What is vitamin D?
Vitamin D is a fat-soluble vitamin that acts more like a hormone in the body. Your skin produces it when exposed to sunlight, and it is found in small amounts in foods like oily fish and eggs. It is critical for absorbing calcium, maintaining bone density, supporting the immune system, and healthy hormone levels including testosterone.
How it works
Vitamin D is converted by the liver and kidneys into its active form, which regulates calcium and phosphate in the body. It binds to vitamin D receptors found in nearly every tissue, influencing bone, muscle, immune cells, and more. Low levels are linked to poor bone health, low mood, frequent illness, and reduced strength.
What the research says
- Corrects deficiency that affects a large share of the population, including in Australia
- Supports bone density and reduces fracture risk, especially alongside calcium
- Low levels are associated with reduced muscle strength and performance
- Plays a role in immune function, with deficiency linked to more frequent infections
- Adequate levels support healthy testosterone, though megadosing does not boost it further
Effective dose
1000–2000 IU per day is a sensible maintenance dose for most people. Those who are deficient may need more under guidance from a doctor, who can test your levels with a simple blood test. Vitamin D3 (cholecalciferol) is the preferred form as it raises blood levels more effectively than D2. Take it with a meal containing fat for best absorption.
What to check on the label
Look for vitamin D3 rather than D2. Check the dose in IU or micrograms (1000 IU equals 25mcg). Many cheap multivitamins contain only a token amount. A standalone D3 supplement, or a D3 and K2 combination, is usually better value and more effective.
Safety
Vitamin D is safe at recommended doses. Because it is fat-soluble, it can build up in the body, so very high doses over long periods can cause toxicity. Stick to sensible doses unless a doctor advises otherwise based on your blood levels. Getting tested is the smartest way to know how much you actually need.