You track your macros. You weigh your food. You time your carbs around sessions. You eat more vegetables than most of your friends and you have a decent idea of what your protein intake looks like in a typical week. And yet, endurance training creates micronutrient demands that even a "perfect" diet often does not meet. The gap is not in your effort. It is in the way endurance training depletes specific nutrients at rates the general dietary recommendations were never designed for.
This article walks through the five micronutrient deficiencies most prevalent in endurance athletes: iron, magnesium, vitamin B6, vitamin C, and vitamin D. For each one, the prevalence data, the cellular mechanism, the symptoms most athletes recognise in themselves, the food sources and why they fall short, and what the evidence says about supplementation. The article also includes a comparison table of recommended daily intake for the general population versus athlete-optimal levels, which is one of the most useful single references in endurance nutrition. For the broader picture of how race-week and daily diet fits together, see the Race-day nutrition guide.
Why are endurance athletes more prone to micronutrient deficiencies?
Endurance athletes deplete specific micronutrients faster than the general population for four reasons that compound across a typical training year. Each one is small in isolation. Together they push intake requirements meaningfully above the recommended daily allowances most public health guidelines are built on.
The first is increased turnover. Hard training accelerates the metabolic reactions that depend on enzymatic cofactors. Magnesium, the B-vitamins, and several trace minerals are consumed at higher rates simply because the cells using them are working harder.
The second is sweat losses. Magnesium, sodium, calcium, zinc, and several other minerals are lost in sweat. For a heavy-sweating athlete doing 10 to 20 hours of training per week in warm conditions, the cumulative loss is meaningful.
The third is exercise-induced disruption of absorption. Iron is the clearest example: hepcidin, a hormone that regulates iron absorption, rises 3 to 6 hours after exercise and remains elevated for several hours, blocking iron uptake exactly when post-training meals are eaten. Research published in Sports Medicine, has documented this hepcidin response in trained athletes across multiple studies.
The fourth is mechanical loss. Footstrike hemolysis (red blood cells damaged by the impact of running) and gastrointestinal microbleeding during long efforts both contribute to ongoing iron losses that are not present in non-athletes.
This is why "the same diet that works for a non-athlete" often falls short for someone training seriously. The general dietary recommendations were not designed for someone doing two-a-days during a marathon build.
How does iron deficiency affect endurance performance?
Iron is the most-researched micronutrient deficiency in endurance sport, and the deficiency with the most direct performance cost. Iron is the central atom in haemoglobin, which transports oxygen in the blood; it is the central atom in myoglobin, which stores oxygen in muscle; and it is a critical component of the electron transport chain inside the mitochondria, where ATP is produced. Cut iron, and you cut all three.
Research published in the European Journal of Applied Physiology indicates that 15 to 35% of male endurance athletes and up to 35 to 50% of female endurance athletes show some level of iron deficiency at any given point in a season. The female prevalence is driven by the combination of menstrual iron losses and the training-induced losses that affect all endurance athletes.
Symptoms athletes recognise: heart rate elevated at submaximal paces that used to feel easy, persistent fatigue that does not resolve with a rest day, perceived exertion creeping up at familiar paces, breathlessness on hills that used to be manageable, and a vague sense that the "engine" is not running at full capacity. Iron deficiency without anaemia (low ferritin but normal haemoglobin) is harder to spot than full anaemia but can still impair performance measurably.
The dietary sources are familiar: red meat, organ meat, oysters and shellfish, lentils, beans, leafy greens, and fortified cereals. The problem is that heme iron (from animal sources) absorbs at roughly 15 to 35% efficiency, while non-heme iron (from plants) absorbs at 2 to 20% efficiency, and exercise-induced hepcidin elevation further reduces absorption in the hours after training. A vegetarian endurance athlete in a high-volume training block faces a structural absorption gap that food alone often cannot close. For the underlying mitochondrial mechanism iron supports, see the Mitochondria guide.
Is magnesium the most overlooked nutrient in endurance sport?
Magnesium is the most overlooked micronutrient in endurance training, and arguably the one with the most direct cellular relevance. Magnesium is a cofactor in over 300 enzymatic reactions in the human body, with ATP synthesis at the centre of that list. Every molecule of ATP that powers your muscles requires magnesium to be biologically active. Below adequate magnesium status, the entire energy production pathway runs at reduced capacity.
Research published in Magnesium Research by Nielsen and Lukaski indicates that sweat losses during prolonged exercise can deplete 10 to 20% of daily magnesium intake per session in heavy-sweating conditions. Combined with the low intake of magnesium-rich foods in many modern diets (modern wheat varieties contain less magnesium than older varieties; processing strips magnesium from grains), the result is that an estimated 50 to 70% of endurance athletes have suboptimal magnesium status according to dietary intake surveys.
