How is level of vitamin d in the body lowered?

Many causes: Lack of exposure to sunlight, effects of diseases(crohn disease), deficiency of vitamin d in diet, exposure to toxic substances, harmful chemicals, side effects of certain medicinesusa.Thanks.

Various reasons: But commonest is poor nutrition and poor exposure to sunlight.

VIT D: Is made in the skin and the light spectrum needed is best between noon and three pm daily for 15-20 minutes -without sunscreen. Sunscreen blocks vit d synthesis by 95%. Staying indoors and not being in sun between 10 am and 4 pm reduces our exposure. As said before...Poor diet and chronic illnesses ..And living above the 34th parallel all contribute.Serum levels are optimal at 50-70.