Two-day weekly intermittent fasting produced dramatically different longevity and metabolic outcomes across 10 genetically distinct mouse strains, with effects varying by sex and genetic background across metabolic, blood, and immune parameters. The Collaborative Cross mice revealed that genetic variation fundamentally determines whether intermittent fasting extends lifespan or potentially causes harm. This finding challenges the assumption that intermittent fasting benefits apply universally. The genetic diversity of these mouse strains mirrors human genetic variation more closely than typical laboratory mice, making these results particularly relevant for human translation. The discovery that fasting response is genetically predetermined suggests that personalized approaches based on genetic profiling may be necessary for safe and effective intermittent fasting protocols. While intermittent fasting has gained popularity as a longevity intervention, this research indicates that individuals may need genetic screening to determine optimal fasting regimens. The work represents a crucial step toward precision nutrition, where dietary interventions are tailored to genetic makeup rather than applied broadly across populations.