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Metabolic Flexibility Assessment

Measuring and estimating your fuel-switching ability

How RQ is Actually Measured

RQ = VCO₂ / VO₂

Volume of CO₂ produced ÷ Volume of O₂ consumed

The Gold Standard: Indirect Calorimetry

RQ is measured using a metabolic cart — specialized equipment that analyzes the gases you breathe in and out.

How it works:
1. You breathe into a mask or hood connected to gas analyzers
2. Machine measures exactly how much O₂ you consume
3. Machine measures exactly how much CO₂ you produce
4. Computer calculates RQ in real-time
5. From RQ, it determines your fuel mix (% carbs vs % fat)

Where to Get RQ Tested

Location Test Name Cost
University exercise physiology labs Resting Metabolic Rate (RMR) test $75-150
High-end gyms/fitness centers Metabolic testing (PNOĒ, Korr) $100-200
Sports performance centers VO2max + RQ analysis $150-300
Hospitals/clinics Clinical indirect calorimetry $200-400

What the Test Looks Like

Resting RQ Test Protocol:
• Arrive fasted (8-12 hours, water okay)
• Lie down in quiet room for 15-20 min to relax
• Breathe normally into mask/hood for 15-30 min
• Machine continuously measures your gases
• Report shows your RQ, fuel mix, and calorie burn

Interpreting Your Resting RQ

Resting RQ Fuel Mix What It Means
0.70 - 0.75 85-100% fat Excellent fat burning, highly flexible
0.75 - 0.80 65-85% fat Good flexibility, healthy metabolism
0.80 - 0.85 50-65% fat Moderate flexibility, room for improvement
0.85 - 0.90 35-50% fat Carb-dependent, reduced flexibility
0.90 - 1.00 0-35% fat Metabolically inflexible, stuck on carbs
Why is resting RQ important?
At rest, you SHOULD be burning mostly fat. If you're burning mostly carbs while sitting still, you're:
  • Wasting limited glycogen for nothing
  • Not accessing your fat stores
  • Creating more ROS than necessary
  • Likely experiencing energy swings

Blood Marker Proxies for Metabolic Flexibility

Since most people can't easily measure RQ, certain blood markers serve as proxies for metabolic flexibility. These are available from standard blood tests.

Key Markers and What They Tell You

1. Fasting Glucose

Measures blood sugar after 8+ hour fast. High fasting glucose suggests you're not burning fat efficiently overnight.

70-85 mg/dLOptimal — excellent overnight fat burning
85-95 mg/dLGood — healthy range
95-100 mg/dLBorderline — flexibility declining
>100 mg/dLElevated — likely metabolically inflexible

2. Fasting Insulin

Often not included in standard panels — you may need to request it. High fasting insulin = chronically elevated = can't access fat stores.

2-5 μIU/mLOptimal — insulin sensitive, flexible
5-8 μIU/mLGood — healthy range
8-12 μIU/mLElevated — developing resistance
>12 μIU/mLHigh — significant insulin resistance

3. HOMA-IR (Calculated)

Homeostatic Model Assessment of Insulin Resistance. Combines fasting glucose and insulin for a better picture.

HOMA-IR = (Fasting Glucose × Fasting Insulin) / 405
(glucose in mg/dL, insulin in μIU/mL)
< 1.0Optimal — highly insulin sensitive
1.0 - 1.5Good — healthy sensitivity
1.5 - 2.0Early resistance — flexibility declining
2.0 - 2.5Moderate resistance
> 2.5Significant insulin resistance

4. Triglyceride/HDL Ratio

From standard lipid panel. This ratio correlates strongly with insulin resistance and metabolic flexibility.

TG/HDL Ratio = Triglycerides ÷ HDL
(both in mg/dL)
< 1.0Optimal — excellent metabolic health
1.0 - 1.5Good — healthy metabolism
1.5 - 2.5Borderline — some inflexibility
2.5 - 3.5Elevated — metabolic dysfunction
> 3.5High — significant metabolic problems

5. HbA1c

3-month average glucose exposure. As discussed, this reflects total time spent with elevated glucose.

< 5.0%Optimal — minimal glucose exposure
5.0 - 5.4%Good — healthy, flexible
5.4 - 5.7%Borderline — some inflexibility
5.7 - 6.4%Pre-diabetic range
≥ 6.5%Diabetic range
The Metabolic Flexibility Panel:
Ask your doctor for these tests together:
  • Fasting glucose
  • Fasting insulin (often need to request specifically)
  • Lipid panel (includes triglycerides, HDL)
  • HbA1c
Total cost: Usually covered by insurance, or $50-150 out of pocket

HOMA-IR Calculator

Calculate your insulin resistance index using your fasting glucose and fasting insulin values.

mg/dL
μIU/mL
Optimal (<1.0) Good (1.0-1.5) Borderline (1.5-2.5) Resistant (>2.5)
What HOMA-IR tells you:
HOMA-IR estimates how much insulin your pancreas needs to produce to keep your blood sugar in check. Higher values mean your cells are more resistant to insulin — a sign of metabolic inflexibility.

Typical Values by Metabolic State

Category Typical Glucose Typical Insulin HOMA-IR
Metabolically flexible athlete 80 mg/dL 3 μIU/mL 0.6
Healthy adult 90 mg/dL 6 μIU/mL 1.3
Early metabolic dysfunction 98 mg/dL 10 μIU/mL 2.4
Insulin resistant 105 mg/dL 15 μIU/mL 3.9
Type 2 diabetic 130 mg/dL 20 μIU/mL 6.4

Triglyceride/HDL Ratio Calculator

This ratio from your standard lipid panel is a strong predictor of metabolic flexibility and cardiovascular risk.

mg/dL
mg/dL
Optimal (<1.0) Good (1.0-1.5) Borderline (1.5-2.5) High (>3.5)
Why TG/HDL ratio matters:
• High triglycerides = excess energy being stored as fat (often from carbs)
• Low HDL = poor fat transport/metabolism
• The combination indicates metabolic inflexibility better than either alone
• Also predicts small, dense LDL particles (the harmful type)

What Improves This Ratio

To lower triglycerides: Reduce refined carbs, reduce fructose, time-restricted eating

To raise HDL: Exercise, healthy fats (olive oil, avocado), moderate alcohol (optional), weight loss

Subjective Metabolic Flexibility Assessment

No blood test required. Answer these questions based on your daily experience to estimate your metabolic flexibility.

1. How do you feel if you skip breakfast?

2. How is your energy between meals (no snacking)?

3. Do you experience "hangry" episodes (irritable when hungry)?

4. Can you exercise in the morning without eating first?

5. How do you feel 2-3 hours after a carb-heavy meal?

6. How often do you crave sweets or carbs?

7. How do you wake up in the morning?

8. Could you fast for 16 hours (e.g., 8pm to 12pm next day)?

9. What's your exercise endurance like?

10. How stable is your body weight?