Supplements for Endurance Athletes: The Evidence-Based Performance Stack

Supplements for Endurance Athletes: The Evidence-Based Performance Stack

Endurance sport is a demanding discipline. Whether you're logging 50-mile weeks preparing for a marathon or spending hours in the saddle for a gran fondo, the physiological stress you place on your body creates specific nutrient demands that a standard diet — even a good one — often fails to meet. Inflammation climbs. Iron stores get depleted. Mitochondria take a beating. And recovery windows shrink.

The supplement industry is predictably loud about what you need. But most products aimed at endurance athletes are either under-dosed, built around marketing rather than mechanisms, or formulated for gym athletes rather than those whose primary adaptation goal is aerobic efficiency. This article cuts through that noise with a clinically grounded look at the supplements that genuinely move the needle for endurance performance — with real study citations, real doses, and guidance on how to build a stack that's personalized to your physiology.

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Why Endurance Athletes Have Different Nutritional Needs

A sedentary adult and a runner logging 50+ miles per week share the same basic biochemistry — but the scaling of demand is radically different. Sustained aerobic output increases:

• Oxidative stress: prolonged exercise elevates reactive oxygen species (ROS), which damage cellular membranes, mitochondria, and DNA (Powers & Jackson, Physiological Reviews, 2008; PMID: 18400713)
• Inflammatory cytokine production: particularly IL-6 and TNF-alpha, which spike post-long-run and remain elevated during high-volume training blocks
• Sweat-mediated micronutrient losses: iron, zinc, magnesium, and sodium are all lost at rates that exceed typical dietary intake in high-volume athletes
• Mitochondrial turnover: the body must continuously build new mitochondria to meet aerobic energy demands — a process that depends on B vitamins, CoQ10, and antioxidant cofactors

Understanding these mechanisms helps explain why the supplements discussed below are relevant — not as performance enhancers in the doping sense, but as critical inputs for a system that's running at an unusually high metabolic rate.

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Iron Athlete Supplement: The Most Overlooked Performance Variable

Iron deficiency is the single most common nutritional deficiency in endurance athletes, affecting an estimated 15–35% of female runners and 3–11% of male runners (Peeling et al., British Journal of Sports Medicine, 2008; PMID: 17921185). What makes iron particularly insidious in this population is that athletes can be iron-depleted — with low ferritin stores and impaired oxygen transport — while still maintaining hemoglobin levels within normal reference ranges. This is called iron deficiency without anemia, and standard complete blood count panels will miss it entirely.

The mechanism matters here. Iron is the core mineral in hemoglobin, which carries oxygen from lungs to working muscles. It's also essential for myoglobin (the oxygen-storage protein in muscle tissue) and for cytochrome enzymes in the mitochondrial electron transport chain. Even marginal iron insufficiency can therefore impair VO2 max, increase perceived exertion at submaximal intensities, and slow recovery.

A 2011 double-blind RCT in iron-deficient non-anemic female athletes found that iron supplementation (100mg iron sulfate daily for six weeks) significantly improved VO2 max, exercise efficiency, and time-to-exhaustion compared to placebo (Burden et al., International Journal of Sport Nutrition and Exercise Metabolism, 2015; PMID: 25811137). Ferritin testing is essential before supplementing — iron loading in athletes who are not deficient can cause oxidative harm. This is exactly the kind of decision that should be driven by lab data, not guesswork.

For athletes who have confirmed low ferritin (ideally below 30 ng/mL for performance-relevant depletion), supplemental iron in forms with good bioavailability and low GI side effects — such as ferrous bisglycinate — is appropriate. Target serum ferritin above 50–70 ng/mL for optimal aerobic function. If you're unsure what your ferritin level looks like, understanding your iron lab values and what they mean for energy is a good starting point before building any iron protocol.

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Marathon Runner Supplement Protocol: The Full-Stack View

Marathon training creates a particularly concentrated set of nutritional demands because of the combination of high weekly mileage, glycogen depletion, and the inflammatory load of repeated long runs. The following supplements have meaningful clinical evidence in endurance-specific contexts:

Vitamin D3 + K2

Vitamin D deficiency is prevalent in athletes who train predominantly indoors or in northern latitudes. Low vitamin D is associated with increased injury risk, impaired immune function, and reduced muscle protein synthesis. A meta-analysis in Nutrients (2017) found that vitamin D supplementation significantly improved muscle strength and reduced stress fracture risk in athletes (Książek et al., PMID: 28282857). The synergy with K2 (as MK-7) ensures calcium is directed to bone rather than soft tissue — an important consideration for athletes with high calcium turnover.

