The Vegan Endurance Runner With Normal Hemoglobin and Low Ferritin
A 34-year-old woman has eaten a vegan diet for three years and is training for her first half marathon: 55–60 km of running per week plus two 45-minute resistance sessions. She is 168 cm, 62 kg, and reports six weeks of fatigue, hair shedding, and two colds. Her physician orders labs: ferritin 18 ng/mL (reference 15–150), hemoglobin 12.9 g/dL (12.0–15.5), MCV 88 fL, CRP 3.2 mg/L, serum B12 185 pg/mL (200–900), and 25-OH vitamin D 22 ng/mL. A three-day food recall averages 78 g protein, 2,100 kcal, 600 mg calcium, and 18 g fiber daily. She takes a generic multivitamin containing no iron, no B12, and no vitamin D, drinks two cups of black tea with meals, and gets most of her protein from beans, lentils, and tofu. She wants to know whether she should 'just eat more iron' and whether her training is safe to continue.
1. How should you interpret the iron panel, given that ferritin is inside the reference range and CRP is mildly elevated?
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Ferritin of 18 ng/mL is inside the printed range but not normal for a menstruating endurance athlete. Practical thresholds: iron stores are considered depleted below roughly 30 ng/mL, and iron-deficient erythropoiesis begins in the low-to-mid 20s with falling transferrin saturation. Hemoglobin and MCV are still normal, so this is deficiency without anemia - the stage at which intervention works best.
CRP of 3.2 mg/L matters because ferritin is an acute-phase reactant: inflammation inflates it, so the true store picture is likely worse than 18 suggests. Some clinical pathways therefore apply a higher ferritin cutoff (for example <100 µg/L) when CRP is elevated.
The reasoning chain: low ferritin + symptoms + heavy training load + tea with meals (tannins inhibit non-heme absorption) + no heme iron = treat and investigate. Recommend physician-supervised oral iron, roughly 45–65 mg elemental iron on alternate days rather than daily - alternate-day dosing avoids the hepcidin rise that blunts next-day absorption - taken with a vitamin C source and away from tea, coffee, calcium, and the multivitamin. Recheck ferritin with CRP in 8–12 weeks. Ask about menstrual losses and GI symptoms. Dietarily: legumes paired with citrus or pepper at the same meal, fortified cereal, cast-iron cookware, and tea between meals rather than with them.
2. What protein intake and distribution would you set for her, and why does the source matter?
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Current intake is 78 g/day at 62 kg, or 1.26 g/kg - enough to meet the RDA, not enough for 55–60 km/week. Endurance athletes are supported at 1.2–1.6 g/kg/day, and because most of her protein is intact plant food with lower digestibility (phytate, fiber, and limiting amino acids lower the DIAAS score), plan at the top of that range: 1.6–1.8 g/kg, about 99–112 g/day. That is an extra 25–35 g - one soy-based meal plus one fortified plant milk or pea/rice blend.
Distribution matters for muscle protein synthesis: three to four feedings of 0.3–0.4 g/kg (19–25 g) spaced 4–5 hours, each delivering roughly 2.5 g leucine. Soy and soy isolate are complete proteins; for other sources, pair legumes with grains across the day and lean on tofu, tempeh, seitan, and fortified plant milk. Place 20–25 g within an hour or two after runs.
Also question total energy: 2,100 kcal against this training volume plus lifting is plausibly low, and fatigue with hair shedding fits low energy availability (RED-S). Raise that possibility rather than assuming iron alone explains her symptoms.
3. Outline the B12, vitamin D, and calcium plan, and state what falls outside your scope.
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B12 at 185 pg/mL is borderline-low, not reassuring. Vegans get reliable B12 only from fortified foods or supplements, so she needs cyanocobalamin at 250 µg daily or 2,500 µg once weekly, with a recheck at 8–12 weeks. Seaweed, spirulina, and fermented foods are not dependable sources.
Vitamin D at 22 ng/mL is insufficient. Recommend 1,000–2,000 IU/day of D3 with a fat-containing meal, recheck in about three months, and target 30–50 ng/mL. Calcium at 600 mg needs to roughly double: fortified plant milk, calcium-set tofu, and low-oxalate greens such as kale, bok choy, and broccoli, with a split supplement of no more than 500 mg per dose if food falls short. Also address iodine (iodized salt), zinc in a phytate-heavy diet, and algal EPA/DHA at 250–500 mg/day.
Scope: food and supplement guidance is within a nutritionist's lane. Interpreting labs and setting an iron dose belong to her physician, and this symptom cluster plus a borderline B12 warrants a referral rather than a supplement stack. Document what you recommended, coordinate with her physician, and keep training unchanged until the iron and energy picture is clarified.