Greetings Primals!
As the weather warms and the sun begins to stay with us longer each day, an ancient biological system wakes up inside the human body! For most of human history, sunlight was more than light and warmth. When our ancestors stepped into the sun after a successful hunt or a long migration, their bodies received a signal that conditions were good! Food had been secured and the tribe was safe. Energy could now be directed toward rebuilding and strengthening the body.
Modern science now shows that one of the most important signals activated by sunlight is the production of vitamin D. What many people do not realize is that vitamin D is not simply a vitamin from food. It functions primarily as a hormone that helps regulate many of the body’s most important biological systems!
In Today’s Issue:
• Why vitamin D is actually a powerful hormone, not just a vitamin
• How sunlight and cholesterol in your skin work together to create it
• What vitamin D does inside your cells to strengthen immunity, hormones, and muscle
• Why sunlight signaled success for our ancestors and triggered recovery and growth
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WEEKLY DEEP DIVE
Vitamin D production begins when sunlight touches the skin. Ultraviolet B light interacts with a cholesterol compound stored in the skin called 7-dehydrocholesterol. The energy from sunlight converts this compound into vitamin D3.
From the skin, vitamin D3 enters the bloodstream and travels to the liver, where it is converted into a storage form known as 25-hydroxyvitamin D. The process continues in the kidneys, which convert it into its fully active hormonal form called calcitriol. At this point vitamin D becomes a powerful signaling molecule that travels throughout the body and influences many biological systems.
Vitamin D belongs to a family of hormones known as steroid hormones. These hormones are built from cholesterol and include testosterone, estrogen, and cortisol. Cholesterol is not simply a substance circulating in the blood! It is a critical building block the body uses to manufacture hormones that regulate metabolism, recovery, and immune function.
When sunlight touches the skin, the body begins converting a cholesterol compound into vitamin D3. The liver and kidneys then activate it into a powerful hormone that supports immunity, bone strength, muscle recovery, and hormonal balance.
Once activated, vitamin D enters cells throughout the body and binds to vitamin D receptors. These receptors function like switches that help regulate gene activity and coordinate important physiological processes.
Vitamin D strengthens the immune system and helps immune cells identify and respond to pathogens. It supports bone health by helping the body absorb calcium and maintain proper mineral balance. It also contributes to muscle strength, coordination, and recovery after physical activity. In addition, vitamin D plays a role in hormone regulation and helps manage inflammation throughout the body.
In many ways vitamin D acts as a master signal that helps the body maintain strength and long term health.
Understanding this system also helps explain why sunlight was such an important environmental signal during human evolution. If our ancestors were resting in the sun with exposed skin, several things were likely true. Food had been secured, immediate threats had passed, and the body had the resources needed to rebuild and prepare for future challenges.
Sunlight therefore became a signal that conditions were safe for growth and recovery. The production of vitamin D helped communicate that message throughout the body.
UVB sunlight interacts with a cholesterol compound in the skin to begin vitamin D production. This sunlight triggered reaction starts the pathway that leads to one of the body’s most important hormones. The process begins the moment skin meets the sun.
The Primal Takeaway
For our ancestors, sunlight meant victory! It meant the hunt had gone well and the tribe could pause long enough to recover. In that moment sunlight hitting the skin was not just warmth. It signaled that resources were available and the body could invest energy into rebuilding itself!
The photons striking your skin begin a cascade that activates hormonal systems designed to strengthen the body. Be intentional with your movement and eat like the hunt was successful. Then, step outside and allow the sunlight to start your recovery!
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References
Holick MF. Vitamin D deficiency. New England Journal of Medicine. 2007.
National Institutes of Health Office of Dietary Supplements. Vitamin D Fact Sheet for Health Professionals.
Bikle DD. Vitamin D metabolism, mechanism of action, and clinical applications. Chemistry & Biology. 2014.
