Energy yield per gram on complete oxidation Among the listed macronutrients, which substrate yields the maximum energy per gram when fully oxidized in aerobic metabolism?

Introduction / Context:
Nutritional biochemistry compares the energy content of macronutrients to understand fuel selection in cells and whole-body metabolism. Energy density per gram explains why adipose tissue is the body’s primary long-term energy store.

Given Data / Assumptions:

• Lipids (triacylglycerols) are more reduced than carbohydrates and proteins.
• Standard physiological values: fat ~9 kcal/g, carbohydrate ~4 kcal/g, protein ~4 kcal/g.
• Complete oxidation refers to conversion to CO2 and H2O (plus urea for protein nitrogen disposal).

Concept / Approach:
The more reduced a fuel (higher hydrogen to oxygen ratio), the more electrons it donates to NAD+ and FAD during catabolism, generating more ATP via oxidative phosphorylation. Triacylglycerols deliver long acyl chains producing abundant acetyl-CoA, NADH, and FADH2.

Step-by-Step Solution:
Assess reduction state: fats > proteins ≈ carbohydrates.Catabolic pathways: β-oxidation for fats vs glycolysis for carbs vs deamination + TCA entry for proteins.ATP yield per gram follows reduction state; therefore fats have the highest energy density.

Verification / Alternative check:
Bomb calorimetry and metabolic balance studies support ~9 kcal/g for fat versus ~4 kcal/g for carbohydrate and protein under physiological conditions.

Why Other Options Are Wrong:
Protein: significant energy used for urea synthesis and lower reducing equivalents per gram.Glycogen/Starch: polysaccharides are hydrated and less reduced; energy density is about half that of fat.Organic acids: partially oxidized substrates have lower potential energy.

Common Pitfalls:
Confusing total ATP per molecule with ATP per gram; forgetting the energetic cost of nitrogen disposal in protein catabolism.

Final Answer:
Fat.