Niacin (vitamin B3) is a precursor for NAD+. Considering NAD+ is required by multiple dehydrogenases in energy metabolism, which integrated outcome would you predict in a person with severe niacin deficiency?

• A. Immediate and specific collapse of bone structure because NAD+ is directly incorporated into hydroxyapatite crystals as a structural component; thus niacin deficiency primarily presents with fractures before any metabolic symptoms.
• B. Complete inability to hydrolyze dietary proteins in the gut because NAD+ is the principal acid in the stomach required for protein digestion; therefore small intestinal bacterial overgrowth is the main presentation.
• C. Impaired glycolysis and TCA cycle dehydrogenase activity leading to reduced ATP production and clinical symptoms like fatigue and muscle weakness, because insufficient NAD+ limits oxidation–reduction reactions critical for macronutrient catabolism.
• D. Enhanced fatty acid beta-oxidation and increased ATP production due to compensatory upregulation of all mitochondrial enzymes in response to niacin deficiency, which would cause hyperactivity and weight loss primarily from adipose tissue.

Answer: (C)