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The vast majority of peak adult bone mass is accumulated by the time longitudinal growth is complete. As peak bone mass is an important determinant of future fracture risk, the goal of the current calcium recommended dietary allowance during youth is to provide a calcium intake that allows individuals to reach their full genetic potential for acquiring skeletal mass. The advent of controlled trials of calcium supplementation and total body bone mass measurements in children and adolescents provide the first direct way of determining the amount of calcium necessary to achieve optimal skeletal accretion. These studies indicate that the current RDAs are insufficient to support optimal bone mass gain during growth and development. Based on the recent intervention trials, recommendations are made for an RDA of 1250 mg during childhood and 1450 mg during adolescence. These values are consistent with established calcium balance intake thresholds for growth during pre-adolescence and adolescence.

Calcium flux and mode of uptake were investigated in an underground dwelling mole-rat, Cryptomys damarensis, fed diets of varying Ca content. The amount of dietary Ca positively influenced the amounts ingested, absorbed and retained. The linear relationship between ingested and absorbed Ca was significantly (P less than 0.001) correlated, implying that this process is nonsaturable. When mole-rats were fed a diet low in Ca, apparent fractional absorption of Ca was high (85.88%). This increased still further when the diet was changed to a food of greater Ca content (96.13%, carrots; 96.97%, gemsbok cucumber). Mineral homeostasis is regulated at the intestinal level in most mammals. Regardless of dietary Ca content, uptake of 45Ca (examined via the everted gut sac technique) was passive, confirming that absorption is via a nonsaturable process. Plasma Ca concentrations were not tightly regulated, yet when fed the diet with the highest Ca content, mole-rats were not hypercalcemic. Regardless of diets, Ca apparent fractional retention was positive, and approached physiological maxima (greater than 97%). Cryptomys damarensis, in using highly efficient modes of mineral uptake and retention, is therefore capable of fully exploiting the limited food resources of their arid ecotope.

An attempt was made to estimate average calcium requirements for various age groups by gender. The factorial method that sums calcium accretion and endogenous losses modified by fractional absorption was used during growth and calcium intakes to achieve calcium balance was used for adults. These estimates exceed the current RDA for females aged 6-16 y and older than 30 y and for males aged 6-30 y. They also exceed current calcium intakes in the U.S. during adolescence for both sexes and for females above the age of 11 y. A number of factors influence calcium requirements with age. Requirements for calcium accretion is dominant until peak daily calcium retention during the adolescent growth spurt is achieved. The rate of calcium accretion with age varies widely, depending on the method of estimation. Urinary calcium losses increase until young adulthood. Contributions of endogenous fecal calcium and sweat calcium to obligatory calcium losses are largely unknown during growth. After growth ceases, calcium requirements increase with menopause, unless estrogen-treated, and with age as calcium absorption efficiency decreases. Data required to estimate calcium requirements during growth are insufficient to precisely define dietary requirements with age. Determining calcium intakes which optimize calcium retention, and therefore, bone mass as has been estimated by Matkovic and Heaney (1992) may be a sound approach. The requirements estimated by this approach will be difficult to achieve given current dietary intakes in the U.S.

Calcium balance and its major components (true calcium absorption, urinary calcium excretion and intestinal calcium secretion) were assessed by a 6-d balance technique in young (6-wk-old) rats fed a diet containing 7.2 g Ca/kg diet. Following either oophorectomy (oophx) or sham operation, the balance study was repeated at 9, 12 and 15 wk of age. Calcium balance decreased with age but remained positive in each group [mean (pooled SEM): sham, (6 wk) 0.86 to (15 wk) 0.27 (0.03) mmol/d (P < 0.0001); oophx, (6 wk) 0.91 to (15 wk) 0.22 (0.03) mmol/d (P < 0.0001)]. Analysis of variance of the change in calcium balance indicated that there was a significantly greater reduction in calcium balance in the oophx group compared with the sham-operated group. Intestinal calcium absorption decreased in each group with age [sham: (6 wk) 46.3% to (15 wk) 22.6% (1.5%) (P < 0.0001); oophx: (6 wk) 48.2% to (15 wk) 21.2% (1.5%) (P < 0.0001)]. There was a marked rise in intestinal calcium secretion in the oophx group compared with the sham-operated group at 6 wk post-operation (12 wk of age) [oophx: 0.33 (0.02) mmol/d; sham: 0.23 (0.02) mmol/d (P < 0.01)]. Urinary calcium excretion was not affected by either age or oophorectomy. We conclude that oophorectomy in young rats leads to a reduction in calcium accumulation, which is mainly the result of an increase in intestinal calcium secretion.

Eight young adult males were subjects in a 51-day metabolic study conducted to examine the effects of level of protein and of phosphorus intake on urinary calcium and calcium balance. Two levels of protein (50-150 g) were given at each of two levels of phosphorus intake (1,010 and 2,525 mg). Dietary calcium and magnesium were maintained at 500 and 350 mg, respectively. Raising the protein intake from 50 to 150 g caused a calciuresis at both phosphorus intakes, but the actual increase in urinary calcium was 71 mg/day greater at the low than at the high phosphorus intake and calcium balance was changed from 24 to -116 mg/day at the low phosphorus intake and from 8 to -25 mg/day at the high. When the phosphorus intake was raised, urinary calcium decreased from 156 to 93 mg/day at the low protein intake and from 334 to 200 mg/day at the high protein intake and the markedly negative calcium balance found at the high protein intake was greatly improved. Simultaneous increases in protein and phosphorus intakes caused a 28% increase in urinary calcium whereas the increase in protein intake alone caused a 115% increase.

The widespread opinion that both protein and phosphorus cause calcium loss is examined. Controlled human studies show that commonly used complex dietary proteins, which have a high phosphorus content, do not cause calcium loss in adult humans. Similarly, a phosphorus intake of up to 2000 mg/d does not have adverse effects on calcium metabolism; however, the type of phosphate contained in carbonated beverages may not behave in the same manner. In contrast, a diet low in protein and phosphorus may have adverse effects on calcium balance in the elderly. Studies with adults suggest that high protein foods do not cause calcium loss.