Changes in Calcium Metabolism during the Life Span

The total body calcium content of the newborn infant is approximately 0.75 mol (30 g), which increases during growth to approximately 1000 g in adult women and 1200 g in adult men. This represents an average daily accumulation of approximately 2.5-3.7mmol (100-150mg) from infancy to adulthood.

The efficiency of calcium absorption is highest during infancy (approximately 60%), and the amount absorbed from breast milk does not appear to be affected by calcium consumed in solid foods. During the growth spurt of adolescence, calcium retention and accretion increase to peak at approximately 200-300 mg per day in girls and boys, respectively. It involves the action of growth hormone, IGF-1, and sex steroids. The onset of menstruation in girls is associated with a rapid decline in bone formation and resorption. Intestinal calcium absorption is predictably more efficient during the growth spurt and also decreases subsequently. Importantly, it is thought that calcium intakes during the period of growth can affect the peak bone mass achieved and therefore influence the amount of bone mineral remaining when osteoporosis begins in later life. Bone mass may continue to accumulate up to approximately age 30 years, although the amount gained is relatively small after age 18 years.

During pregnancy, a relatively small amount of calcium, approximately 625-750 mmol, is transported to the fetus. Most of this calcium is thought to be obtained through greater efficiency of maternal intestinal calcium absorption, possibly induced by increases in 1,25(OH)2D3 production. For this reason, a higher calcium intake during pregnancy is probably not required.

Most studies have reported that there is no increase in intestinal calcium absorption during lactation even when dietary intake of the mineral is relatively low. Changes in biochemical markers and kinetic studies using isotopes indicate that the source of much of the calcium secreted in breast milk is the maternal skeleton, as well as more efficient renal reabsorption and subsequently lower urinary excretion of the mineral. Bone calcium is restored at the end of lactation as the infant is weaned, when ovarian function returns and menstruation resumes. At this time, intestinal calcium absorption increases, urinary calcium remains low, and bone turnover rates decline to normal levels. There is no strong evidence that lactation per se or maternal calcium intake during lactation affect later risk of osteoporosis in women. Thus, there is no strong rationale for increasing maternal calcium intake during lactation. Breast milk calcium concentration is relatively unaffected by maternal intake, and it remains stable throughout lactation.

Menopause begins a period of bone loss that extends until the end of life. It is the major contributor to higher rates of osteoporotic fractures in older women. The decrease in serum estrogen concentrations at menopause is associated with accelerated bone loss, especially from the spine, for the next 5 years, during which approximately 15% of skeletal calcium is lost. The calcium loss by women in early menopause cannot be prevented unless estrogen therapy is provided. Calcium supplements alone are not very helpful in preventing postmeno-pausal bone loss. Upon estrogen treatment, bone resorption is reduced and the intestinal calcium absorption and renal reabsorption of calcium are both increased. Similarly, amenorrheic women have reduced intestinal calcium absorption, high urinary calcium excretion, and lower rates of bone formation (compared to eumenorrheic women). In both men and women, there is a substantial decline in intestinal absorption of calcium in later life.

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