My first winter in NYC was very rough and I didn't understand why at first. I got a blood test at the doc and he said my vitamin D was very low. I committed to walking in the sun at least a half hour a day, weather permitting, and I never really had issues again. Part of the issue was I was spending a lot of time walking outside but not in sunlight due to the tall buildings. Just my anecdotal 2c
In the winter months in NYC, the UV intensity is too low to cause vitamin D production in the skin.