Vitamin D - Why and How?

Examples of Vitamin D supplements in various forms and dosages (no affiliation)

Here is what I learned on Magnesium and how I apply that knowledge to myself. I am not a doctor. Not medical advice. Double check everything with your own research.

Why vitamin D is important

• Enhances #calcium absorption in the small intestine.
• Regulates parathyroid hormone (PTH), which maintains calcium balance.
• Bone health: Helps absorb calcium and phosphorus. Deficiency can lead to #osteopoenia / #osteroporosis.
  • Pubmed: 2010: Vitamin D and Bone Health; Potential Mechanisms
    • Stimulates bone-forming cells and promotes bone mineralization.
  • JBMR 2020: Effects of Supplemental Vitamin D on Bone Health Outcomes in Women and Men in the VITamin D and OmegA‐3 TriaL (VITAL)
• Immune system:
  • Pubmed 2012: Vitamin D and the Immune System
    • Vitamin D receptors (VDRs) are found in nearly all immune cells.
    • Regulates genes involved in immune response.
    • Before antibiotics, doctors used sun exposure and cod liver oil to treat tuberculosis.
    • Expressed in bone marrow, brain, and epithelial tissues (colon, breast, etc.), suggesting widespread physiological roles.
  • Possible that lower winter Vitamin D levels contribute to higher rates of colds, flu, and other infections.
  • May lower the risk of autoimmune diseases like multiple sclerosis and rheumatoid arthritis
  • Many Vitamin D supplementation trials for immune function and autoimmune diseases have failed.
• Mood regulation. Deficiency is associated with depression / seasonal depression / anxiety.
  • 2022 Study: The effect of vitamin D supplementation on depressive symptoms in adults: A systematic review and meta‐analysis of randomized controlled trials.
    • However this study lay have confounding variables. People with higher vitamin D may also be more active outdoors which may be a common cause for elevated vitaminD and elevated mood.
• Mental health: Deficiency is linked with depression.
• Muscle function. Deficiency can lead to weakness and fatigue.
• Many more. Vitamin D is involved in dozens of chemical reactions in the body.
  • Although classified as a vitamin, Vitamin D functions as a hormone.

Why the science is unclear

• Most studies use a fixed dose supplementation (e-g: 2000 IU/ day)
• They don’t measure the actual levels of vitamin D in patients!
• Do they properly take the supplementation?
• Do they absorb it?
• Do they (or the control group) get vitamin D from other sources like the sun?
• Vitamin D levels fluctuate with seasons…

Natural sources

• In theory the skin can produce vitamin D from sun exposure.
  • But that works if you spend your days outside with your shirt off
  • Most western adults spend their days inside or covered (or worse: use sun blocking cream)
  • Most western adults are deficient
• Dietary sources:
  • Fatty fish is the best source but 100g of Salmon bring only ± 500 IU.
  • Eggs: only ± 44 IU per egg.

Lab tests

• Vitamin D benefits follow an inverted U curve: Too much or too little is problematic.
• Most common test is for 25-Hydroxyvitamin D3 ( 25(OH)D ) in the blood serum (approx 16€)
  • “Normal” levels are 30 to 100 ng/mL.
  • Optimal levels are >30 or >50 ng/mL (Dr Boz) and <60 or <80. Dr Ford Brewer allows up to 90. No clear consensus (Dr Attia)
  • My own target is 50 - 60 ng/mL
  • 25-Hydroxyvitamin D2+D3 is equivalent to 25-Hydroxyvitamin D3 if you’re not taking D2 supplementation (and you should indeed take D3 supplementation (of natural source) rather than D2, which is made synthetically ans is less potent (Dr Berg))
• The active form is 1,25-dihydroxyvitamin D3 ( 1,25(OH)₂D ) but it’s more expensive to test (approx 26 €) the form above is considered to be a good proxy.
  • Produced from 25(OH)D via hydroxylation in the kidney.
  • Highly regulated and may not always correlate directly with 25(OH)D levels.
  • Test 25-Hydroxyvitamine D3 until you’re in the optimal range
  • Then test 1,25-dihydroxyvitamine D3 to double check

Supplementation

❗ RISK of toxicity

• Vitamin D is fat soluble, i-e NOT water soluble. Therefore excess cannot be easily excreted through urine and will accumulate in the body.
• You must NOT overshoot!
• Also, a dosage that may be good for you in winter may be too much in the summer depending on how much you produce your own vitamin D through sun exposure.
• Pubmed: 2018: Vitamin D Toxicity–A Clinical Perspective: Hypercalciuria (excess calcium in urine) -> frequent urination, Hypercalcemia (high blood calcium) -> Kidney Stones, Potential bone loss from excessive calcium release, Neuropsychiatric issues, Gastrointestinal distress, Cardiovascular complications, Kidney dysfunction.

• I believe oily forms make more sense than dry tablet forms because Vitamin D is soluble in fat.
  • I don’t even trust the dry tablets to work.
  • I prefer the droplets because you ingest less of filler/container material (which has negative nutritional value)
• With 2000 IUs / day, 99% of people will reach at least 20 ng/mL and 90% will be above 30 ng/mL. No safety concerns at this level, but this is below the optimal range.
• Levels > 10 000 to > 15 000 IU/day pose risk.
• Some say toxicity only happens if taking 100,000 IU/day for several months…
• The Pharmacological Basis of Therapeutics, Fourth edition, pahe 1687 said “The maintenance dose usually ranges between 50,000 and 250,000 units daily”.
• Current textbooks recommend 400 to 600 IU/day.
• People supplementing with high doses should regularly check their levels.
• For bone health: it’s often recommended to combine with vitamin K2

❗ Magnesium dependency

Dr Berg: https://www.youtube.com/watch?v=hrqr3IIoTOs, provided for information but poorly sourced:

• “For vitamin D to work, it’s dependent on #magnesium”
• “If you have a Magnesium deficiency and you take a lot of vitamin D, you are going to exagerate that deficiency”
  • I think my lab tests confirm that. Early 2025 I upped my vitamin D supplementation a lot and upped my Mg intake only a little. After 6 weeks my magnesium levels were down, despite the increased supplementation!
• “The more vitamin D you take the more Magnesium you are going to need. Both of them work synergistically together”

❗ Zinc dependency

Dr Berg: https://www.youtube.com/watch?v=hrqr3IIoTOs, provided for information but poorly sourced:

• The requirement for #zinc goes up when you take more vitamin D supplementation

My experience

• Winter in Luxembourg:
  • 2024: About 1000 IU/day -> 31 ng/mL
  • 2025: About 3000 IU/day and eating more fish -> 40 ng/mL
  • 2025: About 5000 IU/day -> 52 ng/mL
    • I’m going to stay at 500 IU/day and see what happens with more sun exposure as we’re going out of the winter…
  • 2022: Previous best winter: 44 ng/mL but I did not track how much I was supplementing at the time (but most probbaly max 3000 IU/day)
• Summer in Luxembourg:
  • 2018: 36 ng/mL without supplementation