There is no substitute. Take your Vitamin D3 + K2 in the winter, great idea—but we still need sunshine. Human beings had no problem getting regular sunshine until the past few generations. With the shift away from an agricultural society, populations, in general, now spend the majority of their time indoors.
Turns out, it is a misconception that people should stay out of the sun and/or slather themselves with sunscreen. In fact, the #1 correlate for cancer risk is how far from the Equator you are! The further away, the less sun, the higher the cancer risk. High Vitamin D levels actually correlate to low cancer risk.
Lack of sunshine is also a perfect recipe for depression. In the last two decades, a diagnosis of “Seasonal Affective Disorder” is practically an epidemic. That’s because people who live far from the Equator, with long, overcast winters, are living in conditions that few humans ever have—totally indoors. Our biology demands sunshine and everything that comes with it—hands in the black dirt, grounding us, feet in the grass, breathing clean air, and feeling the warm sun on our skin.
Don’t wash that Vitamin D off when you come inside. It takes hours for it to internalize, as hormone, and work with calcium to build bone.