Which statement best defines an essential amino acid in human nutrition?

• A. An amino acid that is required for protein synthesis but can always be made in adequate quantities from glucose in the liver
• B. An amino acid that the human body synthesizes only during fasting and therefore is not needed in the diet
• C. An amino acid that is found only in animal proteins and never in plant-based foods
• D. An amino acid that cannot be synthesized in sufficient amounts by the human body and therefore must be obtained from the diet

Answer: (D)

Verified step by step guidance

1. Understand the definition of essential amino acids: These are amino acids that the human body cannot synthesize in sufficient quantities to meet physiological needs.
2. Recognize that because the body cannot produce these amino acids adequately, they must be obtained through dietary sources to support protein synthesis and other metabolic functions.
3. Distinguish essential amino acids from non-essential amino acids, which the body can synthesize from other compounds such as glucose or other amino acids.
4. Evaluate the given statements by comparing them to the correct definition: the essential amino acid must be obtained from the diet due to insufficient endogenous synthesis.
5. Conclude that the statement 'An amino acid that cannot be synthesized in sufficient amounts by the human body and therefore must be obtained from the diet' best defines an essential amino acid in human nutrition.