Vitamin D3, cholecalciferol, is made in the skin when a form of cholesterol (7-dehydrocholesterol) reacts with UVB ultraviolet light with wavelengths between 290 and 315 nanometers, as seen in Figure 4-1. Since UVB is absorbed
Figure 4-1 The activation of vitamin D.
by the atmosphere, more vitamin D is made when the sun is high. Fifteen minutes of summer sun in a bathing suit makes an average of 20,000 IU of vitamin D— 100 times the adequate daily intake. Since vitamin D is stored for long periods, this may be enough vitamin D to last for 100 days. UVB does not penetrate glass, so time in a closed car is not helpful. Sunlight exposure provides most people with their entire vitamin D requirement.
Young adults and children can make all of the vitamin D they need by spending just a few minutes in the sun three times a week. Older people have a slightly diminished capacity to synthesize vitamin D, so they need a little more sun. Also, many older people use sunscreen and wear protective clothing, which limits vitamin D production. Sunscreen with an SFP factor of 8 curtails production of vitamin D by 95 percent.
In higher latitudes, there is not enough UVB for vitamin D production in the skin for part of the year. Above 40 degrees north, approximately the latitude of Boston or San Francisco, there are four months without enough UVB to make vitamin D. Further north, in Canada, there are five months without much UVB. Vitamin D is stored in the liver for long periods of time. Even in higher latitudes, ten minutes of sun on the arms and face just three times weekly in the spring, summer, and fall will provide enough vitamin D for the whole year.
Except for summer, it is important that this exposure be between 11:00 AM and 2:00 PM so that more UVB will be available. It requires twice the sunning time at 9:15 AM or 2:15 PM than at noon to provide the same amount of UVB exposure
.Animals also produce vitamin D. Vitamin D is produced in fur and feathers. Animals lick or preen to absorb the vitamin.
Dark-skinned people living at higher latitudes may need extra vitamin D because their higher level of skin pigmentation may retard the absorption of UVB rays. Also, the weather in higher latitudes reduces daylight hours in winter and may require protective clothing which covers the skin. This is not a problem in tropical areas, where enough vitamin D is produced despite dark skin color. People with very dark skin may only produce one-sixth as much vitamin D as fair-skinned people do in the same amount of time.
Figure 4-1 The activation of vitamin D.
by the atmosphere, more vitamin D is made when the sun is high. Fifteen minutes of summer sun in a bathing suit makes an average of 20,000 IU of vitamin D— 100 times the adequate daily intake. Since vitamin D is stored for long periods, this may be enough vitamin D to last for 100 days. UVB does not penetrate glass, so time in a closed car is not helpful. Sunlight exposure provides most people with their entire vitamin D requirement.
Young adults and children can make all of the vitamin D they need by spending just a few minutes in the sun three times a week. Older people have a slightly diminished capacity to synthesize vitamin D, so they need a little more sun. Also, many older people use sunscreen and wear protective clothing, which limits vitamin D production. Sunscreen with an SFP factor of 8 curtails production of vitamin D by 95 percent.
In higher latitudes, there is not enough UVB for vitamin D production in the skin for part of the year. Above 40 degrees north, approximately the latitude of Boston or San Francisco, there are four months without enough UVB to make vitamin D. Further north, in Canada, there are five months without much UVB. Vitamin D is stored in the liver for long periods of time. Even in higher latitudes, ten minutes of sun on the arms and face just three times weekly in the spring, summer, and fall will provide enough vitamin D for the whole year.
Except for summer, it is important that this exposure be between 11:00 AM and 2:00 PM so that more UVB will be available. It requires twice the sunning time at 9:15 AM or 2:15 PM than at noon to provide the same amount of UVB exposure
.Animals also produce vitamin D. Vitamin D is produced in fur and feathers. Animals lick or preen to absorb the vitamin.
Dark-skinned people living at higher latitudes may need extra vitamin D because their higher level of skin pigmentation may retard the absorption of UVB rays. Also, the weather in higher latitudes reduces daylight hours in winter and may require protective clothing which covers the skin. This is not a problem in tropical areas, where enough vitamin D is produced despite dark skin color. People with very dark skin may only produce one-sixth as much vitamin D as fair-skinned people do in the same amount of time.
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