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Good nutrition is essential for good health and the body uses nutrients to produce energy and regulate its basic functions. Nutrients are of two types – micronutrients and macronutrients. Vitamins and minerals are known as micronutrients because the body requires a very small amount of these; whereas carbohydrates, proteins, lipids and water are known as macronutrients because they need to be consumed in large quantities (McGuire & Whitney, 2007). Vitamins are organic compounds, not produced in the body, but required in small quantities to perform some basic body functions like growth, reproduction and homeostasis. They function as a catalyst in many metabolic pathways (Bhagavan & Ha, 2011). This paper presents the classification and sources of essential vitamins, their functions and symptoms of hyper and hypovitaminosis of each vitamin.
There are 13 essential vitamins. Based on their solubility, vitamins are classified into two groups – water-soluble vitamins and fat-soluble vitamins. There are 9 water-soluble vitamins which include Vitamin C and the eight B vitamins. The fat-soluble ones are the vitamins A, D, E and K. The water-soluble vitamins dissolve in water and are not stored in the body. Any excess is excreted in urine. They are primarily used as coenzymes in metabolism (Sizer, Piche & Whitney, 2011).
The fat-soluble vitamins dissolve in lipids and are stored in the body. They are absorbed in the small intestine and are circulated in the lymph. In blood they are found attached either to transport proteins or lipoproteins. They play a key role in cell growth and maturation and regulation of gene expression (McGuire & Beerman, 2007).
B vitamins are found abundantly in dairy products, meat, fish, whole grains, tomatoes and leafy green vegetables like spinach. Vitamin B6 is also found in chickpeas, potatoes and bananas. Major dietary sources of vitamin C are fresh fruits, especially citrus fruits and green vegetables. All fat-soluble vitamins are found in fatty portions of meat like beef liver. Other food sources of vitamin A are carrots, pumpkins, squashes, sweet potatoes and milk. Vitamin D is found in whole milk, butter, egg yolk, fish oil and mushrooms. Nuts, seeds and vegetables are good sources of vitamin E. Vitamin K is present in fish, eggs and vegetables (McGuire & Beerman, 2007).
The Recommended Daily Intake (RDI) of vitamin A is 5000 IU (International Unit); RDI for vitamin D is 400 IU; for vitamin E – 30 IU and for vitamin K – 80 µg. The RDI values for water-soluble vitamins are 1.5 mg. for thiamin, 1.7 mg. for riboflavin, 20 mg. for niacin, 400 µg. for folate, 6 µg. for vitamin B12, 2 mg. for vitamin B6, 300 µg. for biotin, 10 mg. for pantothenic acidand 60 mg. for vitamin C (Sizer, Piche & Whitney, 2011)
Benefits of Vitamins
Vitamin C helps in collagen formation. Collagen forms the intercellular substance in bone, cartilage and vascular epithelium. It also helps the body to absorb iron and folate from food, boosts immunity by stimulating the formation of antibodies and acts as antioxidant. Thiamin helps to produce energy from carbohydrates in the body and is also needed for nerve function. Riboflavin acts as a coenzyme in energy metabolism, maintains normal vision and is required for biosynthesis and activation of other compounds like folate and vitamins A, B4 and K and some neurotransmitters like dopamine. It also protects cell membranes from damage due to oxidation.
Niacin acts as a coenzyme in energy metabolism, facilitates the maintenance, repair and replication of DNA strands and also aids protein synthesis and cholesterol metabolism. Vitamin B6 acts as a coenzyme in protein metabolism and is required for synthesizing serotonin, dopamine and heme. Pantothenic acid is part of a coenzyme in energy metabolism. Biotin acts as a coenzyme in energy cycle and also helps in regulation of gene expression. Folate helps in maintenance and repair of DNA and normal growth and development of nerve tissue in fetus. Vitamin B12 is required for activation of folate and also as a coenzyme in energy production (McGuire & Beerman, 2007).
