Vitamin D3 (± K2)
Cholecalciferol, Vitamin K2, MK-7, Ergocalciferol, Vitamin D2, Vitamin K
Forms: D3 (cholecalciferol) alone or with vitamin K2 (MK-7); the added value of K2 is unproven, and K2 counteracts warfarin.
Fat-soluble vitamin made in the skin from sunlight; deficiency is common in people who get little sun.
Legal status: Sold freely as a supplement and as a licensed medicine for deficiency; not prohibited in sport.
What it gives
What people use it for and what the sources report. Strength is a general category from the literature: mild, clear or strong.
Purpose
Taken to correct or prevent deficiency for bone, muscle and immune health. Marketed as a testosterone booster, but a meta-analysis of trials found no rise in testosterone. K2 is often added 'to direct calcium', with little proof.
What people seek and what studies show
Correcting a true deficiency supports bone health. In the large VITAL trial of adults not selected for deficiency, it did not lower fractures, cancer or heart events, nor improve physical performance. In athletes a meta-analysis found no significant strength gain; the latest meta-analysis found no significant drop in respiratory infections. K2 trials on artery calcification are mixed; none shows fewer heart events.
Effects
Evidence: Approved medicine — studied in people for its medical use
Sought effects by goal (as reported)
Possible side effects
Possible does not mean certain: whether they appear depends on dose, duration, individual response and monitoring. Below — what to watch for and how to notice it early.
1 possible side effect
level · how often it is reportedWhat the levels mean
Level: general category from the literature, not your personal risk
How often
- common
- reported often
- possible
- reported in some people
- rare
- reported occasionally
How to lower the risks
For 1 of 1 possible side effect there are specific ways to lower the chance: everyday measures, supplements and options a doctor may consider. No measure removes a risk completely.
Low-level side effects
How each option helps (3)
- Fluid & electrolyte management — Good hydration eases kidney stress from heavy training and heat; dehydration plus NSAIDs is a classic cause of acute kidney injury.
- Regular blood tests — Creatinine reads high in muscular people and with creatine; cystatin C and a urine test give a truer picture of the kidneys.
- Working with a doctor — Investigates protein in the urine or falling kidney function and can start treatment that slows kidney damage.
Check-ups and blood tests
Which tests and check-ups to do and when. This is a guide — agree the exact schedule with your doctor.
Myth & fact
“Vitamin D boosts testosterone.”
Randomized trials, including in men with low testosterone, found no rise. Correcting a real deficiency is worthwhile for bones and muscles, not as a hormone booster.
Combinations
Thiazides make the kidneys hold on to calcium while vitamin D increases its absorption, so blood calcium can climb: thirst, nausea, constipation, confusion and strain on the kidneys. Calcium and 25(OH)D tests show it.
HydrochlorothiazideSources
Possible side effects by body system, combinations, how to lower the risks and which tests to do — free.
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