C4A and infection: To examine BA synthesis, we measured 7α-hydroxy-4-cholesten-3-one (or C4).28 C4 mirrors the activity of a rate-limiting enzyme in BA synthesis pathway (Cyp7a1) and is secreted from liver during conversion of cholesterol to BAs.28 Notably, serum C4 concentration is a highly specific marker of primary BA synthesis.29 Prior to infection, serum C4 was slightly more in HFD-fed mice compared to RD-fed mice, suggesting a role for dietary modifications in inducing hepatic BA synthesis.