NPPA and Skeletal muscle atrophy: It is generally well-understood that high protein and fat intake may increase the availability of TRP in the plasma and hence upregulate the KP, and vice versa.151 Differences in KP concentrations, including lower levels of TRP and KYN, may underpin heightened disease risk, including skeletal muscle atrophy,152 cognitive decline152 and higher mortality in cardiovascular disease,153 for example.