Vitamin D, the sunshine vitamin for strong bones and teeth, turns out to be a multi-tasker! Since its discovery in the early 1920s, this important fat-soluble vitamin was labeled simply as “the antirachitic vitamin” (prevents rickets). Not any more. A major discovery of how it functions as a hormone in the body when converted into its active form by the liver has spurred intense research which is revealing much more about this amazing vitamin.
We now know the active form of vitamin D is essential for the regulation of calcium and phosphorus balance in the body, the absorption of calcium and phosphorus from food in the digestive tract, proper neuromuscular function, normal growth and development and normal bone and tooth formation and maintenance. Recent medical research suggests vitamin D may also provide protection from hypertension, cancer, and several autoimmune diseases.
Vitamin D deficiency leads to malfunction in tissues that require sufficient levels of vitamin D and deficiencies of calcium and phosphorus which both require vitamin D for proper absorption by the body. Calcium and phosphorus are the main minerals that make up our bones and teeth. In our bloodstream, calcium opposes phosphorus as a buffer system to maintain the acid-alkaline balance of the blood. These minerals also play an important role in the function of the neuromuscular system and are critical for milk production in the nursing of infants.
Vitamin D deficiency is common in celiac disease, largely due to fat malabsorption, or disruption of the ability to absorb fat from the small intestine. Vitamin D, being a fat-soluble vitamin (means it dissolves in fat), is lost in the fat that is lost in stool.
Vitamin D Absorption.
We absorb vitamin D in two ways: by consuming it in food and by exposing our skin to sunlight.
1. Vitamin D in food is absorbed in the duodenum, (first part if the small intestine), along with digested fat, by simple diffusion through the intestinal lining with no need of a transport carrier to get into the bloodstream. Emphasis is placed on digested fat, presuming that bile from the liver is sufficient to emulsify it, lipase (fat enzymes) from the pancreas is sufficient to digest it and our intestinal lining is healthy enough to absorb it. In other words, a gluten-free diet in itself cannot restore absorption if problems exist/persist with fat absorption. The doctor can determine malabsorption of fat by ordering a lab study that simply measures fat in the stool.
2. Vitamin D from the skin is absorbed directly into the bloodstream, providing we get out in the sunshine and expose our skin to its ultraviolet rays for 10 to 15 minutes at least 3 times per week. Our ability to produce this vitamin in our skin declines after age 40. By age 70, skin can produce only half as much as it could at age 20.
In addition, vitamin D is stored in the body for about 6 months, but low stores of vitamin D are common in celiac disease and in the general population as well. Why is this? Most people have a poor diet that lacks nutrients. Poor dietary consumption that does not provide for adequate daily absorption cannot provide for adequate storage either.
Blood levels representing sun exposure from the summer months peak in early December, then plunge in January. Levels continue to drop to reach their lowest levels in April, reflecting poor winter absorption from skin when the sun is low and the skin is covered. Levels slowly rise toward the end of April as weather improves and people get outside more.
Skin production of vitamin D in either infants or adults may also be insufficient due to environmental factors including geography (natural lack of sunshine in latitudes further north or south of the equator and higher elevations), customs (clothing that completely covers the skin/ swaddling babies), housing (not being able to get outside) or debility (bedridden).
Active vitamin D occurs in two forms: vitamin D2 and vitamin D3. “Vitamin D”, without a subscript, refers to either D2 or D3.
- Vitamin D2, called ergocalciferol, is an activated substance obtained from certain plants like fungi. More than 50 years ago it was discovered that irradiating yeast activated the natural ergosterol in the plant to produce ergocalciferol, which has subsequently became an important dietary source. It is added to milk and other food products to enrich them with vitamin D.
