Vitamin & Deficiency
Checker
Select your symptoms and discover which vitamins you may be missing — with personalised food sources, daily requirements, and supplement recommendations.
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Why Vitamin Deficiencies Are More Common Than You Think
Vitamin deficiencies are not a developing-world problem — they are widespread in modern Western populations consuming abundant calories but nutritionally poor diets. Processed food, indoor lifestyles, soil depletion, and absorption issues combine to make deficiency a norm rather than an exception.
Fat-Soluble vs Water-Soluble Vitamins
Understanding the fundamental difference between fat-soluble and water-soluble vitamins determines how you supplement, how quickly deficiency develops, and the risk of toxicity from excessive intake.
| Type | Vitamins | Storage | Deficiency Timeline | Toxicity Risk | Key Consideration |
|---|---|---|---|---|---|
| Fat-Soluble | A, D, E, K | Liver and fatty tissues — stored for weeks to months | Slower — reserves provide buffer over weeks to months | Higher — toxicity possible | Must be consumed with dietary fat for absorption; upper limits matter |
| Water-Soluble | C, B1, B2, B3, B6, B9, B12 | Minimal storage — excess excreted in urine daily | Faster — depletion within days to weeks of poor intake | Lower — excess excreted | Daily intake essential; high doses generally safe but not always |
Vitamin D — The Most Widespread Modern Deficiency
Vitamin D is technically a hormone, not a vitamin — the body synthesises it from cholesterol when skin is exposed to UVB radiation. This distinction explains why dietary sources alone rarely achieve optimal status, and why deficiency is so widespread in indoor, northern-latitude populations.
☀️ The Sunlight Problem
In the UK and northern US, UVB radiation sufficient for vitamin D synthesis only exists from approximately April to September. Between October and March, skin synthesis is essentially zero regardless of outdoor time. This means 6 months of every year, dietary or supplemental vitamin D is the only source — making 2,000–4,000 IU daily supplementation appropriate for most adults during winter months.
📊 The Testing Gap
Standard blood tests measure 25(OH)D — the storage form. The “normal” range (30–100 ng/mL) is too broad to be clinically useful. Optimal levels for bone health, immune function, and cancer prevention are consistently associated with 40–60 ng/mL in research — levels rarely achieved without supplementation in indoor populations.
✅ What Vitamin D Does
Vitamin D regulates over 200 genes — including those controlling calcium absorption for bone density, immune cell differentiation (T-cells, macrophages), insulin secretion, cardiovascular function, and mood regulation via serotonin pathway modulation. Its influence extends far beyond “bone health” — the traditional narrow framing of vitamin D’s role.
✅ D3 + K2 Together
Vitamin D3 increases calcium absorption, which needs to be directed into bones rather than arterial walls. Vitamin K2 (MK-7 form) activates matrix Gla-protein and osteocalcin — directing calcium to bones and away from arteries. D3 and K2 are physiological partners. High-dose D3 supplementation without K2 may theoretically increase arterial calcification risk in some individuals.
The B Vitamins — Eight Nutrients, One System
The eight B vitamins function as co-enzymes in energy metabolism, neurotransmitter synthesis, DNA production, and red blood cell formation. They are interdependent — deficiency in one often impairs the function of others. Understanding which are most commonly deficient and in whom guides targeted supplementation.
| B Vitamin | Primary Function | Deficiency Risk Group | Key Symptom | Priority Level |
|---|---|---|---|---|
| B1 (Thiamine) | Glucose energy metabolism | Alcohol users, elderly, those with poor diet | Fatigue, nerve damage, confusion | Moderate |
| B2 (Riboflavin) | Energy production, FAD cofactor | Vegans, elderly, athletes | Cracked lips, bloodshot eyes, skin inflammation | Moderate |
| B3 (Niacin) | NAD/NADP for 400+ reactions | Alcohol users, corn-staple diets | Pellagra: skin lesions, diarrhoea, dementia | Moderate |
| B6 (Pyridoxine) | Amino acid metabolism, neurotransmitters | Elderly, oral contraceptive users | Depression, confusion, skin rashes, anaemia | Moderate |
| B9 (Folate) | DNA synthesis, cell division | Women of childbearing age, elderly | Megaloblastic anaemia, birth defects (neural tube) | High |
| B12 (Cobalamin) | Nerve function, red blood cells | Vegans, vegetarians, over-50s, metformin users | Pernicious anaemia, irreversible nerve damage, dementia risk | Very High |
Symptom-to-Vitamin Reference Map
Many deficiency symptoms overlap across multiple vitamins — making pattern recognition more diagnostic than any single symptom. This map shows which vitamin combinations to consider for each common symptom cluster.
