Using a proper loading dose, most people reach optimal levels within 60-90 days. Maintenance dosing then sustains these levels long-term.

It’s very difficult. You’d need to eat oily fish daily, and most foods contain minimal vitamin D. Supplementation is necessary for most UK residents, especially October-March.

If you spend significant time outdoors with skin exposed, you may produce enough naturally. However, many people still fall short—especially those with indoor jobs or darker skin. Testing confirms your status.

These are two units for measuring vitamin D. UK labs typically use nmol/L. To convert: nmol/L ÷ 2.5 = ng/ml. Our calculator uses nmol/L (UK standard).

Yes—vitamin D is safe alongside most supplements. It works synergistically with vitamin K2, magnesium, and omega-3s. Take with food for best absorption.

Vitamin D3 (cholecalciferol) is a specific form of vitamin D—and the most effective for supplementation. Vitamin D2 (ergocalciferol) is less effective. Always choose D3.

To calculate your personalised vitamin D dose, you need three key pieces of information:

1. Your current 25(OH)D level from a blood test (in nmol/L)
2. Your target level (optimal range is 150-200 nmol/L)
3. Your body weight in kg

The calculation uses this clinically-validated formula:

Daily Dose (IU) = (Target Level – Current Level) × 10,000 × (Body Weight ÷ 70) ÷ Days

For example, if your current level is 40 nmol/L, your target is 162 nmol/L, you weigh 80kg, and you want to reach your target in 60 days:

• Shortfall: 162 – 40 = 122 nmol/L (÷ 2.5 = 48.8 ng/ml)
• Weight ratio: 80 ÷ 70 = 1.14
• Total IU needed: 48.8 × 10,000 × 1.14 = 556,320 IU
• Daily dose: 556,320 ÷ 60 = 9,272 IU per day

Our free calculator does this automatically based on your inputs.

Research suggests approximately 60 IU per kilogram of body weight per day to maintain optimal vitamin D levels of around 125 nmol/L (50 ng/ml) in most adults without sun exposure.

Quick reference by body weight:

Important: These are maintenance doses for someone already at optimal levels. If you’re deficient, you’ll need a higher loading dose initially—which our calculator determines based on your current blood level.

Heavier individuals require proportionally more vitamin D because it’s fat-soluble and distributes throughout body tissue. Research shows obese individuals may need 2-3 times more vitamin D than normal-weight people to achieve the same blood levels.

On average, 1,000 IU of vitamin D3 daily raises blood levels by approximately 10-12.5 nmol/L (4-5 ng/ml) over 2-3 months in a 70kg adult.

However, this varies significantly based on:

• Body weight: Heavier individuals see smaller increases per 1,000 IU
• Starting level: Those with lower levels may see larger initial increases
• Age: Older adults often absorb less efficiently
• Genetics: Some people are “low responders” to vitamin D
• Fat absorption: Conditions affecting fat absorption reduce uptake

Practical example:

• A 70kg person at 50 nmol/L taking 4,000 IU daily for 3 months would likely reach approximately 90-100 nmol/L
• A 100kg person at the same starting level taking the same dose would likely only reach 70-80 nmol/L

This is why weight-adjusted dosing—like our calculator provides—is more accurate than fixed doses.

The NHS recommendation of 400 IU (10 mcg) daily is a minimum population-level guideline designed to prevent severe deficiency diseases like rickets and osteomalacia—not to achieve optimal health.

Key differences:

The NHS recommendation is conservative for several reasons:

1. It must be safe for everyone, including those sensitive to vitamin D
2. It assumes some dietary and sun exposure contribution
3. It’s based on preventing deficiency, not optimising function
4. Regulatory caution around higher dose recommendations

Research increasingly shows that 2,000-5,000 IU daily is needed for most UK adults to reach truly optimal levels (150-200 nmol/L), particularly during winter months when sun exposure provides no vitamin D.

Loading Dose: A higher initial dose taken for a limited period (typically 60-90 days) to rapidly correct a deficiency and reach your target blood level.

Maintenance Dose: A lower ongoing dose taken after reaching your target to sustain optimal levels long-term.

