Fat-Soluble Vitamins; Vitamin D

 Vitamin D

Vitamin D food sources

Vitamin D differs from the other nutrients in that the body can synthesize it, with the help of sunlight, from a precursor that the body makes from cholesterol. Therefore, vitamin D is not an essential nutrient; given enough time in the sun, people need no vitamin D from foods. 

Also known as calciferol, vitamin D comes in two major forms. Vitamin D2 derives primarily from plant foods in the diet. Vitamin D3 derives from animal foods in the diet and from synthesis in the skin. These two forms of vitamin D are similar and both must be activated before they can fully function. 

To make vitamin D, ultraviolet rays from the sun hit a precursor in the skin and convert it to pre-vitamin D3, which is converted to vitamin D3 with the help of body's heat. To activate vitamin D _whether made in the body or consumed from the diet _two hydroxylation reactions must occur. First, the liver adds an OH group, and then the kidneys add another OH group to produce the active vitamin. Diseases affecting either the liver or the kidneys can interfere with the activation of vitamin D and produce symptoms of deficiency.

Vitamin D Recommendation & Sources:

Only a few foods contain vitamin D naturally. Fortunately, the body can make vitamin D with the help of a little sunshine. In setting dietary recommendations, however, the DRI Committee assumed that no vitamin D was available from skin synthesis.  Some research suggests that the vitamin D RDA is insufficient, especially for African Americans.

Vitamin D in Foods:

The Dietary Guidelines advise consumers to drink vitamin D fortified milk. The fortification of milk and other foods with vitamin D is the best guarantee that people will meet their needs. Despite vitamin D fortification, the average intake in the United States falls short of recommendations. Egg yolks, and oily fish such as salmon, mackerel, and sardines are the best natural sources of vitamin D. 

Meeting vitamin D needs is difficult without adequate sunshine, fortification or supplementation. Importantly, feeding infants and young children nonfortified "healthy beverages", instead of milk or infant formula can create severe nutrient deficiencies, including rickets.  

Vitamin D From the Sun:

Sunlight; major source of vitamin D

Most of the world's population relies on natural exposure to sunlight to maintain adequate vitamin D nutrition. The sun imposes no risk of vitamin D toxicity; prolonged exposure to sunlight degrades the vitamin d precursor in the skin, preventing its conversion to the active vitamin.

Prolonged exposure to sunlight can, however, prematurely wrinkle the skin and cause skin cancer. Sunscreen help reduce these risks, but sunscreens with a sun protection factor of 8 and higher can also reduce vitamin D synthesis. For most people, exposing hands, face, and arms on a clear summer day for 5 to 10 minutes two or three times a week should be sufficient to maintain vitamin D nutrition. Avoiding sun exposure completely may be harmful to health.

The pigments of dark skin provide some protection from the sun's damage, but they also reduce vitamin d synthesis. Dark-skinned people require more sunlight exposure than light-skinned people _perhaps as much as 4 to 6 times longer. People who stay in the shade and wear long-sleeved clothing are twice as likely to develop vitamin D deficiency as those who rarely do so. Vitamin d deficiency is especially prevalent in the winter and in the Arctic and Antarctic regions of the world. To ensure an adequate vitamin D status, supplements may be needed. The body's vitamin D supplies from summer synthesis alone are insufficient to meet winter needs.

Depending on the radiation used, the UV rays from tanning lamps and tanning beds may also stimulate vitamin D synthesis. The potential hazards of skin damage, however, may outweigh any possible benefits. The Food and Drug Administration (FDA) warns that if the lamps are not properly filtered, people using tanning booths risk burns, damage to the eyes and blood vessels, and skin cancer.

Vitamin D from Supplements:

Vitamin D Supplements

Some people may benefit from taking vitamin D supplements. Vitamin D can be found in multivitamin-mineral supplements as well as a high-dose supplement. As a single supplement, vitamin D3 is less expensive, more commonly available, and more effective than vitamin D2. 

Functions of Vitamin D in the Body:

Though called a vitamin, the active form of vitamin D is actually a hormone _a compound manufactured by one part of the body that travels through the blood and triggers a response from another part of the body. Like vitamin A, vitamin D has a binding protein that carries it to the target organs; most notably, the intestines, the kidneys, and the bones. All respond to vitamin D by making the minerals needed for bone growth and maintenance available.

Vitamin D in Bone Growth:

Vitamin D is a member of a large and cooperative bone-making and maintenance team composed of nutrients and other compounds, including vitamins A and K; the hormones parathyroid hormone and calcitonin; the protein collagen; and the minerals calcium, phosphorus, magnesium, and fluoride. Vitamin D's specific role in bone health is to assist in the absorption of calcium and  phosphorus, thus helping to maintain blood concentration of these minerals. The bones grow denser and stronger as they absorb and deposit these minerals. 

Vitamin D raises blood concentrations of bone minerals in three ways. When the diet is sufficient, vitamin D enhances mineral absorption from the GI tract. When the diet is insufficient, vitamin D provides the needed minerals from other sources; reabsorption by the kidneys and mobilization from the bones into the blood. Vitamin D may work alone, as it does in the GI tract, or in combination with parathyroid hormone, as it does in the bones and kidneys. 

