Constitutive causal effects of the six cancers on endogenous antioxidants were also detected: NSCLC was suggestively associated with increased CAT levels; SCLC showed a suggestive association with increased GST levels; BC was nominally correlated with decreased albumin levels; PC showed a positive causal relationship with GPX; and retinol was the only exogenous antioxidant showing a causal association with NSCLC. Here, CAT is linked to cancer.