Example Protocol:

Why this matters:

If you’re severely deficient (below 50 nmol/L), taking only a maintenance dose of 2,000 IU/day would take 6-12 months to reach optimal levels. A proper loading dose gets you there in 2-3 months.

Our calculator determines your exact loading dose based on how far you need to raise your levels and over what timeframe.

After completing your loading phase:

Retest annually (or every 6 months initially)

Retest your 25(OH)D levels

Confirm you’ve reached your target

Reduce to a maintenance dose

Between October and late March in the UK, the sun sits too low in the sky for UVB rays to reach your skin at the intensity required for vitamin D synthesis—regardless of how much time you spend outdoors.

The science:

Vitamin D is produced when UVB rays (wavelength 290-320nm) hit your skin. For this to happen, the sun must be at an angle greater than approximately 45° above the horizon.

In the UK:

• April-September: Sun angle sufficient for vitamin D production (typically 11am-3pm)
• October-March: Sun angle too low—no vitamin D production possible
• Peak production: June-July between 11am-3pm

Even in summer, many UK residents don’t make enough because:

• Indoor work during peak hours
• Clothing covering most skin
• Sunscreen blocking UVB rays
• Cloud cover reducing UV intensity
• Air pollution filtering UVB
• Darker skin tones requiring longer exposure

The recommendation: All UK adults should consider supplementing with vitamin D year-round, and especially from October to March when sunlight cannot provide any vitamin D.

Optimal range: 150-200 nmol/L (60-80 ng/ml)

This is significantly higher than the NHS minimum threshold of 25 nmol/L, which only prevents deficiency diseases.

UK Reference Ranges:

Why aim for 150-200 nmol/L?

Research shows this range is associated with:

• Optimal immune function
• Lowest all-cause mortality
• Best testosterone levels in men
• Lowest cancer risk
• Optimal muscle function
• Best mood and cognitive function

Note: UK labs typically report in nmol/L. To convert to ng/ml (used in American research), divide by 2.5.

Conversion formulas:

• nmol/L = ng/ml × 2.5
• ng/ml = nmol/L ÷ 2.5

Quick conversion table:

Why this matters:

Most UK labs report in nmol/L, while much American research uses ng/ml. When reading research or comparing international guidelines, ensure you’re comparing like-for-like.

Our calculator uses nmol/L (UK standard) so you can enter your blood test result directly.

While vitamin D testing is available through several routes, a standalone vitamin D test only tells part of the story. For meaningful results that actually guide your health decisions, context matters.

Your options:

1. NHS (Free)

• Request through your GP
• Usually only tests if clinically indicated
• Results take 1-2 weeks
• Limitation: Tests vitamin D in isolation—doesn’t assess cofactors, absorption markers, or related health indicators

2. Basic Home Finger-Prick Tests (£29-£49)

• Convenient postal tests
• Results in 2-5 days
• Limitation: Single biomarker with generic reference ranges. You get a number but no guidance on what it means for you or how to optimise effectively

3. Comprehensive Functional Blood Chemistry Analysis with Functional Aesthetics (Premium)

This is the approach serious about their health choose. Rather than testing vitamin D alone, a Functional Blood Chemistry Analysis examines your vitamin D alongside:

• Cofactor status: Magnesium RBC, zinc, calcium—without these, vitamin D can’t activate properly
• Thyroid markers: Vitamin D and thyroid function are intimately connected
• Inflammatory markers: High inflammation increases vitamin D utilisation
• Metabolic health: Blood glucose, insulin, lipids—the full picture
• Hormonal status: Including testosterone, which vitamin D directly influences
• Liver and kidney function: Required for vitamin D activation

Why this matters:

You might have “adequate” vitamin D on paper but still experience deficiency symptoms because:

• Your magnesium is depleted (vitamin D stays inactive)
• Chronic inflammation is burning through your stores
• Poor thyroid function is affecting utilisation
• You’re missing other critical nutrients

A standalone vitamin D number without this context is like checking your car’s fuel gauge while ignoring the engine warning lights.

