🏃 Endurance & work capacity

What actually decides this outcome, in order of size

The same ergogenic aid produces a large effect in one person and nothing in another, and the variable that separates them is not genetics. Ranked by how much of the outcome each one owns:

1. Training status, which is the moderator in the pooled evidence. An umbrella review of beetroot juice separated professional athletes from healthy individuals precisely because the answer differs between them Tian 2025. The trained athlete has already up-regulated the pathway the supplement acts on, which leaves less room; the untrained participant has more room and produces the larger published effect.
2. Iron status, which is the commonest correctable limit and the most often missed. Intravenous iron improved fatigue in non-anemic women with low ferritin Vaucher 2012, and ferritin thresholds for iron deficiency have been derived from physiological criteria rather than inherited from a reference range Addo 2022. In endurance athletes the losses are real and the reference range is misleading.
3. Which physiological limit you are actually hitting, because the four pathways address different ones. Oxygen delivery, mitochondrial capacity, proton buffering and fuel availability fail at different durations and feel different. A 90-second effort and a four-hour effort do not share a limiting step, and no supplement crosses that boundary.
4. That the buffering agents live in a narrow time window. A systematic review and multilevel meta-analysis found no ergogenic effect of beta-alanine on repeated sprint ability Liang 2026, and combining bicarbonate with beta-alanine has been examined as a question in its own right Curran-Bowen 2024. Buying a buffer for a marathon is buying the wrong duration.
5. Fitness itself, which is the only variable on this hub with a mortality association attached. Cardiorespiratory fitness measured on treadmill testing tracks long-term mortality Harb 2021, and exercise recommendations for healthy longevity have been formalized Izquierdo 2024. That is the reason to train that survives whether or not any supplement works.

The order to run these in, and what has to be true first

Correct the deficiency, establish the limit, then buy for the duration you actually race. This sequence exists because the commonest cause of a stalled endurance athlete is on a blood panel rather than on a shelf.

1. One panel first, and read ferritin against physiology rather than the printed range. Complete Blood Count (CBC) with Differential with Ferritin and the Iron Panel (Iron, TIBC, Transferrin Saturation), plus hs-CRP (High-Sensitivity C-Reactive Protein) because ferritin is an acute-phase reactant. Thresholds derived from physiological criteria sit above the lower reference limit most laboratories print Addo 2022, and the randomized evidence for treating low ferritin is in non-anemic people Vaucher 2012.
2. Add the cheap exclusions in the same draw. Vitamin D (25-Hydroxy), TSH (Thyroid-Stimulating Hormone) with Free T4 (Thyroxine), Comprehensive Metabolic Panel (CMP) and HbA1c (Hemoglobin A1c). Between them these cover most of what makes a previously fit person stop improving without an obvious injury.
3. Establish the limit before buying for it. A field test at two durations, one around 90 seconds and one over 30 minutes, tells you which end of this hub you live at. That distinction decides which of the four pathways is yours and it costs a training session.
4. Then route to exactly one pathway. Mitochondrial density and the exercise-mimetic shelf is Exercise mimetics & mitochondrial biogenesis. Nitric oxide and blood flow is Oxygen delivery & nitric oxide, where the training-status moderator matters most Tian 2025. Proton handling in the one-to-four-minute window is Buffering & fatigue resistance. Fuel and its transport is Substrate & fuel availability.
5. Electrolytes and fluid strategy before any ergogenic aid. Exercise-associated hyponatremia has an international consensus statement behind it Hew-Butler 2015, it is caused by drinking too much rather than too little, and it is the one acute danger on this hub that has actually killed endurance athletes.
6. Iron only if the panel says so, and never on symptoms alone. Iron is the supplement on this hub with both the largest upside in deficiency Vaucher 2012 and a genuine downside in excess, which is the entire argument for testing first.
7. Beta-Alanine and Sodium Bicarbonate only for the duration they act on. The negative repeated-sprint result Liang 2026 and the combination question Curran-Bowen 2024 together define where these belong, which is efforts long enough to accumulate protons and short enough that substrate is not the limit.
8. L-Carnitine L-Tartrate and the fuel shelf last. Oral carnitine's effect on exercise variables has been reviewed Mielgo-Ayuso 2021, and plasma carnitine is a poor guide to the muscle pool, which is the argument made in full at Substrate & fuel availability.

What gets bought for this that cannot move it

The category that fails is the ergogenic aid bought by the athlete who has already trained the adaptation. Nitrate supplementation works partly by improving efficiency in muscle that is not already maximally efficient, which is why the umbrella review had to separate professional athletes from healthy individuals to report a coherent answer Tian 2025. The better trained you are, the smaller the expected effect. That is not a disappointing footnote; it is the structure of the evidence.