Recommended dose: 2,000–5,000 IU D3 daily, paired with 90–200mcg K2 (MK-7). Vitamin D3 and K2 synergy for bone and immune health explains how these two nutrients work together and why dose matters.

Magnesium Glycinate

Magnesium is lost heavily in sweat and urine during intense training. It's a cofactor for over 300 enzymatic reactions, including ATP synthesis, muscle contraction, and nervous system regulation. Athletes with low magnesium show elevated lactate levels at submaximal exercise intensities — meaning harder perceived effort for the same output (Zhang et al., Nutrients, 2017; PMID: 28594349). Magnesium glycinate is preferred for athletes because it has high bioavailability and avoids the laxative effect common with magnesium oxide or citrate at therapeutic doses.

Recommended dose: 300–400mg elemental magnesium as glycinate daily, ideally in the evening to also support sleep quality and overnight recovery.

Ashwagandha (KSM-66)

Cortisol is chronically elevated in athletes during heavy training blocks. Ashwagandha root extract — particularly the KSM-66 standardized form at 600mg daily — has been shown in multiple RCTs to significantly reduce cortisol and improve cardiorespiratory endurance. A 2015 RCT of 50 male athletes using 300mg KSM-66 twice daily for 12 weeks found significant improvements in VO2 max, time-to-exhaustion, and recovery markers compared to placebo (Choudhary et al., Journal of the International Society of Sports Nutrition, 2015; PMID: 26609282). The evidence for clinical-grade ashwagandha supplementation in stress and performance contexts is now among the strongest of any adaptogen.

Beta-Alanine

Beta-alanine is a precursor to carnosine, a dipeptide that buffers hydrogen ions in muscle during high-intensity exercise — delaying the acidosis that causes fatigue. Meta-analyses confirm that beta-alanine improves performance in bouts lasting 1–4 minutes and in repeated high-intensity efforts within longer events (Hobson et al., Amino Acids, 2012; PMID: 22270875). For marathon runners, this is most relevant in late-race surges and threshold training sessions. Standard dose: 3.2–6.4g daily in divided doses to minimize paresthesia (the harmless tingling side effect).

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Omega-3 for Athletes: Inflammation, Recovery, and More

Omega-3 fatty acids — specifically EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) — are among the most studied supplements in the athletic population, and for good reason. Their primary relevance to endurance athletes comes through three mechanisms:

1. Reduction of exercise-induced inflammation: EPA and DHA compete with arachidonic acid for cyclooxygenase enzymes, producing less inflammatory eicosanoids. This translates to reduced muscle soreness and faster recovery between sessions.
2. Improved oxygen efficiency: A 2020 systematic review in Sports Medicine found that omega-3 supplementation (≥2g EPA+DHA daily) reduced muscle damage markers and delayed-onset muscle soreness (DOMS) following eccentric exercise — highly relevant for downhill running and track sessions (Tsuchiya et al., Journal of Human Kinetics, 2020; doi.org/10.2478/hukin-2019-0114).
3. Cardiovascular and lung function: DHA in particular is incorporated into cell membrane phospholipids, improving red blood cell deformability — the ability of red cells to squeeze through narrow capillaries — which may enhance oxygen delivery to working muscle.

For endurance athletes, a combined EPA+DHA dose of 1,500–3,000mg daily appears to be the effective range for anti-inflammatory and recovery benefits. For a deeper look at how omega-3 EPA and DHA ratios affect performance and inflammation, the distinction between fish oil forms (ethyl ester vs. triglyceride) also matters for bioavailability.

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CoQ10 Endurance: Powering the Mitochondrial Engine

Coenzyme Q10 (CoQ10) — and its more bioavailable reduced form, ubiquinol — is a molecule that sits at the heart of mitochondrial ATP production. It functions as an electron carrier in the mitochondrial respiratory chain and as a fat-soluble antioxidant. For endurance athletes whose performance is fundamentally limited by aerobic power output, CoQ10 status is directly relevant.

