- Biomarker 01
- HbA1c
- Biomarker 02
- Fasting Glucose
Quick answer.
HbA1c measures 90-day average blood sugar; fasting glucose is a point-in-time snapshot. For longevity, track both: HbA1c reveals hidden glucose spikes, while fasting glucose shows overnight metabolism. Target HbA1c under 5.3% and fasting glucose under 90 mg/dL for optimal healthspan.
Key differences.
- Time window — A1c: 90-day average; FBG: Single point-in-time.
- Captures postprandial spikes — A1c: Yes - includes all glucose exposure; FBG: No - only measures overnight fasting state.
- Fasting required — A1c: No; FBG: Yes - 8-12 hour fast.
- Affected by recent diet — A1c: Minimal - reflects 3-month trend; FBG: High - last meal impacts results.
When to use each.
HbA1c
- Diabetes diagnosis and monitoring
- Tracking long-term glucose control
- Assessing metabolic health trajectory
- Biological age calculation
Fasting Glucose
- Quick metabolic health check
- Detecting acute glucose issues
- Part of routine metabolic panel
- Monitoring fasting metabolism
Use both
- Comprehensive metabolic assessment
- Detecting hidden postprandial hyperglycemia
- Pre-diabetes screening
- Longevity optimization protocol
Side-by-side.
| Feature | A1c | FBG |
|---|---|---|
| What it measures | Glycated hemoglobin percentage | Blood glucose concentration |
| Optimal for longevity | < 5.3% | < 90 mg/dL |
| Lab normal range | < 5.7% | < 100 mg/dL |
| Diabetes threshold | ≥ 6.5% | ≥ 126 mg/dL |
| How often to test | Every 3-6 months | Annually or more |
| Cost | ~$15-50 | ~$5-20 |
FAQs.
Can HbA1c be normal but fasting glucose high?
Yes. This pattern suggests recent glucose dysregulation that hasn't yet affected your 90-day average. It can occur with acute stress, illness, or recent dietary changes. If persistent, it may indicate early insulin resistance.
Can fasting glucose be normal but HbA1c high?
Yes, and this is concerning. It means you have good overnight glucose control but significant postprandial (after-meal) spikes. Your fasting test misses these spikes, but HbA1c captures them. This pattern requires dietary intervention.
Which test is better for longevity tracking?
HbA1c is superior for longevity because glycation (sugar-protein bonding) is a fundamental aging mechanism. HbA1c directly measures this process. However, tracking both provides the most complete picture.
Why do some doctors only order fasting glucose?
Fasting glucose is cheaper, faster, and part of standard metabolic panels. Many doctors use it for initial screening and only order HbA1c if fasting glucose is elevated. For proactive health optimization, requesting both is worthwhile.
Verdict.
For longevity optimization, HbA1c is the superior marker because it captures total glucose exposure including postprandial spikes. However, fasting glucose provides valuable insight into overnight metabolism. The optimal approach: track HbA1c every 3-6 months, with fasting glucose in your annual panel. If HbA1c is above 5.3%, add a continuous glucose monitor (CGM) for 2 weeks to identify specific dietary triggers.