Human adult amino acid requirements: [1-13C]leucine balance evaluation of the efficiency of utilization and apparent requirements for wheat protein and lysine compared with those for milk protein in healthy adults.
BACKGROUND
There is considerable debate about the human lysine requirement and the consequent nutritional value of wheat protein.


OBJECTIVE
We used a novel [1-(13)C]leucine balance protocol to examine whether adaptive mechanisms to conserve lysine allow wheat to be utilized more efficiently than expected according to current estimates of lysine requirements and wheat utilization.


METHODS
Wheat and milk proteins were compared in 6 adults infused for 9 h with L-[1-(13)C]leucine in the postabsorptive state (0-3 h), who were fed half-hourly with low-protein (2% of energy, 3-6 h) and isoenergetic higher-protein (12-13% of energy, 6-9 h) meals providing maintenance energy intakes. From acute measurements of [1-(13)C]leucine balance, we predicted nitrogen balance, the metabolic demand for protein, the efficiency of postprandial protein utilization (PPU), and the requirements for wheat protein and lysine.


RESULTS
Leucine balance was higher after the milk than after the wheat feeding because of the greater inhibition of proteolysis by milk. PPU, calculated as the ratio of Deltanitrogen balance to Deltanitrogen intake between the low-protein and higher-protein periods, was 0.68 +/- 0.06 for wheat and 1.00 +/- 0.09 for milk (P </= 0.001). The estimated average wheat protein requirement (0. 6/PPU) was 0.89 +/- 0.08 g*kg(-)(1)*d(-)(1), indicating a lysine requirement of 23.2 +/- 2.0 mg*kg(-)(1)*d(-)(1). The measured PPU for wheat, 0.68 +/- 0.06, was higher than the value calculated from wheat lysine intake and milk protein lysine deposition, 0.26 +/- 0. 02, and higher than predicted by most published estimates of lysine requirements, apart from a value of 19 mg/kg indicated by nitrogen balance studies.


CONCLUSIONS
The results show that adaptive mechanisms of lysine conservation allow wheat protein to be utilized more efficiently than expected.
Millward
D J
DJ
Centre for Nutrition and Food Safety, School of Biological Sciences, University of Surrey, Guildford, United Kingdom. d.millward@surrey.ac.uk
Fereday
A
A
Gibson
N R
NR
Pacy
P J
PJ
D000328:Adult; D001786:Blood Glucose / Q000378:metabolism; D001944:Breath Tests; D002245:Carbon Dioxide / Q000378:metabolism; D002247:Carbon Isotopes; D004044:Dietary Proteins / Q000008:administration & dosage*; D004435:Eating; D005260:Female; D006801:Humans; D007930:Leucine / Q000097:blood*; D008239:Lysine / Q000008:administration & dosage*; D008297:Male; D008894:Milk Proteins / Q000008:administration & dosage; D009751:Nutritional Requirements; D014674:Plant Proteins, Dietary / Q000008:administration & dosage; D013997:Time Factors; D014908:Triticum
D001786:Blood Glucose; D002247:Carbon Isotopes; D004044:Dietary Proteins; D008894:Milk Proteins; D014674:Plant Proteins, Dietary; D002245:Carbon Dioxide; D007930:Leucine; D008239:Lysine