Questions: Dietary Protein, Amino Acids, and Nitrogen Balance

Question 1

A person consuming only rice as their sole protein source would most likely become deficient in which essential amino acid, and why is the traditional pairing of rice and beans nutritionally sound?

• A. Methionine — beans are high in methionine, compensating for rice's deficit
• B. Lysine — beans are rich in lysine, which is the limiting amino acid in rice
• C. Tryptophan — beans provide tryptophan that rice cannot supply
• D. Phenylalanine — beans and rice together provide complementary aromatic amino acids

Answer: (B)

Rice is limiting in lysine, the essential amino acid present in smallest amount relative to requirement. Legumes like beans are high in lysine but limiting in methionine — which rice provides. Together they form a complementary protein pair that covers all essential amino acids. This is the nutritional basis for rice-and-bean staple combinations found across many cultures.

Question 2

A person in positive nitrogen balance is breaking down more body protein than they are synthesizing.

• T. True
• F. False

Answer: False

Positive nitrogen balance means nitrogen intake (from dietary protein) exceeds nitrogen excretion (via urine, feces, sweat). Since protein contains approximately 16% nitrogen, positive balance indicates the body is retaining nitrogen — net protein synthesis is occurring. This happens during growth, pregnancy, and recovery from illness or injury. Negative nitrogen balance, not positive, indicates net protein catabolism.

Question 3

Why can't humans synthesize the nine essential amino acids, even though the body can synthesize the other eleven?

Model answer: Humans lack the enzymes required to build the carbon skeletons of essential amino acids from scratch. These biosynthetic pathways exist in plants, bacteria, and fungi but were lost during vertebrate evolution, presumably because dietary intake was reliable enough that maintaining the metabolic machinery was a net cost. The eleven nonessential amino acids can be synthesized from common metabolic intermediates using pathways humans do retain.

Distinguishing between 'cannot synthesize' and 'conditionally essential' is important: conditionally essential amino acids (like glutamine and arginine) can normally be synthesized in adequate amounts but may become insufficient under physiological stress such as illness or rapid growth.