Blood tests can help you and your doctor see whether a change in eating pattern, such as intermittent fasting, is affecting markers like blood glucose, cholesterol, and liver enzymes. This guide explains which tests are commonly discussed, when repeat testing is typically done, what the common markers mean, and how to avoid misreading results.
It is general education only. Which tests you need, and how to interpret them, depends on your health history, so decide on testing with your doctor. Reference ranges vary between laboratories and countries; always use the range printed on your own lab report.
Table of Contents
A Typical Baseline Panel
If you and your doctor decide a baseline is useful before you start fasting (or while you already are), these are commonly included:
- Metabolic panel: glucose, electrolytes (sodium, potassium), kidney function (creatinine, eGFR, urea/BUN), and often liver enzymes, albumin, and calcium
- HbA1c: reflects average blood glucose over roughly the previous 2-3 months
- Lipid panel: total cholesterol, LDL, HDL, triglycerides, and non-HDL cholesterol
- Complete blood count: screens for anemia and other blood abnormalities
- TSH (thyroid stimulating hormone): a screening test for thyroid function, if your doctor thinks it is relevant
- Blood pressure: not a blood test, but a useful baseline, measured in the clinic or with a validated home cuff
- Weight and waist measurement: simple reference points for later comparison
Depending on your situation, your doctor may also suggest fasting insulin, vitamin D, ferritin, or vitamin B12. Baseline testing is especially important if you have diabetes, kidney or liver disease, or take regular medication. See fasting and medications.
When to Retest
There is no fixed schedule; your doctor will base it on your results and health. A common pattern is:
- Around 3 months after a significant change: lipids and any marker that was abnormal at baseline. HbA1c reflects roughly the previous 2-3 months, so retesting it much sooner is less informative.
- At your usual check-ups: whatever your doctor normally monitors
- Sooner if needed: if you have new symptoms, if you take medication that depends on food intake, or if your doctor asks for closer monitoring
Metabolic Markers
Fasting glucose
- Commonly used thresholds (after an overnight fast): below 100 mg/dL (5.6 mmol/L) is considered normal; 100-125 mg/dL (5.6-6.9 mmol/L) suggests prediabetes; 126 mg/dL (7.0 mmol/L) or higher on repeated testing is consistent with diabetes
- Some studies of intermittent fasting report modest improvements in people with raised glucose, often alongside weight loss; results vary
- Some people who fast or eat low-carbohydrate diets see morning glucose readings a little higher than expected. Discuss any unexpected result with your doctor rather than drawing conclusions yourself
HbA1c
- Commonly used thresholds: below 5.7% (39 mmol/mol) normal; 5.7-6.4% (39-47 mmol/mol) prediabetes; 6.5% (48 mmol/mol) or higher consistent with diabetes
- Can be affected by anemia and some other conditions, so it is interpreted alongside other results
Fasting insulin and HOMA-IR
- Fasting insulin is not part of most routine panels, and there is no universally agreed normal range; interpret it using your lab’s reference range
- HOMA-IR is an estimate of insulin resistance calculated from fasting glucose and insulin: (glucose in mg/dL × insulin in µU/mL) / 405, or (glucose in mmol/L × insulin) / 22.5
- Cutoffs for HOMA-IR vary between studies and populations. It is mainly a research tool and is not used on its own to diagnose anything
Lipids
Studies of intermittent fasting show mixed effects on blood lipids:
- Triglycerides: the marker most often reported to improve, particularly with weight loss
- LDL cholesterol: variable; it falls in some people and rises in others, sometimes noticeably. See our cholesterol guide
- HDL cholesterol: changes are generally small and inconsistent
Your doctor will interpret lipids in the context of your overall cardiovascular risk (age, blood pressure, smoking, diabetes, family history), not as isolated numbers.
Liver Function
- ALT: an enzyme that can be raised in fatty liver disease and other liver conditions
- AST: interpreted together with ALT
- GGT: often raised with alcohol use and fatty liver
- ALP: relates to the liver and bile ducts, and also to bone
Normal ranges for these enzymes differ between laboratories. In people with fatty liver disease, weight loss by various methods, including some fasting approaches, has been associated with improvements in liver enzymes. Raised liver enzymes need medical assessment; see fasting and fatty liver.
