DescriptionDietary sulfur amino acid restriction (SAAR) increases food intake and energy expenditure and improves body composition. Dietary SAAR activates the integrated stress response (ISR), which orchestrates transcriptional changes through activating transcription factor 4 (ATF4). Fibroblast growth factor 21 (FGF21) is a hepatokine and ATF4 target which affects food and fluid intake, energy expenditure, and body composition. To determine whether ATF4 is needed to increase food and fluid intake and energy expenditure, improve body composition, and alter hepatic gene expression during SAAR, male and female wild type (WT) and Atf4-/- (KO) mice were maintained on a high fat (60% fat by Kcal) control (0.86% Met, 0% Cys) or high fat SAAR (0.12% Met, 0% Cys) diet for 5 weeks. SAAR increased hepatic Fgf21 and water intake independently of ATF4 in males and females. In males, SAAR increased food intake and energy expenditure independently of ATF4, while females showed no response. SAAR improved body composition in WT males but not females and did not prevent increased adiposity in male KO mice. ATF4 is required for protection from adiposity but not for increased food and water intake, energy expenditure, or hepatic Fgf21 expression during SAAR. FGF21 does not work exclusively through an ATF4-FGF21 axis and may be driving ATF4-independent responses to SAAR.