What is Vitamin D?
Vitamin D is fat-soluble vitamin that is needed to be healthy and maintain strong bones. Known as the "sunshine vitamin", vitamin D is formed when skin is exposed to the sun's ultraviolet rays and is also found in dietary supplements and certain foods.
There are two major types of vitamin D in humans. Vitamin D3 (cholecalciferol) is the type produced in the body in response to exposure to the sun's ultraviolet B rays. Vitamin D2 (ergocalciferol) is synthesized in plants. Both types must be converted in the liver and kidneys to the active form, 1,25 dihydroxyvitamin D, to be utilized in the body.
Health Benefits of Vitamin D
The main function of vitamin D is to help the body absorb calcium and phosphorus in the small intestine. Calcium is needed to support bone mineralization (hardening of bones), cell functions, and proper nerve and muscle function.
People who have a vitamin D deficiency may develop soft, weakened, and brittle bones, a condition known as rickets in children and osteomalacia in adults. Vitamin D is promoted by mainstream clinicians for it's role in balancing calcium and phosphorus and for bone health. Additionally, there are a number of promising areas of vitamin D research beyond bone disorders.
1) Heart Health
According to the Health Professional Follow-Up study, which checked blood levels of vitamin D in almost 50,000 men who were healthy and followed them for 10 years, men who were vitamin D deficient were twice as likely to have a heart attack as men with adequate vitamin D levels.
Supplementation with 1,000 IU of vitamin D, or higher serum vitamin D levels, may have a slightly lower risk of cardiovascular disease and complications.
According to observational studies and preliminary lab studies, higher vitamin D and calcium intake and status may be linked with lower cancer risk (especially colorectal cancer), although it is difficult to separate the effect of the two because of the way vitamin D affects calcium levels. According to a meta-analysis published in the American Journal of Preventative Medicine, those with the highest level of vitamin D were less at a 50% lower risk of colorectal cancer.
A four year study published in 2007 examined the use of calcium (1,400-1,500mg daily), vitamin D3 (1,100IU daily) or a placebo in 1,179 women over 55. The women who took calcium and vitamin D had significantly less risk for all types of cancer combined, as well as the women with higher vitamin D levels at the start of the study. Not all studies have been positive, as a Women's Health Initiative study published in 2006 did not find a lower cancer risk in those who took vitamin D (vitamin D intake was lower, at 400IU per day).
3) Colds and Flu
The flu virus causes the most illness in the winter months, leading some researchers to hypothesize that influenza may be related to vitamin D levels. Vitamin D levels are lowest during the winter. Also observational studies have found that people who have low vitamin D levels are more likely to get respiratory infections or report having a recent cold or upper respiratory tract infection.
A study published in the American Journal of Clinical Nutrition examined the use of vitamin D (1,200 daily) or a placebo in nearly 340 children during the winter months. After four months, researchers found that the rate of influenza type A was approximately 40 percent lower than in the placebo group, with no significant difference in influenza type B rates.
4) Weight Loss
The evidence on vitamin D for overweight/obese adults is mixed. According to a study published in Nutrition Journal, 25mcg daily of vitamin D for 12 weeks in overweight and obese women induced a statistically significant reduction in fat mass compared to those taking a placebo.
A 2013 study in Clinical Nutrition examined 4000IU of vitamin D daily plus resistance training for 12 weeks and failed to find significant changes in fat mass in those taking vitamin D.
- Osteoporosis prevention and treatment
- Multiple sclerosis
- Dental cavities and gum disease prevention
- Skin conditions including psoriasis, acne and eczema
- Fatigue, low energy
- Pain (such as back pain, knee pain, neuropathy)
- Mood disorders including depression, seasonal affective disorder
- Autoimmune disorders such as diabetes, rheumatoid arthritis, Crohn's disease, ulcerative colitis
- Thyroid and kidney disease
Vitamin D in Foods and Supplements
The main source of vitamin D comes from exposure to the sun's UVB rays. The American Academy of Dermatology advises that we obtain vitamin D from foods and supplements rather than UV exposure, because of the risk of skin cancer.
