A Dietitian's Guide to Macro Tracking for Athletes

Bone Turnover Markers (Serum CTX and P1NP) in the Sports RD Workup: Why the Paired Resorption-and-Formation Panel Catches the Active Bone-Loss Signal DXA Reads Six Months Late, the Draw-Protocol Discipline That Controls the Diurnal and Meal-State Variance the Standard Reference Ranges Ignore, and the SOAP Assessment Structure That Documents the Coupled-Uncoupled Read So the Referral Starts at the Right Question

A 22-year-old female collegiate cross-country runner presents to the sports RD on referral with two consecutive tibial stress reactions on the same tibia across the last twelve months, a DXA from three months ago flagged 'within reference for age,' a computed energy availability of 33 kcal/kg FFM/day, a serum 25-hydroxyvitamin D of 38 ng/mL, and a regular menstrual cycle. The referring note reads 'rule out low energy availability and calcium/vitamin D intake.' The DXA is not wrong — it is the wrong tool for the question the presenting picture is asking. DXA integrates bone-mineral-density history across many months to years of prior turnover; it cannot see what the athlete's remodeling coupling is doing this week or this training block. Two consecutive stress reactions on the same tibia in twelve months is the signal that the resorption-formation coupling has shifted toward net resorption, and the paired serum C-terminal telopeptide (CTX) and procollagen type 1 N-terminal propeptide (P1NP) panel is the workup that catches it at the intervention window that still reverses cleanly. This is the bone-turnover-marker interpretation guide for the sports RD: what CTX and P1NP actually measure and why they read a real-time signal DXA cannot, the fasted-morning-no-recent-training-same-lab-same-assay draw-protocol discipline that controls the diurnal and meal-state variance the reference ranges quietly absorb, the four intake presentations that should trigger the paired panel regardless of the DXA read, the coupled-versus-uncoupled interpretation framework that separates the healthy loaded skeleton from the LEA-driven or hypogonadism-driven net-resorption pattern, the SOAP assessment structure that documents the coupling read so the endocrinology or sports-medicine referral starts at the correct question, and the paired energy-availability-plus-mechanical-loading intervention loop the sports RD holds across the 12-to-16-week reassessment cadence.

September 3, 2026

Hemoglobin A1c and Fasting Glucose Interpretation in Non-Diabetic Athletes: Why the Paired Panel Catches the Reduced-Carbohydrate-Tolerance Pattern CGM Alone Misses, the Athlete-Specific Functional Thresholds That Shift Below the Standard Non-Diabetic Reference Band, and How the Sports RD Documents the Metabolic-Flexibility Read So the Follow-Up Conversation Starts at the Correct Question

A 38-year-old male masters marathoner presents to the sports RD on referral with stalled performance across two training cycles despite textbook periodization, a CGM two-week wear showing mean sensor glucose of 98 mg/dL in the upper-normal band, a fasting glucose of 97 mg/dL flagged as within reference against the standard 70-99 mg/dL band, and an HbA1c of 5.6 percent flagged as within reference against the standard non-diabetic ceiling of 5.7 percent. Both markers cleared their thresholds. The primary read the panel as glucose metabolism unremarkable and referred for a dietary review. The primary's read is not wrong on the individual markers — the interpretation of the paired panel is incomplete. The pattern the athlete presents is the reduced-carbohydrate-tolerance signal that the endurance-athlete population develops at a rate the general-population reference ranges were not calibrated to catch, and it has direct implications for how the fueling plan should be restructured, how the training-load intensity distribution should be programmed, and how the sports RD documents the read. This is the paired fasting-glucose-plus-HbA1c interpretation guide for the sports RD working with non-diabetic athletes — what the two markers actually measure and why they read a different signal from CGM, the athlete-specific interpretive ceilings that shift below the standard reference band (95 mg/dL fasting glucose and 5.4 percent A1c as the functional targets), the four intake presentations that should trigger the paired panel regardless of the primary panel, the differential that separates the endurance-athlete reduced-carbohydrate-tolerance pattern from early insulin resistance from acute-training-response distortions from sports-anemia-driven A1c artifacts, the SOAP assessment structure that names the metabolic-flexibility differential explicitly so the referring-physician conversation starts at the correct question, and the paired dietary-plus-training-load intensity-distribution intervention loop the sports RD holds across the 10-to-12-week reassessment cadence.

September 1, 2026

Reverse T3 and Low-T3 Syndrome in the Overtrained Athlete: Why the "Normal" Thyroid Panel Misses the Non-Thyroidal-Illness Pattern, the Free T3-to-Reverse T3 Ratio the Sports RD Should Compute at Every Intake With an Overtraining Signal, and How the SOAP Assessment Documents the Read So the Endocrinology Consult Starts at the Right Question

A 34-year-old female triathlete presents to the sports RD in week 14 of an Ironman build with the presentation every experienced sports RD recognizes on sight: fatigue disproportionate to training load, resting heart rate drifted up 8 bpm across four weeks, cold intolerance in a warm gym, weight gain despite unchanged intake and unchanged training log, three weeks of dampened workout responses her coach has attributed to "needing to push through," and a normal TSH and free T4 panel her primary drew last week that reads "thyroid function within reference — please rule out iron, sleep, and nutritional causes." The panel is not wrong. It is incomplete. The pattern is non-thyroidal illness syndrome — the low-T3 physiologic downregulation that overtraining, low energy availability, and prolonged catabolic stress produce through preferential 5'-deiodinase conversion of T4 to reverse T3 rather than to T3 — and it is invisible on the standard TSH-plus-T4 workup because both markers can hold in reference while the peripheral tissue is running on the wrong active hormone. This is the reverse T3 workup for the sports RD: when to trigger it (overtraining presentation with normal TSH/T4, LEA signal at intake, RED-S differential, unexplained recovery-failure pattern), how to order it (free T3, reverse T3, free T4, TSH — same draw), the free T3-to-reverse T3 ratio computation the referring physician usually skips and the interpretive thresholds that separate physiologic overtraining downregulation from primary hypothyroid pathology, the SOAP assessment structure that names the non-thyroidal-illness differential explicitly so the endocrinology consult starts at the correct question rather than repeating the normal-TSH read, and the paired training-plus-energy-availability intervention loop the sports RD holds while the panel reassesses.

August 27, 2026