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?

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.

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.

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.

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.