Vitamin D or Cholecalciferol + Food Sources of Vitamin D, Role in the Body, and Deficiency and Toxicity
What is vitamin D?
Vitamin D is a fat soluble vitamin and is somewhat unique in relation to the other vitamins because the body can produce it in adequate amounts with the assistance of the sun. In fact, many researchers feel that since certain cells can produce vitamin D and because it then circulates and affects tissue throughout the body, it might be better classified as a hormone than a vitamin. However, the body’s ability to make vitamin D relies upon exposure to sunlight (ultraviolet B light), and not everyone receives adequate exposure. Furthermore, direct exposure to sunlight is not recommended due to the increased risk of skin cancers. For this reason, vitamin D will maintain its position as a vitamin.
What foods provide vitamin D?
There are two possible ways of supplying the body with vitamin D: through diet and exposure to the sunlight. In the human diet, the richest sources of vitamin D are vitamin D-fortified milk and milk products, tuna, salmon, margarine (vitamin D fortified), herring, and vitamin D-fortified cereals (see ). Vitamin D in foods appears fairly stable in various cooking and storing procedures. Vitamin is available in nutrition supplements in the form of cholecalciferol. International Units (IU) are often used to express vitamin D levels on packaging whereby 1 µg is equal to 40 IU of vitamin D. Thus 10 µg would be equal to 400 IU which is commonly used in supplements and provides 100% of the Daily Value (DV).
How much vitamin D do we need?
The RDA for adults 50 years of age and younger as well as pregnant women is 5 µg ( 200 IU) of vitamin D daily. One µg is the equivalent of 40 IU of vitamin D. For adults over the age of 50 and 70 the RDA increases to 10 and 15 µg (400 IU) daily. See the DRI/RDA Table for recommended levels of vitamin D for children and teens.
How much sunlight is required to make vitaminD?
People with lighter skin color require as little as 10 min of sun exposure to make adequate amounts of vitamin D. However this requires direct sunlight exposure to skin during mid day. However, sunscreen with SPF 8 or higher significantly reduces the process and the necessary exposure is increased for people with darker skin color seems to increase relative to the degree of darkness. This also means that a person will make less and less vitamin D as they tan longer or over several days, as in a vacation at the beach. This helps to protect people from potentially making too much vitamin D.
Interestingly, the ability to make vitamin D appears to be stronger during youth and decreases as humans get older. For this reason, the need for vitamin D from food and or supplements increases as we get older (50+). This is one reason why many nutrition scientists are calling for even higher recommended intake levels for aging people.
What processes are involved in making vitamin D?
The process of making vitamin D can be simplified to three primary locations within the body. First, within the skin a derivative of cholesterol called 7-dehydrocholesterol is converted to another substance called cholecalciferol. In order for this to occur, 7-dehydrocholesterol must be exposed to ultraviolet (UV) radiation from the sun or other UV sources (e.g., tanning beds). As mentioned, the efficiency of this conversion appears to decrease as UV exposure time increases. Once cholecalciferol has been produced it leaves the skin region and circulates in the blood with the help of a transport protein called vitamin D binding protein (DBP). In the cholecalciferol form, vitamin D is only minimally active in the body.
The activity of vitamin D depends on its ability to be recognized by vitamin D receptors in specific cells. In order for cholecalciferol to become more attractive to the vitamin D receptor, it must undergo more changes in its molecular design.The first change takes place in the liver as circulating cholecalciferol is removed and modified to become 25-hydroxycholecalciferol. This form of vitamin D is released by the liver and reenters the blood. This form of vitamin D is a little more attractive to vitamin D receptors and some of the effects of vitamin D are realized. 25-dihydroxycholecalciferol can circulate to the kidneys and be modified further to the most potent form of vitamin D-1,25-dihydroxycholecalciferol or calcitriol, which is released back into circulation in this form, vitamin D is exceptionally attractive to vitamin D receptors strategically located within certain cells in the body.
Does the vitamin D in foods need to be processed in the body?
The vitamin D in foods is fairly well absorbed across the wall of the small intestine. Because this form of vitamin D is fat soluble (water insoluble), it will require the same digestive and absorptive considerations as other lipid substances. This includes the presence of bile and the incorporation into chylomicrons. This vitamin D will eventually make it to the liver and must also undergo the same modifications in the liver and kidney as did the vitamin D made from cholesterol in the skin.
What does vitamin D do in the body?
In order for a cell to be influenced by vitamin D it must possess the vitamin D receptor (VDR) which is located in the nucleus of cells. This further strengthens the argument that vitamin D is more like a hormone than a vitamin. Remember that a hormone must bind with a specific receptor in order to be active. While researchers continue to discover vitamin D receptors in various tissues throughout the body, most of the attention has centered around the bone, kidneys, and intestines. Vitamin D is classically recognized as being principally involved in bone and calcium metabolism although newer functions of vitamin D are being added to the list.
What happens if too little vitamin D is consumed?
Deficiency of vitamin D can occur when a combination of factors is present. If vitamin D intake and/or absorption is low and an individual does not receive adequate exposure to sunlight, the potential for a vitamin D deficiency is present. Depending on the stage in life, vitamin D deficiency can results in:
Can too much vitamin D be consumed?
Of the vitamins, vitamin D has one of the lowest levels of intakes above recommendations that could give rise to side effects. Many of the manifestations appear to be related to vitamin D’s increasing calcium absorption, which in turn results in too much calcium in the blood. Prolonged hypercalcemia (elevated blood calcium) can affect muscle cell activity, which includes the heart, and can induce nausea, vomiting, mental confusion, and lead to calcium deposition in various tissues throughout the body. While the Tolerable Upper Limit has been set at 5X the AI for adults more recent research suggests that the threshold for potential side effects of excessive intake could indeed be at double that level.
Luckily, as exposure to sunlight increases the body’s ability to make vitamin D decreases. Also, as the level of active vitamin D increases, kidney cells produce less and less of the converting enzyme needed to make more active vitamin D. These mechanisms attempt to decrease the potential for toxicity.Vitamin D We may be more sensitive to vitamin D toxicity than other vitamins when looking at the intake level associated with signs and symptoms.