What you get with Functional Aesthetics:

✓ Comprehensive blood panel (50+ biomarkers) ✓ Functional range interpretation—not just “normal” vs “abnormal” ✓ Personalised analysis of how your results interact ✓ Specific supplementation recommendations based on YOUR biochemistry ✓ Protocol adjustments for your goals (energy, performance, longevity, hormone optimisation) ✓ Follow-up support to track progress

When to test:

February-March is ideal timing—captures your lowest annual levels after the UK’s vitamin D winter

Before starting any supplementation protocol (establish true baseline)

After 60-90 days (verify your protocol is working)

Every 6-12 months (ongoing optimisation)

Yes—vitamin K2 is strongly recommended when taking vitamin D, especially at doses above 2,000 IU daily.

The reason:

Vitamin D increases calcium absorption from your gut. Vitamin K2 then directs that calcium to your bones and teeth (where you want it) rather than your arteries and soft tissues (where you don’t).

Without adequate K2, high-dose vitamin D could potentially contribute to arterial calcification over time.

Recommended K2:D3 ratio:

100 mcg of vitamin K2 (MK-7) per 10,000 IU of vitamin D3

Or simplified: 10 mcg K2 per 1,000 IU D3

Practical dosing:

K2 forms:

• MK-7 (menaquinone-7): Longest-lasting, most studied—preferred form
• MK-4 (menaquinone-4): Shorter half-life, requires higher doses

Important: If you take blood-thinning medications like warfarin, consult your doctor before taking vitamin K2, as it can affect blood clotting.

Magnesium is essential for vitamin D activation. Without adequate magnesium, vitamin D remains inactive in your body—even if blood levels appear adequate.

How it works:

Vitamin D goes through two conversion steps to become active:

1. Liver converts D3 → 25(OH)D (what blood tests measure)
2. Kidneys convert 25(OH)D → 1,25(OH)2D (active form)

Both conversion steps require magnesium-dependent enzymes. If you’re magnesium deficient, these conversions are impaired.

Additionally:

• Vitamin D increases magnesium excretion
• High-dose vitamin D can deplete magnesium levels
• Magnesium deficiency is common (estimated 50%+ of UK adults)

Recommended magnesium intake with vitamin D:

50-100 mg of magnesium per 1,000 IU of vitamin D3

Or approximately 300-400 mg magnesium daily total intake.

Best forms of magnesium for supplementation:

• Magnesium glycinate: Best absorbed, gentle on stomach, calming
• Magnesium citrate: Well absorbed, may have mild laxative effect
• Magnesium malate: Good for energy and muscle function
• Avoid: Magnesium oxide (poorly absorbed)

Timing: Many people take magnesium in the evening as it can support relaxation and sleep quality.

Take vitamin D with your largest meal containing fat—typically breakfast or dinner.

Why with food?

Vitamin D is fat-soluble, meaning it requires dietary fat for optimal absorption. Studies show 50% better absorption when vitamin D is taken with a fat-containing meal versus on an empty stomach.

Best practices:

1. With a meal containing healthy fats (eggs, avocado, olive oil, nuts, fatty fish)
2. Consistency matters more than timing—same time each day helps establish routine
3. Morning or midday may be preferable for some people as vitamin D can theoretically affect melatonin production (though evidence is limited)

What about splitting doses?

If your calculated dose exceeds 10,000 IU daily, consider splitting it:

• 5,000 IU with breakfast
• 5,000 IU with dinner

This improves absorption and maintains more stable blood levels.

Avoid:

• Taking on an empty stomach
• Taking with fibre supplements (can reduce absorption)

Taking with mineral oil or fat-blocking medications

Daily dosing is generally preferred over weekly bolus doses.

