Northern elephant seal skeletal muscle, blubber, and plasma were collected from 5 adult females early and late in a 5-week molting fast (mean 33 days apart; see Sample Key in supplementary files). Proteins were isolated from muscle using the Trizol method and blubber using Qiazol with the Qiagen RNeasy Lipid Mini kit (see Khudyakov et al., 2018, doi: 10.1242/bio.036731). Plasma samples were used raw (without depletion of highly abundant proteins). Muscle, blubber, and plasma proteins were denatured using 1% sodium deoxycholate, 8 M urea, and 5 mM DTT in 50 mM ammonium bicarbonate and digested in-solution using trypsin (1:50 trypsin to protein ratio).