Vitamin A is vital for vision, cell growth and reproduction. It maintains the protective barriers of the body- skin and epithelial tissues in good health and is required to produce lymphocytes and antibodies. Vitamin A is also important for healthy bones as it promotes bone formation. Vitamin D maintains bone health by regulating blood calcium and phosphorus levels and also stimulates cell maturation. Vitamin E is an antioxidant and helps to protect against cell damage. Vitamin K plays a vital role in the clotting mechanism of blood and is also required for synthesizing some important bone proteins (Sizer, Piche & Whitney, 2011)
Hypervitaminosis is a condition caused by excessive intake of vitamins, especially the fat-soluble vitamins since they are stored in the body. Intake of dietary supplements can cause toxicity. Hypervitaminosis A causes headache, dry and scaly skin, loss of hair, bone and joint pain, abnormal bone growth, liver and nerve damage, anorexia and birth defects. Vitamin D toxicity can cause hypercalcemia (excess of calcium in blood) and hypercalciuria (elevated calcium in urine), leading to calcifications in soft tissues, especially kidneys. Though toxicity of vitamin E is rare, it causes adverse effects like increased level of serum lipids, decrease in serum thyroid hormone level and impaired blood coagulation. High intake of vitamin K has no known toxic effects (McGuire & Beerman, 2007).
Water-soluble vitamins are not stored in the body and are excreted in urine. Therefore, they generally do not cause toxicity except for niacinand vitamin C. Niacin, when taken as nicotinic acid, can cause “niacin flush” – dilation of capillaries, causing tingling and flushing of skin. Hypervitaminosis C can cause gastrointestinal upset and diarrhea, kidney stones and bladder irritation.It interferes with anticoagulant drug therapy and can cause iron overload in people with abnormal iron metabolism, as in haemochromatosis (Blake, 2007).
Hypovitaminosis is caused by deficiency of vitamins. Thiamin deficiency causes beriberi. There is wasting of muscles, leg cramps, tenderness, numbness in feet and legs and sometimes severe edema in the limbs. There may be respiratory problems and enlargement of the heart, causing heart failure. Riboflavin deficiency, or ariboflavinosis, causes inflammation of mouth and tongue, cheilosis, anemia and weakness. Niacin deficiency results in pellagra. The early signs are fatigue, listlessness, loss of appetite and weight and general poor health. It is followed by sore mouth, tongue and throat and difficulty in swallowing. The tongue and lips become abnormally red. Dermatitis occurs on exposed skin surfaces, and neurological symptoms include confusion, hallucinations and dementia. The symptoms of pellagra are the four ‘D’s- diarrhea, dermatitis, dementia and death (McGuire & Beerman, 2007).
Riboflavin deficiency causes cracking and redness of corners of mouth, sore throat, glossitis, inflammation of eyes, photosensitivity and skin rashes. Folate deficiency causes macrocytic anemia and neural tube defects in newborns. Deficiency of vitamin B12 causes pernicious anemia. Vitamin B6 deficiency causes anemia and general symptoms like weakness, insomnia, confusion, irritability and depression. It may also cause dermatitis, convulsions and weakening of immune response. Biotin deficiency causes depression, skin irritation and loss of muscle control. Though pantothenic acid deficiency is rare, it causes ‘burning feet syndrome’. Symptoms are tingling in feet and legs and weakness. Vitamin C deficiency causes scurvy, a condition characterized by bleeding gums, skin irritation and poor wound healing (McGuire & Beerman, 2007).
Hypovitaminosis A causes VADD (vitamin A deficiency disorder) with symptoms of night blindness, xerophthalmia (dry eyes) and hyperkeratosis (dry, scaly skin). Deficiency of vitamin D causes rickets in children and osteomalacia and osteoporosis in adults. Vitamin E deficiency causes muscle pain, neuromuscular problems, loss of coordination and hemolytic anemia, where membranes of red blood cells weaken and rupture. Deficiency of vitamin K causes excessive bleeding and prolonged clotting time (McGuire & Beerman, 2007).
Vitamins are vital for growth and development. With further research, many new benefits of vitamins are being found, such as their role in the treatment of cancer and in delaying aging. The daily diet should include food that supplies adequate quantities of all macro and micronutrients. Vitamins derived from food rarely ever cause toxicity. Vitamin supplements can be useful in certain special situations, but dietary supplements are not a substitute for a well balanced diet.