Requirements of Humans and Animals

Based on the fact that 99% of total body calcium is contained in the skeletal system, it must be concluded that Ca requirement is almost completely determined by the demands of the skeletal system. Conversely, this implies that, in a healthy organism, the only variable which might have any critical impact on calcium homeostasis is the absolute amount of calcium absorbed from the intestinal lumen. In other words, maintenance of normal extracellular calcium levels in case of low intake or of malabsorp- 

Pregnancy and lactation are two critical periods, since the fetus as well as the newborn secure the calcium needed for development of their skeletal system completely from maternal sources - that is, the mother s skeletal system. The organism also has a particular need for calcium during skeletal growth and development in order to achieve an optimal peak bone mass (Johnston et al. 

1992) and to minimize the risk of subsequent osteoporotic fractures during later life (Matkovic et al. 1979). 

Many women experience substantial calcium losses due to rapid bone turnover after the menopause. Hence, intake of calcium that would cause the calcium balance to remain positive should be increased to - 1500 mg per day (Heaney etal. 1978). Apart from the menopause, bone loss in the elderly results probably also from malabsorption of calcium (Ireland and Fordtran 1973, Gallagher etal. 1979). In addition, other factors that are common in the elderly may threaten their nutritional status, including social isolation, impaired cognition, or the physical inability to either obtain or prepare food (Barrett-Connor 1989). 

Malabsorption of calcium is often the consequence of hypovitaminosis D, which is observed in gastrointestinal, hepatic, and renal diseases (Haussler et al. 1976, Fraser 1980, Iqbal 1994), as well as in some endocrine disorders such as hyperthyroidism (Peerenboom et al. 1984). Alcoholism (Kra-witt 1975) and long-term treatment with certain drugs, particularly glucocorticoids (Klein et al. 1977) or anti-convulsants (Hahn and Halstead 1979), also have negative effects on calcium absorption from the intestine. 

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