. 2021 Jun 8;13(6):1976. doi: 10.3390/nu13061976

Table 2.

Changes in plasma free amino acid levels throughout the oral phenylalanine or serine supplementation period in healthy adults.

Essential Amino Acids (mmol/L)	Trial	Phenylalanine (n = 23)	Serine (n = 30)
Histidine	Baseline	$84.0 \pm$ 1.6	$86.5 \pm$ 1.4
	3 g/day	$85.1 \pm$ 1.6	$84.5 \pm$ 1.9
	6 g/day	$83.8 \pm$ 1.4	$82.5 \pm$ 1.6
	9 g/day	$82.0 \pm$ 1.1	$82.0 \pm$ 1.6
	12 g/day	$84.3 \pm$ 1.7	$83.3 \pm$ 1.6
Isoleucine	Baseline	$63.7 \pm$ 2.1	$74.6 \pm$ 3.9
	3 g/day	$66.7 \pm$ 2.5	$72.3 \pm$ 3.9
	6 g/day	$72.1 \pm$ 5.0	$71.4 \pm$ 3.0
	9 g/day	$66.3 \pm$ 3.1	$74.5 \pm$ 4.5
	12 g/day	$69.7 \pm$ 2.7	$75.4 \pm$ 3.6
Leucine	Baseline	$124.4 \pm$ 3.8	$138.8 \pm$ 6.8
	3 g/day	$127.4 \pm$ 3.6	$137.6 \pm$ 6.9
	6 g/day	$133.6 \pm$ 5.8	$131.7 \pm$ 3.9
	9 g/day	$124.4 \pm$ 4.9	$137.1 \pm$ 7.2
	12 g/day	$131.1 \pm$ 4.3	$140.3 \pm$ 5.5
Lysine	Baseline	$197.3 \pm$ 4.5	$203.7 \pm$ 8.4
	3 g/day	$201.1 \pm$ 5.1	$193.8 \pm$ 5.6
	6 g/day	$194.6 \pm$ 5.7	$191.8 \pm$ 4.9
	9 g/day	$191.4 \pm$ 5.7	$196.5 \pm$ 5.5
	12 g/day	$207.4 \pm$ 6.0	$204.9 \pm$ 6.5
Methionine	Baseline	$28.3 \pm$ 0.9	$29.2 \pm$ 1.1
	3 g/day	$28.5 \pm$ 0.8	$27.7 \pm$ 0.9
	6 g/day	$28.7 \pm 0$ .3	$27.8 \pm$ 0.7
	9 g/day	$27.5 \pm$ 1.1	$28.2 \pm$ 0.9
	12 g/day	$28.1 \pm$ 0.6	$28.7 \pm$ 1.1
Phenylalanine	Baseline	$61.9 \pm$ 1.7	$62.2 \pm$ 1.5
	3 g/day	$65.1 \pm$ 2.7	$61.6 \pm$ 1.5
	6 g/day	$73.6 \pm$ 5.2	$60.9 \pm$ 1.3
	9 g/day	$68.6 \pm$ 3.7	$60.2 \pm$ 1.8
	12 g/day	$69.4 \pm$ 3.8	$59.3 \pm$ 1.3
Threonine	Baseline	$133.0 \pm$ 4.4	$138.8 \pm$ 5.2
	3 g/day	$131.3 \pm$ 4.0	$128.7 \pm$ 4.7
	6 g/day	$124.7 \pm$ 5.0	$131.2 \pm$ 3.5
	9 g/day	$123.8 \pm$ 5.0	$127.3 \pm$ 3.8
	12 g/day	$124.7 \pm$ 3.7	$135.2 \pm$ 4.6
Tryptophan	Baseline	$52.1 \pm$ 1.2	$59.3 \pm$ 2.1
	3 g/day	$53.9 \pm$ 1.3	$56.8 \pm$ 2.7
	6 g/day	$54.0 \pm$ 1.4	$57.4 \pm$ 1.6
	9 g/day	$52.2 \pm$ 1.6	$56.5 \pm$ 2.1
	12 g/day	$52.0 \pm$ 1.3	$54.7 \pm$ 2.0
Valine	Baseline	$237.0 \pm$ 5.7	$261.7 \pm$ 10.1
	3 g/day	$238.4 \pm$ 5.9	$254.5 \pm$ 11.1
	6 g/day	$249.0 \pm$ 8.4	$248.6 \pm$ 7.9
	9 g/day	$238.3 \pm$ 8.0	$252.5 \pm$ 10.6
	12 g/day	$251.8 \pm$ 7.5	$259.8 \pm$ 8.3
Non-Essential Amino Acids, Cystine and Homocysteine (mmol/L)	Trial	Phenylalanine (n = 23)	Serine (n = 30)
Alanine	Baseline	$379.8 \pm$ 17.9	$410.9 \pm$ 14.8
	3 g/day	$391.5 \pm$ 15.6	$384.2 \pm$ 17.9
	6 g/day	$384.8 \pm$ 20.0	$391.8 \pm$ 17.3
	9 g/day	$378.0 \pm$ 18.2	$396.4 \pm$ 16.0
	12 g/day	$381.8 \pm$ 15.4	$385.1 \pm$ 16.5
