Final answer:
Individuals with darker skin require up to two hours of sunlight exposure to synthesize several days' worth of vitamin D, while those with lighter skin need significantly less time, about 15-20 minutes of exposure a few times a week. Melanin in darker skin acts as a protective filter against UV radiation but also necessitates longer exposure for adequate vitamin D synthesis.
Step-by-step explanation:
To synthesize several days' worth of vitamin D, individuals with dark skin tones may require up to 6 times more exposure to UV radiation from sunlight compared to individuals with light skin. People with lighter skin may produce sufficient vitamin D with as little as 15-20 minutes of sunlight exposure a few times a week, whereas for those with darker skin, it may require around two hours of exposure to direct sunlight to synthesize an equivalent amount of vitamin D. This is because of the higher melanin content in the skin of darker-skinned individuals, which acts as a natural protector against UV radiation but also makes the synthesis of vitamin D less efficient. Diet can also provide vitamin D, especially from sources like cod liver oil, egg yolk, and fortified foods such as milk and orange juice. However, endogenous production through sun exposure often plays a significant role in meeting the body's daily requirement of vitamin D, which stands at around 100 units, though lactating women may need up to 400 units/day. Excessive sun exposure, on the other hand, can deplete the body's vitamin B9 levels, particularly in those with light skin, by 50% after just an hour under intense sunlight. Dark-skinned individuals, while they can also sunburn, have more protection due to the melanin in their skin. It's also critical to avoid excessive UV exposure to reduce the risk of skin damage and skin cancer. The synthesis of melanin peaks about 10 days after initial sun exposure, which gives some level of adaptive protection against UV radiation.