Do You Really Need to Fast Before a Blood Test? A Study of 9.8 Million Results Says Often Not

You know the drill. The appointment card says “fasting labs.” So you skip breakfast, skip your coffee, drag yourself to the clinic at 7 a.m. and sit in the waiting room getting hungrier and crankier by the minute. If you have diabetes, you might feel shaky. If you are bringing an elderly parent, you are probably managing their mood too.

A large new study suggests that for most blood tests, all that suffering may not be changing the numbers much at all.

The research, published September 28 in JAMA Internal Medicine, analyzed nearly 9.8 million laboratory results to measure exactly how fasting time affects routine tests. The answer: for a handful of tests, fasting matters. For most of the rest, the differences were small enough that they may not justify the hassle.

Here is what the researchers found, what it means for you and your family, and what you should ask the next time a doctor orders bloodwork.

What the Study Looked At

The team, led by Sunghwan Shin of Asan Medical Center and the University of Ulsan College of Medicine in Seoul, reviewed lab data from 101,148 adult outpatients at a university hospital in South Korea between February 2021 and December 2023, according to Medical Dialogues. In total, they examined 9,755,547 test results across 372 laboratory items.

Patients were sorted into three groups based on how long they had gone without eating: less than 8 hours, 8 to 12 hours, and more than 12 hours. The researchers then compared results against established performance standards that labs use to decide whether a difference is big enough to matter.

The clever part came next. Comparing different people is tricky, because the people who fast longer may differ in age, health or the reason they are being tested. So the team also looked at the same patients who had been tested at different fasting times. That within-person comparison is what makes the findings more convincing.

The Results: Most Tests Barely Moved

When the researchers compared groups of different people, 27 of 121 tests showed differences beyond acceptable error limits. But when they looked at the same patients over time, only 10 tests showed consistent differences, Medical Dialogues reported. That gap suggests many apparent “fasting effects” were really differences between patients, not the food.

The tests that did change after a shorter fast of less than 8 hours were:

• Glucose: about 10.8 percent higher
• Triglycerides: about 20.1 percent higher
• Gamma-glutamyl transferase (GGT), a liver enzyme: about 14.8 percent higher
• Lipase, a pancreas enzyme: about 36.4 percent higher

Two values moved the other way. Total bilirubin and vitamin D ran lower after shorter fasts, meaning they tended to rise with longer fasting, according to a summary of the study by a physician-run health site, Dr. Kumar Discovery.

And one finding should make every early-morning patient smile: fasting longer than 12 hours provided no additional benefit. If you were told to fast “at least 12 hours” and stretched it to 14 or 16 to be safe, you were not helping your results.

What the Researchers Concluded

The authors argued that “flexible, analyte-specific fasting protocols could reduce patient burden while maintaining test accuracy.” In plain English, that means fasting rules should be matched to the specific test, not applied to every blood draw by default.

They also suggested that at least 8 hours of fasting may still be needed for glucose and lipid measurements, while many other tests can tolerate shorter fasts.

The journal paired the study with a commentary under the blunt title “Time to Stop Fasting Before Blood Sampling.” The commentary’s authors, from Copenhagen University Hospital, made a simple point: most people spend most of their day in a nonfasting state, so nonfasting results may better reflect what is really happening in the body.

That idea is not brand new. In 2016, the European Atherosclerosis Society and the European Federation of Clinical Chemistry and Laboratory Medicine issued a joint consensus statement saying nonfasting samples are acceptable for routine cholesterol testing in most people. Many labs and doctors have been moving in that direction for years. This study adds a much broader look across dozens of other common tests.

The Limits of the Study

Before anyone throws out their fasting instructions, the caveats matter.

This was a single-center, retrospective study, meaning researchers looked back at existing records rather than running a controlled experiment. The fasting times were reported by patients and not independently verified. And the patients skewed older, which may limit how well the results apply to younger adults or children, Medical Dialogues noted.

It was also done in South Korea, where typical diets differ from American ones. A breakfast of rice and soup may affect triglycerides differently from a bacon-egg-and-cheese or a large sweet coffee drink. That does not erase the findings, but it is a reason to expect follow-up research in other populations.

Most importantly, the study did not test what happens when doctors change their instructions. It measured differences in numbers, not differences in diagnoses or patient outcomes.

Why Food Changes Some Tests and Not Others

It helps to understand why fasting rules exist at all.

When you eat, your body breaks carbohydrates down into sugar, and blood glucose rises for a while before insulin brings it back down. That is why a fasting glucose test, one of the standard ways to screen for diabetes, is defined by an overnight fast. A glucose reading taken an hour after a bagel tells a different story than one taken after a night’s sleep.

Triglycerides work in a similar way. After a meal with fat in it, the gut packages that fat and sends it into the bloodstream, and triglyceride levels can stay elevated for hours. That is why triglycerides showed one of the biggest jumps in the study.

Many other common tests, such as a complete blood count or markers of kidney function, measure things that do not swing much with a single meal. That is consistent with what the researchers saw. The study did not try to explain every individual result, including why lipase moved as much as it did, which is another question for future research.

Questions to Ask at Your Next Appointment

If you or a family member has bloodwork coming up, these questions can save a hungry morning:

• Which of the tests on this order actually require fasting?
• If I cannot fast, can I still come in and tell the lab what I ate?
• How should I handle my morning medications?
• Is it safe for me, given my health conditions, to skip breakfast?
• If a result comes back borderline, would you repeat it fasting before making a decision?

What This Means for Your Family

For most families, the practical lesson is simple: ask questions.

Ask which tests need fasting. If your order includes glucose, a lipid panel with triglycerides, or tests related to the liver or pancreas, your doctor may still want you to fast. If it is mostly a blood count, kidney function or other routine markers, fasting may not be necessary. Your doctor or the lab can tell you.

Do not overdo it. The study found no benefit to fasting past 12 hours. A fast that stretches from dinner the night before to a late-morning appointment can leave you lightheaded and does not make results better.

Be careful if you have diabetes. People who take insulin or certain diabetes medications can be at risk of low blood sugar when they skip meals. Never change medication timing or skip food without talking to the doctor who ordered the test.

Water is usually fine. Most fasting instructions allow plain water, and staying hydrated can make the blood draw easier. Check your specific instructions.

Tell the lab what you ate. If you did eat, say so. Doctors can often interpret results with that information in mind instead of making you come back.

For older parents and grandparents especially, skipping meals is not a small thing. Falls, dizziness and confusion are real risks for some seniors. If a fasting requirement is causing stress, it is worth a phone call to ask whether a nonfasting test would work just as well.

The Bigger Picture

Medicine is full of habits that made sense once and were never revisited. Fasting before every blood test is a good example. It became routine partly because some tests really did need it, and it was easier to tell everyone to fast than to sort out which tests mattered.

Large data studies like this one make it possible to check those habits against reality. That kind of re-examination is how care gets better and less burdensome, the same way public health guidance has shifted in other areas, from changing views on moderate drinking to a broader focus on men’s mental health and getting real help.

No single study changes medical practice overnight. But if follow-up research confirms these findings, the 7 a.m. hungry crowd in the lab waiting room could get a lot smaller.

This article is for general information and is not medical advice. Talk with your doctor before changing how you prepare for any lab test.