Nutrition Science and Everyday Application - beta v 0.1

518 ALICE CALLAHAN, PHD, HEATHER LEONARD, MED, RDN, AND TAMBERLY POWELL, MS, RDN
that decreases exposure to UV rays can interfere with vitamin D synthesis, including the
following: 2
• Geographic latitude and season. Your location on Earth and the time of the year
affects your exposure to UV radiation from the sun. Exposure to UV light is greatest
at the equator and declines as you move further north or south. Likewise, in the
summer months, the sun is directly overhead for a greater part of the day, so you
have more opportunities to synthesize vitamin D. In the winter, the sun stays lower
in the sky, day length is shorter, and cloud cover is more likely to block the sun’s
rays, all of which decrease opportunities to synthesize vitamin D. North of about 35
degrees latitude, vitamin D synthesis is inadequate for at least a few months
during the winter, because the sun simply doesn’t get high enough in the sky to
provide enough UV radiation on earth’s surface. For example, in Boston and at the
California-Oregon border (42nd parallel north), vitamin D synthesis occurs only
from March until October. However, in Los Angeles (34th parallel north), vitamin D
synthesis occurs year round. Ozone and air pollution can also block UV rays and
decrease vitamin D synthesis.
• Skin pigmentation. Darker skin pigmentation, caused by greater melanin
production in the skin, decreases UV light absorption. This helps to protect the skin
from damage from UV radiation—a helpful adaptation for those living closer to the
equator—but it also reduces synthesis of vitamin D. People with darker skin
pigmentation need to spend more time in the sun in order to synthesize the same
amount of vitamin D as lighter-skinned people.
• Age. The efficiency of vitamin D synthesis declines with age. In addition, older
adults often spend less time outside so may receive less exposure to sunlight.
• Sun-protective behavior. While some UV light exposure is needed to synthesize
vitamin D, UV radiation is also carcinogenic, and too much exposure increases the
risk of skin cancer. It’s wise to protect your skin from UV radiation by applying
sunscreen, covering up with clothing and a hat, finding shade, and avoiding sun
exposure in the middle of the day. People who are highly vigilant in these sunprotective
behaviors or simply aren’t able to go outside during the day (UV rays
don’t penetrate glass) may not get enough UV light for vitamin D synthesis.
With so many factors affecting UV radiation exposure, many people are unable to synthesize
enough vitamin D for at least part of the year. Because vitamin D is fat-soluble, liver and
adipose storage can supply the body for a while. Beyond that, dietary sources and
supplements may be needed to meet the vitamin D requirement. 2
DIETARY SOURCES OF VITAMIN D
Only a few foods are naturally good sources of vitamin D. These include fatty fish such as
salmon, tuna, and mackerel, as well as fish liver oil (e.g., cod liver oil). Smaller amounts are
found in egg yolks, cheese, and beef liver. Additionally, some mushrooms grown in UV light
can be a good source of vitamin D.
Most cow’s milk is fortified with vitamin D in the U.S. and Canada, but other dairy products
such as ice cream and cheese are not. Fortified orange juice, soymilk and other plant-based