The optimal vitamin D range seems to be between 35 and 50ng/ml. Anything below 20ng/ml is apparently dangerously low.
Vitamin D is found naturally in very few foods; primary sources are the sun and mushrooms. They are also found in fatty fish, and fortified dairy products, but seeing as I’m vegan those are not an option.
Even if you’re not vegan, it’s suggested most people should be taking a supplement of around 1,000 - 2,000 IU daily to achieve adequate blood levels and gain protection from osteoporosis, cancer, and heart disease. Look for supplements that provide D3 (cholecalciferol) rather than D2 (ergocalciferol).
I’ve written about Vitamin D for my own reference, please do your own research before changing your diet or taking supplements.