Impact of habituated dietary protein intake on fasting and postprandial whole-body protein turnover and splanchnic amino acid metabolism in elderly men: a randomized, controlled, crossover trial

Research output: Contribution to journalArticlepeer-review


  • Grith Højfeldt
  • Jacob Bülow
  • Jakob Agergaard
  • Ali Asmar
  • Peter Schjerling
  • Lene Simonsen
  • Jens Bülow
  • Gerrit Van hall

Colleges, School and Institutes

External organisations

  • Bispebjerg Hospital
  • Copenhagen University Hospitals
  • University of Copenhagen


Background: Efficacy of protein absorption and subsequent amino acid utilization may be reduced in the elderly. Higher protein intakes have been suggested to counteract this.

Objectives: We aimed to elucidate how habituated amounts of protein intake affect the fasted state of, and the stimulatory effect of a protein-rich meal on, protein absorption, whole-body protein turnover, and splanchnic amino acid metabolism.

Methods: Twelve men (65–70 y) were included in a double-blinded crossover intervention study, consisting of a 20-d habituation period to a protein intake at the RDA or a high amount [1.1 g · kg lean body mass (LBM)−1 · d−1 or >2.1 g · kg LBM−1 · d−1, respectively], each followed by an experimental trial with a primed, constant infusion of D8-phenylalanine and D2-tyrosine. Arterial and hepatic venous blood samples were obtained after an overnight fast and repeatedly 4 h after a standardized meal including intrinsically labeled whey protein concentrate and calcium-caseinate proteins. Blood was analyzed for amino acid concentrations and phenylalanine and tyrosine tracer enrichments from which whole-body and splanchnic amino acid and protein kinetics were calculated.

Results: High (compared with the recommended amount of) protein intake resulted in a higher fasting whole-body protein turnover with a resultant mean ± SEM 0.03 ± 0.01 μmol · kg LBM−1 · min−1 lower net balance (P < 0.05), which was not rescued by the intake of a protein-dense meal. The mean ± SEM plasma protein fractional synthesis rate was 0.13 ± 0.06%/h lower (P < 0.05) after habituation to high protein. Furthermore, higher fasting and postprandial amino acid removal were observed after habituation to high protein, yielding higher urea excretion and increased phenylalanine oxidation rates (P < 0.01).

Conclusions: Three weeks of habituation to high protein intake (>2.1 g protein · kg LBM−1 · d−1) led to a significantly higher net protein loss in the fasted state. This was not compensated for in the 4-h postprandial period after intake of a meal high in protein.

This trial was registered at as NCT02587156.


Original languageEnglish
Pages (from-to)1468-1484
Number of pages17
JournalThe American journal of clinical nutrition
Issue number6
Early online date25 Jul 2020
Publication statusPublished - Dec 2020


  • habitual protein intake, recommended protein intake, protein turnover, protein breakdown, whole-body protein turnover, intrinsically labeled proteins, stable-isotope tracers, whey protein, caseinate protein