A randomized controlled trial reported fewer fractures in elderly women taking 1000 mg/day calcium with vitamin D compared with placebo. As a nutrition scientist, evaluate two limitations that could bias these results and how they would affect interpretation.

• A. Potential limitations include poor adherence to supplementation (which would bias results toward the null if not accounted for) and lack of control for baseline vitamin D status (which could confound results because absorption depends on vitamin D; either could over- or underestimate the true effect), therefore interpret with caution
• B. Limitations include the impossibility of randomization in nutritional studies and the fact that fractures are unrelated to bone density; these automatically make results meaningless
• C. The primary bias is that calcium increases exercise in patients which artificially lowers fracture risk, and therefore calcium should be banned as a supplement
• D. The only limitation is that calcium cannot affect bone density, so any observed benefit must be entirely due to chance and the trial is invalid

Answer: (A)