Vitamin D obtained from sun exposure, food, and supplements is biologically inert and must undergo two hydroxylations in the body for activation.
The first occurs in the liver and converts vitamin D to 25-hydroxyvitamin D [25(OH)D], also known as calcidiol or ergocalciferol or vitamin D2.
The second occurs primarily in the kidney and forms the physiologically active 1,25-dihydroxyvitamin D [1,25(OH)2D], also known as calcitriol or cholecalciferol or vitamin D3. As supplement is preferable the D3 active form.
Vitamin D promotes calcium absorption in the gut and maintains adequate serum calcium and phosphate concentrations to enable normal mineralization of bone. It is also needed for bone growth and bone remodeling. Without sufficient vitamin D, bones can become thin, brittle, or misshapen. Vitamin D sufficiency prevents rickets in children and osteomalacia in adults. Together with calcium, vitamin D also helps protect older adults from osteoporosis.
Vitamin D has other roles in the body, including modulation of cell growth, neuromuscular and immune function, and reduction of inflammation.
The RDAs range is 200 IU/5 mcg – 800 IU/20 mcg:
|Table 1: Recommended Dietary Allowances (RDAs) for Vitamin D|
|0-12 months*||400 IU (100 mcg)||400 IU (100 mcg)|
|1-13 years||600 IU (15 mcg)||600 IU (15 mcg)|
|14-18 years||600 IU (15 mcg)||600 IU (15 mcg)||600 IU (15 mcg)||600 IU (15 mcg)|
|19-50 years||600 IU (15 mcg)||600 IU (15 mcg)||600 IU (15 mcg)||600 IU (15 mcg)|
|51-70 years||600 IU (15 mcg)||600 IU (15 mcg)|
|>70 years||800 IU (20 mcg)||800 IU (20 mcg)|
* Adequate Intake (AI)
Very few foods in nature contain vitamin D, mainly fish:
|Table 2: Selected Food Sources of Vitamin D|
|Food||IUs per serving*||Percent DV**|
|Cod liver oil, 1 tablespoon||1.360||340|
|Swordfish, cooked, 3 ounces||566||142|
|Salmon (sockeye), cooked, 3 ounces||447||112|
|Tuna fish, canned in water, drained, 3 ounces||154||39|
|Milk, nonfat, reduced fat, and whole, vitamin D-fortified, 1 cup||115-124||29-31|
|Yogurt, fortified with 20% of the DV for vitamin D, 6 ounces||80||20|
|Sardines, canned in oil, drained, 2 sardines||46||12|
|Egg, 1 large (vitamin D is found in yolk)||41||10|
|Cheese, Swiss, 1 ounce||6||2|
* IUs = International Units; ** DV = Daily Value.
Vitamin D Deficiency
Nutrient deficiencies are usually the result of dietary inadequacy, impaired absorption and use, increased requirement, or increased excretion. A vitamin D deficiency can occur when usual intake is lower than recommended levels over time, exposure to sunlight is limited, the kidneys cannot convert vitamin D2 to its active form D3, or absorption of vitamin D from the digestive tract is inadequate. Vitamin D-deficient diets are associated with milk allergy, lactose intolerance, ovo-vegetarianism, and veganism.
In adults, vitamin D deficiency can lead to osteomalacia, resulting in weak bones. Symptoms of bone pain and muscle weakness can indicate inadequate vitamin D levels, but such symptoms can be subtle and go undetected in the initial stages.
Groups at Risk of Vitamin D Inadequacy
The American Academy of Pediatrics recommends that exclusively and partially breastfed infants be supplemented with 400 IU of vitamin D per day.
Older adults are at increased risk of developing vitamin D insufficiency in part because, as they age, skin cannot synthesize vitamin D as efficiently, they are likely to spend more time indoors, and they may have inadequate intakes of the vitamin. As many as half of older adults in the United States with hip fractures could have inadequate serum levels of vitamin D.
People with limited sun exposure
Homebound individuals, women who wear long robes and head coverings for religious reasons, and people with occupations that limit sun exposure are unlikely to obtain adequate vitamin D from sunlight. Ingesting RDA levels of vitamin D from foods and/or supplements will provide these individuals with adequate amounts of this nutrient.
