Food and nutrition.pdf

102 Function of FoodThe skeleton of a living person is physiologically different from thedry skeleton in a grave or museum. The bones are living tissues, consistingmainly of a mineralized protein collagen substance. In the living bodythere is continuous turnover of calcium. Bone is laid down and resorbedall the time, in people of all ages. Bone cells called osteoclasts take up orresorb bone, while others, termed osteoblasts, lay down or form new bone.The bone cells in the mineralized collagen are called osteocytes.Up to full growth or maturity (which has usually taken place by age18 to 22 years), new bone is formed as the skeleton enlarges to its adultsize. In young adults, despite bone remodelling, the skeleton generallymaintains its size. However, as persons get older there is some loss ofbone mass.A complex physiological system maintains proper calcium andphosphorus levels. The contrQl involves hormones from the parathyroidgland, calcitonin and the active form of vitamin D (l,25-dihydroxycholecalciferol).Small but highly important amounts of calcium are present inextracellular fluids, particularly blood plasma, as well as in various bodycells. In serum most of the calcium is in two forms, ionized and proteinbound. Laboratories usually measure only total plasma calcium; thenormal range is 8.5 to 10.5 mg/dl (2.1 to 2.6 mmol/litre). A drop in thelevel of calcium to below 2.1 mmol/litre is termed hypocalcaemia and canlead to various symptoms. Tetany (not to be confused with tetanusresulting from the tetanus bacillus), characterized by spasms andsometimes fits, results from low levels of ionized calcium in the blood.Dietary SourcesAll the calcium in the body, except that inherited from the mother,comes from food and water consumed. It is especially necessary to haveadequate quantities of calcium during growth, for it is at this stage thatthe bones develop.The foetus in the mother's uterus has most of its nutritionalrequirements satisfied, for in terms of nutrition the unborn child is almostparasitic. If the mother's diet is poor in calcium, she draws extra suppliesof this mineral from her bones.An entirely breastfed infant will obtain adequate calcium frombreastmilk as long as the volume of milk is sufficient. Contrary to popularbelief, the calcium content of human milk varies rather little; 100 ml ofbreastmilk, even from an undernourished mother on a diet very low incalcium, provides approximately 30 mg of calcium (Table 18). A lactatingmother secreting 1 litre will thus lose 300 mg of calcium per day.Cows' milk is a very rich source of calcium, richer than human milk.