The evidence:

Recent research suggests that daily dosing is more effective and potentially safer than equivalent weekly or monthly doses:

Why daily is better:

1. More stable blood levels: Avoids peaks and troughs
2. Better mimics natural production: Sun exposure provides small daily amounts
3. Safer for some outcomes: Large intermittent doses (50,000+ IU) have been associated with increased fall risk in elderly populations
4. Better absorption: Smaller doses absorb more efficiently than large bolus doses

Practical compromise:

If daily dosing is difficult, alternate-day dosing is acceptable. For example:

• Instead of 4,000 IU daily → 8,000 IU every other day

Avoid:

Yearly mega-doses (associated with adverse outcomes in studies)

Monthly mega-doses (300,000+ IU) unless under medical supervision

10,000 IU daily is considered the “No Observed Adverse Effect Level” (NOAEL) for most healthy adults according to the Endocrine Society and independent research reviews.

The evidence:

• The Endocrine Society states 10,000 IU/day is the safe upper limit
• Long-term studies show no toxicity at this level in healthy adults
• Some researchers argue even higher doses are safe when monitored

However, important caveats:

1. Duration matters: 10,000 IU is typically appropriate for a loading phase (60-90 days), not necessarily lifelong maintenance
2. Monitor your levels: At doses above 4,000 IU daily, periodic blood testing is advisable
   • Target: 150-200 nmol/L
   • Upper safe limit: 250 nmol/L
   • Concern level: >375 nmol/L (>150 ng/ml)
3. Include cofactors: At this dose, ensure adequate vitamin K2 (100+ mcg) and magnesium (400mg)
4. Individual variation: Some people are more sensitive to vitamin D. Start conservatively and monitor.
5. Contraindications: Avoid high doses if you have:
   • History of kidney stones
   • Sarcoidosis or other granulomatous diseases
   • Primary hyperparathyroidism
   • Hypercalcaemia

Recommended approach:

Retest annually

Use a loading dose (potentially 10,000+ IU) to reach your target

Retest after 60-90 days

Reduce to maintenance dose (2,000-5,000 IU) once optimal

Vitamin D toxicity is rare but can occur with sustained intake above 40,000 IU daily or blood levels exceeding 375 nmol/L (150 ng/ml).

Warning signs of excess vitamin D (hypercalcaemia):

Early symptoms:

• Nausea and vomiting
• Loss of appetite
• Constipation
• Increased thirst
• Frequent urination
• Fatigue and weakness

Advanced symptoms:

• Confusion
• Bone pain
• Kidney stones
• Irregular heartbeat
• Muscle weakness

Blood test indicators:

Risk factors for toxicity:

• Taking very high doses (>40,000 IU/day) for extended periods
• Certain granulomatous diseases (sarcoidosis, tuberculosis)
• Some lymphomas
• Manufacturing errors in supplements (rare but documented)

Prevention:

Don’t assume more is better—target 150-200 nmol/L, not maximum possible

Don’t exceed 10,000 IU daily without medical supervision

Test blood levels after loading phase

Include vitamin K2 to help manage calcium

Reduce dose once target level achieved

Yes—research conclusively shows that overweight and obese individuals require significantly higher vitamin D doses to achieve the same blood levels as normal-weight people.

The guidelines:

Why higher doses are needed:

1. Volumetric dilution: Vitamin D distributes throughout body tissue. More tissue = more dilution
2. Sequestration in fat: Vitamin D is fat-soluble and gets “trapped” in adipose tissue, reducing circulating levels
3. Reduced enzyme activity: Obese individuals show lower expression of the CYP2R1 enzyme needed to activate vitamin D
4. Larger distribution volume: Blood, muscle, liver, and fat volumes are all increased

Practical example:

A 100kg person might need 6,000-8,000 IU daily to achieve the same blood level that a 70kg person achieves with 4,000 IU.

Our calculator accounts for this by including body weight in the dosing formula—ensuring your calculated dose is appropriate for your size.

Note: Even with higher doses, vitamin D toxicity risk doesn’t increase proportionally because the vitamin continues to be distributed into larger tissue volumes.

If you have a history of calcium kidney stones, approach vitamin D supplementation with caution and medical supervision.

The concern:

Vitamin D increases calcium absorption from the gut. In susceptible individuals, this can increase urinary calcium (hypercalciuria), potentially contributing to calcium stone formation.

