Why Might Increasing Levels of Tryptophan in the Bloodstream Not

Why might increasing levels of tryptophan in the bloodstream not increase brain serotonin levels as would be expected?

A) It doesn't matter how much tryptophan is present, serotonin is made at a constant rate from brain stores of tryptophan.
B) Tryptophan cannot cross the blood-brain barrier under any circumstances.
C) Tryptophan competes with other amino acids for transport across the blood-brain barrier.
D) Insulin is required for transport of tryptophan across the blood-brain barrier.

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