RHR: The Only Fasting Metric That Matters
Cut Through the Noise
Section titled “Cut Through the Noise”Heart rate variability. Stress scores. Sleep metrics. Recovery scores. Body battery. Readiness. Modern wearables produce a deluge of data.
For dry fasting, most of it is noise.
One metric cuts through everything: Resting Heart Rate (RHR).
If you understand how to read your RHR during a dry fast, you understand when you’re optimized and when you’re in trouble. Everything else is secondary.
Why RHR Is the Master Metric
Section titled “Why RHR Is the Master Metric”Direct Window to Autonomic State
Section titled “Direct Window to Autonomic State”Your resting heart rate reflects the balance between:
- Sympathetic nervous system (fight or flight) → Raises HR
- Parasympathetic nervous system (rest and digest) → Lowers HR
During normal function, these systems balance. Your RHR reflects this balance.
What RHR Tells You During Fasting
Section titled “What RHR Tells You During Fasting”| RHR Status | What It Means | Implication |
|---|---|---|
| At baseline | Autonomic balance maintained | Green zone, continue |
| Slightly elevated (+5-10%) | Mild sympathetic activation | Yellow zone, monitor |
| Significantly elevated (+20-50%) | Sympathetic overdrive | Red zone, intervene |
No other single metric provides this direct a window into your physiological state.
Establishing Your Baseline
Section titled “Establishing Your Baseline”Before the Fast
Section titled “Before the Fast”Record your RHR for 2-3 days before starting. Most wearables (Garmin, Oura, Whoop, Apple Watch) track this automatically.
Your baseline is:
- Morning RHR (before getting out of bed)
- Average RHR during normal sedentary activity
- Overnight RHR (lowest point during sleep)
Example baseline:
- Waking RHR: 58 bpm
- Daytime resting: 62-68 bpm
- Overnight low: 52 bpm
Individual Variation
Section titled “Individual Variation”RHR varies significantly between individuals:
- Athletes: 40-55 bpm
- Healthy adults: 55-70 bpm
- Less conditioned: 70-85 bpm
Your threshold is relative to YOUR baseline, not population averages.
Reading RHR During the Fast
Section titled “Reading RHR During the Fast”Day 1-2: Adaptation Phase
Section titled “Day 1-2: Adaptation Phase”Normal pattern:
- RHR may increase slightly (5-10%) as body adjusts
- This is adaptive stress—expected and manageable
- Usually returns to baseline by end of day 2
Watch for: RHR rising more than 15-20% — could indicate unusual stress response
Day 3-4: The Critical Window
Section titled “Day 3-4: The Critical Window”This is where unsupplemented fasters diverge from Magnesium Method fasters.
Unsupplemented pattern:
- RHR begins steady climb
- +15-20% by day 3
- +30-50% by day 4-5
- Continues rising until forced termination
Magnesium Method pattern:
- RHR stays at or near baseline
- Minimal drift, quickly corrected with magnesium dose
- No crisis trajectory
Day 5+: Extended Phase
Section titled “Day 5+: Extended Phase”If RHR is at baseline: You’re optimized. The fast can continue based on other factors.
If RHR is elevated: Crisis is developing. Increase magnesium immediately.
