Most vitamin D is produced by skin exposure to sunlight. However, many people do not produce the necessary amounts of vitamin D because the ability of the sun to contribute to the synthesis of vitamin D by the skin is affected by the season, the time of day at which the skin is exposed to the sun (exposure between 11am and 3pm is necessary), smog/environmental pollution (most residents live in large cities), the skin's melanin content (people with dark skin phototypes synthesize smaller amounts of vitamin D after exposure to sunlight) and the use of sunscreen (SPF 8 and above). Even during the summer when vitamin D synthesis is more feasible, it is estimated that 45% of the population have plasma vitamin D levels below 40nmol/L (vitamin D deficiency) and 75% fail to achieve the “late” levels of 75nmol/L. The naturally occurring vitamin D3 in this product is of the same chemical form as that naturally synthesized in the body following exposure of the skin to sunlight. Lamberts Vitamin D 4000iu should be used only on the advice of a healthcare professional.