Vitamins: Vitamin A (Retinol) Overview

Classification of Vitamins

• Vitamins can be classified into two categories:
  • Fat-soluble vitamins: can be absorbed from the intestine along with lipids, can be stored in the body for a long time, and can accumulate in the body to the point of toxicity
  • Water-soluble vitamins: absorbed with water from the intestinal tract, typically remain in the body only a short time before being excreted in the urine

Essential Vitamins

• Cannot be produced by the body
• Must be obtained through the diet
• No single food item or nutrient class provides all the essential vitamins
• Absence of essential vitamins in the diet can lead to specific deficiency diseases

Vitamins

• Vitamin A (Retinol):
  • Fat-soluble
  • RDI: 900
  • Sources: provitamin, carotene, liver, egg yolk, butter, and milk
  • Function: necessary for rhodopsin synthesis, normal health of epithelial cells, and bone and tooth growth
  • Deficiency symptoms: rhodopsin deficiency, night blindness, retarded growth, skin disorders, and increased infection risk
• Vitamin B1 (Thiamine):
  • Water-soluble
  • RDI: 1.2 mg
  • Sources: yeast, grains, and milk
  • Function: involved in carbohydrate and amino acid metabolism, necessary for growth
  • Deficiency symptoms: beriberi, muscle weakness, neuritis, and paralysis
• Vitamin B2 (Riboflavin):
  • Water-soluble
  • RDI: 1.3 mg
  • Sources: green vegetables, liver, wheat germ, milk, and eggs
  • Function: component of flavin adenine, dinucleotide, involved in citric acid cycle
  • Deficiency symptoms: eye disorders and skin crackling, especially at corners of the mouth
• Pantothenic acid:
  • Water-soluble
  • RDI: 16 mg
  • Sources: liver, yeast, green vegetables, grains, and intestinal bacteria
  • Function: constituent of coenzyme-A, glucose production from lipids and amino acids, steroid hormone synthesis
  • Deficiency symptoms: neuromuscular dysfunction and fatigue
• Biotin:
  • Water-soluble
  • RDI: 30 μg
  • Sources: liver, yeast, eggs, and intestinal bacteria
  • Function: fatty acid and nucleic acid synthesis, movement of pyruvic acid into citric acid cycle
  • Deficiency symptoms: mental and muscle dysfunction, fatigue, and nausea
• Vitamin B6 (Pyridoxine):
  • Water-soluble
  • RDI: 17 mg
  • Sources: fish, liver, yeast, tomatoes, and intestinal bacteria
  • Function: involved in amino acid metabolism
  • Deficiency symptoms: dermatitis, retarded growth, and nausea
• Folate:
  • Water-soluble
  • RDI: 0.4 mg
  • Sources: liver, green leafy vegetables, and intestinal bacteria
  • Function: nucleic acid synthesis, hematopoiesis, prevents birth defects
  • Deficiency symptoms: macrocytic anemia, spina bifida
• Vitamin B12 (Cobalamins):
  • Water-soluble
  • RDI: 2.4 μg
  • Sources: liver, red meat, milk, and eggs
  • Function: necessary for red blood cell production, some nucleic acid and amino acid metabolism
  • Deficiency symptoms: pernicious anemia, nervous system disorders
• Vitamin C (Ascorbic acid):
  • Water-soluble
  • RDI: 90 mg
  • Sources: citrus fruit, tomatoes, and green vegetables
  • Function: collagen synthesis, general protein metabolism
  • Deficiency symptoms: scurvy, defective bone formation, poor wound healing
• Vitamin D (Cholecalciferol, Ergosterol):
  • Fat-soluble
  • RDI: 10 μg
  • Sources: fish liver oil, enriched milk, and eggs, provitamin D converted by sunlight to cholecalciferol in the skin
  • Function: promotes calcium and phosphorus use, normal growth, and bone and tooth formation
  • Deficiency symptoms: rickets, poorly developed weak bones, osteomalacia, bone reabsorption
• Vitamin E (Tocopherol, Tocotrienols):
  • Fat-soluble
  • RDI: 15 mg
  • Sources: wheat germ, cottonseed, palm, and rice oils, grain, liver, and lettuce
  • Function: prevents oxidation of cell membranes and DNA
  • Deficiency symptoms: hemolysis of red blood cells
• Vitamin K (Phylloquinone):
  • Fat-soluble
  • RDI: 120 μg
  • Sources: alfalfa, liver, spinach, vegetable oils, cabbage, and intestinal bacteria
  • Function: required for synthesis of a number of clotting factors
  • Deficiency symptoms: excessive bleeding due to retarded blood clotting

Provitamins and Coenzymes

• Provitamins: parts of a vitamin that the body can assemble or modify into a functional vitamin
  • Examples: beta carotene, 7-dehydrocholesterol, tryptophan
• Coenzymes: combine with enzymes to make the enzymes functional
  • Examples: vitamins B2, B3, biotin, and pantothenic acid, folate, and Vitamin B12

History of Vitamins

• First identified at the beginning of the 20th century
• Found to be associated with certain foods that were known to protect people from diseases such as rickets and beriberi
• 1941: The first Food and Nutrition Board established the Recommended Dietary Allowances (RDAs)

Recommended Dietary Allowances (RDAs)

• Nutrient intakes that are sufficient to meet the needs of nearly all people in certain age and gender groups
• Established for different-aged males and females, starting with infants and continuing on to adults
• Also set for pregnant and lactating women
• RDAs have been reevaluated every 4-5 years and updated, when necessary, on the basis of new information