The present invention relates to methods of determining the levels of 3-methylhistidine before and after a meal in a companion animal, wherein the meal is a pet foodstuff having a particular protein to fat ratio, which is useful in decreasing the levels of 3-methydlhistidine post-prandially and having the beneficial effects as described herein. The pet foodstuff comprises a ratio of protein to fat of 1:0.27 to 1:0.63 on a gram:gram as fed or dry matter basis. The pet foodstuff described is fed to a companion animal for use in reducing and/or preventing sarcopenia. The present invention also relates to methods of feeding the pet foodstuff described and/or dietary regimes to provide the companion animal with the benefit of reducing and/or preventing sarcopenia.