Symptoms athletes recognise: poor sleep quality despite reasonable sleep hygiene, suppressed HRV that does not respond to rest, muscle cramps in the calves or feet, eye twitches, persistent low-grade fatigue, and increased perceived stress. The symptoms are unspecific enough that most athletes blame overtraining, age, or stress rather than checking magnesium status.
Food sources include leafy greens (spinach, swiss chard), nuts (almonds, cashews, brazil nuts), seeds (pumpkin, sunflower), whole grains, legumes, and dark chocolate. The challenge is hitting 400 to 600 mg per day consistently across the training year while training volume is depleting stores. For supplementation, magnesium glycinate, magnesium citrate, and magnesium taurate are well-tolerated bioavailable forms; magnesium oxide is the form most likely to cause GI distress and the form most cheap multivitamins use.
Why do endurance athletes need more vitamin B6 than the general population?
Vitamin B6 (pyridoxine) is a cofactor in over 100 enzymatic reactions, including glycogen breakdown into glucose during exercise, amino acid metabolism, neurotransmitter synthesis, and haemoglobin formation. For an endurance athlete, B6 is involved in nearly every step of the chain from "stored carbohydrate" to "muscle contraction."
The prevalence data is less dramatic than for iron or magnesium, with roughly 8 to 30% of athletes showing suboptimal B6 status depending on diet quality. The mechanism, however, is clear: increased training load increases B6 turnover, particularly for glycogen breakdown via glycogen phosphorylase, an enzyme that requires B6 to function. Athletes with restricted diets (vegan, very low-fat, very low-protein) and those in heavy training blocks are at highest risk.
Symptoms are subtle and easily attributed to other causes: reduced energy metabolism efficiency, slightly slower glycogen utilisation, low-grade fatigue, and sometimes mood effects through the neurotransmitter pathway. B6 deficiency rarely presents dramatically. It usually presents as a quiet drag on performance and recovery.
Food sources: poultry, fish (tuna, salmon), potatoes (one medium baked potato has about 0.5 mg), bananas, chickpeas, fortified cereals. The general RDA is 1.3 to 1.7 mg per day. Athletes in high-volume training likely benefit from 1.5 to 2.5 mg per day. Above roughly 100 mg per day, sustained intake can cause peripheral neuropathy, so this is not a nutrient to mega-dose.
How does training increase your vitamin C requirements?
Endurance athletes deplete vitamin C 30 to 50% faster than sedentary controls. The reason is the oxidative load of training: ROS production during exercise consumes circulating antioxidants, including vitamin C, at elevated rates. Recovery between sessions requires the replenishment of those stores. Run on chronic vitamin C insufficiency and the antioxidant defence system runs short of one of its core inputs.
Vitamin C's role goes beyond direct antioxidant activity. It enhances non-heme iron absorption (which matters for vegetarian athletes and anyone struggling with iron status), supports collagen synthesis in tendons and connective tissue (which matters for any athlete prone to tendon issues), regenerates other antioxidants in the body, and supports immune function during high training loads when athletes are most prone to upper respiratory infections.
The dose question is where the science gets more nuanced. For general health, the RDA is 75 to 90 mg per day. For endurance athletes, intake of 200 to 500 mg per day is commonly recommended and well-tolerated. Above roughly 1,000 mg per day, the evidence becomes mixed: high doses of isolated vitamin C have been shown in controlled trials to blunt some training adaptations, the same finding discussed in the OLEUS guide to plant-based antioxidants. The takeaway: enough to cover the elevated need, not so much that it interferes with adaptation.
Food sources are easy: citrus, peppers, kiwi, berries, broccoli, acerola cherries (one of the most vitamin C-dense fruits in nature). A varied diet usually hits the basic intake. A supplementation strategy aimed at the upper end of the athlete-relevant range is what most serious endurance athletes use.
Why is vitamin D deficiency so common in endurance athletes?
Vitamin D insufficiency affects 30 to 60% of athletes depending on latitude, season, and indoor training time. At latitudes above 35° north (which includes most of Europe), the sun is too low in the sky for skin synthesis of vitamin D for several months of the year. Athletes who train primarily indoors, who use sunscreen consistently outdoors, who have darker skin, or who live in winter conditions are at higher risk regardless of dietary intake.
