Using Glucose, Lactate, and Ketones to Determine Readiness
Endurance athletes spend enormous effort measuring output — pace, watts, heart rate, HRV — yet the most honest readiness signals often come when the body is doing nothing at all. A rest day, several hours after eating, can reveal whether recovery is truly happening or merely being hoped for.
Consider this rest-day snapshot taken three hours after lunch:
Glucose: 107 mg/dL
Lactate: 1.2 mmol/L
Ketones: 0.2 mmol/L
On the surface these look like ordinary numbers. In combination, they form a metabolic fingerprint that tells a deeper story about fuel use, clearance, and nervous-system tone. This is not about chasing perfection — it’s about recognizing patterns that indicate when the engine is idling smoothly versus when it’s quietly overheating.
Lactate 1.2 mmol/L — The “Quiet Engine” Signal
For endurance athletes, resting or mid-day lactate values typically fall between 0.8 and 1.5 mmol/L when recovery is on track. A value of 1.2 on a rest day is about as clean as it gets.
What it implies physiologically:
Minimal glycolytic backlog — yesterday’s hard efforts have been processed.
Muscle repair largely caught up — lactate isn’t lingering from tissue stress.
Low nervous-system load — sympathetic drive isn’t pushing glucose into rapid turnover.
Strong mitochondrial throughput — aerobic pathways are handling energy demands efficiently.
Lactate is uniquely valuable because it reflects metabolic strain, not just fuel presence. Heart rate can be influenced by caffeine, adrenaline, or emotion. Lactate is far less easily fooled. When it sits around 1.2 on a rest day, the message is simple: the engine is idling smoothly.
Ketones 0.2 mmol/L — Evidence of Fuel Flexibility
Ketones often get misinterpreted as a binary sign of ketosis versus non-ketosis. In performance physiology, low-level ketones are more interesting than high ones. A reading of 0.2 mmol/L three hours after a meal suggests:
Insulin rose and fell appropriately — no prolonged suppression of fat oxidation.
Fat metabolism restarted quickly once post-meal glucose was handled.
No “carb lock-in” — the body is not dependent on constant carbohydrate flow.
Metabolic flexibility is intact — dual-fuel capability remains available.
For endurance athletes, this flexibility is gold. It means long-duration energy systems can come online without a dramatic hormonal shift. The presence of small ketones between meals is not about dieting — it’s about substrate agility.
Glucose 107 mg/dL — Normal Post-Meal Settling
Three hours after lunch, 107 mg/dL is entirely reasonable, especially if the meal contained moderate carbohydrates. The key is not the absolute number, but the trajectory it represents.
At this point in digestion, a value near 100 implies:
Liver output is calm — no excessive glucose release.
No prolonged spike — insulin has already done its primary job.
Hormones are tapering, not lingering high.
A gradual drift downward is likely without effort or intervention.
For athletes, this indicates adequate fuel availability without evidence of metabolic stress or rebound hypoglycemia. In other words, the system is balanced — neither flooded nor depleted.
Why the Pattern Matters More Than the Number
Individually, each metric provides a clue. Together, they create a cohesive picture:
Glucose 107 → Fuel present, stabilizing.
Lactate 1.2 → Clearance strong, no metabolic residue.
Ketones 0.2 → Fat oxidation active, flexibility preserved.
This triad signals:
Good insulin sensitivity — glucose normalizes efficiently.
Effective clearance capacity — lactate does not accumulate.
Fuel versatility — carbohydrate and fat systems coexist.
Low stress chemistry — no signs of sympathetic overdrive.
This is the physiology of a successful deload or rest day — not dramatic, not flashy, just quietly effective. Recovery is not merely the absence of training; it is the presence of metabolic harmony.
What It Predicts for the Next Morning
When evening stress stays low and sleep is adequate, this profile often precedes:
Calm morning energy rather than jittery urgency.
Fasting lactate around 1.2–1.4 mmol/L.
Morning glucose near 90–100 mg/dL.
“Light legs” sensation with steady mood and focus.
These are not guarantees, but they are strong probabilities. The body is signaling that recovery is finishing and readiness is building — the exact transition endurance athletes seek before a quality session or race block.
Practical Coaching Takeaway
Athletes frequently look for a single green light — a perfect HRV score or a fast warm-up split. True readiness is rarely announced so loudly. More often, it shows up as quiet consistency across metabolic markers.
A rest-day profile of low lactate, small ketones, and normalized glucose is textbook evidence of a system that has processed stress, restored balance, and retained fuel options. It doesn’t promise a personal best tomorrow, but it strongly suggests the internal environment required for one is forming.
In endurance sport, durability wins seasons. And durability is built not in the loud workouts, but in these quiet metabolic moments when the engine runs clean, the fuels are flexible, and the body whispers, “I’m ready when you are.”
Train well,
Joe