Vitamin D in Other Roles:

Scientists have discovered many other tissues that respond to vitamin D. In the brain and nerve cells, vitamin D protects against cognitive decline and shows the progression of Parkinson disease. Vitamin D in muscle cells encourages growth in children and preserves strength in adults. Vitamin D signals cells of the immune system to defend against infectious diseases. Vitamin D may also regulate the cells of the adipose tissue in ways that might influence the development of obesity.

In many cases, vitamin D enhances or suppresses the activity of genes that regulate cell growth. As such as, it may be valuable in treating a number of diseases. Recent research suggests that vitamin D may protect against heart disease, type 2 diabetes, inflammation, brain disorders, macular degeneration, hypertension, and some cancers. Even so, evidence does not support vitamin D supplementation to improve health beyond correcting deficiencies. 

Vitamin D Deficiency:

Overt signs of vitamin D deficiency are relatively rare, but vitamin D insufficiency is remarkably common. An estimated 16 percent of the US population has low blood levels of vitamin D. Most vulnerable are blacks, especially overweight and obese black children. Factors that contribute to vitamin D deficiency include dark skin, breastfeeding without supplementation, lack of sunlight, and not using fortified milk. In vitamin D deficiency, production of calbindin, a calcium-binding transport protein that binds calcium in the intestinal cells and it requires vitamin D for its synthesis, slows. Thus, even when calcium in the diet is adequate, it passes through the GI tract unabsorbed, leaving the bones undersupplied. Consequently, a vitamin D deficiency creates a calcium deficiency and increases the risks of several chronic diseases and osteoporosis. Vitamin D-deficient adolescents do not reach their peak bone mass.

Rickets:

The vitamin D-deficiency disease mostly occur in children. Worldwide, the prevalence of the vitamin D-deficiency disease rickets is extremely high, affecting more than half of the children in some countries. In the United States, rickets is not common, but when it occurs, black children and adolescents _especially females and overweight teens _are the ones most likely to be affected. To prevent rickets, the American Academy of Pediatrics recommends a supplement for all infants, children, and adolescents who do not receive enough vitamin D.

In rickets, the bones fail to calcify normally, causing growth impairments and skeletal abnormalities. The bones become so weak that they bend when they have to support the body's weight. A child with rickets who is old enough to walk characteristically develops bowed legs (in rickets, the poorly formed long bones of the legs bend outward as weight-bearing activities such as walking begin), often the most obvious sign of the disease. Another sign is the beaded ribs (in rickets, a series of beads develop where the cartilages and bones attach) that result from the poorly formed attachments of the bones to the cartilage. A rare type of rickets, not caused by vitamin D deficiency, is known as vitamin D- refractory rickets.

Osteomalacia:

Osteo means "bone" and malacia means "softening" , it is a bone disease characterized by softening of the bones. Symptoms include bending of the spine and bowing of the legs. The disease occurs most often in adult women. In adults, the poor mineralization of bone results in the painful bone disease osteomalacia. The bones become increasingly soft, flexible, brittle and deformed. 

Osteoporosis:

The word "osteoporosis" means 'porous bone'. It is a disease that weakens bones, and if anyone have it, than he/she are at a greater risk for sudden and unexpected bone fractures. Osteoporosis means that you have less bone mass and strength.  Any failure to synthesize adequate vitamin D or obtain enough from foods sets the stage for a loss of calcium from the bones, which can result in painful fractures. Most of these are fractures of the hip, wrist and spine. 

The Elderly:

Vitamin d deficiency is especially likely in older adults for several reasons. For one, the skin, liver, and kidneys lose their capacity to make and activate vitamin D with advancing age. For another, older adults typically drink little or no milk __the main dietary source of vitamin D. And finally, older adults typically spend much of the day indoors, and when they do venture outside, many of them cautiously wear protective clothing or apply sunscreen to all sun-exposed areas of the skin. Dark-skinned adults living in northern areas are particularly vulnerable. All of these factors increase the likelihood of vitamin D deficiency and its consequences : bone losses, osteoporotic fractures, and muscle weakness. Vitamin D supplementation helps raise blood levels, reduce bone loss, improve muscle performance, and lower the risks of falls and fractures in elderly persons.

Vitamin D Toxicity:

  Vitamin D clearly illustrates how nutrients in optimal amounts support health, but both inadequacies and excesses create harm. Vitamin D is among the most likely of the vitamins to have toxic effects when consumed in excessive amounts. The amounts of vitamin D made by the skin and found in foods are well within the safe limits set by the UI, but supplements containing the vitamin in concentrated form should be kept out of the reach of children and used cautiously by adults. Excess vitamin D raises the concentration of blood calcium. Excess blood calcium tends to participate in the soft tissue, forming stones, especially in the kidneys, where calcium is concentrated in an effort to excrete it. Calcification may also harden the blood vessels and is especially dangerous in the major arteries of the brain, heart, and lungs, where it can cause death.