Micronutrients for performance: iron, vitamin D and magnesium

Summary: This article delves into three key micronutrients—iron, vitamin D, and magnesium—that influence energy, recovery, and physical and cognitive performance. You'll learn how to identify deficiencies, which tests to order, how to correct them with diet and supplements, important interactions, and practical recommendations for athletes, active individuals, and the general population.

Index

• Why these micronutrients matter
• Quick summary: functions and warning signals
• Iron: types, functions, signs of deficiency and correction
• Vitamin D: synthesis, role, testing and supplementation
• Magnesium: why it's essential and how to optimize it
• How to identify deficiencies: tests and diagnosis
• Plan to correct deficiencies: diet, supplementation, and monitoring
• Special populations: athletes, women, vegans, the elderly
• Interactions, absorption, and practices to optimize bioavailability
• Practical examples: menus and supplementation protocols
• Risks, safety, and when to see a doctor
• Frequently Asked Questions
• References and recommended readings

Why these micronutrients matter

Micronutrients do not provide calories, but they are essential for the body to convert food into energy, repair tissues, maintain neuromuscular function, and regulate the immune system. Among them, iron, vitamin D, and magnesium stand out for their direct impact on:

• Transport and use of oxygen (iron).
• Muscle and bone function, as well as hormonal regulation (vitamin D).
• Muscle contraction and relaxation, electrolyte balance, energy metabolism and protein synthesis (magnesium).

In athletes and active individuals, even a slight deficiency can lead to fatigue, slower recovery, cramps, reduced performance, and an increased risk of injury. In the general population, chronic deficiencies negatively impact quality of life and long-term health.

Quick summary: functions and warning signals

Micronutrient	Key functions	Signs of deficiency
Iron	Oxygen transport (hemoglobin), storage (ferritin), energy production	Fatigue, exercise intolerance, paleness, tachycardia, reduced performance
Vitamin D	Calcium homeostasis, bone health, immune and muscle function	Muscle pain, proximal weakness, increased risk of fractures, low energy
Magnesium	Enzymatic reactions, muscle and nerve function, energy metabolism	Cramps, spasms, fatigue, arrhythmias in severe cases

Iron: types, functions, signs of deficiency and correction

What is iron and why is it critical?

Iron is an essential mineral for the synthesis of hemoglobin (which transports oxygen in the blood), myoglobin (which stores oxygen in muscle), and numerous enzymes involved in energy metabolism. Adequate iron intake contributes to aerobic performance, endurance, and muscle recovery.

Forms of iron in the diet

• Heme iron: present in meats, fish and poultry; it is better absorbed.
• Non-heme iron: present in plants (legumes, cereals, vegetables) and supplements; its absorption is more dependent on the composition of the food.

Requirements and recommendations

Iron requirements vary according to age, sex, and physiological state (pregnancy, lactation). Guidelines may differ by country; a summary reference is that of the National Institutes of Health (NIH):

• Adult men: ~8 mg/day
• Women of childbearing age: 18 mg/day (greater losses due to menstruation)
• Pregnancy: 27 mg/day

Source: NIH Office of Dietary Supplements. Iron — SDG

Signs and symptoms of iron deficiency

Iron deficiency typically progresses in stages: loss of iron stores (low ferritin), decreased transport (low transferrin or saturation), and, in advanced stages, iron deficiency anemia (low hemoglobin). Symptoms may include:

• Persistent fatigue and lack of stamina
• Difficulty maintaining intensity during training
• Paleness, dizziness, headache
• Brittle nails, smooth tongue, or glossitis
• Feeling of cold, tachycardia

In athletes, increased losses (menstrual bleeding, intestinal microhemorrhages in runners, sweat) and a diet low in bioavailable iron increase the risk. Review on iron and exercise: Peeling et al., 2009 .