- Vitamin D3, called cholecalciferol, is made in the skin exposed to ultraviolet-B (UVB) rays from sunlight and is derived from animal sources, mainly fish liver oils and egg yolks. It is a prohormone, meaning it will be used to form active vitamin D in the body.Either through the skin or by way of the digestive tract, absorbed vitamin D travels in the bloodstream bound to a protein called vitamin D plasma binding protein (DBP) to the liver where it is made into the active form 25(OH)D, necessary for its functions. It is further converted to the more active form, mainly in the kidneys. This conversion can also be made by a wide variety of tissues at the cellular level because of its important role in proper cell growth.
Vitamin D Functions
The active form of vitamin D affects connective tissue and the tissues of the intestines, kidneys, parathyroid gland, bone, skin, breast and pancreas. Here are known functions within the body:
1. Vitamin D promotes intestinal absorption of calcium and phosphorus.
- Vitamin D stimulates synthesis of calcium-binding protein and the enzyme, alkaline phosphatase, for active transport (carrying) of ionized calcium through the small intestinal lining into the bloodstream.
- Vitamin D stimulates intestinal absorption by active transport of ionized phosphorus into the bloodstream.
2. Vitamin D maintains normal calcium and phosphorus levels in the blood.
- When dietary calcium fails to supply sufficient blood levels, vitamin D mobilizes calcium from bone by inducing preosteoclasts (an immature bone cell) to become mature osteoclasts (mature bone cells). The mature osteoclast releases hydrochloric acid and enzymes to dissolve bone and thereby frees its calcium stores into the bloodstream.
- When dietary phosphorus is inadequate, vitamin D mobilizes phosphate from bone to free its phosphorus stores into the bloodstream.
- Vitamin D increases the reabsorption of calcium and phosphorus in the kidney to prevent loss in urine.
3. Vitamin D is needed in spermatogenesis (formation of sperm) for male reproduction.
4. Vitamin D improves skeletal (voluntary) muscle function.
5. Vitamin D may decrease the risk of high blood pressure by preventing the inappropriate activation of a blood pressure mechanism in the kidneys called the renin-angiotensin system. Angiotensin is a substance that increases blood pressure by inducing small arteries to constrict, thus requiring more force to pump blood through them which elevates blood pressure, and increasing sodium and water retention.
6. Vitamin D may prevent cancer by inhibiting uncontrolled cell growth that may lead to cell mutations characteristic of diseases like cancer.
7. Vitamin D increases natural immunity through the production of a peptide in macrophages (white blood cells) critical for destroying the microbe that causes tuberculosis according to a new research paper co-authored by Dr. Barry Bloom, Dean of the School of Public Health, Harvard University.
Leon H. Rottman, Ph.D., Professor Emeritus, University of Nebraska – Lincoln notes, “attention is again focusing on vitamin D as a tuberculosis preventive. In the 1800s sunlight and vitamin D-rich cod-liver oil were the treatments of choice for tuberculosis. With the development of antimycobacterial drugs, these earlier treatments, which had proved quite effective in many cases, were set aside and largely forgotten.”
8. Vitamin D may prevent development of autoimmunity.
- Scientific evidence shows lower incidence of multiple sclerosis in areas where sunshine is greater and people eat vitamin D rich fish.
- Preliminary research investigating risk of developing type 1 diabetes mellitus in infants receiving vitamin D supplementation found that infants given vitamin D supplementation in the form of calcitriol or cod liver oil during the first year of life are less likely to develop type 1 diabetes than infants fed lesser amounts of vitamin D.
- Preliminary research investigating insulin sensitivity in adults with type 2 diabetes mellitus given vitamin D supplementation demonstrated improved insulin sensitivity.
- It has been shown in other studies that active vitamin D stimulates the beta cells of the pancreas to meet demands for insulin.
9. Vitamin D improves some cases of seasonal-related depression and psoriasis.
10. Vitamin D appears to preserve cognitive function in the elderly.
Vitamin D Deficiency
In celiac disease, vitamin D deficiency develops when inflammation and swelling of the small intestinal lining hampers its ability to digest and absorb fat, thus disallowing vitamin D to pass through into the bloodstream as well as failure to absorb fat.
