You have the intuition of a "lack" but don't know where to start? This guide synthesizes the signs to identify, the right tests (and their limits), the most common deficiencies, and above all simple steps to act: diet, reasoned supplementation, precautions and follow-up. The idea is not to pile up products: we prioritize, we measure, we reassess.
Symptoms of nutritional deficiencies, grouped by systems and organs
- Neurological / cognition: persistent fatigue, concentration problems, paresthesias (B12, magnesium).
- Hematological: pallor, shortness of breath, exertional tachycardia (iron), smooth tongue/mouth pain (B12/folates).
- Dermatological / appendages: dry skin, hair loss, brittle nails (zinc, iron, D).
- Muscular: night cramps, weakness, diffuse pain (magnesium, D).
- Immune: repeated infections, slow healing (zinc, D).
- Thyroid / metabolism: feeling cold, weight gain, fatigue (insufficient iodine).
These signs are non-specific; a targeted assessment guides diagnosis.
Most common nutritional deficiencies and most exposed populations
Common deficiencies — typical symptoms, reference test, indicative threshold and trial duration
| # | Deficiency | Typical symptoms | Main test | Reference threshold | Trial duration |
|---|---|---|---|---|---|
| 1 | Iron | Fatigue, pallor, shortness of breath | Ferritin | Low ferritin | 8–12 weeks |
| 2 | Vitamin D | Diffuse pain, weakness | 25‑OH vitamin D | Low value | 8–12 weeks |
| 3 | Vitamin B12 | Paresthesias, smooth tongue | Serum B12 ± holoTC | Low B12 | 8–12 weeks |
| 4 | Zinc | Skin/nails/hair, immunity | Serum zinc | Low zinc | 4–8 weeks |
| 5 | Magnesium | Cramps, nervousness | Serum magnesium ± 24h urinary magnesium | Low indicators | 4–8 weeks |
| 6 | Iodine | Hypothyroid signs | Urinary iodine | Low ioduria | 8–12 weeks |
Higher risk populations: vegetarians/vegans (B12, iron, D, omega‑3), women of childbearing age (iron), seniors (D, B12), endurance athletes (iron, D, magnesium), dietary restriction situations.
Useful tests and reference thresholds to guide deficiency assessment
- Ferritin: central marker of iron status; interpretation with inflammatory context (CRP). See iron.
- 25‑OH vitamin D: reference test for vitamin D; target according to age and context. See vitamin D.
- Serum zinc: useful but sensitive to inflammation and fasting. See zinc.
- Urinary iodine: indicator of recent iodine intake at individual/population level. See urinary iodine.
- Serum magnesium: not very sensitive; 24h urinary magnesium can refine assessment. See magnesium.
Interpreting a test
Cross the clinical context (symptoms, diet) with the biomarker. Recheck at 8–12 weeks after correction when relevant.
Causes and risk factors of main nutritional deficiencies
- Insufficient intake: restrictive diets, ultra-processed, seasonality (sun exposure for D).
- Malabsorption: digestive diseases, bariatric surgery, medications (PPIs, metformin — depending on context).
- Increased losses: heavy periods, endurance sports, digestive bleeding.
- Increased needs: pregnancy/breastfeeding, growth, convalescence.
When to consult and how to prioritize specialized medical advice
- If marked or persistent symptoms, or at-risk profile (pregnancy, pathology).
- If abnormal tests or difficult to interpret (e.g. elevated CRP with ferritin).
- In case of failure of well-conducted dietary correction.
Supplements and doses: realistic benchmarks, trial duration, digestive tolerance
- Iron: only if confirmed deficiency; adapt form and tolerance; review ferritin. See iron and our iron deficiency symptoms.
- Vitamin D: adjust to age and status (25‑OH); respect limits. See vitamin D and our vitamin D guide.
- B12: choose adapted form; doses according to deficit and cause; recheck. See our B12 deficiency symptoms.
- Zinc: avoid prolonged high doses; monitor copper/iron; short cycles. See zinc and our zinc guide.
- Magnesium: prefer tolerated forms (bisglycinate/citrate); adjust to laxative effect; 4–8 weeks. See magnesium and which magnesium form to choose.
- Omega‑3 (EPA/DHA): aim for about 250 mg/day overall; take with meal. See omega‑3 and our omega-3 guide.
Precautions, interactions and daily safety of supplement use
- Anticoagulants/antiplatelets: caution with omega‑3; beyond 3 g/day marine, bleeding risk increases (see safety threshold).
- Minerals: zinc/iron/copper can interact; stagger intakes and avoid excess.
- Pregnancy/breastfeeding: adapt to situation; professional advice recommended.
- Quality: favor products controlled against contaminants and clearly dosed.
Safety
Check your treatments and your total doses (especially fat-soluble vitamins). If in doubt, seek advice.
