If you’re going to do something significant to your metabolism, knowing what changed and what didn’t is the difference between informed practice and hopeful guessing. The right baseline panel + targeted re-tests over months tells you whether fasting is doing what you wanted, whether it’s producing unexpected effects, and whether the protocol matches your physiology.
This guide covers which markers actually matter, the timing for tests, expected directions of change, and the markers that get over-tested in fasting communities without much value.
Table of Contents
The Baseline Panel
Get these before starting (or as a baseline reference if you’ve been fasting):
- Comprehensive metabolic panel: includes glucose, electrolytes, kidney function (creatinine, eGFR, BUN), liver enzymes (ALT, AST), albumin, calcium
- HbA1c: 3-month average glucose
- Fasting insulin: often missing from standard panels but very informative; calculate HOMA-IR with glucose
- Lipid panel: total cholesterol, LDL, HDL, triglycerides, non-HDL, and TG/HDL ratio
- TSH (thyroid stimulating hormone): screen thyroid function
- Vitamin D (25-OH): commonly low; needs to be in range
- Complete blood count: screen for anemia, infection
- Blood pressure (home cuff or in-office)
- Body composition reference (DEXA if accessible, otherwise weight + waist + photos)
Follow-Up Cadence
- 8-12 weeks after starting: repeat lipids, HbA1c (won’t fully reflect changes yet), fasting insulin, liver enzymes, blood pressure
- 6 months: full panel re-test
- 12 months and annually: full panel + advanced markers if indicated
- Anytime symptoms emerge: targeted testing
Metabolic Markers
Fasting glucose
- Normal: 70-99 mg/dL (3.9-5.5 mmol/L)
- Pre-diabetes: 100-125 mg/dL
- Diabetes: 126+ mg/dL
- Expected change with fasting: typically improves over 8-12 weeks for elevated baselines; may transiently rise during fasting itself due to gluconeogenesis (don’t panic at this)
HbA1c
- Normal: under 5.7%
- Pre-diabetes: 5.7-6.4%
- Diabetes: 6.5%+
- Expected change: 0.2-0.5% reduction over 3 months for elevated baselines
Fasting insulin
- Optimal: under 7 mIU/L
- Acceptable: under 12 mIU/L
- Insulin resistance suggested: above 15 mIU/L
- Expected change: reductions of 30-50% common over 3-6 months
HOMA-IR
- Calculated: (fasting glucose × fasting insulin) / 405 (US units) or 22.5 (mmol/L)
- Optimal: under 1.0
- Insulin resistance: above 2.5
Lipids
The typical pattern after sustained fasting:
- Triglycerides: drop 15-30% (most reliable improvement)
- HDL: rise 5-15%
- LDL: variable (most see modest reduction; some see substantial rise — see cholesterol guide)
- TG/HDL ratio: improves substantially in most
The TG/HDL ratio is probably the most useful single lipid metric for tracking fasting effects. Aim for under 2 (US units) or under 0.87 (mmol/L).
Liver Function
- ALT: normal under 25-40 U/L; sensitive to fatty liver
- AST: normal under 25-40 U/L
- GGT: normal under 40-50 U/L; elevated in fatty liver, alcohol use
- ALP: normal 40-130 U/L
Fasting typically improves all liver enzymes when elevated baseline reflects fatty liver. Improvements often visible within 8-12 weeks.
Hormones
- TSH: normal 0.4-4.0 mIU/L (some advocate tighter ranges); fasting generally doesn’t change this in healthy adults but worth tracking
- Free T3, Free T4: if TSH abnormal or symptoms; can shift modestly with fasting
- Cortisol (morning serum): baseline if cortisol questions; usually within normal range with fasting
- Testosterone (men): typically maintained or modestly improved with moderate fasting; can drop with aggressive caloric restriction
- Estradiol (women): baseline if cycle changes occurring
- Sex hormone binding globulin (SHBG): sometimes useful alongside testosterone
Advanced/Optional Markers
- ApoB: better predictor of cardiovascular risk than LDL-C; useful especially if lipid pattern is unusual
- Lipoprotein(a): measure once in life; genetically determined
- hs-CRP: inflammation marker; should drop with fasting in elevated baselines
- Homocysteine: cardiovascular risk; fasting doesn’t typically affect this
- Ferritin: iron stores; relevant for energy and women’s health
- Vitamin B12: particularly for vegans/vegetarians
- IGF-1: longevity-relevant; fasting reduces; not commonly tested clinically
- Coronary artery calcium score (CAC): imaging, not blood; useful for cardiovascular risk stratification, especially with unusual lipids
Testing Tips
- Test at the same time of day for comparisons
- Test in your typical fasting state (e.g., morning after typical overnight fast)
- Don’t test mid-extended-fast — values shift transiently
- Don’t test the day after a high-fat meal (triglycerides spike)
- Don’t test the morning after intense exercise (some markers shift)
- Direct-to-consumer testing services can be useful for tracking when GP testing is limited
- Bring previous results to comparison appointments — trends matter more than single values
Interpretation Cautions
- Single values are noisy; trends over multiple tests matter more
- Reference ranges are population averages, not individual targets
- Some markers (LDL particularly) require individual interpretation in fasting context
- Symptoms always trump numbers — feeling well with mediocre numbers beats feeling unwell with great numbers
- Don’t over-test — quarterly is plenty for most metrics
- Doctor familiar with metabolic medicine may add value for complex patterns
Frequently Asked Questions
Should I test fasting glucose mid-fast?
If curious, yes — but understand that fasting glucose typically rises transiently during a fast due to gluconeogenesis. Don’t panic at numbers higher than your fed baseline.
How often should I test?
Baseline before starting; 8-12 weeks; 6 months; then annually. More frequent testing of specific markers if you’re actively tracking a particular condition (diabetes, fatty liver).
My doctor won’t order fasting insulin. What now?
Direct-to-consumer testing services (LetsGetChecked, Quest, etc.) can order it. Often paid out of pocket but informative.
Should I get a CGM?
Useful for diabetes patients and informative for non-diabetics curious about glucose responses. See our CGM guide.
Do I need to fast 12 hours before a lipid panel?
Modern guidelines accept non-fasting lipid panels for general screening. For tracking changes, fasting (12-14 hours) gives more reproducible numbers.
What if my numbers got worse?
Consider: am I genuinely adherent to the protocol? Has another factor changed (sleep, stress, alcohol)? Is this a transient pattern or sustained? Discuss with doctor before assuming the protocol is failing.
The Bottom Line
Get a comprehensive baseline panel before starting fasting practice. Re-test at 8-12 weeks, 6 months, and annually. Focus on metabolic markers (HbA1c, fasting insulin, HOMA-IR), lipids (especially TG/HDL ratio), and liver enzymes. Add advanced markers (ApoB, hs-CRP, Lp(a)) for nuance. Track trends over multiple tests, not single snapshots. Numbers inform; symptoms decide.