Laboratory tests to evaluate iron

• Serum ferritin: reflects reserves; low values ​​indicate deficiency. Important: it is an acute-phase reactant and can be elevated with inflammation; interpret in context.
• Hemoglobin (Hb): identifies anemia, but may be normal in early stages of deficiency.
• Hematocrit (Hct): complementary to Hb.
• Serum iron, total iron-binding capacity (TIBC), and % transferrin saturation: assess available transport.
• Reticulocytes: provide information about the bone response to supplementation.

Typical interpretation (guideline values): Ferritin <30 ng/mL suggests iron deficiency in adults; some experts use <50 ng/mL in symptomatic athletes. Consult your local laboratory for reference ranges.

Dietary strategies to increase iron

Optimizes intake and absorption:

• Prefer sources of heme iron: lean meats, liver (in moderation), fish and poultry.
• It includes sources of non-heme iron: legumes (lentils, chickpeas), seeds (sesame, pumpkin), tofu and fortified cereals.
• Combine non-heme iron with vitamin C (citrus fruits, peppers, strawberries) to improve absorption.
• Avoid drinking strong coffee or tea with iron-rich foods (tannins reduce absorption).
• Be careful with foods rich in calcium at the same time: high doses of calcium can compete with iron.

Iron supplementation: types and guidelines

There are multiple iron salts in supplements: ferrous sulfate, ferrous fumarate, ferrous gluconate, ferrous bisglycinate, among others. Key differences:

• Ferrous sulfate: high elemental concentration, traditionally used; higher incidence of GI effects.
• Fumarate and gluconate: alternatives with different elemental amounts.
• Iron bisglycinate (chelated): is often associated with better gastrointestinal tolerance and good absorption in some people.

Practical tips:

• Take iron with a source of vitamin C to improve absorption.
• If gastrointestinal side effects are a problem, try chelated forms or smaller divided doses.
• Avoid taking iron with calcium or antacids.
• Supplementation should be individualized and monitored with tests (ferritin, Hb).

Duration of treatment and monitoring

In confirmed iron deficiency, treatment usually continues until ferritin levels normalize and are maintained (often 3–6 months or longer). More severe iron deficiency anemia may require more intensive regimens and more frequent monitoring. Typical re-evaluation includes ferritin and complete blood count after 4–8 weeks to assess response.

Avoid overdose

Excessive iron can be toxic. Do not take high doses of iron supplements without medical supervision. In countries with fortification programs, monitor total iron intake to avoid accumulation.

Vitamin D: synthesis, role, testing and supplementation

Main functions of vitamin D

Vitamin D is a prohormone with effects on the regulation of calcium and phosphorus, bone health, muscle function, and immunity. Vitamin D receptors are found in numerous tissues, which explains its broad impact.

Summary and sources

• Cutaneous synthesis: The skin produces cholecalciferol (vitamin D3) after exposure to UVB radiation. Factors such as latitude, season, time of day, skin pigmentation, sunscreen use, and clothing affect synthesis.
• Food sources: fatty fish (salmon, mackerel), liver, egg yolks and fortified foods (milk, cereals).
• Supplements: vitamin D3 (cholecalciferol) or vitamin D2 (ergocalciferol). D3 is the preferred form for most people because of its effectiveness in raising serum 25(OH)D levels.

Diagnostic test

The most commonly used measure is 25-hydroxyvitamin D (25(OH)D). Approximate interpretation:

• <20 ng/mL (<50 nmol/L): deficiency
• 20–30 ng/mL (50–75 nmol/L): insufficiency
• >30 ng/mL (>75 nmol/L): sufficient for most

References: NIH Office of Dietary Supplements. Vitamin D — SDG

Impact on performance and muscle function

Studies suggest that adequate vitamin D levels are associated with improved muscle function, a reduced risk of falls in older adults, and a possible influence on muscle recovery. However, the evidence regarding performance improvements in athletes without deficiency is mixed. Review: Owens et al., 2015 .

