Enhanced protein breakdown after eccentric exercise in young and older men

The effects of eccentric exercise on whole body protein metabolism were compared in five young untrained [age 24 ± 1 yr, maximal O₂ uptake (V̇O(2max)) = 49 ± 6 ml · kg^-1 · min^-1] and five older untrained men (age 61 ± 1 yr, V̇O(2max) = 34 ± 2 ml · kg^-1 · min^-1). They performed 45 min of eccentric exercise on a cycle ergometer at a power output equivalent to 80% V̇O(2max) (182 ± 18 W). Beginning 5 days before exercise and continuing for at least 10 days after exercise, they consumed a eucaloric diet providing 1.5 g · kg^-1 · day^-1 of protein. Leucine metabolism in the fed state was measured before, immediately after, and 10 days after exercise, with intravenous L-[1-¹³C]leucine as a tracer (0.115 μmol · kg^-1 · min^-1). Leucine flux increased 9% immediately after exercise (P < 0.011) and remain elevated 10 days later, with no effect of age. Leucine oxidation increased 19% immediately after exercise and remained 15% above baseline 10 days after exercise (P < 0.0001), with no effect of age. In the young men, urinary excretion of 3-methylhistidine per gram of creatinine did not increase until 10 days postexercise (P < 0.05), but in the older men, it increased 5 days after exercise and remained high through 10 days postexercise (P < 0.05), averaging 37% higher than in the young men. These data suggest that eccentric exercise produces a similar increase in whole body protein breakdown in older and young men, but myofibrillar proteolysis may contribute more to whole body protein breakdown in the older group.