Before phototropic plants began bending toward sunlight, before the bikini-clad beach denizens began tanning en masse, and before the first house cat followed the sliver of sunlight around the room all afternoon, our primitive, microscopic marine forebears were flourishing by converting the sun’s energy into chemical energy usable by biological life. You’re probably aware of photosynthesis, the process by which plants, algae, and other organisms do it and produce byproducts like oxygen, but even the unicellular archaea that do not produce oxygen utilize sunlight for energy. And if you aren’t obtaining energy directly from the sun, you’re probably eating the organisms that do. Either way, sunlight directly or indirectly supports all life (well, except for the chemoautotrophs living in deep sea hydrothermal vents feeding off of inorganic energy sources like iron, ammonia, or sulfur).
Humans don’t directly consume sunlight, though. We aren’t breatharians or walking, talking photosynthesizers. Sunlight might feel good and it might brighten up our moods, but we don’t consume it for metabolic sustenance. It does, however, provide the raw material for the 7- dehydrocholesterol in our skin to manufacture the ever-important vitamin D3, or cholecalciferol.
For some organisms, like the naked mole rat with its natural, “preferred” vitamin D deficiency, vitamin D is unimportant. But for most, including man, vitamin D is vital. You might even say it’s the most important vitamin of all. It’s not really even a vitamin, in fact; it’s a secesteroid, a hormonal precursor that closely resembles steroids like cortisol, testosterone, and cholesterol. Most vitamins are exogenous and stem from outside sources. Vitamin D is a different sort of “vitamin.” We mammals make it in-house.
As an equatorial people, we homo sapiens all spent well over half our collective development time bathed in tropical sun, almost year-round. Those of us who flitted off to the South Pacific and other equatorial regions got plenty of the stuff, too. Northerners even developed fair skin with less melanin, which maximized their ability to produce vitamin D from less sunlight. We’ve clearly evolved with the availability of sunlight – and therefore vitamin D3 – as a central stressor to our development.
As you know, the Primal Blueprint is about looking for that perfect storm of evolutionary concordance and modern scientific confirmation. I try to find the eye of the storm and stay there. When it moves, I move with it.
To evaluate whether a food or nutrient or behavior is suitable or not, I:
Vitamin D checks out on all levels. We’re not just talking about some exogenous substance which our ancestors may have occasionally come across and which may confer benefits, like bone marrow or polyphenols. This is vitamin D, the active form of which – called calcitriol – is produced in our livers in response to cholecalciferol derived from UV-B radiation. That’s sunlight, a resource readily available to our equatorial ancestors, the importance of which is indicated by the presence of naturally-occurring vitamin D receptors in the body’s cells.
Our bodies clearly consider vitamin D essential, but why? What does it do?
For something as readily available on any given sunny day, it’s also, surprisingly, in short supply these days. We’re told to avoid the sun at all costs or, if we absolutely must venture out into the light, to apply a layer of sunscreen so thick that an equal amount of something as inert as lite mayo would be just as effective. Between office jobs without a view or window panes that allow UV-A but not UV-B rays, humans just aren’t giving their skin enough opportunities to create vitamin D3. And if it’s an essentiality, it’s probably best to provide your body plenty of opportunities to make (or consume) vitamin D.
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