Which statement about essential amino acids is true?

• A. There are eight essential amino acids that the body cannot synthesize.
• B. There are ten essential amino acids that the body cannot synthesize.
• C. There are eleven essential amino acids that the body cannot synthesize.
• D. There are nine essential amino acids that the body cannot synthesize and must be obtained from the diet.

Answer: (D)

Essential amino acids are the ones the body cannot synthesize in sufficient amounts, so they must come from the diet. In adults, there are nine of them: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Because the body cannot make these enough on its own, dietary intake is required for growth, maintenance, and repair. The numbers eight, ten, and eleven are not the standard count for adults, though some sources note conditional essentiality for certain amino acids in specific situations (for example, arginine can be conditionally essential for infants). The widely accepted statement is that there are nine essential amino acids that must be obtained from the diet, so this choice is correct. To apply this, include complete proteins or combine foods to ensure all nine essential amino acids are provided, especially on plant-based diets.