Supplementation recommendations

Doses vary according to baseline and target levels. General guidelines:

• People with confirmed deficiency (25(OH)D <20 ng/mL): repletion protocols may use 1,000–4,000 IU/day or higher intermittent doses as medically indicated.
• Maintenance: 800–2,000 IU/day is common, depending on sun exposure and diet.
• Caution: Doses above 4,000 IU/day should be monitored to avoid long-term toxicity (hypercalcemia).

If you're looking for a supplement option, Foodelis Vitamin D3 is available in different strengths. Consult your doctor to personalize your dosage.

Monitoring and response time

After starting supplementation, wait 8–12 weeks to reassess 25(OH)D levels. Adjust the dose according to the results and individual needs. Document any symptoms and adverse effects (nausea, polyuria, dizziness), although these are rare at recommended doses.

Magnesium: why it's essential and how to optimize it

Features and why they matter for performance

Magnesium is involved in more than 300 enzymatic reactions, including ATP production, protein synthesis, muscle contraction, heart rhythm regulation, and electrolyte balance. A deficiency affects muscle contraction and relaxation, which can contribute to cramps, spasms, and fatigue.

Review on magnesium and physical activity: Cuciureanu and Vink, 2011 (among others).

Food sources of magnesium

• Nuts and seeds (almonds, cashews, pumpkin seeds)
• Legumes (beans, lentils)
• Leafy green vegetables (spinach)
• Whole grains
• Fish and some dairy products

Requirements and signs of deficiency

Recommended daily doses vary: in adults, around 300–420 mg/day depending on sex and age. Deficiency may manifest as:

• Muscle cramps, spasms, or fasciculations
• Fatigue, weakness
• Arrhythmias or palpitations in severe cases
• Sensitivity to stress and sleep disorders

Important: Serum magnesium represents only about 1% of total body magnesium, so the test may not reflect tissue deficiency. Some clinics use red blood cell magnesium or functional tests, although these are not always standardized.

Magnesium supplements: forms and recommendations

Common forms:

• Magnesium oxide: high elemental content but lower bioavailability and may cause a laxative effect.
• Magnesium citrate: good absorption, sometimes used for laxative effect.
• Glycinate or bisglycinate: chelated forms with good tolerance and less laxative effect.
• Malate or threonate: options for certain objectives (threonate is of interest for cognitive effects in research).

Practical dosage: 200–400 mg/day is the usual range for adults, adjusted according to dietary intake, symptoms, and tolerance. If a laxative effect is needed, citrate or oxide may be used; if better tolerance is desired, glycinate is usually preferred.

If you are considering supplementation, check out Foodelis Magnesium Complex for formats and concentrations.

Interactions and precautions

• Some antibiotics (quinolones, tetracyclines) may interact with magnesium; leave several hours between doses.
• Severe renal insufficiency increases the risk of magnesium accumulation and toxicity; in these cases, supplementation should be medically supervised.

How to identify deficiencies: tests and diagnosis

When to suspect deficiency

Consider evaluating whether you have:

• Persistent fatigue with no clear explanation.
• Loss of athletic performance or stagnation.
• Frequent muscle cramps or weakness.
• Symptoms of anemia (dizziness, paleness) or risk factors (heavy menstruation, strict vegetarian diet).
• Limited sun exposure or risk factors for low vitamin D (dark skin, living at high latitudes, strict use of sunscreen).

Recommended tests per nutrient

• Iron: serum ferritin, complete blood count, serum iron, TIBC, and % saturation. In complex cases, blood loss assessment (gynecological, gastrointestinal).
• Vitamin D: 25(OH)D.
• Magnesium: serum magnesium; in cases with high clinical suspicion, consider erythrocyte magnesium or functional assessment.

Always interpret results together with your doctor, especially if there is inflammation, infection, chronic diseases or pregnancy, which can modify parameters.

Complementary clinical evaluation

Don't underestimate a good medical history and physical exam: dietary patterns, medication use, digestive symptoms that affect absorption (celiac disease, inflammatory bowel disease), chronic losses, and high-volume physical activities.

