Vitamin D is an essential nutrient that primarily aids calcium and phosphorus absorption, playing a role in bone growth and density, bone healing, and immune system function.
We get this vitamin naturally from sunlight and certain foods. In South Africa, the daily recommended allowance is 15 μg/day (600 IU/day) for adults. Certain foods, such as oily fish and eggs, do contain vitamin D, but they don’t contain significant amounts, so it is unlikely for a person to consume enough of it to be sufficient. There are also vitamin D-fortified foods available in SA (mainly margarines and cereals), but these must also be consumed in moderation as they are considered to be processed foods.
Most of us get our ‘sunshine’ vitamin from sunlight When your skin is exposed to the sun, vitamin D synthesis takes place when the ultraviolet B (UVB) rays hit cholesterol in the skin cells. So we know that we need to spend some time in the sun, but how much sun do we need for optimal health? The short answer is that it’s difficult to tell, as there are a host of variables to be considered:
1. Time of day and atmospheric conditions
At noon, the sun is at its highest point, and its UVB rays are most intense. That means you need less time in the sun to make sufficient vitamin D. Clouds and smog, however, reduces your skin’s exposure to UV rays.
2. Skin exposure
While it’s vital to protect yourself from skin cancer, it takes very little unprotected sun exposure for your body to start producing vitamin D. The more skin you expose, the quicker your body will start producing the vitamin. Sitting indoors behind a glass window doesn’t count as sun exposure as the glass blocks the sun’s UVB rays. For those who are always covered when they go outside, or spend most of their daylight hours indoors, they need to be especially cognisant of their vitamin D levels.
3. Skin pigmentation
The colour of your skin is determined by a pigment called melanin which acts as a natural sunscreen and absorbs the sun’s UV rays to defend against sunburn and skin cancers. This means that darker-skinned people will need to spend a longer time in the sun than lighter-skinned people to produce the same amount of vitamin D.
High-SPF sunscreens are designed to filter out most of the sun’s UVB radiation to protect against skin cancers. UVB wavelengths are also the specific wavelengths that trigger vitamin D production. Nonetheless, the US-based Skin Cancer Foundation report that clinical studies have never found that everyday sunscreen use leads to vitamin D insufficiency. In fact, the prevailing studies show that people who use sunscreen daily can maintain their vitamin D levels. One of the explanations for this may be that no matter how much sunscreen you use or how high the SPF, some of the sun’s UV rays reach your skin. An SPF 15 sunscreen filters out 93 percent of UVB rays, SPF 30 keeps out 97 percent, and SPF 50 filters out 98 percent. This leaves anywhere from 2 to 7 percent of solar UVB reaching your skin, even with high-SPF sunscreens. And that’s if you use them perfectly.
5. Geographical area
The further away you live from the equator, the less vitamin D you’ll make. The reason is that, although the sun is closest to the earth in the winter, the sun’s rays are entering at a more oblique angle (zenith angle), causing the UVB photons to pass through the ozone for a greater distance and hence more UVB photons are absorbed by the ozone layer.
Maximum vitamin D production occurs in the summer months, whereas depending on the latitude, little to no vitamin D is made in winter months. In the southern hemisphere, for example, residents of Cape Town, South Africa, can make far less vitamin D from the sun during their winter months (June through August) than they can during their spring and summer.
The amount of 7-dehydrocholesterol in the epidermis of skin (which influences vitamin D3 production) is relatively constant until later in life, when it begins to decline, so a 70-year-old who is exposed to the same amount of sunlight as a 20-year-old, only makes 25% of the vitamin D3 that the 20-year-old makes.
Vitamin D3 is fat soluble and is stored in the body fat. Any excess vitamin D3 that is produced can be stored in the body fat and used during the winter, when little vitamin D3 is produced in the skin. For obese individuals, the fat can be an irreversible sink for vitamin D and they are at increased risk for vitamin D deficiency.
While there is concern about sunlight exposure causing skin damage, skin cancer and wrinkling, researchers have found that it is reasonable to allow some sun exposure without sun protection, for production of adequate amounts of vitamin D3. Though, if you’re unable to get sunshine every day, supplementation is an option.