Evidence is mixed:

• Some studies show no increased stone risk with moderate vitamin D supplementation
• Others suggest possible increased risk, particularly with calcium co-supplementation
• Much depends on the underlying cause of your stones

If you have a history of kidney stones:

1. Consult your GP or urologist before starting vitamin D supplementation
2. Get baseline testing: 25(OH)D, calcium, and 24-hour urinary calcium
3. Start with lower doses: 1,000-2,000 IU daily initially
4. Monitor closely: Retest calcium and urinary calcium after 3 months
5. Stay hydrated: Adequate fluid intake is essential
6. Consider citrate forms: Potassium citrate can help prevent stone formation

Important distinctions:

• Calcium oxalate stones (most common): Moderate vitamin D is usually acceptable
• Calcium phosphate stones: May require more caution
• Uric acid stones: Vitamin D is generally not a concern

Key point: Vitamin D deficiency itself is associated with stone formation in some studies. The goal is achieving optimal—not excessive—levels with appropriate monitoring.

Vitamin D deficiency symptoms often develop gradually and may be attributed to other causes. In the UK—where deficiency affects up to 50% of adults—recognising these signs is important.

Common symptoms:

Physical:

• Persistent fatigue despite adequate sleep
• Muscle weakness or pain
• Bone pain (especially lower back, hips, legs)
• Frequent infections and slow recovery
• Hair loss or thinning
• Slow wound healing
• Unexplained weight gain

Mental/Emotional:

• Low mood or depression
• Anxiety
• Brain fog and poor concentration
• Seasonal mood changes (SAD)
• Irritability
• Sleep disturbances

Performance indicators:

• Reduced exercise capacity
• Slower recovery from workouts
• Decreased strength over time
• Increased injury frequency
• Poor endurance

Populations at higher risk in the UK:

• Those with darker skin
• People who spend most time indoors
• Shift workers
• Those who cover most skin when outside
• People over 65
• Those with digestive conditions affecting absorption
• Vegans and vegetarians
• Obese individuals

Important: These symptoms are non-specific and can indicate many conditions. A blood test is the only way to confirm vitamin D deficiency.

With proper dosing, most people reach optimal levels within 60-90 days. However, symptom improvement may begin within 2-4 weeks.

Timeline expectations:

Factors affecting recovery speed:

Faster recovery:

• Higher loading doses
• Good absorption (healthy gut)
• Taking with fat
• Including cofactors (K2, magnesium)
• Consistent daily dosing

Slower recovery:

• Lower doses
• Gut absorption issues
• Taking without food
• Missing cofactors
• Inconsistent dosing
• Higher body weight (larger distribution volume)
• Older age

Critical point: Using only NHS-recommended doses (400 IU) to correct deficiency would take 6-12+ months. A proper loading dose protocol achieves the same result in 2-3 months.

Verify with testing: Always retest 25(OH)D levels after completing your loading phase to confirm you’ve reached your target before reducing to maintenance dosing.

If your vitamin D levels remain low despite supplementation, several factors may be responsible:

1. Dose too low

• NHS-recommended 400 IU is insufficient to correct deficiency
• You likely need 4,000-10,000 IU daily based on your weight and starting level
• Use our calculator to determine your personalised dose

2. Absorption issues

• Taking supplements without fat reduces absorption by up to 50%
• Gut conditions (IBS, Crohn’s, coeliac) impair absorption
• Gastric bypass surgery significantly reduces absorption
• Some medications interfere with absorption

3. Body weight

• Overweight/obese individuals need 1.5-3× higher doses
• Standard doses are calculated for 70kg—adjust upward if heavier

4. Wrong form of vitamin D

• Vitamin D2 is less effective than D3
• Ensure you’re taking vitamin D3 (cholecalciferol)

5. Insufficient duration

• Levels rise gradually—need 2-3 months to see full effect
• Testing too early won’t show final result

6. Genetic factors

• Some people are “low responders” due to genetic variations
• May need 2× typical doses to achieve same levels

7. Increased utilisation

• Chronic illness increases vitamin D consumption
• High stress/inflammation depletes vitamin D faster

Solutions:

Consult a healthcare provider if levels remain low despite high doses

Recalculate dose using a weight-adjusted formula

Always take with a fat-containing meal

Use vitamin D3, not D2

Include magnesium (required for activation)

Allow 3 months before retesting

Consider liposomal forms if absorption is suspected

VITAMIN D & SPECIFIC HEALTH GOALS

Yes—research shows that correcting vitamin D deficiency can significantly increase testosterone levels in men.