In training, intense aerobic exercise increases oxidative stress in a way that can deplete CoQ10 tissue levels. A 2008 RCT of trained cyclists found that 300mg CoQ10 supplementation over eight weeks significantly reduced markers of oxidative stress and improved time-trial performance compared to placebo (Cooke et al., Journal of the International Society of Sports Nutrition, 2008; PMID: 18307826). While effect sizes are modest in well-trained athletes with adequate CoQ10 status, the compound becomes substantially more important in athletes over 35 (where endogenous synthesis declines), those taking statins (which block CoQ10 synthesis), and anyone with cardiovascular strain.

At Ones, CoQ10/Ubiquinol is available at 200mg — a clinically relevant dose aligned with the research literature. For masters endurance athletes in particular, this is one of the highest-priority additions to a performance formula. The dose matters: many retail supplements contain 30–50mg, which falls well short of the 100–300mg range studied in performance contexts.

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What This Means for Your Formula

Building an effective endurance supplement stack isn't just about knowing which ingredients work — it's about knowing which ones you need based on your actual physiology. An athlete with ferritin at 80 ng/mL needs a very different iron approach than one at 15 ng/mL. An athlete with optimal vitamin D from year-round sun exposure needs different D3 dosing than a northern-hemisphere winter trainer.

This is exactly where Ones is built differently from off-the-shelf sport nutrition products. Ones functions as an AI health practitioner: it analyzes your blood work, wearable data (HRV, sleep scores, resting heart rate trends), and health history to identify the specific gaps in your performance stack. Rather than giving every endurance athlete the same 12-capsule formula, Ones calibrates which of 70+ clinical-grade ingredients belong in your formula, at what dose, and in what priority order.

For endurance athletes specifically, Ones draws from:

• CoQ10/Ubiquinol at 200mg — dosed to match the ranges used in performance RCTs, particularly relevant for masters athletes and anyone with cardiovascular stress markers
• Omega-3 (EPA/DHA) — calibrated to your inflammatory load and dietary intake assessment, targeting the 1,500–3,000mg combined EPA/DHA range shown to reduce exercise-induced inflammation
• KSM-66 Ashwagandha at 600mg — the exact standardized form and dose used in the VO2 max and cortisol RCTs, available individually or as part of the Adrenal Support System Blend for athletes in heavy training blocks
• Magnesium Complex — Ones' proprietary Magnesium Complex blend is built around highly bioavailable forms including magnesium glycinate, addressing the sweat-loss demands of high-mileage training
• Vitamin D3 + K2 (MK-7) — paired at doses guided by your actual serum 25(OH)D levels rather than one-size-fits-all recommendations

Formulas come in 6, 9, or 12-capsule plans, so the stack scales with your training phase — higher volume blocks can warrant more comprehensive formulas, while base training phases may require fewer active ingredients.

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Key Takeaways

• Iron deficiency is endemic in endurance athletes and frequently missed by standard CBC panels — serum ferritin below 30–50 ng/mL can impair VO2 max and exercise economy even without clinical anemia; always test before supplementing
• CoQ10 at 200mg supports mitochondrial ATP production and reduces oxidative stress; particularly important for masters athletes and those on statins where endogenous CoQ10 synthesis is compromised
• Omega-3 at 1,500–3,000mg EPA+DHA daily reduces exercise-induced inflammation, DOMS, and supports red blood cell deformability for improved oxygen delivery
• KSM-66 Ashwagandha at 600mg has RCT-level evidence for improving VO2 max and reducing cortisol in athletic populations — making it one of the most evidence-backed adaptogens for endurance use
• Magnesium and Vitamin D3+K2 are foundational for high-volume athletes due to sweat losses, musculoskeletal integrity demands, and the immune suppression associated with prolonged aerobic training
• Personalized dosing based on lab data — not generic sport nutrition formulas — is the most effective approach; Ones builds your stack from actual blood work and wearable metrics so every capsule has a reason to be there

Always consult a qualified healthcare provider before beginning any new supplement protocol, particularly if you have underlying health conditions or are taking medications.