Vitamin D's role for athletes extends beyond calcium homeostasis and bone health. It supports muscle function (vitamin D receptors are present in skeletal muscle), immune function (low vitamin D is associated with higher rates of upper respiratory infection in athletes), and may contribute to aerobic capacity, though the evidence on direct performance effects is mixed. Research published in Sports Medicine has summarised the evidence linking low vitamin D status to higher injury risk and impaired recovery in athletes.
Symptoms athletes notice: stress fractures or bone-related injuries, more frequent illness during high training loads, sometimes low mood, and reduced muscle strength relative to baseline. The deficiency is silent for many athletes until a bone injury forces a blood test.
Food sources are limited: fatty fish (salmon, mackerel, sardines), egg yolks, fortified dairy and plant milks. The general RDA is 600 to 800 IU per day. Athletes, particularly those at high latitudes or training indoors, often need 1,000 to 4,000 IU per day to maintain optimal serum 25(OH)D status. This is one of the few micronutrients where blood testing (rather than estimating from intake) is genuinely useful.
Athlete daily intake: RDA versus what endurance training actually requires
| Nutrient | General RDA | Endurance athlete target | Best food sources | Common gap driver |
|---|---|---|---|---|
| Iron | 8 mg (M), 18 mg (W) | 17 to 23 mg (M), 23 to 30 mg (W) | Red meat, oysters, lentils, leafy greens | Hepcidin response post-exercise, menstrual losses, footstrike hemolysis |
| Magnesium | 310 to 420 mg | 500 to 600 mg | Leafy greens, nuts, seeds, whole grains, dark chocolate | Sweat losses, low dietary intake in modern diets |
| Vitamin B6 | 1.3 to 1.7 mg | 1.5 to 2.5 mg | Poultry, fish, potatoes, bananas, chickpeas | Increased glycogen-breakdown enzyme demand |
| Vitamin C | 75 to 90 mg | 200 to 500 mg | Citrus, peppers, kiwi, berries, acerola | Oxidative load of training depletes stores |
| Vitamin D | 600 to 800 IU | 1,000 to 4,000 IU | Fatty fish, egg yolks, fortified dairy, sunlight | Latitude, indoor training, winter, sunscreen |
Three patterns hold across the table. The endurance athlete target is meaningfully higher than the general RDA for every nutrient. Food sources alone can hit some of these targets some of the time, but rarely all of them all of the time. The gap is structural, not a sign of poor effort.
Research from Peter Peeling and colleagues at the University of Western Australia has shown that the post-exercise hepcidin response can reduce iron absorption by roughly 30 to 40% in the hours after training, with the effect strongest in the 3 to 6-hour window. The practical implication is that timing iron-rich meals further from hard training, or supplementing iron with vitamin C, can meaningfully improve absorption.
How serious endurance athletes cover their micronutrient gaps
The athletes who train at high volumes for years without falling into deficiency patterns share three habits. The first is a varied whole-food diet anchored on real food: red meat or fish a few times a week, generous leafy greens, nuts and seeds, fruits, legumes, dairy or fortified plant alternatives, and minimal ultra-processed food. The diet is the foundation. Nothing replaces it.
The second is targeted testing. Serial athletes get a blood panel once or twice a year covering at minimum: ferritin (iron stores), 25(OH)D (vitamin D status), and full blood count. Many add B12, magnesium (RBC magnesium is more useful than serum magnesium), and zinc. Testing turns guessing into informed decisions, and it is how athletes catch developing deficiencies before they become performance limiters.
The third is a daily foundation supplement that covers the structural gaps the diet does not. The OLEUS Daily Shot was built around exactly the deficiencies named in this article. Each shot delivers 2.1 mg of iron, 56.25 mg of magnesium, 1.4 mg of vitamin B6 (100% nutrient reference value), 15 mg of vitamin C from acerola extract, and 5 mcg (200 IU) of vitamin D3 (100% nutrient reference value), alongside the full B-vitamin complex at 100% nutrient reference value each, plus 50 mg of oleuropein for mitochondrial support and 100 mg of Siberian ginseng as an adaptogen. The doses are designed as a daily foundation alongside food, not as a megadose replacement for testing or diagnosed deficiency.
The Daily Shot was never designed to fix a diagnosed deficiency. It was designed to be the baseline most endurance athletes do not have: the cellular cofactors and trace nutrients that elevated training volume systematically depletes. Food first. Testing where it matters. Daily Shot for what neither covers.
OLEUS Performance Lab
For athletes with confirmed deficiencies, particularly iron deficiency anaemia or vitamin D deficiency at low serum levels, targeted prescription-strength supplementation under medical supervision is the right next step. The Daily Shot is a daily floor, not a treatment.