The evidence:

A landmark 2011 study published in Hormone and Metabolic Research found:

• Men given 3,332 IU vitamin D daily for one year increased:
  • Total testosterone: 10.7 → 13.4 nmol/L (25% increase)
  • Bioactive testosterone: 5.21 → 6.25 nmol/L (20% increase)
  • Free testosterone: 0.222 → 0.267 nmol/L (20% increase)
• The placebo group showed no change

Why vitamin D affects testosterone:

1. Vitamin D receptors in testes: The cells producing testosterone have vitamin D receptors
2. Enzyme regulation: Vitamin D influences enzymes involved in steroid hormone synthesis
3. Supports Leydig cells: Vitamin D is required for optimal testicular function

Key points:

• Effect is most pronounced when correcting deficiency
• Men already at optimal vitamin D may not see additional testosterone increase
• Correlation exists between vitamin D and testosterone levels in population studies
• Optimal vitamin D range for testosterone: 125-175 nmol/L (50-70 ng/ml)

Practical application:

If you’re a man experiencing symptoms of low testosterone (low energy, reduced libido, muscle loss, mood changes), checking and optimising your vitamin D levels is a sensible first step before considering other interventions.

Yes—vitamin D deficiency is one of the most common and overlooked causes of persistent fatigue, and supplementation often produces significant improvement.

The connection:

Vitamin D is involved in:

• Mitochondrial function: Energy production at the cellular level
• Muscle function: Weakness contributes to feelings of tiredness
• Sleep regulation: Deficiency disrupts sleep architecture
• Mood regulation: Low mood reduces perceived energy
• Thyroid function: Vitamin D supports thyroid hormone production

What the research shows:

• Multiple studies show fatigue improvement with vitamin D supplementation
• One study found 89% of participants with unexplained fatigue were vitamin D deficient
• Improvement often noticed within 2-4 weeks of starting adequate supplementation

Timeline for energy improvement:

Important considerations:

• Fatigue has many causes—vitamin D is just one possibility
• Severe deficiency (<25 nmol/L) typically causes more pronounced fatigue
• Other deficiencies (iron, B12, thyroid) should also be checked
• Adequate sleep, nutrition, and stress management remain essential

Recommendation: If you experience persistent fatigue, test your vitamin D level. If below 75 nmol/L, optimising to 150-200 nmol/L often provides meaningful improvement in energy levels.

Yes—vitamin D plays a critical role in immune function, and deficiency significantly impairs immune response.

How vitamin D supports immunity:

1. Innate immunity (first-line defence):

• Enhances antimicrobial peptide production (defensins, cathelicidin)
• Improves macrophage function
• Supports physical barrier integrity

2. Adaptive immunity (targeted response):

• Modulates T-cell and B-cell function
• Reduces excessive inflammatory responses
• Supports antibody production

3. Immune regulation:

• Prevents overactive immune responses
• Reduces autoimmune disease risk
• Balances pro- and anti-inflammatory pathways

Research evidence:

• Meta-analyses show vitamin D supplementation reduces respiratory infection risk by 12-42%
• Greatest benefit seen in those with deficiency (<50 nmol/L)
• Daily or weekly dosing more effective than large monthly doses
• Optimal immune function requires levels >75 nmol/L

UK relevance:

The “vitamin D winter” (October-March) coincides with peak cold and flu season. Maintaining optimal levels through winter supplementation may reduce infection frequency and severity.

Recommended levels for immune function:

• Minimum: 75 nmol/L (30 ng/ml)
• Optimal: 100-150 nmol/L (40-60 ng/ml)

Consider higher (150-200 nmol/L) during illness or immune challenge

Vitamin D3 (cholecalciferol) in an oil-based or liposomal formulation is optimal for most people.

