During winter and cloudy days in Ohio I struggle due to lack of sun, and I know it’s because my body can’t generate enough vitamin D. I don’t take much vitamin D supplements, and here NetSweat notices that you can more than double your dosage of vitamin D:

Yes, you can increase your consumption of Vitamin D through foods such fortified dairy products, but probably not enough. New research suggests you need at least 800 international units, though the Cooper Institute recommends taking 1,000 international units. To put these numbers in perspective, the U.S. government currently recommends a daily intake of 400 international units - a huge difference.