Proprotein convertase, subtilisin/kexin type 9 (PCSK9), a key regulator of 
plasma LDL-cholesterol (LDL-c) and cardiovascular risk, is produced in liver and 
secreted into plasma where it binds hepatic LDL receptors (LDLR), leading to 
their degradation. PCSK9 is transcriptionally activated by sterol response 
element-binding protein (SREBP)-2, a transcription factor that also activates 
all genes for cholesterol synthesis as well as the LDLR. Here we investigated 
the relationship between plasma PCSK9 levels and the lathosterol-to-cholesterol 
ratio, a marker of cholesterol biosynthesis, in 18 healthy subjects during a 48 
h fast. In all individuals, plasma PCSK9 levels declined steadily during the 
fasting period, reaching a nadir at 36 h that was ∼58% lower than levels 
measured in the fed state (P < 0.001). Similarly, the lathosterol-to-cholesterol 
ratio declined in parallel with plasma PCSK9 concentrations during the fast, 
reaching a nadir at 36 h that was ∼28% lower than that measured in the fed state 
(P = 0.024). In summary, fasting has a marked effect on plasma PCSK9 
concentrations, which is mirrored by measures of cholesterol synthesis in 
humans. Inasmuch as cholesterol synthesis and PCSK9 are both regulated by 
SREBP-2, these results suggest that plasma PCSK9 levels may serve as a surrogate 
marker of hepatic SREBP-2 activity in humans.