Vitamin D is a fat-soluble vitamin, that is, it is stored in body fat. It is found in both animal and plant sources. It mainly regulates calcium absorption, and thus indirectly bone mineralization and strength.
What Is Vitamin D
Vitamin D is manufactured in skin by conversion of its precursor molecule, an intermediate product of cholesterol, in the presence of ultraviolet component in sunlight, hence it is also called sunshine vitamin. Ten to fifteen minutes of exposure to sunlight twice a week suffices for the production of required amounts of vitamin D.
Since the body makes vitamin D, some maintain vitamin D is not actually a vitamin but a hormone, because by definition a “vitamin” is one that the body cannot make and needs to be supplied in food. Calcitriol is the active form of vitamin D, to which it is converted in two steps, the first step occurring in liver and the second step in kidney.
The other forms of vitamin D are Vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D aids absorption of calcium and thus helps maintain the strength of bones. Its deficiency in children leads to rickets, and in adults to osteomalacia.
Food Sources Of Vitamin D
Vitamin D is found in dairy products like cheese, butter and cream. It is also found in egg yolk, oily fish (salmon, tuna and mackerel); fortified milk, breakfast cereals and margarine. The body also manufactures the vitamin on exposure to sunlight for at least 10 minutes.
Functions Of Vitamin D
Vitamin D helps in absorption of calcium and phosphorus from the gastrointestinal tract. It is actually the active form, calcitriol, that is responsible for absorption of calcium from diet. Vitamin D induces the calcium-binding protein calbindin that brings about calcium absorption from intestines.
Along with parathyroid hormone it regulates the balance of calcium in the body (calcium homeostasis). This effect of vitamin D on calcium absorption and homeostasis indirectly helps bone growth and strength, failing which bones become susceptible to fractures.
In addition, it has a role in secretion of insulin, synthesis as well as secretion of thyroid and parathyroid hormones, and inhibits production of interleukin and immunoglobulins (antibodies). Vitamin D is also thought to play a role in protecting against osteoporosis (decrease in bone mass), high BP (hypertension), cancer, and some autoimmune diseases.
The recommended daily allowances (RDA) for vitamin D are as follows (all units are in IU or international units per day, with the value in brackets being in micrograms/day): Infants (less than 1 year) – 400 (5); children (ages 1 to 14) – 600 (15); adults below 70 years – 600 (15); adults over 70 years – 800 (20); during pregnancy and lactation – 600 (15).
The safe upper limits for vitamin D intake are 1500 IU per day in infants, 3000 IU per day in children, and 4000 IU per day in adults. 1 microgram of vitamin D3 is equivalent to 40 international units of vitamin D.
Deficiency Symptoms Of Vitamin D (Vitamin D Deficiency)
Vitamin D deficiency is due to either inadequate exposure to sunlight, inadequate intake in diet, or impaired absorption. So those who stay mainly indoors are at risk. Intestinal diseases like celiac disease and Crohn’s disease can affect absorption of vitamin D and lead to its deficiency.
Chronic liver and kidney disease can interfere with the conversion of vitamin D into its active form calcitriol, which too can manifest with deficiency symptoms. Drugs like phenytoin and phenobarbitone, which are given in epilepsy or seizures, affect the metabolism of vitamin D and lead to its deficiency.
Vitamin D deficiency manifests in children as rickets and in adults as osteomalacia due to deficient calcification of bone. Since vitamin D is also necessary for absorption of phosphorus, its deficiency can lead to hypophosphatemia (low phosphorus), leading to varied symptoms as discussed below.
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Children with rickets exhibit delayed development and decreased muscle tone. Skull is thin and deformed (craniotabes). Wrists are widened; there is swelling or beading of the ends of ribs (rickety rosary), and there is a groove in rib cage (Harrison’s sulcus). Deformities of long bones (bowed legs), spine, chest and pelvis may occur.
Severe rickets may be associated with spasms of hands, feet and vocal cords with high-pitched voice or crying (hypocalcemic tetany). Lowered calcium level can also lead to epileptic seizures. Measurement of vitamin D, alkaline phosphatase and parathyroid hormone levels, X-rays help in diagnosis.
Underlying cause should be treated. 10 to 50 micrograms of vitamin D daily or 0.25 to 1.5 micrograms of calcitriol (active form of vitamin D) daily and calcium supplementation of 500 to 1000 mg daily are required. Treatment should be continued for a period of 3 to 4 months. Maintenance levels of vitamin D may need to be continued in those at risk and those with underlying disease predisposing to deficiency.
Osteomalacia in adults can be asymptomatic. Malaise, bone pain and fractures occur with the progression of the disease. Muscle weakness causes the patient walk peculiarly with a wide base (waddling gait) and there will be difficulty in climbing stairs or getting up from a chair. Bones and muscles are tender (pain on applying mild pressure or pressing). Diagnosis and treatment are similar to that of rickets. If chronic failure is present, calcitriol should be given rather than just vitamin D.
In hypophosphatemia there is muscle weakness, confusion, convulsions and hallucinations. Weakness of diaphragm may cause difficulty in breathing. It can be associated with low calcium levels and associate symptoms like rickets and tetany. Rickets due to hypophosphatemia responds well to therapy with phosphate supplements of 2–4 g daily, combined with active form of vitamin D, that is, calcitriol.
Overdose Of Vitamin D
Vitamin D taken in excess can be toxic. Excessive exposure to sunlight does not lead to increased production of vitamin D in the body as there is limited uptake of vitamin D precursors by the skin. Since excess vitamin D can lead to increased absorption of calcium, it raises the blood levels of calcium leading to deposits of calcium in heart and lungs, and even kidney stones.
The other symptoms of toxicity are general weakness, nausea and vomiting, decreased appetite, excessive thirst, irritability, increased blood pressure (hypertension), and kidney malfunction. Treatment is decrease in intake of vitamin D. Intravenous fluids and corticosteroids may be required.
How To Store Vitamin D
Vitamin D tablets should be stored below 25 degree centigrade, and in original packing, away from moisture and light.