Chemistry Reference
In-Depth Information
bone strength (Rubin and Rubin, 2006). The bone calcium deposition rate
in vivo (V
o
+) during early prepuberty (8.3 yr) was estimated at 1504 mg/d;
puberty (10.2 yr), 1952 mg/d (Abrams et al., 2000); menarche, 3000 mg/d;
and postmenarche (60 months), 1000 mg/d (Abrams et al., 1996). Accord-
ingly, the rate of calcium deposition is much higher than either calcium
consumption (
1200 mg/d) or the gut absorption (
400 mg/d) observed in
human studies (Bronner and Abrams, 1998; Abrams et al., 2000). Skeletal
tissue is replaced every 10-12 yr, on average (Heaney, 2006), because the
skeleton is a metabolically active organ and must undergo continuous remo-
deling throughout life to adapt its internal microstructure to changes in the
mechanical and physiological environment. Furthermore, bone is renewed
continuously to repair micro-damage to minimize the risk of fracture. If the
bone repairing function is slower than micro-damage accumulation, a ''stress
fracture'' (very small, incomplete fracture) finally occurs (Garcia-Aznar et al.,
2005).
Factors that contribute to skeletal fragility include suboptimal calcium
intake, genetics, lifestyle, smoking, decrease in sex hormone production and
certain medications. Cases of rickets caused by calcium deficiency have been
reported in developed (Davidovits et al., 2006) and developing (Thacher
et al., 2006) countries. Age-related osteoporosis is a major metabolic bone
disease of unknown but apparent multifactorial etiology with nutritional,
lifestyle, genetic and endocrine components. It is characterized by compro-
mised bone strength that predisposes a person to increased fracture risk. A
diagnosis of osteoporosis requires both low bone mass and a preexisting loss
of bone tissue (Harvey et al., 2006). A distinguishing characteristic of this
disease, compared to osteomalacia, is a normal mineral/collagen ratio (Looker
et al., 1993). One in two women and one in five men who are 50 yr of age
will have an osteoporotic fracture in their remaining lifetime (Harvey et al.,
2006).
Calcium intake recommendations vary widely worldwide, with that of
the US National Academy of Sciences (NAS) among the highest. The NAS
established AIs, rather than EARs or RDAs, for Ca (Table 10.2). This
decision was based on several concerns, including uncertainties in the preci-
sion and significance of balance studies needed to determine a desirable
retention model and lack of concordance between mean Ca intake and
experimentally derived values predicted to achieve a desirable level of Ca
retention (Food and Nutrition Board: Institute of Medicine, 1997). NAS
recommendations for pregnancy and lactation are specific for the age of the
mother. The dietary reference nutrient intake values for calcium in the United
Kingdom are lower than in the United States: ages 0-12 months, 525 mg
(17.1 mmol); 1-3 yr, 350 mg (11.4 mmol); 4-6 yr, 450 mg (14.6 mmol);
7-10 yr, 550 mg (17.9 mmol); 11-18 yr (male), 1000 mg (32.5 mmol);