Plan to correct deficiencies: diet, supplementation, and monitoring

Step 1 — Confirm with evidence

Don't assume a deficiency based on symptoms alone. Perform basic tests to confirm the diagnosis before starting prolonged treatment.

Step 2 — Simultaneous dietary intervention

Optimize your diet to provide natural sources and improve absorption:

• Iron: combine with vitamin C, prefer heme iron if it is compatible with your diet, prepare legumes with processes that reduce phytates (soaking, sprouting).
• Vitamin D: Promote safe sun exposure and rich or fortified foods; for those not exposed, supplementation is the main route.
• Magnesium: include nuts, seeds, green vegetables, and whole grains.

Step 3 — Targeted and safe supplementation

If a deficiency is confirmed, use an appropriate supplement. General recommendations:

• Iron: Choose the appropriate form (bisglycinate if there is GI intolerance), take with vitamin C, and avoid with calcium/tea/coffee. Monitor ferritin levels.
• Vitamin D: repletion according to baseline level; adjustment with 25(OH)D monitoring every 8–12 weeks if therapeutic doses are used.
• Magnesium: choose form according to tolerance and objective; adjust dose gradually to minimize GI effects.

Step 4 — Re-evaluation and maintenance

Monitor clinical and analytical response. Once parameters normalize, consider maintaining a combined strategy of diet and maintenance supplement dosage if necessary. Re-evaluate periodically according to context.

Special populations

Athletes and people with high training volume

• Increased risk of iron deficiency (especially in women) due to increased losses and demands. Monitor ferritin levels regularly if symptoms are present or training volume is high.
• Vitamin D can affect muscle function; monitor levels during intense training and in seasons with low sun exposure.
• Magnesium levels may decrease due to sweating and increased requirements; pay attention to replenishment during prolonged training.

Women of childbearing age

Due to menstrual blood loss, women of childbearing age are at increased risk of iron deficiency. Supplementation should be individualized; regular screening may be a good practice for those with heavy periods or intense training.

Vegetarians and vegans

• Higher risk of low heme iron and possible lower intake of vitamin D and magnesium if the diet is not planned.
• Strategies: optimize plant-based sources rich in iron, use cooking techniques that increase bioavailability (soaking, sprouting), combine with vitamin C, and consider supplements if evidence indicates it.

Older people

Increased risk of vitamin D deficiency due to decreased cutaneous synthesis and magnesium deficiency due to decreased intake/absorption; in addition, polypharmacy may interact with nutrient absorption.

Interactions, absorption, and practices to optimize bioavailability

Factors that decrease iron absorption

• Tea and coffee (tannins)
• Calcium and some antacids
• Phytates in cereals and legumes (mitigate with soaking/germination/fermentation)

Factors that improve absorption

• Vitamin C (ascorbic acid): combine citrus fruits, bell pepper, kiwi or supplement together.
• Animal proteins (meat factor effect) help increase the absorption of non-heme iron.

Drug interactions

• Iron may reduce the absorption of certain antibiotics and levothyroxine; space out doses.
• Magnesium interferes with some antibiotics and may potentiate the effects of neuromuscular blocking agents.
• Vitamin D increases calcium absorption; in patients with sarcoidosis or other granulomatous diseases, the conversion may cause hypercalcemia.

Practical examples: menus and supplementation protocols

Sample daily menu to provide iron, vitamin D and magnesium

Designed for an active omnivorous person. Adjusts for calorie requirements, preferences, and restrictions.

• Breakfast: Wholegrain oats with vitamin D fortified milk, a tablespoon of pumpkin seeds (magnesium) and strawberry slices (vitamin C to absorb iron for the rest of the day).
• Mid-morning: Natural yogurt with almonds and a banana.
• Lunch: Spinach salad (magnesium) with lentils (non-heme iron), raw red pepper (vitamin C) and grilled salmon fillet (vitamin D, heme iron).
• Snack: Hummus with carrot sticks and an orange.
• Dinner: Chicken breast or marinated tofu with quinoa (magnesium), steamed broccoli and sweet potato.
• Before bed (if needed): magnesium glycinate 200 mg supplement if cramps or poor recovery occur.

