>>723160218
Our bodies are pretty maladapted for absorbing dietary vitamin D. In fact, some people basically can't metabolize it at all. We evolved in a sun-rich environment and when our ancestors expanded across the planet it was quicker and easier for evolution to adjust melanin levels than retool our metabolism.
If you're dark-skinned living in a northern climate you're just fucked. There's a reason black people don't move that far north.
There are different kinds of vitamin D though and they aren't interchangeable. Your body uses the UV radiation from sunlight to synthesize Vitamin D3 from cholesterol. This process is extremely fast, taking only 5-30 minutes to give you a day's worth of D3, which is why you can step out on a break and come back in feeling recharged. It's D3 specifically.
To adjust for the lower sunlight exposure in autumn/winter, you need to both find alternate sources of Vitamin D3, and lower your cholesterol intake. You want to avoid generic Vitamin D supplements since these are a mix of different Vitamin D subtypes and are designed specifically for elderly people with metabolic issues, not healthy young adults.