Vitamin D3 vs D2:

Delivery formats ranked:

1. Liposomal liquid: Best absorption, especially for those with gut issues
2. Softgel in oil: Excellent absorption, convenient
3. Oil drops: Good absorption, flexible dosing
4. Tablets with fat: Good if taken with fatty meal
5. Dry tablets/capsules: Adequate but lower absorption

What to look for:

✓ Vitamin D3 (cholecalciferol), not D2 ✓ Oil-based formulation (MCT oil, olive oil) ✓ Combined with K2 (MK-7 form) ✓ Third-party tested for purity ✓ Dosage that matches your calculated needs ✓ Minimal unnecessary additives

Vegan options:

Vitamin D3 derived from lichen is available for vegans—equally effective as lanolin-sourced D3.

Avoid:

✗ Vitamin D2 unless vegan with no D3 option ✗ Dry powder formulations (lower absorption) ✗ Products with excessive fillers/additives ✗ Doses that don’t match your calculated needs

No—it’s virtually impossible to achieve optimal vitamin D levels through diet alone in the UK.

Why food isn’t enough:

The average UK diet provides only 100-200 IU of vitamin D daily—far short of the 2,000-5,000+ IU needed for optimal levels.

Vitamin D content in foods:

To get 4,000 IU from food, you’d need to eat daily:

• 400-700g of wild salmon, OR
• 100 egg yolks, OR
• 10 litres of fortified milk

Additional challenges:

• UK milk is not routinely fortified with vitamin D (unlike USA)
• Few foods naturally contain significant vitamin D
• Cooking reduces vitamin D content
• Absorption varies based on fat content of meal
• Modern diets typically lack vitamin D-rich foods

Bottom line:

Food can contribute to vitamin D intake but cannot provide adequate amounts alone. Supplementation is necessary for most UK residents, particularly October through March.

Vitamin D has a half-life of approximately 2-3 weeks in the body. This means your levels decrease by about half every 2-3 weeks when supplementation stops.

Typical decline without supplementation:

Why levels fall quickly:

1. Ongoing utilisation: Your body constantly uses vitamin D for thousands of processes
2. No sun production (UK): October-March provides no vitamin D synthesis
3. Metabolism: Vitamin D is broken down and excreted continuously
4. Limited storage: While some is stored in fat, it’s released slowly

Implications:

Levels should be retested if you’ve stopped supplements for 8+ weeks

Don’t assume one loading course “fixes” you long-term

Maintenance supplementation is necessary year-round in the UK

Vitamin D is generally safe with most medications, but some interactions exist:

Medications that MAY require dose adjustment or monitoring:

Generally safe combinations:

• Blood pressure medications (most)
• Thyroid medications
• Diabetes medications
• Antidepressants
• Proton pump inhibitors (though absorption may be reduced)

Always inform your doctor if you’re starting high-dose vitamin D supplementation, especially if you take:

• Any calcium-affecting medications
• Multiple medications
• Medications for kidney disease
• Cardiac medications

Yes, vitamin D deficiency is fully reversible with proper supplementation. However, levels will NOT stay elevated without ongoing supplementation or sun exposure.

What reverses permanently:

• Blood levels (return to optimal with supplementation)
• Most deficiency symptoms (energy, mood, muscle function)
• Bone density (with adequate calcium and time)

What requires ongoing maintenance:

• Blood levels must be maintained through continuous supplementation
• Stop supplementing = levels decline within weeks
• UK residents typically need year-round supplementation

The maintenance reality:

Think of vitamin D like fuel in a tank:

• Loading dose = filling an empty tank
• Maintenance dose = regular top-ups to keep it full
• Without top-ups, the tank gradually empties

Long-term strategy:

Adjust as needed: Based on test results and season

Loading phase: Calculate dose to reach target (60-90 days)

Retest: Confirm target reached

Maintenance phase: Reduce to 2,000-5,000 IU daily

Annual retest: Ideally in late winter when levels lowest