People with inflammatory bowel disease and other conditions causing fat malabsorption
Because vitamin D is a fat-soluble vitamin, its absorption depends on the gut’s ability to absorb dietary fat. Individuals who have a reduced ability to absorb dietary fat might require vitamin D supplementation. Fat malabsorption is associated with a variety of medical conditions, including some forms of liver disease, cystic fibrosis, celiac disease, and Crohn’s disease, as well as ulcerative colitis when the terminal ileum is inflamed. In addition, people with some of these conditions might have lower intakes of certain foods, such as dairy products fortified with vitamin D.
People who are obese or who have undergone gastric bypass surgery
A body mass index ≥30 is associated with lower serum vitamin D levels compared with non-obese individuals; people who are obese may need larger than usual intakes of vitamin D to achieve vitamin D levels comparable to those of normal weight. Obesity does not affect skin’s capacity to synthesize vitamin D, but greater amounts of subcutaneous fat sequester more of the vitamin and alter its release into the circulation. Obese individuals who have undergone gastric bypass surgery may become vitamin D deficient over time without a sufficient intake of this nutrient from food or supplements, since part of the upper small intestine where vitamin D is absorbed is bypassed and vitamin D mobilized into the serum from fat stores may not compensate over time.
Vitamin D and Health
More than 40 million adults in the United States have or are at risk of developing osteoporosis, a disease characterized by low bone mass and structural deterioration of bone tissue that increases bone fragility and significantly increases the risk of bone fractures. osteoporosis is an example of a long-term effect of calcium and vitamin D insufficiency. Adequate storage levels of vitamin D maintain bone strength and might help prevent osteoporosis in older adults, non-ambulatory individuals who have difficulty exercising, postmenopausal women, and individuals on chronic steroid therapy.
Laboratory and animal evidence as well as epidemiologic data suggest that vitamin D status could affect cancer risk. Strong biological and mechanistic bases indicate that vitamin D plays a role in the prevention of colon, prostate, and breast cancers. Taken together, however, studies to date do not support a role for vitamin D in reducing the risk of cancer.
A growing body of research suggests that vitamin D might play some role in the prevention and treatment of type 1 and type 2 diabetes, hypertension, glucose intolerance, multiple sclerosis, and other medical conditions. However, most evidence for these roles comes from in vitro, animal, and epidemiological studies, not the randomized clinical trials considered to be more definitive.
Health Risks from Excessive Vitamin D
Vitamin D toxicity can cause non-specific symptoms such as anorexia, weight loss, polyuria, and heart arrhythmias. More seriously, it can also raise blood levels of calcium which leads to vascular and tissue calcification, with subsequent damage to the heart, blood vessels, and kidneys. The use of supplements of (1,000 mg/day) and vitamin D (400 IU) by postmenopausal women was associated with a 17% increase in the risk of kidney stones over 7 years in the Women’s Health Initiative. While symptoms of toxicity are unlikely at daily intakes below 10,000 IU/day, the FNB pointed to emerging science from national survey data, observational studies, and clinical trials suggesting that even lower vitamin D intakes and serum 25(OH)D levels might have adverse health effects over time.
Long-term intakes above the UL increase the risk of adverse health effects:
|Table 3: Tolerable Upper Intake Levels (ULs) for Vitamin D|
|0-6 months||1.000 IU (25 mcg)||1.000 IU (25 mcg)|
|7-12 months||1.500 IU (38 mcg)||1.500 IU (38 mcg)|
|1-3 years||2.500 IU (63 mcg)||2.500 IU (63 mcg)|
|4-8 years||3.000 IU (75 mcg)||3.000 IU (75 mcg)|
|9-18 years||4.000 IU (100 mcg)||4.000 IU (100 mcg)||4.000 IU (100 mcg)||4.000 IU (100 mcg)|
|19+ years||4.000 IU (100 mcg)||4.000 IU (100 mcg)||4.000 IU (100 mcg)||4.000 IU (100 mcg)|
Reference: National Institute of Health USA 2018
Vitamin D 800 IU is a pure pharma-grade vitamin D in its active form. The tablet dosage/day of 800 IU is effective for health protection of muscular skeletal as well as immune system without the risks of vitamin D Upper Limits Intakes.
60 divisible tablets.
1 tablet (800 IU) a day.