Practical RHR Monitoring
Section titled “Practical RHR Monitoring”Garmin
Section titled “Garmin”Where to find it:
- Garmin Connect app → Health Stats → Heart Rate
- Look at “Resting Heart Rate” trend
- Check “All-Day Stress” for additional context
What to watch:
- Daily RHR trend line
- Red/orange zones on stress graph
- “Body Battery” declining faster than expected
Oura Ring
Section titled “Oura Ring”Where to find it:
- Oura app → Readiness tab → Resting Heart Rate
- Trend view shows multi-day pattern
- Compare to your personal baseline range
What to watch:
- RHR above your typical range
- Nighttime HR not dropping as low as usual
- HRV dropping (secondary confirmation)
Apple Watch
Section titled “Apple Watch”Where to find it:
- Health app → Heart → Resting Heart Rate
- Shows daily measurements and trend
What to watch:
- Trend moving upward over fast duration
- Notifications about elevated heart rate
- Resting readings significantly above baseline
Where to find it:
- Whoop app → Recovery → Heart Rate section
- Shows resting and sleeping heart rate
What to watch:
- Recovery score declining
- RHR trend moving up
- Strain accumulating without recovery
The Three-Zone System Using RHR
Section titled “The Three-Zone System Using RHR”Establish Your Zones
Section titled “Establish Your Zones”Based on your personal baseline, define:
| Zone | RHR Range | Example (58 bpm baseline) |
|---|---|---|
| GREEN | Baseline ±5% | 55-61 bpm |
| YELLOW | +5-15% above baseline | 61-67 bpm |
| RED | +20%+ above baseline | 70+ bpm |
Response Protocol
Section titled “Response Protocol”| Zone | Status | Action |
|---|---|---|
| GREEN | Optimal | Continue current protocol |
| YELLOW | Warning | Increase magnesium to 800mg/day; reassess in 4-6 hours |
| RED | Crisis approaching | Immediate 800-1200mg magnesium; if no improvement in 6 hours, consider ending fast |
RHR vs. Other Metrics
Section titled “RHR vs. Other Metrics”Why Not Just Use Stress Score?
Section titled “Why Not Just Use Stress Score?”Stress scores are derived from HRV and RHR. They’re useful but:
- Algorithm varies by device
- Can be affected by motion artifacts
- Less directly interpretable
RHR is the raw data. Stress score is one interpretation of that data.
Why Not HRV?
Section titled “Why Not HRV?”Heart rate variability is valuable but:
- More variable day-to-day
- Affected by measurement conditions (position, breathing)
- Requires understanding of personal patterns
- Different devices calculate differently
For simplicity and reliability during fasting, RHR is cleaner.
Why Not Sleep Metrics?
Section titled “Why Not Sleep Metrics?”Sleep quality matters, but:
- Delayed indicator (you find out the next day)
- Affected by many non-fasting factors
- Less actionable in real-time
RHR gives you immediate feedback. Sleep confirms the pattern.
Case Study: RHR Tracking in 165-Hour Fast
Section titled “Case Study: RHR Tracking in 165-Hour Fast”The Pattern Observed
Section titled “The Pattern Observed”With Magnesium Method:
- Day 1: RHR at baseline
- Day 2: RHR at baseline
- Day 3: Slight elevation detected → 400mg magnesium → returned to baseline
- Day 4: RHR at baseline
- Day 5: RHR at baseline
- Day 6: RHR at baseline
- Day 7: Slight fatigue (physical exertion) but RHR still stable
Compare to previous unsupplemented fasts:
- Day 1-2: Baseline
- Day 3: +10%
- Day 4: +25%
- Day 5: +40-50%, forced termination
Same person. Same baseline. Different protocol. Different RHR trajectory.
Troubleshooting RHR Readings
Section titled “Troubleshooting RHR Readings”False Positives (RHR Appears Elevated but You’re Fine)
Section titled “False Positives (RHR Appears Elevated but You’re Fine)”- Just finished physical activity
- Ate something (if refeeding)
- Caffeine intake
- Anxiety about something unrelated to the fast
- Poor sleep previous night
Solution: Check RHR in morning before getting up, or after 15+ minutes of complete rest.
False Negatives (RHR Appears Fine but You’re Struggling)
Section titled “False Negatives (RHR Appears Fine but You’re Struggling)”- Individual variation (some people don’t show HR changes until very late)
- Medications affecting heart rate (beta blockers)
- High baseline fitness masking stress
Solution: Cross-reference with stress score and subjective state. If you feel terrible, believe your body.
Key Takeaways
Section titled “Key Takeaways”- RHR is the single most useful metric for dry fasting guidance
- Establish YOUR baseline before the fast (2-3 days)
- Define personal zones based on percentage above baseline
- Monitor daily and adjust magnesium based on zone
- RHR rising + not correcting = end the fast
The Bottom Line
Section titled “The Bottom Line”You don’t need a degree in data science to track your dry fast effectively.
One number. Your resting heart rate. Compare it to your baseline.
If it’s climbing and magnesium doesn’t bring it back down, your body is telling you to stop.
Listen.
For the complete biometric-guided protocol, see The Death of the Acidosis Crisis.
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