What is it about vitamin D that lends itself to such impressive study results? Can vitamin D really help prevent the common cold?
What Is Vitamin D?
Vitamin D is a fat-soluble vitamin that can be found in just a few food sources. It can also be synthesized (made) in the human body as a result of exposure to the ultraviolet (UV) rays in sunlight. A fat-soluble vitamin is one that can be dissolved in fats and oils, is absorbed along with fats in the diet, and is stored in fat tissue in the body.
The Function of Vitamin D
A primary function of vitamin D is to promote calcium absorption, which is necessary for healthy bones. This is one reason Vitamin D is added to milk products: it ensures that the calcium in milk is readily absorbed by the body, which promotes healthy bone growth.
Vitamin D supplementation in U.S. milk products began as an effort to prevent rickets (a childhood disease involving soft, distorted bones, often resulting in bow-legs, from vitamin D deficiency). Vitamin D also helps protect against osteoporosis in older people.
Vitamin D is also used by the body to:
Promote cell growthPromote neuromuscular (nerves and muscles) functionReduce inflammationInfluence immune function
Vitamin D and the Immune System
The immune system defends the body against foreign organisms such as bacteria, viruses, and parasites. Not only does the immune system kill foreign invaders, but it also develops a protective ability (acquired immunity) to prevent future infections.
Vitamin D has been shown to have many effects on immune cells, enhancing the body’s ability to fight infection and decrease inflammation. Vitamin D has also been found to regulate acquired immune response (also called the adaptive immune response). A deficiency of vitamin D is linked to increased susceptibility to infection.
Historical Use
In the past, vitamin D was unintentionally used to treat infections, such as tuberculosis, before antibiotics were available. Tuberculosis patients were sent to long-term care centers called sanitariums. They were treated with sunlight, which was thought to kill tuberculosis, when in fact the sunlight was producing vitamin D in the body. Vitamin D, not the sunlight, is now thought to be the causative factor in the positive response the tuberculosis patients realized from sunlight exposure.
Another common treatment for tuberculosis was cod liver oil, which is rich in vitamin D. Cod liver oil has been used for many years as a preventative measure to protect from infections.
Studies on Vitamin D to Prevent Respiratory Infections
A systematic review of 25 controlled studies published in BMJ found that vitamin D supplementation “reduced the risk of acute respiratory infection among all participants,” according to the study authors. The study also found that those who have low vitamin D levels, and who take vitamin D3 supplements daily or weekly (rather than in one large dose), realized the highest level of benefits when it came to preventing acute respiratory infections.
The type of infections that are considered acute respiratory infections include:
The common cold Ear infections Bronchitis Tonsillitis Pneumonia
Sources of Vitamin D
Food
Foods rich in vitamin D include:
The Harvard report adds that taking a supplemental dose of 1,000 to 2,000 IU per day of vitamin D is optimal. This is particularly appropriate for those who have reason to believe that they have low levels of vitamin D (such as dark-skinned people who do not get the optimal benefits of sunlight exposure and those who live in Northern climates, or who otherwise do not get enough sunlight exposure).
Flesh from fatty fish (such as salmon and mackerel)Fish liver oils (such as cod liver oil)
Foods with small amounts of vitamin D include:
Beef liverCheeseEgg yolksSome mushrooms (vitamin D2)
Fortified foods provide most of the vitamin D in the American diet. These include:
MilkBreakfast cerealsSome orange juice, yogurt, and margarine brandsSome plant-based milk products (such as almond, soy, or oat milk)
Sun
It’s not always easy to get all of your required vitamin D from food, but the body (in humans and animals) is capable of making vitamin D when the skin is exposed to sunlight.
When ultraviolet B (UVB) light rays from sunlight penetrate the skin, it triggers the synthesis of vitamin D3 in the body. UVB rays convert a protein in the skin called 7-DHC into vitamin D3.
Most people get at least some of their vitamin D supplies from sunlight exposure. But there are factors that influence the absorption of sunlight and, subsequently, the conversion of ultraviolet light rays to vitamin D. These factors include:
SeasonTime of dayAmount of cloud coverLevel of environmental smogConcentration of skin melanin (dark-skinned people receive less ultraviolet light penetration than light-skinned people)Use of sunscreen (which blocks the absorption of UV rays)
Some of the vitamin D produced by the skin during warm weather months is stored in the liver and fat tissue for later use. In this way, even in northern winter climates, people are able to utilize stored vitamin D rather than relying completely on dietary sources. Those with limited sunlight exposure should be sure to eat vitamin-D-rich foods or take a vitamin D supplement.
Supplements
There are two types of vitamin D supplements available for purchase: these are vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D2 comes from plant sources (such as mushrooms), while vitamin D3 is from animal sources. Sunlight stimulates the synthesis of D3 and is also found in animal sources (such as fatty fish).
Although some experts debate which type of vitamin D supplement is more effective at raising levels of vitamin D in the human body, there is evidence that D3 may be better. A 2012 meta-analysis of randomized controlled trials comparing D2 and D3 supplements found that D3 led to more of an increase in blood levels of the vitamin, and this effect lasted longer than with D2.
