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Decreasing bone density is a major problem for elderly individuals and can result from many factors. Bone density loss may result in skeletal fractures that might be avoidable with proper preventative measures.

To understand how supplements might help maintain or improve bone density, a group of medical students performs a study on a group of otherwise healthy elderly male individuals.

The 300 participants aged 65-75 were evenly split into three treatment groups that received two pills nightly for three years:

(1) Placebo: 2 cellulose pills
(2) Calcium supplements: pill containing 500 mg of calcium in the form of calcium citrate malate + placebo pill
(3) Calcium supplement + vitamin D supplement: 2 pills, 1 containing 500 mg of calcium citrate malate + 1 containing 700 IU of cholecalciferol

Diets were monitored to ensure comparable intake of macronutrients and calcium. At the end of the study, bone density of the femoral neck was determined, as were plasma levels of ionized calcium, and 25-hydroxy vitamin D (calcidiol).

The table below shows clinical results from the study as average changes from initial values in each treatment (standard errors given in parentheses). Significant differences from placebo are indicated with bold font.

Placebo Calcium Calcium + Vitamin D
Bone density - 1.35 (0.02) - 1.09 (0.03) + 1.18 (0.01)
Ionized calcium + 0.0 (0.01) + 0.0 (0.01) + 0.15 (0.02)
Calcidiol - 2.43 (0.88) - 1.99 (0.82) + 11.9 (0.95)

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Which of the following statements gives the most fundamental reason for the results seen in plasma ionized calcium?


The calcium supplements were not properly absorbed by the small intestines.


Calcium was reabsorbed by the kidney at higher rates in the ‘calcium’ group.


Vitamin D is necessary for proper absorption of calcium in the small intestines.


Placebo individuals apparently had a higher intake of calcium in their diets.

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