| Symptom | Primary Suspect | Secondary Suspects | Confirm With |
|---|---|---|---|
| Persistent fatigue | B12, Iron, Vitamin D | B1, B9, Vitamin C | Ferritin, B12, 25(OH)D blood test |
| Hair loss / thinning | Biotin (B7), Iron, Zinc | Vitamin D, B2, B3 | Ferritin, thyroid panel, Vitamin D |
| Night blindness | Vitamin A | Zinc (needed to mobilise Vitamin A) | Retinol blood test |
| Cracked corners of mouth | B2 (Riboflavin), B6 | Iron, Zinc | B vitamin complex test |
| Numbness / tingling | B12 | B6 (paradoxically — both deficiency AND toxicity), B1 | B12 blood test (methylmalonic acid for accuracy) |
| Bone / joint pain | Vitamin D, Calcium | Vitamin K2, Magnesium | 25(OH)D, DEXA scan if persistent |
| Depression / low mood | Vitamin D, B9 (Folate) | B12, Omega-3, Iron | 25(OH)D, B12, Ferritin, TSH (thyroid) |
| Bleeding gums / bruising | Vitamin C | Vitamin K | Vitamin C blood test; dietary assessment |
| Muscle cramps | Magnesium, Potassium | Calcium, B1, Vitamin D | RBC magnesium (not serum — serum is misleading) |
Testing Your Vitamin Levels — What to Request
Symptom-based assessment is a useful screening tool — but blood testing provides the objective confirmation needed for accurate supplementation. Knowing which tests to request (and their limitations) prevents both under-treatment and unnecessary supplementation.
☀️ Vitamin D — 25(OH)D
The gold standard test. Request “25-hydroxyvitamin D” specifically — some labs only test the active form (1,25(OH)2D), which doesn’t reflect stores. Optimal range: 40–60 ng/mL (100–150 nmol/L). Below 20 ng/mL is deficient; below 30 ng/mL is insufficient. Most GPs will test this if requested.
🔴 B12 — Methylmalonic Acid
Standard serum B12 has poor sensitivity — borderline results (150–300 pg/mL) may miss functional deficiency. Request methylmalonic acid (MMA) or homocysteine alongside B12 — these are more sensitive markers of cellular B12 deficiency. Elevated MMA confirms deficiency even when serum B12 looks “normal.”
⚙️ Iron — Ferritin Not Just Hb
Haemoglobin tests miss early iron deficiency — ferritin falls long before haemoglobin does. Request a full iron panel: ferritin, serum iron, TIBC, and transferrin saturation. Ferritin below 30 ng/mL produces fatigue and hair loss even with normal haemoglobin. Optimal ferritin for symptoms: 50–100 ng/mL.
🥜 Magnesium — RBC Not Serum
Serum magnesium is an unreliable marker of magnesium status — the body maintains serum levels at the expense of cellular stores. Request red blood cell (RBC) magnesium for a more accurate reflection of true magnesium status. Deficiency is extremely common (50%+ of adults) but frequently missed by standard serum testing.
🌿 Folate — Red Cell Folate
Red cell folate is a more accurate reflection of tissue folate status than serum folate, which fluctuates rapidly with recent dietary intake. Particularly important in women planning pregnancy and those on methotrexate. MTHFR gene testing may be indicated if folate supplementation doesn’t normalise levels.
🐟 Omega-3 Index
Not a vitamin, but closely related to deficiency syndromes. The Omega-3 Index measures EPA+DHA as a percentage of red blood cell fatty acids. Optimal: above 8%. Most adults measure 4–5% — associated with increased cardiovascular and cognitive risk. This test is available from private labs and reveals a common correctable deficiency.
Food First — The Most Nutrient-Dense Sources
Whole food sources of vitamins are generally superior to supplements — they provide vitamins alongside cofactors, enzymes, and synergistic nutrients that improve absorption and utilisation. These are the highest-density food sources for each vitamin group.
