NME2.15: vitamin absorption and deficiencies14/02/08

Learning Outcomes

Describe the routes of absorption of the main vitamins

  • A vitamin is defined as an organic substrate required in small amounts for normal growth and health that cannot be synthesised by the body
  • There are currently 13 identified vitamins that are broadly categorised as either water soluble or fatsoluble
  • Vitamins A, C and E are antioxidants
  • Fat soluble vitamins can be stored and overdoses can be toxic
  • They are absorbed as part of mixed micelles during fat digestion
  • Water soluble vitamins cannot be stored and are readily excreted
  • They are absorbed by facilitated diffusion or receptor-mediated transcytosis
  • Facilitated diffusion of water soluble vitamins involves their hydrolysis, transport and intracellular modification
  • An example of transcytosis of a water soluble vitamin is that of vitamin B12:
  • Binds to haptocorrin in the gut
  • Haptocorrin digested in the intestine
  • Binds to intrinsic factor (IF)
  • Complex binds to receptor in terminal ileum
  • B12 liberated and internalised
  • Binds to transcoblaamin II
  • Transported across basal membrane to blood

Outline the consequences of vitamin deficiencies

  • Fat soluble vitamins are:
  • Vitamin A – retinal pigment; found in fish and meat; deficiency causes night blindness
  • Vitamin D – involved in calcium absorption; found in fish and meat; deficiency causes rickets
  • Vitamin E – antioxidant; found in most foods; deficiency causes peripheral neuropathy
  • Vitamin K – cofactor for synthesis of factors II, VII, IX and X; found in plants; deficiency causes haemorrhagic disease
  • Water soluble vitamins are:
  • Vitamin B1–coenzyme in decarboxylation of pyruvate and keto-acids; found in pulses; deficiency causes beriberi
  • Vitamin B2– form flavin nucleotides; found in most foods; deficiency causes sore mouth and normocytic anaemia
  • Vitamin B3– forms NAD/NADP; found in meat and pulses; deficiency causes pellagra[TG1]
  • Vitamin B5– forms coenzyme-Aas part of acetyl-CoA, crucial in metabolism
  • Vitamin B6– involved in synthesis of amino acids and transamination; found in most foods; deficiency causes normocytic anaemia
  • Vitamin B7 – carnitine; crucial in transport of fatty acids across inner mitochondrial membrane; deficiency causes pellagra
  • Vitamin B12– promotes formation of RBCs; found in animal products; deficiency causes pernicious anaemia
  • Folate – synthesis of purines and thymine; found in greens; deficiency causes megaloblastic anaemia
  • Vitamin C – involved in formation of collagen; found in fruit; deficiency causes scurvy

[TG1]Skin lesions, depression