Vitamin D capsule. Vitamin D is found in foods such as oily fish, milk, liver and egg yolk, but the main source is exposure to sunlight. It is necessary for the absorption of calcium and phosphorous in the gut and for a healthy immune system. Deficiencies in vitamin D lead to rickets (improper bone formation) in children, and osteomalacia (bone softening) and osteoporosis (brittle bone disease) in adults. Patients with vitamin D deficiency have also been shown to be at higher risk of death from acute respiratory infections, including Covid-19, the disease caused by the new coronavirus SARS-CoV-2.