| Vitamin | Top Food Sources | Bioavailability | Practical Serving |
|---|---|---|---|
| Vitamin A | Beef liver, sweet potato, carrots, kale, spinach | Retinol (animal) — high; Beta-carotene (plant) — variable (genetic) | 100g beef liver = 5× weekly RDA |
| Vitamin D | Fatty fish (salmon, sardines), egg yolks, fortified milk, mushrooms (UV-exposed) | D3 from animal sources > D2 from plant sources | 100g salmon ≈ 600–1,000 IU — still below optimal dose |
| B12 | Clams, beef liver, salmon, sardines, eggs, dairy | Animal sources very high; plant sources: zero (without fortification) | 85g clams = 1,400% RDA; eggs = 20% per egg |
| Folate (B9) | Chicken liver, edamame, lentils, spinach, asparagus, avocado | Food folate 85% absorbed; folic acid (supplement) ≈ 100% | 200g lentils ≈ 90% RDA |
| Vitamin C | Red bell pepper, kiwi, citrus, strawberries, broccoli, Brussels sprouts | High from whole food; degrades with heat/storage | 1 red pepper = 150% RDA; 1 orange = 80% |
| Vitamin K | Kale, spinach, broccoli, Brussels sprouts, parsley, fermented foods (K2) | K1 (plant): moderate; K2 MK-7 (natto, fermented): very high | 30g kale = 150% K1 RDA; natto = best K2 source |
Choosing the Right Supplement Form
Not all supplement forms are equally absorbed. The same vitamin available in multiple chemical forms can produce dramatically different blood level responses — making form selection important for effective supplementation.
✅ Vitamin D3 vs D2
Vitamin D3 (cholecalciferol — animal-derived) raises and maintains serum 25(OH)D levels approximately 3× more effectively than D2 (ergocalciferol — plant/yeast-derived). D3 is the form produced by sun exposure. For vegans: algae-derived D3 provides the same form without animal products.
✅ Magnesium Glycinate vs Oxide
Magnesium oxide (cheapest, most common) has approximately 4% bioavailability — most passes through unabsorbed. Magnesium glycinate (chelated form) has 80%+ bioavailability, minimal laxative effect, and is the appropriate form for addressing deficiency. Magnesium citrate is intermediate — higher absorption than oxide but can cause loose stools.
✅ Folate vs Folic Acid
10–15% of people have the MTHFR gene variant that impairs folic acid conversion to active methylfolate. For these individuals, methylfolate (5-MTHF) supplements bypass the conversion and are significantly more effective. If standard folic acid supplementation hasn’t resolved deficiency, methylfolate is the appropriate switch.
✅ B12 Forms
Cyanocobalamin (cheapest) must be converted to active forms by the body — this conversion is impaired in some individuals. Methylcobalamin and adenosylcobalamin are the active forms — directly usable without conversion. For neurological symptoms or older adults with suspected absorption issues, sublingual methylcobalamin (bypasses stomach acid) is preferred.
High-Risk Groups for Vitamin Deficiency
Certain populations face significantly elevated deficiency risk due to dietary restrictions, physiological changes, medication use, or malabsorption conditions. Identifying your risk group guides proactive testing and supplementation priorities.
| Risk Group | Primary Deficiency Risk | Reason | Priority Supplements |
|---|---|---|---|
| Vegans / Vegetarians | B12, D3, Iron, Zinc, Omega-3, Calcium | B12 absent from plant foods; D3 largely from animal sources; non-haem iron lower absorption | B12 methylcobalamin daily; algae D3; zinc; algal DHA |
| Adults Over 50 | B12, D3, B9, Magnesium, Calcium | Gastric acid decline (B12 absorption); less outdoor time (D3); reduced nutrient absorption generally | B12 sublingual; D3 2,000–4,000 IU; B-complex; magnesium glycinate |
| Women of Childbearing Age | Iron, Folate, D3, Iodine | Monthly blood loss (iron); folate essential before and during pregnancy; iodine widely deficient | Ferritin test first; 400mcg folate (or methylfolate); D3 |
| Pregnancy | Folate, Iron, D3, Omega-3 DHA, Iodine | Dramatically increased demands for foetal development | 400–600mcg folate; D3 1,000–2,000 IU; DHA 200mg; prenatal iron |
| Metformin Users | B12 | Metformin directly reduces B12 absorption in the ileum — progressive depletion with long-term use | Annual B12 testing; methylcobalamin supplementation if borderline |
| Malabsorption Conditions | All fat-soluble (A, D, E, K); B12; Iron | Coeliac, Crohn’s, IBD, bariatric surgery — all reduce absorption of multiple nutrients | Comprehensive blood panel; gastroenterologist input; often high-dose supplementation needed |
Vitamin Interactions — What Works Together & What Doesn’t
Vitamins and minerals interact in ways that can either enhance or inhibit absorption. Understanding these relationships prevents accidental interference between supplements taken simultaneously.
✅ Beneficial Combinations
Vitamin D3 + K2 (MK-7): D3 increases calcium absorption; K2 directs it to bones. Vitamin C + Iron (non-haem): C dramatically improves non-haem iron absorption from plant foods — consume together at the same meal. Vitamin D + Magnesium: magnesium is required to convert vitamin D to its active form — deficiency in one impairs the other.
