Vitamin D3 supplements. Box and capsules of vitamin D3 (cholecalciferol). Vitamin D is found in foods such as oily fish, milk, liver and egg yolk. It is necessary for the absorption of calcium and phosphorous in the gut. Deficiencies in vitamin D lead to rickets (improper bone formation) in children, and osteomalacia (bone softening) and osteoporosis (brittle bone disease) in adults.