For decades, patients have been advised to avoid eating for eight to twelve hours before blood tests. This familiar instruction often means skipping breakfast, arranging early appointments and spending the morning feeling hungry.

However, new research suggests that fasting may be unnecessary for many routine laboratory investigations.

A large South Korean study, published in JAMA Internal Medicine, examined millions of blood test results and found that most measurements changed very little according to how long patients had gone without food.

Why Do Doctors Recommend Fasting?

Eating temporarily changes the concentration of certain substances circulating in the bloodstream. Glucose rises after meals, while triglycerides, a type of blood lipid, can also increase.

Traditionally, doctors have recommended fasting to minimise these fluctuations and make laboratory results easier to interpret.

Over time, the practice became common for numerous routine tests, even when evidence supporting fasting was limited.

Researchers wanted to establish which measurements genuinely require an empty stomach and whether longer fasting periods improve accuracy.

What Did The Researchers Discover?

Scientists analysed approximately 9.8 million laboratory results from 101,148 adults treated at 670 hospitals across South Korea.

Participants were divided into three groups according to fasting duration: fewer than eight hours, eight to twelve hours and more than twelve hours.

The researchers compared measurements across these groups and examined repeated tests from the same individuals to reduce the influence of differences between patients.

Most routine results, including blood cell counts, kidney function markers, electrolytes and many liver measurements, showed little clinically meaningful variation.

Among 121 commonly performed tests examined in detail, only a small number displayed changes substantial enough to affect interpretation.

Which Blood Tests Were Affected?

Several measurements remained sensitive to recent food intake.

Compared with longer fasting periods, tests performed after fewer than eight hours without food showed glucose concentrations approximately 10.8% higher.

Triglyceride levels were around 20.1% higher, while lipase, an enzyme involved in lipid digestion, increased by approximately 36.4%.

Certain liver-related measurements also showed differences.

These findings indicate that fasting can remain important when doctors need specific baseline measurements, particularly for diagnosing conditions involving glucose metabolism.

Does Fasting Longer Improve Accuracy?

One particularly interesting finding concerned prolonged fasting.

The researchers detected no clinically meaningful differences between results obtained after eight to twelve hours without food and those collected after more than twelve hours.

This suggests that extending a fast beyond the usual recommended period offers little additional benefit for the tests investigated.

Long fasting periods can also create unnecessary discomfort, particularly for older patients or people managing certain medical conditions.

What Do Medical Experts Think?

Dr Liv Tybjærg Nordestgaard, a clinical biochemistry researcher at Copenhagen University Hospital, and her colleagues discussed the findings in an accompanying editorial.

They explained that most people spend much of their day in a nonfasting state, meaning blood samples collected after eating may better reflect ordinary physiological conditions.

The experts also highlighted how unnecessary fasting requirements can complicate appointments and place additional burdens on patients.

Their commentary supports reconsidering routine fasting instructions while preserving them for tests where food intake meaningfully affects interpretation.

Should Patients Skip Fasting?

Despite the promising findings, researchers emphasised that further studies involving more diverse populations are needed before clinical recommendations change. The analysis was observational, and fasting duration was recorded during routine hospital care.

Patients should therefore continue following their doctor's or laboratory's instructions. Requirements depend on the specific tests ordered, medical history and diagnostic purpose.

The research points towards a more personalised approach in which fasting is requested only when clinically necessary. For millions of patients, that could eventually mean simpler appointments, fewer unnecessary restrictions and a more convenient experience.