Table 1.

Concentrations, variability, and contribution to the 1-factor solution of 15 amino acids.

Amino Acid	Median (SD) Concentration* μM	%CV Between Individuals	Mean %CV (Range) Within Individuals	Mean %CV Analytical Variability	Factor Loading
1. Glycine	313.2 (73.4)	23.0	10.9 (3.1 – 27.8)	3.2	0.53
2. Alanine	481.6 (104.8)	21.5	14.4 (5.1 – 31.4)	5.1	0.53
3. Serine	116.6 (24.8)	21.2	11.5 (1.9 – 27.5)	7.2	0.49
4. Proline	231.6 (63.5)	26.9	17.9 (5.3 – 32.4)	2.9	0.66
5. Valine	275.4 (72.7)	25.8	14.6 (5.7 – 27.4)	6.2	0.68
6. Leucine/Isoleucine	196.1 (62.4)	30.2	21.6 (4.0 – 51.2)	5.2	0.65
7. Methionine	31.4 (8.4)	26.4	21.9 (5.3–46.5)	6.6	0.80
8. Histidine	86.7 (20.0)	22.5	10.2 (3.3 – 24.4)	12.2	0.55
9. Phenylalanine	76.2 (16.0)	20.5	14.2 (4.4 – 26.1)	3.0	0.75
10. Tyrosine	82.7 (21.4)	25.3	17.2 4.8 – 35.0)	3.1	0.71
11. Aspartate/Asparagine	21.4 (6.0)	26.9	20.9 (7.4 – 65.8)	8.1	0.23
12. Glutamate/Glutamine	100.0 (27.5)	26.5	14.2 (2.1 – 31.8)	10.5	0.36
13. Ornithine	77.5 (19.7)	25.7	16.8 (2.1 – 49.5)	7.6	0.48
14. Citrulline	41.3 (13.6)	32.1	13.9 (5.7 – 28.0)	7.4	0.26
15. Arginine	125.8 (26.6)	20.9	13.1 (4.8 – 29.2)	12.5	0.55

Variability was expressed as %CV, calculated as 100*SD/mean. Lower limits of quantification were 5 μM for glycine and alanine, and 1 μM for remaining amino acids. Variability between individuals, or total variability, was calculated based on all values (all subjects at all time points) obtained in the study. Variability within individuals was calculated based on determining each individual’s %CV over four sample collection time points. Analytical variability, or error attributable to the instrument, was determined by measuring two quality control mixtures (diluent spiked with high or low concentrations of each AA) each day before and after assaying samples. %CV=100*standard deviation/mean