The buffering shelf is the clearest case of a real effect sold outside its window. Raising muscle carnosine helps efforts limited by intracellular proton accumulation and does nothing in efforts limited by substrate or by oxygen delivery, and the pooled repeated-sprint result is negative Liang 2026. A cyclist buying it for a century ride is paying for a mechanism that will not be engaged.

And the most dangerous mistake available from this hub is a fluid one. Exercise-associated hyponatremia arises from drinking more than is lost, it presents as confusion and collapse late in a long event, and it has a consensus statement because it has killed people Hew-Butler 2015. Electrolyte products are sold against cramp, which is not the same problem and has a much weaker evidence base.

If the picture is different, so is the hub. Fatigue at rest rather than in training is Energy & fatigue, where a normal panel is the commonest and most useful result. A falling performance with disturbed sleep and low mood is Mood & stress resilience plus a training-load review. Joint pain limiting volume is Joints & bone. And breathlessness disproportionate to fitness, chest tightness or exertional syncope is a clinician before the next session.

How you would know it was working, on a real read-out and a real timescale

The prediction this hub makes is that a correctable deficiency shows up on the panel and a real ergogenic effect shows up on a fixed test at the right duration. If Ferritin was low and rises and the test does not improve, the iron was not the limit. If a supplement improves a 90-second test and not a 30-minute one, it is working exactly as its mechanism predicts.

• Ferritin with hs-CRP (High-Sensitivity C-Reactive Protein) as a pair, at baseline and 12 weeks. Ferritin rises with inflammation, so a normal value in an athlete with a raised CRP proves nothing Addo 2022. Twelve weeks because a new red cell cohort and refilling stores take about that long Vaucher 2012.
• Complete Blood Count (CBC) with Differential with the same draws, for hemoglobin and red cell indices. A falling mean cell volume alongside a falling ferritin is iron deficiency arriving; the two together are more informative than either alone.
• Two fixed field tests, at 90 seconds and at 30 minutes, repeated every 6 weeks. Two durations because they interrogate different limiting steps, which is how a reader learns which pathway they are actually on rather than which one they bought from.
• Comprehensive Metabolic Panel (CMP) with sodium once, and again after any long event with symptoms. Hyponatremia is a sodium measurement and nothing else diagnoses it Hew-Butler 2015, which is why the panel belongs on this hub rather than only on the fluid page.
• Vitamin D (25-Hydroxy) and TSH (Thyroid-Stimulating Hormone) once, to be normal. Both are cheap, both produce a plateau that looks like overtraining, and a normal pair closes those questions for a season.

What will fool you. Fitness improves on its own through a training block, so anything started at the beginning of one takes the credit for the block. Taper produces the largest single-week performance gain most athletes ever see and it is not a supplement. Beetroot's effect depends on oral nitrate-reducing bacteria, so antibacterial mouthwash abolishes it and a null result may be a mouthwash result Tian 2025. Beta-alanine tingling is not an effect Liang 2026. And ferritin drawn within a few days of a hard session reads high because it is an acute-phase reactant Addo 2022.

Sources read for these sections

• Harb SC. Associations between cardiorespiratory fitness, sex and long term mortality amongst adults undergoing exercise treadmill testing. International Journal of Cardiology 2021 · PMID 34363868
• Tian C. Effects of Beetroot Juice on Physical Performance in Professional Athletes and Healthy Individuals: An Umbrella Review. Nutrients 2025 · PMID 40573069
• Vaucher P. Effect of iron supplementation on fatigue in nonanemic menstruating women with low ferritin: a randomized controlled trial. Canadian Medical Association Journal 2012;184(11):1247-54 · PMID 22777991
• Addo OY, et al. Physiologically based serum ferritin thresholds for iron deficiency in women of reproductive age who are blood donors. Blood Advances 2022 · PMID 35404995
• Liang W. No ergogeniceffect of beta-alanine on repeated sprint ability: a systematic review and multilevel meta-analysis of randomized controlled trials. Frontiers in Nutrition 2026 · PMID 41971372
• Curran-Bowen T. Sodium bicarbonate and beta-alanine supplementation: Is combining both better than either alone? A systematic review and meta-analysis. Biology of Sport 2024;41(4):3-20 · PMID 38952910
• Mielgo-Ayuso J. Effect of Acute and Chronic Oral l-Carnitine Supplementation on Exercise Performance Based on the Exercise Intensity: A Systematic Review. Nutrients 2021;13(12):4359 · PMID 34959912
• Izquierdo M. Global consensus on optimal exercise recommendations for enhancing healthy longevity in older adults (ICFSR). Journal of Nutrition Health and Aging 2024 · PMID 39743381
• Hew-Butler T, et al. Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Clinical Journal of Sport Medicine, 2015 · PMID 26102445