Vitamin D deficiency disallows the proper absorption of calcium and phosphorus from food. It is estimated that only 10–15% of calcium and 50–60% of phosphorus are absorbed. The resulting failure of available calcium to meet calcium needs of the body triggers the parathyroid glands to compensate by increasing production and secreting of parathyroid hormone (PTH). PTH acts to restore and maintain normal blood calcium levels by drawing calcium from the bones and increasing reabsorption of calcium in the kidney to prevent loss while lowering phosphorus levels by passing it out in the urine.
Calcium deficiency stemming from vitamin D deficiency impairs the ability of muscles to contract, nerves to conduct impulses and blood vessels to regulate blood pressure. If the level of calcium in the blood drops significantly (hypocalcemia), tetany, convulsions, and laryngospasm can develop. These dramatic symptoms involve severe muscle spasms that may be life-threatening. Inadequate calcium contributes to osteoporosis as bone gives up its calcium to provide for essential functions. Low calcium is shown to be involved in colon cancer.
In pregnancy, pre-eclampsia develops (hypertension, protein spilling in urine, and swelling). Sudden weight gain, headaches and visual disturbances can rapidly advance to eclampsia with potential death of the baby and mother.
Vitamin D deficiency is involved in impaired metabolic functions, male reproduction and in prolonged deficiency, cataract development. It has been implicated in psoriasis, hypertension, insulin secretion, and autoimmunity.
Here is a deeper look at conditions that develop when vitamin D deficiency is ongoing:
1. Muscle weakness, lack of energy and pain are early symptoms in both children and adults due to inadequate calcium. In a study investigating 150 consecutive patients referred to a clinic in Minnesota for the evaluation of persistent, nonspecific musculoskeletal pain, 93% had blood levels indicative of vitamin D deficiency. Another study has reported vitamin D deficiency in patients with low-back pain. Vitamin D supplements resulted in pain reduction in many patients after three months.
2. Secondary hyperparathyroidism is due to overstimulation of parathyroid glands.
3. Rickets in children is due to inadequate mineralization (depositing of calcium/phosphorus to build bone in children) of the skeleton resulting in skeletal deformities. Rickets is mostly seen before 18 months of age, especially between the ages of 4 and 12 months.
- Rapidly growing bones are most severely affected, causing bowing in weight-bearing limbs (arms and legs).
- In infants, closure of the fontanels (soft spots) in the skull may be delayed and the rib cage may become deformed (pigeon-chest) due to the pulling action of the diaphragm in breathing.
- The pelvic bone structure is flattened from front to back.
- Tooth development is impaired with delayed eruption, enamel hypoplasia, and early dental caries.
- Poor muscle tone and flabby legs develop.
Osteomalcia in adults is due to demineralization of the skeleton (dissolving of minerals out of bone in adults) resulting in soft bones, muscular weakness and rheumatic type pain. In older children and adolescents, symptoms are similar to those observed in adult osteomalacia, including bone pain, waddling gait, and fatigue.
Osteoarthritis progression of the knee has been associated in research studies with low dietary intake of vitamin D. In participants who took part in the Framingham study, risk for progression increased threefold in those participants in the middle and lower tertiles for both vitamin D intake and serum levels of vitamin D.
Bone fractures are due to failure of bone remodeling.
Risk Factors for Vitamin D Deficiency other than malabsorption
- Dark complexion. The more pigmentation in the skin the less UVB radiation from sun exposure gets through.
- Breastfeeding infants. Human milk is not enough for an infant if it is the only source of vitamin D, putting them at high risk of vitamin D deficiency, particularly if they have dark skin and/or receive little sun exposure.
- Aging. The elderly have reduced capacity to synthesize vitamin D in the skin when exposed to UVB radiation, and are more likely to stay indoors or use sunscreen. Institutionalized adults are at extremely high risk of vitamin D deficiency without supplementation.
- Covering all exposed skin prevents skin absorption.
