee will not affect B12 levels directly, but it may increase digestion speed and impact nutrient absorption. “Coffee with milk means you are taking in calcium, which can inhibit vitamin B12 absorption and produce lower readings,” says Dr Krishnan.

Does caffeine affect thyroid-stimulating hormone test results?

Thyroid-stimulating hormone (TSH) tests are required to diagnose and monitor thyroid diseases such as hypothyroidism and hyperthyroidism. These tests determine the amount of TSH, a hormone generated by the pituitary gland that governs the thyroid function. “While fasting is not necessary for a TSH test, coffee consumption before the test may affect the results,” says the expert.

Caffeine in your cup of coffee is known to stimulate the nervous system and has the potential to interfere with the regulation of hormones. “Caffeine may result in alterations of TSH levels by impacting the hypothalamic-pituitary-thyroid (HPT) axis, which governs thyroid activity. This would result in minute changes in TSH measurements, possibly impacting diagnosis and treatment decisions,” says the expert.

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

Fasting blood tests need a time of abstinence from food and beverages to get the right findings. “For all blood tests that require fasting, it is best to stay away from coffee for a minimum of 8 to 12 hours prior to the test,” says the expert. This prevents caffeine and other substances from affecting the results. “If you take coffee in the morning, keep a gap of at least 12 hours before starting fasting,” says the expert. If you need to take tests in the morning, it is better to skip coffee the night before.

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

The brain and body stimulant gives a short-term increase in alertness and energy, which is why people love to drink coffee or tea in the morning. “After drinking the beverage, caffeine gets absorbed into the bloodstream in 15 to 45 minutes,” says the expert. The time it takes for half of the caffeine to be removed from the body is between 3 and 6 hours in most people. “It may take a maximum of 10 hours or longer to completely remove from the body depending on factors such as age, metabolism, and liver.

When it comes to fasting blood tests, foods and drinks apart from water are excluded. Hydration is important during fasting, as dehydration may make it more difficult to take blood. But have only plain water, as drinking coffee before a fasting blood test can affect the results.