K.C. Hayes, P. Khosla, T. Hajri, A. Pronczuk

It has been known for 40 years that dietary saturated fat (SAT FAT) increases plasma cholesterol, including LDL-C and HDL-C. In humans, where LDL-C is typically > 90 mg/dl this SAT FAT effect largely reflects changes in LDL-C pool size. The original human studies suggested that LDL-C expansion during SAT FAT consumption reflected reduced LDL clearance (LDL receptor activity) in hyperlipemics and increased LDL production rates in normolipemics (LDL-C < 100 mg/dl) . This dual explanation is supported by data from several animal models where specific saturated fatty acids (SFAs) have been the focus. However, the situation is complicated by the fact that polyunsaturated fatty acids (PUFAs) oppose SFAs, i.e. PUFAs decrease LDL-C and increase LDL receptor (LDLr) activity, so the effect of SAT FAT intake may represent the combined influence of increased SFAs and decreased PUFAs. In fact, careful scrutiny of primate data suggests a negligible effect of saturated fat on LDL clearance (and receptor activity) in the absence of dietary cholesterol when PUFA intake is adequate (5-10%en) and the lipoprotein profile is relatively normal (LDL-C < 90 mg/dl), i.e. normolipemic situations at the time of dietary intervention. In such cases increases in LDL-C due to SFAs (particularly 12:0+14:0) appear to reflect LDL overproduction associated with a shift in cholesterol from tissues to the plasma cholesteryl ester (CE) pool (both LDL-C and HDL-C) without altering whole-body cholesterol balance. The reason for this shift, which is accompanied by an increase in the plasma oleic/linoleic CE ratio, is unknown but may reflect a decreased rate of CE hydrolysis by the liver. When individuals or animals are rendered hyperlipemic by other factors (e.g. chronic caloric and dietary cholesterol excesses in humans or by cholesterol feeding in animals) specific SFAs (particularly 16:0) can contribute to decreased LDLr activity initiated by a primary factor, such as dietary cholesterol. However, LDLr down-regulation by dietary cholesterol greatly exceeds any contribution from SFAs.