Written by &you Labs Team
Updated October 21, 2026
Fasting means going without food for a set time. It shifts many of your blood markers in known ways. This happens with the normal overnight fast before a blood test. It also happens with longer fasts some people try on purpose.
Knowing these shifts helps you read your results the right way. It also helps you time your test correctly.
A normal overnight fast, 8 to 12 hours, gives you the most accurate glucose, insulin, and cholesterol results. A longer fast, over 24 hours, causes bigger shifts. Blood sugar drops, then can rise again. Triglycerides fall at first. Ketones rise a lot. Cortisol and uric acid shift too. Most of these changes go back to normal within a few days of regular eating.
What it affects. Glucose, for an accurate fasting reading. Insulin, for an accurate fasting reading. Triglycerides, which are most reliable when fasted. LDL, when your triglycerides run high. The full cholesterol panel, best done fasted.
What it barely affects. Most other markers. HbA1c, a longer-term sugar marker. Your hormones. Vitamin D, B12, and ferritin. Your liver and kidney markers.
Why fasting helps for these tests. Food raises glucose, insulin, and triglycerides for a few hours after you eat. A 8 to 12 hour fast lets these settle back to your true baseline.
Glucose. Drops at first, within the first day. Then it can rise again, as your body makes its own sugar. It can even run low in some people. With a long fast, your fasting glucose can end up higher than expected.
Insulin. Drops a lot. Your body becomes more sensitive to it. This is one reason people try fasting.
Triglycerides. Drop at first. Can rise again with a very long fast.
Ketones. Rise a lot after 16 to 24 hours or more. Your body starts using them as fuel. A fasting ketone test uses different healthy ranges.
Free fatty acids. Rise as your body switches to burning fat.
Cortisol. Rises with a long fast. This is your body's stress hormone.
Growth hormone. Rises with fasting too.
Testosterone. Drops a little in men during a long fast. A stricter fast has a bigger effect.
Uric acid. Often rises during a fast. This can trigger gout in people prone to it.
Inflammation markers. hs-CRP often drops with a sustained fast.
Electrolytes. Stay fairly steady if you drink enough water. A very long fast can shift them.
Fasting is required. Fasting glucose. Fasting insulin. Triglycerides. Calculated LDL, when triglycerides matter. Sometimes homocysteine.
Fasting helps but is not required. HDL and total cholesterol, with a small effect. Some hormone tests.
Fasting is not needed. HbA1c. Most hormones. TSH. Vitamin D, B12, folate, and ferritin. Liver enzymes, though often done fasted for convenience. A full blood count. Hepatitis screening.
Do this. Eat your last meal 8 to 12 hours before your draw. Drink water freely, it does not break your fast. Skip coffee and tea unless your doctor says otherwise, since caffeine shifts several of your blood markers. Keep taking your usual medicines. Avoid alcohol for 24 hours before.
Avoid this. Fasting longer than 12 to 14 hours, since some markers start to shift. Coffee or tea before a fasting test, since they affect the reading. Skipping water, since it can throw off your results. Changing your diet a lot in the day or two before, since that creates a false baseline.
After your draw. Eat right away to break your fast. Then carry on with your normal day.
Time-restricted eating, like a 16:8 schedule. Test during your usual fasting window. Keep the timing steady, relative to your eating window.
Alternate-day fasting. Test on a fasting day, or right after a normal eating day. Staying consistent matters more than which day you pick.
A fast of 24 hours or more. Avoid testing during or right after one of these. The results will not reflect your normal baseline. Wait 24 to 48 hours of regular eating first.
A steady fasting routine, kept up for 12 weeks or more, tends to improve several markers over time:
These changes reflect a long-term habit. They are different from the short-term shifts of a single fast.
People with diabetes. Fasting often needs insulin or medicine changes. Low blood sugar is a real risk. Always talk with your doctor first.
Pregnant or breastfeeding. A long fast is not appropriate. A normal, short pre-test fast is fine.
Children. Fasting rules differ. A long fast is usually not appropriate.
Older adults. Extra care is needed with a long fast. A normal pre-test fast is fine.
A history of an eating disorder. A long fast is not appropriate. Even a normal pre-test fast may need to be adjusted with your doctor's guidance.
A full pre-test guide covers everything else you should know before your draw. You can book your &you Labs panel and get clear fasting instructions ahead of time.
No. Past 12 to 14 hours, your markers start shifting away from a true baseline. The standard 8 to 12 hours works best.
Yes, water is always fine. Black coffee or plain tea usually is not allowed for an accurate fasting test.
Tell the person drawing your blood. Some tests, like glucose, insulin, and triglycerides, may need to be redone. Most other tests are still fine.
HbA1c reflects about 3 months of blood sugar, so a recent fast does not change it much. A sustained fasting routine over months can improve it a little.
Most are fine. Stop high-dose biotin 2 to 3 days before, since it can throw off several tests. Iron supplements are fine to continue.
The &you Labs Team combines medical science with personal care. We help Filipinos understand their health markers with blood work analysis that is easy to follow, whole person support, and health expertise built for Filipino life.
&you Labs Team in Labs
Electrolytes control your hydration, nerves, and blood pressure. Learn what your sodium, potassium, and chloride results mean, in plain language.
&you Labs Team in Labs
Your CBC checks red cells, white cells, and platelets. See the normal ranges for hemoglobin, MCV, RDW, and platelets, and what your numbers point to.
&you Labs Team in Labs
A cystatin C test can read kidney health more accurately than creatinine. Learn when it helps, what the healthy range is, and how to read your result.
.avif&w=256&q=75){kind=logo}