Several amino acids are considered conditionally essential as they are rate limiting for protein synthesis under certain conditions. Individual amino acids are often described as conditionally essential, based on requirements for optimal growth and maintenance of positive nitrogen balance.1

While in extreme circumstances-such as in the absence of certain nonessential amino acids from the diet and the presence of limited amounts of essential amino acids from which to synthesize the nonessential amino acids-any of the amino acids might be considered conditionally essential, including arginine, citrulline, ornithine, proline, cysteine, tyrosine, histidine, and taurine.2-6 However, practically, only seven other amino acids can be considered essential under certain conditions.