- Use of sunscreen whenever outside blocks absorption. A SPF factor of 8 reduces production of vitamin D by 95%.
- Inflammatory bowel disease appears to put people at increased risk of vitamin D deficiency.
- Obesity. Once vitamin D is synthesized in the skin or ingested, it is deposited in body fat stores. Large stores of body fat make it harder to access vitamin D.
Recommended Daily Allowances
Adequate Intake (AI) levels have been established by the U.S. Institute of Medicine of the National Academy of Sciences to prevent deficiencies in vitamin D. The AI is 5 micrograms (200 IU) daily for all individuals (males, females, pregnant/lactating women) under the age of 50, 10 micrograms daily (400 IU) for all individuals aged 50-70, and 15 micrograms daily (600 IU) for those who are over the age of 70.
The daily "upper limit" for vitamin D is 25 micrograms (1,000 IU) for infants up to 12 months of age and 50 micrograms (2,000 IU) for children, adults, pregnant, and lactating women due to toxicities that can occur when taken in higher doses.
Food Sources of vitamin D
Very few foods are natural sources of vitamin D. Foods that do contain vitamin D include fatty fish, fish liver oils, especially cod liver oil, and eggs from hens that have been fed vitamin D. Nearly all the vitamin D intake from foods comes from milk products fortified with both vitamin D2 and D3 and other foods, such as breakfast cereals, that have been fortified with vitamin D.
- Herring, canned, 3 oz. - 840 IU
- Pink salmon, canned with bones, 3 oz. - 530 IU
- Sardines, canned, 3 oz. - 231 IU
- Salmon, Pacific, 3 oz. cooked - 420 IU
- Mackeral, canned, 3 oz. - 214 IU
- Tuna, canned, 3 oz. - 200 IU
- Cow's milk, fortified with vitamin D, 1 cup - 100 IU
- Rice or soy milk, fortified, 1 cup - 100 IU
- Egg yolk, medium - 25 IU
Impact of Storage, Processing, and Cooking
Vitamin D is minimally affected by storage unless the temperature is over 212 degrees F, or cooking is long. As a result, foods containing vitamin D can be warmed without loss of strength. It is not affected by oxidation in processing.
Tests for Vitamin D Deficiency
- In children, x-rays of the wrists and knees are used to show development of rickets.
- In adults, blood tests for vitamin D, calcium and phosphorus levels are used to identify development of osteomalacia. A bone scan is used to show development of osteoporosis.
Make strong bones a priority! If you have muscle cramps, act now. Ask your doctor about blood testing to get a baseline for both initial treatment and monitoring.
Although skin exposure to sunshine is the ideal way to meet vitamin D needs, many persons may not be able to sufficiently absorb ultraviolet rays. Taking a vitamin D supplement or a daily multivitamin, which contains 100% (not more) of vitamin D, is an effective way to meet daily needs. The two forms of vitamin D used for nutritional supplementation are vitamin D2 and vitamin D3. In addition, the gluten-free diet should be rich in calcium. Calcium supplements may be needed until bone health is restored.
Some people with osteomalacia and intestinal problems may need vitamin D in the form of calciferol (vitamin D2) given by a single injection that lasts up to a year before another injection may be needed. Pain reduction results about two weeks after the injection.
Ordinarily, persons whose bone density scan shows decreased axial bone density are advised to obtain 1,200 mg calcium and 400 mg vitamin D daily. Higher doses of vitamin D must only be taken under medical supervision including blood monitoring.
Researchers found that vitamin D supplements can help reduce the risk of falls in residents of nursing homes by up to 53%.
Pregnant women and nursing mothers should avoid vitamin D supplemental intakes greater than 400 IU or 10 micrograms daily unless higher amounts are prescribed by their physicians.
Persons on thiazides (water pill), digoxin or any cardiac glycoside (heart pill) should take supplemental vitamin D only on advice from their doctor and have drug monitoring.
Toxicity of Supplementation
Excessive supplemental intake of 1000 μg (40,000 IU)