Vitamin D3
Cholecalciferol
A fat-soluble vitamin that functions as a hormone precursor, studied for its role in testosterone synthesis, immune function, and the signaling axis that drives male hormonal output.
The Ingredient Explained
Vitamin D3 is not a traditional vitamin in the way most people think of one. Once converted in the body, it behaves more like a hormone, binding to receptors throughout the endocrine system including in the testes, where testosterone is produced. Most men in the modern world are deficient in it. Deficiency is directly associated with lower testosterone levels, and correction of that deficiency has been shown to improve them. Foundation uses 5,000 IU per serving because that's the dose that moves the needle in clinical studies, not the conservative daily value designed to prevent rickets.
Why it's in Foundation
The hypothalamic-pituitary-gonadal axis is the signaling chain that tells your body to produce testosterone. Vitamin D3 receptors are present at every level of that chain. Without adequate D3, the entire system operates below capacity regardless of what else you do. Most men testing low in testosterone are also testing low in Vitamin D3. That's not a coincidence. It's a direct relationship. Foundation addresses it at 5,000 IU because correcting a deficiency requires a therapeutic dose, not a maintenance dose.
Your 90-Day Timeline.
Blood levels of Vitamin D begin to rise. If you were deficient, this correction phase is where the foundational work happens before downstream hormonal effects become measurable.
At consistent supplementation, D3 levels stabilize in the optimal range. The testosterone-supporting effect is sustained as long as supplementation continues.
Most Shoden clinical studies run 8-12 weeks. The combination of sustained cortisol reduction and testosterone support is strongest at this timeframe.
What the research shows
5,000 IU. A therapeutic dose, not a maintenance one.
A 12-month randomized controlled trial showed that men taking 3,332 IU of Vitamin D3 daily experienced significantly higher testosterone levels compared to placebo. Foundation uses 5,000 IU, above that studied threshold, because most deficient men require higher doses to reach optimal serum levels. Cholecalciferol, the D3 form used in Foundation, is the most bioavailable form and the same used in clinical research.
View the human study
Vitamin D3 supplementation and testosterone in men, published clinical trial
How it works with other ingredients
Vitamin K2
Vitamin D3 and K2 are the most well-established nutritional pairing in the research. D3 increases calcium absorption. K2 directs that calcium away from soft tissue and into bone where it belongs. Without K2, high-dose D3 supplementation can drive calcium into the wrong places. Together they work the way D3 was designed to work.
Zinc
Zinc supports testosterone synthesis directly at the testicular level. Vitamin D3 supports the hormonal signaling axis above it. They work at different levels of the same system, both required for the chain to function properly.
Boron
Boron reduces SHBG, freeing bound testosterone for use. It also works alongside Vitamin D3 in bone metabolism. The combination supports both hormonal availability and the structural systems D3 was originally studied for.