Introduction

One of the fat-soluble vitamins, vitamin D constitutes two major forms which are vitamin D2 or ergocalciferol and vitamin D3 or cholecalciferol. Vitamin D2 is synthesised by plants whilst vitamin D3 is synthesised by the body. Vitamin D is naturally present in a number of food sources and dietary supplements, and is also manufactured with the aid of sun exposure. It is produced within the body when ultraviolet-B or UVB rays from sunlight reaches the skin. The liver and the kidney eventually help to convert vitamin D to an active hormone form.

Functions of Vitamin D

Vitamin D plays several key roles in the maintenance of organ systems. For example:

*Vitamin D helps regulate calcium concentrations in the blood. Since it works more like a hormone, its active form, calcitriol acts with the parathyroid hormone or PTH. If the calcium level is low, this would trigger the secretion of PTH, which would stimulate the conversion of vitamin D to calcitriol. Calcitriol would then act to increase calcium absorption in the intestines, calcium resorption in the kidneys and stimulate calcium release from the bones.

*Vitamin D helps maintain blood phosphorus levels. Low levels of vitamin D can result to an overactive parathyroid gland. Also with inadequate phosphorus in the body, bones cannot undergo normal mineralisation.

*It is also said that vitamin D benefits the immune system since the T-cells or immune cells have receptors for vitamin D. It acts by boosting phagocytosis, the bodily functions to combat tumours, modulate neuromuscular and immune functions and reduce inflammation.

*Vitamin D is also responsible for maintaining normal cell growth and function. It may be an important element to the prevention and treatment of cancer. It has also been suggested that vitamin D plays a role in regulating the growth and function of brain cells.

*Research studies suggest that vitamin D, because of its anti-inflammatory effect, plays a role in providing protection against osteoporosis, hypertension, cancer, type 1 diabetes, psoriasis and numerous autoimmune diseases.

Deficiency Conditions

There may be certain underlying causes of vitamin D deficiency. It can result from:

*Inadequate vitamin D intake together with inadequate sun exposure

*Certain disorders that compromise vitamin D absorption

*Conditions that can impair the conversion of vitamin D metabolites such as kidney or liver diseases or hereditary disorders.

Insufficient vitamin D intake can have serious effects on the bones, which can make them thin, brittle or deformed. Vitamin D deficiency often results in conditions like:

*Rickets which is common in children and is characterised by delayed growth and deformity of long bones.

*Osteomalacia, which occurs in adults and results in thinning of the bones. Signs of proximal weakness and bone fragility are familiar characteristics.

*Osteoporosis which is a condition wherein the bone mineral density is reduced and bone fragility is increased.

Lack of vitamin D may also be linked to increased susceptibility of several chronic diseases like high blood pressure, cancer, tuberculosis, periodontal disease, multiple sclerosis, chronic pain, schisophrenia, depression, affective disorders, peripheral arterial disease and a number of autoimmune diseases such as type-1 diabetes.

Deficiency Symptoms in Adults and Infants

Signs of vitamin D deficiency includes muscle aches, myopathy or muscle weakness and bone pain. These symptoms can occur at any age. Pregnant mothers who have vitamin D deficiency can also cause fetal deficiency.

In younger infants, rickets can produce a condition called craniotabes or softening of the skull. In older babies, rickets can impede or delay sitting and crawling and the closure of the fontanels; bossing of the infants' skull and presence of costochondral thickening or what is referred to as 'rachitic rosary'. Children with rickets aged 1-4 years old experience epiphyseal cartilage enlargements on the long bones and delayed walking. Older children and adolescents can experience pain upon walking and in worst cases it can result in 'bowlegs' or 'knock-knees'.

Tetany that is caused by hypocalcemia may go along with vitamin D deficiency in infants and adults. This condition can lead to symptoms such as loss of feeling in the lip or tongue areas and the fingers, facial spasms, and at worst, seizures.

Recommended Daily Dose of Vitamin D

Doses or intake of vitamin D is determined according to age groups. These amounts are vital to maintain normal growth and bone health and also normal calcium metabolism in the body. The adequate intakes or AIs for vitamin D indicated are based on the supposition that the vitamin D is not synthesized by sunlight exposure.

From birth to age 13, the recommended AI for children is 5 mcg or 200 IU; for both males and females aged 14-50 years old, 5 mcg or 200 IU is needed; for both males and females aged 51-70 years of age, 10 mcg or 400 IU is required; both males and females reaching the age of 71 and up require 15 mcg or 600 IU of vitamin D. Pregnant and lactating mothers need 5 mcg or 200 IU of the vitamin.

Vitamin D Food Sources

Only a few food sources contain vitamin D. Best sources of vitamin D are fish meat and fish liver oils. There are also small amounts of vitamin D, in the form of vitamin D3, found in dairy products like cheese and egg as well as beef and liver. Some types of mushrooms also contain varying amounts of vitamin D2.

Common sources of vitamin D include the following:

*Cod liver oil

*Fish like salmon, mackerel, tuna, sardines

*Milk including non-fat, reduced fat, whole or vitamin D fortified

*Margarine

*Cereals

*Egg

*Beef liver

*Swiss cheese

*Fortified orange juice

*Fortified rice or soy beverage

The Need for Increased Amounts

Since obtaining sufficient amounts of vitamin D in the diet can be quite difficult, many people now consume vitamin D fortified foods in order to maintain a healthy dose of the vitamin. There are some groups however who need increased amounts of vitamin D:

*Breastfed infants because vitamin D cannot be supplied by breast milk alone

*Older people due to the fact that synthesis of vitamin D decreases with age and the ability of the kidney to convert vitamin D diminishes

*People with limited sun exposure especially those living in northern latitudes, those wearing robes or head covers or those with occupations that prevent them from having sun exposure

*People with dark skin as more skin pigments like melanin reduces the ability of the skin to produce vitamin D.

*People with fat malabsorption conditions such as Crohn's disease, cystic fibrosis, liver and celiac disease or patients who have undergone surgical removal of any part of the stomach or intestine.

*People who are obese. An increased amount of subcutaneous fat can snatch more of the vitamin D and somehow alter its release in the circulation.

Interaction of Vitamin D with Prescribed Drugs

Vitamin D supplements have the tendency to react with certain types of prescription medications. These include the following:

*Steroids or corticosteroid medications like prednisone which can cause decreased calcium absorption and also damage the vitamin D metabolism process.
*Weight-loss medications such as orlistat and cholesterol-lowering drugs like cholestyramine also decrease the absorption of vitamin D and other fat-soluble vitamins
*Phenobarbital and phenytoin increases vitamin D metabolism and decreases calcium absorption.