What does the OLEUS community look like?
More than 5,000 endurance athletes across Belgium, the Netherlands, Switzerland, and beyond use the OLEUS system as part of their daily nutrition. The use case that comes up most consistently is the one above: a varied whole-food diet plus the Daily Shot each morning, with the Pre-Activity Shot reserved for hard sessions and race days. This is the framework the system was built for.
Frequently asked questions
Should I get blood tests to check my micronutrient status?
Yes, at least once a year, more often if you are in a heavy training block or if you have noticed unexplained fatigue. The minimum useful panel: ferritin, full blood count, 25(OH)D. Add B12 if you are vegetarian or vegan, and add RBC magnesium if you have ongoing cramp or sleep issues. Standard serum magnesium often misses cellular-level deficiency. Iron tests should be taken in the morning, fasted, and at least 48 hours after hard training to avoid post-exercise inflammation skewing the result.
Can I get all my micronutrients from food alone?
For some nutrients, often yes. Vitamin B6, vitamin C, and most of the B-complex can be hit through a varied diet. For iron (particularly in female athletes), magnesium (particularly in heavy-sweating athletes), and vitamin D (particularly at high latitudes or in winter), the structural gap is harder to close with food alone. A varied whole-food diet remains the foundation. Supplementation fills the gaps the diet leaves.
Are multivitamins enough for endurance athletes?
A standard multivitamin at 100% nutrient reference value is fine alongside a varied diet for general nutrient coverage. For endurance-specific gaps, generic multivitamins often miss the doses that matter (vitamin D and magnesium are commonly underdosed, iron is often included at low doses or omitted for safety reasons). A formula designed around the specific gaps endurance training creates, such as the Daily Shot, fits the use case more closely.
What is the difference between RDA and athlete-optimal intake?
The recommended daily allowance is set to prevent deficiency in the general population, not to optimise performance in someone training 10 to 20 hours per week. For most micronutrients, the athlete-optimal intake sits 20 to 100% above the general RDA. The comparison table above shows the gap for the five nutrients most relevant to endurance training.
Why does iron deficiency hit female endurance athletes harder?
Two reasons compound. Menstruation introduces a monthly iron loss that male athletes do not face. Training-induced iron losses (hepcidin response blocking absorption, footstrike hemolysis, GI microbleeds) apply equally to female and male athletes. The combination pushes female endurance athletes into iron deficiency at rates roughly twice that of male athletes. The fix is the same combination as for male athletes (diet, timing, supplementation, testing), simply with a higher baseline need.
The bottom line
Five micronutrients are structurally underdosed in most endurance athletes' diets: iron, magnesium, vitamin B6, vitamin C, and vitamin D. The gap is not a sign of poor effort. It is the predictable result of training that pushes intake requirements above what general dietary guidelines were built for. The fix is a varied diet, targeted testing once or twice a year, and a daily foundation supplement that covers the cofactors training depletes. The OLEUS Daily Shot was built for that role.
Sources
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Sim, M., Garvican-Lewis, L.A., Cox, G.R., Govus, A., McKay, A.K.A., Stellingwerff, T., Peeling, P. (2019). Iron considerations for the athlete: a narrative review. European Journal of Applied Physiology, 119(7), 1463-1478.
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Peeling, P., Dawson, B., Goodman, C., Landers, G., Wiegerinck, E.T., Swinkels, D.W., Trinder, D. (2009). Effects of exercise on hepcidin response and iron metabolism during recovery. International Journal of Sport Nutrition and Exercise Metabolism, 19(6), 583-597.
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Nielsen, F.H., Lukaski, H.C. (2006). Update on the relationship between magnesium and exercise. Magnesium Research, 19(3), 180-189.
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Volpe, S.L. (2015). Magnesium and the athlete. Current Sports Medicine Reports, 14(4), 279-283.
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Peake, J.M. (2003). Vitamin C: effects of exercise and requirements with training. International Journal of Sport Nutrition and Exercise Metabolism, 13(2), 125-151.
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Owens, D.J., Allison, R., Close, G.L. (2018). Vitamin D and the athlete: current perspectives and new challenges. Sports Medicine, 48(Suppl 1), 3-16.
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Larson-Meyer, D.E., Willis, K.S. (2010). Vitamin D and athletes. Current Sports Medicine Reports, 9(4), 220-226.
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NIH Office of Dietary Supplements. Fact Sheets for Health Professionals: Iron, Magnesium, Vitamin B6, Vitamin C, Vitamin D. ods.od.nih.gov