⚠ Competitive Inhibitors
Iron + Calcium (taken together): calcium significantly reduces iron absorption when co-administered — take at different meals. Iron + Zinc (high-dose supplements): compete for the same intestinal transporters — do not supplement both simultaneously. Fat-soluble vitamins compete for absorption at very high doses — spread throughout the day if taking multiple fat-soluble supplements.
✅ Food Enhancers
Fat-soluble vitamins (A, D, E, K) require dietary fat for absorption — always take these supplements with a meal containing fat. Turmeric (curcumin) absorption increases 2,000% with black pepper (piperine). Green tea enhances iron absorption from plant sources in some studies, despite traditional belief that tannins inhibit it.
⚠ Drug-Nutrient Interactions
Proton pump inhibitors (omeprazole) reduce B12 and magnesium absorption. Cholestyramine reduces absorption of all fat-soluble vitamins. Anticonvulsants (carbamazepine, phenytoin) reduce folate and vitamin D levels. Warfarin: vitamin K supplementation can interfere with dose control — always inform your prescriber before supplementing K.
Daily Vitamin Requirements — RDA & Optimal Ranges
The Recommended Dietary Allowance (RDA) represents the intake sufficient to meet the needs of 97.5% of healthy individuals — it is a minimum baseline, not an optimal target. Optimal ranges for many vitamins exceed RDAs, particularly for Vitamin D and Magnesium.
| Vitamin | RDA (Adult) | Upper Limit (UL) | Optimal Research Range | Supplementation Need? |
|---|---|---|---|---|
| Vitamin A | 700–900 mcg RAE | 3,000 mcg (toxicity risk) | RDA generally sufficient if varied diet | Rarely — widespread in varied diets |
| Vitamin D3 | 600–800 IU (15–20 mcg) | 4,000 IU (official) — research suggests 10,000 IU safe | 2,000–4,000 IU daily for most adults; test to confirm | Yes — for most people |
| Vitamin C | 75–90 mg | 2,000 mg | 200–500 mg from food; supplements may help at higher needs | Rarely if diet includes fruit and vegetables |
| Vitamin B12 | 2.4 mcg | None established | Serum level 400–700 pg/mL as target | Yes for vegans, over-50s |
| Folate (B9) | 400 mcg DFE (600 when pregnant) | 1,000 mcg (synthetic) | RDA plus methylfolate if MTHFR variant | Yes for pregnant women |
| Vitamin K2 | 90–120 mcg (total K) | None established for K2 | 100–200 mcg MK-7 if supplementing D3 | Consider with D3 supplementation |
| Vitamin E | 15 mg alpha-tocopherol | 1,000 mg | RDA sufficient from nuts/seeds | Rarely needed with varied diet |
Your Vitamin Optimisation Action Plan
This step-by-step approach systematically identifies and corrects deficiencies — prioritising those with the strongest evidence and broadest health impact, while avoiding unnecessary supplementation.
Step 1 — Assess Risk Group
Identify whether you fall into any high-risk group (vegan, over-50, female/pregnant, medication user, malabsorption condition). This immediately flags which deficiencies are most probable and guides testing priorities before any supplementation decisions.
Step 2 — Test Priority Nutrients
Request: Vitamin D (25-OH), B12 (with methylmalonic acid if borderline), Ferritin (not just haemoglobin), RBC Magnesium, and Folate (red cell). These five tests cover the most common, consequential, and treatable deficiencies — and are the foundation of an evidence-based supplement strategy.
Step 3 — Food First
Before supplementing, assess your diet against the top food sources in Section 07. Dietary improvements provide vitamins alongside synergistic cofactors no supplement delivers. Organ meats (liver), oily fish, eggs, and colourful vegetables collectively address most common deficiencies more effectively than isolated supplements.
Step 4 — Supplement Strategically
If diet cannot achieve optimal levels (particularly Vitamin D, B12 for vegans, and Magnesium), supplement using the correct form (D3 not D2; methylcobalamin not cyanocobalamin; magnesium glycinate not oxide). Start one supplement at a time and retest at 8–12 weeks to confirm levels are responding.
Step 5 — Monitor & Adjust
Retest key nutrients every 6–12 months. Requirements change with seasons (Vitamin D), life stages (pregnancy, menopause), medications, and health changes. Supplement doses that were appropriate at one stage may be inadequate or excessive at another. Regular testing prevents this drift.
Step 6 — Medical Oversight
For persistent symptoms despite apparent dietary adequacy and supplementation, or for fat-soluble vitamin supplementation at higher doses, or for any nutrient-drug interactions — consult your GP or a registered dietitian. Self-directed supplementation is appropriate for mild deficiency; significant or symptomatic deficiency warrants professional guidance.
Please consult a qualified healthcare provider for diagnosis and treatment. This page contains affiliate links — we may earn a commission at no extra cost to you.