We report on research progress on two approaches that may be useful in determining the upper adequacy range for macronutrients such as amino acids. One approach was to attempt to identify “toxic metabolites” that were responsible for toxicity or biomarkers for the toxicity of excessive intake of an amino acid in rats. We found that there was hepatic toxicity that was specifically associated with L-cystine excess, but not with L-cysteine excess. We analyzed urine samples from rats fed basal diets or L-cystine or L-cysteine excess diets and identified 25 peaks from gas chromatography mass spectrometry analysis that were specific for L-cystine excess and also correlated with toxicity markers. Another approach was to try to identify “metabolic limits” by measuring CO2 arising from amino acid excess. Uniformly 13C labeled L-leucine was used as tracer, in diets with added L-leucine fed to rats, and 13CO2 arising from its metabolism was collected over 24 h and the fraction of the ingested L-leucine that was exhaled as CO2 was calculated. The fractional exhalation of 13CO2 increased with increasing L-leucine dose, but showed an inflexion point at —8.9 g/kg body weight, after which it reached a plateau. This suggested that >8.9 g/kg BW, the catabolism of L-leucine changed and this approximately coincided with the dose above which a statistically significant decrease in body weight was seen.