Different approaches to define individual amino acid requirements.

A full review of the strengths and limitations of the various methods used to define amino acid requirements is provided. The focus is on the recent development of carbon oxidation techniques such as indicator amino acid oxidation and 24-h amino acid balance to determine dietary indispensable (essential) amino acid needs in adults. All approaches depend on the change in a metabolic parameter in response to graded intake of the test amino acid. In humans, the within-subject variance is less than the between-subject variance, which has led to an appreciation of the need to study each subject across a range of intakes, above and below the mean requirement level. The data can then be analyzed using two-phase linear regression crossover and a precise population mean requirement can be determined. Several approaches have been used to define the variance of the mean requirement. Finally, a minimally invasive indicator amino acid oxidation model has been developed which allows the determination of dietary essential amino acid requirements in children and other vulnerable populations.