Hormones
Hormone tests are usually only done when there is a specific reason, such as symptoms:
- TSH, free T4, free T3: thyroid tests. Longer fasts and substantial calorie restriction can lower T3 somewhat; this is usually considered an adaptation rather than thyroid disease, but results should be interpreted by your doctor. See fasting and thyroid health
- Morning cortisol: ordered only if your doctor suspects a problem
- Testosterone (men): can fall with large calorie deficits or low body fat
- Sex hormones (women): may be checked if periods become irregular or stop, which is a reason to review your eating pattern with your doctor
Additional Markers
These are sometimes ordered depending on your risk factors and your doctor’s judgment:
- ApoB: reflects the number of cholesterol-carrying particles; increasingly used alongside LDL to assess cardiovascular risk
- Lipoprotein(a): largely genetically determined; often only needs to be measured once
- hs-CRP: a general marker of inflammation; can be raised temporarily by infection or injury
- Ferritin: reflects iron stores
- Vitamin B12: particularly relevant for vegans and some people taking metformin
- Vitamin D (25-OH): often checked if you have risk factors for deficiency
- Coronary artery calcium (CAC) score: an imaging test rather than a blood test, sometimes used to refine cardiovascular risk
Testing Tips
- Follow the instructions from your doctor or lab about whether to fast beforehand and for how long
- For comparisons over time, test at a similar time of day and under similar conditions
- Avoid routine blood tests during or right after an extended fast unless your doctor asks for it, since several values can shift temporarily
- A very heavy meal or alcohol the evening before can raise triglycerides
- Hard exercise in the day or two before a test can temporarily raise some markers, such as liver enzymes and creatine kinase
- Tell your doctor you are fasting and about any supplements you take (for example, biotin can interfere with some lab tests)
- Keep copies of previous results so trends can be compared
Interpretation Cautions
- Single results vary from day to day; trends across several tests are more informative
- Reference ranges describe a typical population, and what matters for you depends on your overall health
- Some markers, such as LDL cholesterol, can move in different directions in different people on the same eating pattern
- Feeling well does not rule out an abnormal result that needs attention; for example, high cholesterol and early diabetes usually cause no symptoms
- More testing is not always better; unnecessary tests can lead to confusion and further tests
- Discuss results with your doctor rather than adjusting diet, supplements, or medication on your own based on them
Frequently Asked Questions
Should I check my glucose in the middle of a fast?
Only if your doctor recommends it, for example if you have diabetes. For people without diabetes, blood glucose typically stays within a normal range during common fasting schedules, and home readings can be hard to interpret. If you have diabetes, especially if you take insulin or sulfonylureas, your care team should tell you how often to check.
How often should I test?
That depends on your health, not on fasting itself. Many healthy adults only need the routine checks their doctor already recommends. People with diabetes, fatty liver disease, abnormal lipids, or food-dependent medications may need closer monitoring when they change how they eat.
Should I ask for a fasting insulin test?
You can ask your doctor whether it would add useful information in your case. It is not a routine screening test in most guidelines, and results can be hard to interpret because there is no standard normal range. Private testing services exist in some countries, but results are most useful when reviewed with a clinician.
Should I get a CGM?
Continuous glucose monitors are well established for managing diabetes. For people without diabetes, they can show glucose patterns, but the readings are harder to interpret. See our CGM guide.
Do I need to fast before a lipid panel?
Many guidelines now accept non-fasting lipid tests for routine screening, but your doctor may request a fasting sample, for example if your triglycerides are high. Follow the instructions you are given, and use the same conditions each time if you are comparing results.
What if my numbers got worse?
Discuss them with your doctor. Consider other things that may have changed, such as weight, alcohol, sleep, illness, medications, or what you eat during your eating window. A result that worsens and stays worse, such as a clear rise in LDL cholesterol, is a reason to review whether your current approach suits you.
The Bottom Line
Blood tests can help show how a change in eating pattern is affecting you, but which tests you need and how often is a decision to make with your doctor. Common markers include glucose and HbA1c, a lipid panel, liver enzymes, kidney function, and a blood count. Look at trends rather than single results, use your lab’s own reference ranges, and remember that feeling well does not rule out a result that needs attention.