REF-473 · San-Millán & Brooks, 2018 · Sports Medicine, · Paper · PubMed

Assessment of metabolic flexibility by measuring blood lactate, fat and carbohydrate oxidation during exercise in professional endurance athletes and less-fit individuals

Fat oxidation and blood lactate are inversely correlated (r = -0.97 in pros, -0.76 across all groups)

Blimited evidenceMeasured on elite to world-class athletesWritten before October 2026, being re-checked

Background (abstract)

Increased muscle mitochondrial mass characterizes elite professional endurance athletes (PAs), whereas higher blood lactate at the same absolute submaximal intensity and lower mitochondrial oxidative capacity characterize individuals with low aerobic power.

Patients with metabolic syndrome (MtS) show a reduced capacity to oxidize lipids, an early transition from fat to carbohydrate oxidation, and elevated blood lactate as power output rises — "metabolic inflexibility". Aim: assess metabolic flexibility across populations with different metabolic characteristics.

Participants

Three groups: professional endurance athletes (PAs), moderately active individuals (MAs) and individuals with metabolic syndrome (MtS). Group sizes are not given in the abstract; the full text gives 22 international-level male professional road cyclists (Pro Tour), 20 moderately active men and 10 men with MtS — all male (full text, checked 2026-09-28: Materials and Methods).

Protocol

Indirect calorimetry (fat and carbohydrate oxidation, FATox / CHOox) and blood lactate [La⁻] measured during a graded leg cycle ergometer test to volitional exhaustion, about 35 W increments every 10 min after a 15-min warm-up below 100 W (full text, checked 2026-09-28: Materials and Methods)

Key Findings (abstract)

FATox was significantly higher in PAs than in MAs and MtS patients (p < 0.01), and [La⁻] was significantly lower in PAs than in the other two groups
FATox and [La⁻] were inversely correlated within every group: PA r = -0.97, MA r = -0.98, MtS r = -0.92 (all p < 0.01)
Across all data points from all populations: r = -0.76 (p < 0.01)
Conclusion: blood lactate accumulation is negatively correlated with fat oxidation and positively with carbohydrate oxidation across populations of widely different metabolic capability; because both lactate and fatty acids are mitochondrial substrates, measuring [La⁻] and FATox during exercise offers an indirect assessment of metabolic flexibility and oxidative capacity

Practical Application

Professional endurance athletes, moderately active people and metabolic-syndrome patients were tested with indirect calorimetry and blood lactate during graded exercise: the pros oxidized significantly more fat and accumulated less lactate at a given power (p < 0.01), and fat oxidation and lactate were tightly inversely correlated in every group (r = -0.97 pros, -0.98 moderately active, -0.92 metabolic syndrome; r = -0.76 pooled) — so lactate plus fat-oxidation rate during exercise is proposed as an indirect index of mitochondrial oxidative capacity, "metabolic flexibility" (San-Millán & Brooks 2018). Meaning: a rightward-shifted lactate curve with high fat oxidation is the hallmark of a trained oxidative engine, and a simple lactate test tracks it.
Rationale for the base's "Zone 2" concept: the intensity range where fat oxidation is high and lactate still low — the mitochondrial-capacity training target. Not from this paper: the exact lactate cut-off used to define that zone is the base's own practice; this abstract gives no threshold value
Lactate–power and fat-oxidation curves as individual phenotype markers (feed SYNTH-13 archetype classification)
Field-relevant: low lactate at a given power + high fat oxidation = the trained "Diesel" signature; MtS-like early lactate rise = metabolic inflexibility
Inference, not tested: the large base of sub-threshold volume in pyramidal/polarized programmes targets this metabolic-flexibility adaptation (cross-sectional study; training was not tested)

Limitations

Cross-sectional — trained vs less-fit comparison, not a training intervention; does not establish causality of training → metabolic flexibility
Group sizes not stated in the abstract; the full text gives n=22 professional cyclists, 20 moderately active and 10 MtS, all male (full text, checked 2026-09-28)
No FatMax intensity or lactate threshold value is reported in the abstract
Lactate-based zone definitions carry measurement variability
How this entry was checked and changed
Why this level

regraded 2026-09-27 from A: cross-sectional / correlational study, group sizes not stated in the abstract

Content correction 2026-09-27
"This work operationalized 'Zone 2' as the intensity where blood lactate first rises ~0.5 mmol/L above baseline" -> not in the abstract; removed, Zone 2 cut-off relabelled as the base's own practice. "FatMax occurs at a higher % of capacity in trained athletes" -> not in the abstract; removed.
Added the abstract's results: FATox higher / lactate lower in pros (p < 0.01); within-group correlations r = -0.97 / -0.98 / -0.92; pooled r = -0.76; three groups incl. metabolic-syndrome patients.
Level A -> B (cross-sectional, group sizes not in the abstract); added Tier/Sport lines and an abstract-based lead.
2026-09-28: lines labelled "full text; not checked" read against the open-access full text (published version on eScholarship, escholarship.org): 1 confirmed (cycle ergometry; stage protocol added), 0 corrected, 0 removed.
2026-09-28: group sizes added from the open-access full text (escholarship.org, Materials and Methods): 22 professional cyclists, 20 moderately active men, 10 men with metabolic syndrome, all male; "professional cohort small" removed.
2026-10-01: cross-reference to SYNTH-13 no longer names an archetype — that model was retired as an unvalidated composite.