Vitamin D rich foods include certain types of fatty fish, such as herring, mackerel, salmon, tuna, and sardines. Egg yolks, cheese, and beef liver provide smaller amounts of vitamin D. Mushrooms provide some vitamin D, with mushrooms that have been exposed to ultraviolet light being higher in vitamin D.
Although there are few foods that naturally contain vitamin D, many common foods are often fortified with vitamin D such as milk, breakfast cereals, soy milk, rice milk (and other plant-based milks), yogurt, orange, and margarine.
Vitamin D supplements are also available as capsules, gummies, liquid, or chewable tablets. Cod liver oil is also still used. Vitamin D in supplements or in enriched foods may be vitamin D2 and D3. Vitamin D3 (cholecalciferol) is the preferred form due to better utilization in the body. In addition to single supplements, multivitamins and calcium supplements provide vitamin D, but the amount varies widely so it's important to read labels.
People who follow a vegetarian or vegan diet should check the source of vitamin D in fortified foods and supplements - while vitamin D3 is widely considered the better utilized form, vitamin D3 is often sourced from animals (primarily sheep's wool) while vitamin D2 comes from plant sources. Vitamin D gummies may also contain gelatin.
Recommended Vitamin D Intake
In the United States, the Institute of Medicine's recommendations for vitamin D intake, which were released in 1997, are as follows:
- Birth to 50 years -- 200 IU (5 mcg)
- Between 51 and 70 years -- 400 IU (10 mcg)
- Over 71 years -- 600 IU (15 mcg)
- Pregnant and nursing women -- 200 IU (5 mcg)
Optimal vitamin D intakes are considered to be much higher, however, with at least 1,000 to 2,000 IU (25-50mcg) recommended for adults. There is growing consensus that reference intakes need to be reassessed, based on increasing evidence that vitamin D deficiency is widespread and also due to research on the complex role of vitamin D in the prevention of many diseases.
Because there are many sources of vitamin D, the best way to measure one's vitamin D level is to get one's level checked with a blood test for a form known as 25-hydroxyvitamin D. In general, vitamin D levels below 30nmol/L (12 ng/mL) are too low for bone health and overall health. A vitamin D level of 50 nmol/L or above is sufficient for most, although vitamin D levels above 125 nmol/L (50 ng/mL) are probably too high.
The safe upper limit of vitamin D is 1,000-1,500 IU/day for infants, 2,500-3,000 IU for children 1-8 years, and 4,000 IU/day for children 9 and older, adults, pregnant and breastfeeding women.
People Who May Be At Risk for Vitamin D Deficiency
People with limited sun exposure
The amount of vitamin D formed from exposure to the sun is affected by season and latitude. In general, in northern cities such as Boston or New York, there aren't enough UVB rays to form vitamin D during the fall and winter months.People who are homebound, women who wear robes and head coverings for religious reasons, and people whose jobs or work hours limit their exposure to sunlight are likely not obtaining significant amounts of vitamin D from sunlight.
People who use sunscreen
The use of sunscreen prevents the formation of vitamin D, too. Even sunscreen with an SPF of 8, an amount found in many daily moisturizers, can greatly reduce vitamin D production. In one research study, nearly half of people in Springfield, Illinois, who always wore sunscreen while outdoors had a vitamin D deficiency.
People with greater skin pigment
People with darker skin have more melanin, the pigment that gives skin its color. Melanin absorbs UV rays, which reduces the skin's ability to produce vitamin D. The more pigment in a person's skin, the more important it is that he or she consume enough vitamin D.
Older people are at greater risk of vitamin D deficiency, as the ability to produce vitamin D dwindles with age. In fact, vitamin D levels in elderly subjects have been found to be approximately 30% of the levels found in young adults.It's estimated that over 50% of adults over age 50 in the United States and Europe are at risk of vitamin D deficiency. One study found that in Boston at the end of the summer, 30% of whites, 42% of Hispanics, and 84% of African-Americans had vitamin D deficiency.
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