Example protocol for mild-to-moderate iron deficiency (guideline)

1. Confirm low ferritin and evaluate Hb.
2. Start oral iron: for example, iron bisglycinate with 30–60 mg of elemental iron daily, depending on tolerance and degree of deficiency. Take with orange or 100–200 mg of vitamin C.
3. Avoid taking with dairy products or coffee; space out these foods by 2 hours.
4. Re-evaluate ferritin and blood count in 6–8 weeks; continue treatment until normal reserves are maintained (3–6 months in total in many cases).

Important: This is a general example. The exact dosage should be determined by a healthcare professional.

Example protocol for vitamin D deficiency

1. Confirm 25(OH)D (e.g., 15 ng/mL).
2. Usual repletion: 1,000–4,000 IU/day of vitamin D3, or alternative high-dose regimen under supervision (e.g., 50,000 IU weekly for 6–8 weeks in indicated cases).
3. Re-evaluate 25(OH)D after 8–12 weeks and adjust to maintenance dose (800–2,000 IU/day or as needed).

Example protocol for magnesium

1. Ensure adequate dietary intake.
2. If symptoms are present and serum magnesium is low or clinically suspected, consider 200–400 mg/day of magnesium in a well-tolerated form (glycinate/bisglycinate) for 6–12 weeks.
3. Monitor symptoms and kidney function. Adjust according to tolerance and response.

Risks, safety, and when to see a doctor

Warning signs

• Significant bleeding, marked paleness, and syncope (seek urgent care).
• Symptoms of hypercalcemia during vitamin D supplementation (nausea, thirst, polyuria, confusion): seek medical attention.
• Severe abdominal pain, persistent vomiting, or black stools at the start of iron supplementation (may indicate intolerance or a complication).

Safety and medical supervision

Always consult a doctor before starting high doses of supplements, especially if you are taking chronic medications, have kidney disease, sarcoidosis, autoimmune diseases, or conditions that affect absorption.

Frequently Asked Questions

Q: Can I take iron and calcium together?

A: It's not ideal. Calcium competes with iron for absorption. It's best to space the intake by at least 2 hours.

Q: How much sun exposure do I need to produce vitamin D?

A: It depends on latitude, season, skin pigmentation, and clothing/sunscreen. In general, 10–30 minutes of direct sun exposure on the arms and legs several times a week may be sufficient for many people, but this varies widely. If in doubt, get a 25(OH)D test.

Q: Are natural supplements better than synthetic ones?

A: "Natural" doesn't always mean better absorption or safety. What's important is the chemical form, the dosage, the product quality, and its suitability for your needs. Look for brands with quality certification and third-party testing.

References and recommended readings

• NIH Office of Dietary Supplements—Iron. https://ods.od.nih.gov/factsheets/Iron-Consumer/
• NIH Office of Dietary Supplements — Vitamin D. https://ods.od.nih.gov/factsheets/VitaminD-Consumer/
• NIH Office of Dietary Supplements—Magnesium. https://ods.od.nih.gov/factsheets/Magnesium-Consumer/
• Peeling P, et al. Iron and exercise: recent developments. Sports Med. 2009. https://pubmed.ncbi.nlm.nih.gov/19513679/
• Owens DJ, et al. Vitamin D and skeletal muscle function. Nutrients. 2015. https://pubmed.ncbi.nlm.nih.gov/25841207/
• Cuciureanu MD, Vink R. Magnesium and stress. Nutrients. 2011. https://pubmed.ncbi.nlm.nih.gov/28837989/
• World Health Organization — Anaemia. https://www.who.int/health-topics/anaemia

Note: The recommendations included are for informational purposes only and do not replace medical advice. Before starting any supplements, especially high doses or if you have a medical condition, consult a healthcare professional.