Arginine	Baseline	$98.0 \pm$ 2.6	$101.7 \pm$ 3.3
	3 g/day	$94.8 \pm$ 3.1	$92.8 \pm$ 2.7
	6 g/day	$88.8 \pm$ 3.1	$90.8 \pm$ 2.7
	9 g/day	$86.5 \pm$ 2.7	$93.2 \pm$ 3.4
	12 g/day	$91.2 \pm$ 2.4	$95.5 \pm$ 3.6
Aspartic acid	Baseline	$2.2 \pm$ 0.2	$2.8 \pm$ 0.3
	3 g/day	$1.5 \pm$ 0.3	$1.7 \pm$ 0.2
	6 g/day	$2.0 \pm$ 0.2	$2.1 \pm$ 0.2
	9 g/day	$2.4 \pm$ 0.2 ^$	$2.3 \pm$ 0.2
	12 g/day	$1.6 \pm$ 0.3	$2.1 \pm$ 0.3
Asparagine	Baseline	$59.4 \pm$ 1.3	$61.1 \pm$ 1.9
	3 g/day	$61.2 \pm$ 1.4	$58.1 \pm$ 1.8
	6 g/day	$60 \pm$ 2.3	$59.2 \pm$ 2.0
	9 g/day	$58.3 \pm$ 1.9	$59.1 \pm$ 1.9
	12 g/day	$58.2 \pm$ 1.4	$57.4 \pm$ 1.9
Cystine	Baseline	$50.3 \pm$ 1.5	$51.2 \pm$ 1.6
	3 g/day	$49.0 \pm$ 1.6	$49.2 \pm$ 1.5
	6 g/day	$47.4 \pm$ 1.5	$48.1 \pm$ 1.4
	9 g/day	$48.5 \pm$ 1.4	$49.8 \pm$ 1.6
	12 g/day	$46.4 \pm$ 1.4	$47.9 \pm$ 1.4
Glutamic acid	Baseline	$32.4 \pm$ 1.7	$39.7 \pm$ 2.2
	3 g/day	$35.5 \pm$ 2.2	$37.0 \pm$ 2.0
	6 g/day	$36.5 \pm$ 2.6	$39.5 \pm$ 1.8
	9 g/day	$35.1 \pm$ 2.4	$40.3 \pm$ 2.0
	12 g/day	$28.2 \pm$ 2.3	$36.6 \pm$ 2.5
Glutamine	Baseline	$559.6 \pm$ 10.5	$527.4 \pm$ 8.6
	3 g/day	$547.3 \pm$ 10.9	$518.4 \pm$ 9.6 ^#
	6 g/day	$522.5 \pm$ 12.4	$495.9 \pm$ 9.5
	9 g/day	$513.6 \pm$ 9.4 *^,#	$502.2 \pm$ 9.6
	12 g/day	$559.4 \pm$ 10.5	$537.9 \pm$ 10.2
Glycine	Baseline	$244.6 \pm$ 8.5	$257.5 \pm$ 12.4
	3 g/day	$248.0 \pm$ 8.5	$250.0 \pm$ 11.3
	6 g/day	$230.2 \pm$ 10.0	$264.8 \pm$ 13.6
	9 g/day	$229.0 \pm$ 9.1	$259.7 \pm$ 12.0
	12 g/day	$228.0 \pm$ 7.4	$269.7 \pm$ 13.0
Proline	Baseline	$164.6 \pm$ 6.8	$186.6 \pm$ 8.5
	3 g/day	$162.1 \pm$ 6.7	$178.4 \pm$ 9.2
	6 g/day	$167.9 \pm$ 10.0	$182.3 \pm$ 8.2
	9 g/day	$157.3 \pm$ 6.2	$182.5 \pm$ 11.0
	12 g/day	$154.5 \pm$ 5.6	$178.2 \pm$ 9.5
Serine	Baseline	$113.9 \pm$ 3.4	$117.3 \pm$ 4.0
	3 g/day	$119.7 \pm$ .9	$122.5 \pm$ 3.6
	6 g/day	$113.0 \pm$ 4.4	$138.7 \pm$ 5.6
	9 g/day	$113.1 \pm$ 3.5	$157.1 \pm$ 7.2 **
	12 g/day	$113.9 \pm$ 3.5	$175.5 \pm$ 12.7 **
Tyrosine	Baseline	$62.3 \pm$ 1.8	$67.2 \pm$ 2.1
	3 g/day	$66.7 \pm$ 2.2	$62.9 \pm$ 2.5
	6 g/day	$72.4 \pm$ 3.6	$63.5 \pm 2.1$
	9 g/day	$67.1 \pm$ 3.7	$61.6 \pm$ 2.8
	12 g/day	$75.0 \pm$ 4.2 *	$62.1 \pm$ 2.4
Homocysteine	Baseline		$12.6 \pm$ 0.6
	3 g/day		$12.8 \pm$ 1.4
	6 g/day		$13.8 \pm$ 2.0
	9 g/day		$13.9 \pm$ 2.2
	12 g/day		$13.4 \pm$ 2.0

Values represent mean ± SEM. One-way ANOVA was followed with the Tukey’s multiple comparisons test between each amino acid intake: * p < 0.05, ** p < 0.01 from baseline, ^$ p < 0.05 from dosage of 3 g/kg, ^# p < 0.05 from dosage of 12 g/kg.