Calcium absorption is likely to be highest in

Question

BlackTom AI · Accepted Answer

Adolescents absorb calcium most efficiently because their bones are undergoing rapid growth and modeling, driven by hormones that increase intestinal calcium uptake.

1) Relevant knowledge points and concepts
- Intestinal calcium absorption is hormonally regulated, primarily by active vitamin D (1,25-dihydroxyvitamin D or calcitriol), with contributions from growth hormone and IGF-1 during growth phases.  
- Physiologic demand: during periods of rapid skeletal growth (infancy, childhood, adolescence, and pregnancy), fractional calcium absorption increases to meet bone mineralization needs.  
- Aging and estrogen deficiency (postmenopause) lower fractional absorption and increase bone resorption.

2) Step-by-step reasoning
- Adolescence: rapid linear growth and bone mass accrual require more calcium; the body responds by upregulating calcitriol-mediated active transport in the gut, so a higher fraction of dietary calcium is absorbed.  
- Middle-aged adults: growth has ceased, so demand and hormonally driven absorption return toward lower, maintenance levels.  
- Postmenopausal women: estrogen deficiency leads to increased bone resorption and often reduced intestinal calcium absorption efficiency; although bone turnover rises, net bone loss occurs because absorption does not increase sufficiently to replace the lost calcium.

3) Quantitative context (illustrative, not exact): fractional calcium absorption is highest in growth phases (adolescents), lower in healthy middle-aged adults, and often lowest in older/postmenopausal women—consistent with observed patterns of bone mass accrual and loss.

4) Why the correct answer is correct
Adolescent males and females have the highest physiological calcium absorption due to elevated demand for bone mineralization and upregulation of the vitamin D–dependent absorption pathway. This directly explains why they absorb more dietary calcium than adults.

5) Why other options are incorrect
- a (postmenopausal women): incorrect because estrogen loss reduces absorption efficiency and causes net bone loss.  
- b (adults in middle age): incorrect because they are in a maintenance phase with lower fractional absorption than adolescents.  
- d (both a and b): incorrect, since neither group has higher absorption than adolescents.

Conclusion: Therefore, the best choice is "adolescent males and females," matching the provided correct answer.

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