The rate of protein digestion affects protein gain differently during aging in humans

Abstract

In young men ingesting protein meals, slowly digested proteins (caseins: CAS) induce a higher protein gain than those that are rapidly digested (whey proteins: WP). Our aim was to assess whether or not this is true in elderly men receiving mixed meals. The effects of meals containing either CAS or two different amounts of WP (WP-iN: isonitrogenous with CAS, or WP-iL: providing the same amount of leucine as CAS) on protein metabolism (assessed by combining oral and intravenous leucine tracers) were compared in nine healthy, elderly (mean +/- S.E.M. age 72 +/- 1 years) and six young men (24 +/- 1 years). In both age groups, WP-iL and WP-iN were digested faster than CAS (P < 0.001, ANOVA). Proteolysis was inhibited similarly whatever the meal and age groups (P = NS). Protein synthesis was higher with WP-iN than with CAS or WP-iL (P < 0.01), irrespective of age (P = NS). An age-related effect (P < 0.05) was found with postprandial leucine balance. Leucine balance was higher with CAS than with WP-iL (P < 0.01) in young men, but not in elderly subjects (P = NS). In isonitrogenous conditions, leucine balance was higher with WP-iN than with CAS (P < 0.001) in both age groups, but the magnitude of the differences was higher in the elderly men (P = 0.05). In conclusion, during aging, protein gain was greater with WP (rapidly digested protein), and lower with CAS (slowly digested protein). This suggests that a 'fast' protein might be more beneficial than a 'slow' one to limit protein losses during aging.

Figures

Figure 1. Experimental design

Young (n = 6) and elderly men (n = 9) ingested mixed meals containing different types of protein: casein (CAS: 0.48 g CAS (kg BM)−1 and 296 μmol leucine (kg BM)−1) or two amounts of whey protein (WP): one was isonitrogenous with CAS (WP-iN) and the other provided the same amount of leucine as CAS (WP-iL). CAS and WP were intrinsically labelled with [5,5,5,2H3]leucine. i.v., intravenous.

Figure 2. Effect of ingestion of CAS, WP-iL and WP-iN on plasma concentrations of glucagon and insulin

Plasma glucagon (A) and insulin concentrations (B) after ingestion of CAS (•), WP-iL (△) and WP-iN (□) by young (n = 6, upper graphs) or elderly men (n = 9, lower graphs). Values are means ±s.e.m.

Figure 3. Effect of ingestion of CAS, WP-iL and WP-iN on plasma concentrations of leucine, amino acids (AAs) and the rate of appearance of exogenous leucine

Plasma concentrations of leucine (A), the sum of essential (B) and total AAs (C) and the rate of appearance of exogenous leucine (Exo Leu Ra, D) after ingestion of CAS (•), WP-iL (△) and WP-iN (□) by young (n = 6, upper graphs) or elderly men (n = 9, lower graphs). Values are means ±s.e.m. The lines at the top of the graphs indicate time-points where values are different (P < 0.05) from baseline within each study.

Figure 4. Effect of ingestion of CAS, WP-iL and WP-iN on the rate of appearance of endogenous leucine, non-oxidative leucine disposal (NOLD) and leucine oxidation

A, rate of appearance of endogenous leucine (Endo Leu Ra; i.e. proteolysis). B, NOLD (i.e. protein synthesis) and C, leucine oxidation (Leu Ox) after ingestion of CAS (•), WP-iL (△) and WP-iN (□) by young (n = 6, upper graphs) or elderly men (n = 9, lower graphs). Results are means ±s.e.m. FFM: fat free mass (kg). The lines at the top of the graphs indicate time-points where values are different from baseline within each study (P < 0.05).

Figure 5. Effect of ingestion of CAS and WP on leucine balance in elderly and younger men

Leucine (Leu) balance over 420 min after ingestion of CAS (white bars) and of WP providing either the same amount of leucine as CAS (black bars: WP-iL; A) or the same quantity of protein as CAS (grey bars: WP-iN; B), and efficiency of posptrandial protein utilization (PPUN; C). Meals were ingested by young (n = 6; left) and elderly men (n = 9; right). FFM, fat-free mass (kg). The results are presented as means ±s.e.m. The lines at the top of the graphs indicate differences between CAS and WP within the age group. *Significant differences between age groups within each meal (P < 0.05).