Triglycerides vs cholesterol: what the difference is and why both are on a lipid panel

The two get lumped together as 'blood fats', but chemically and biologically they are unrelated. Triglyceride is fuel: what you eat as fat and what your liver makes from excess sugar is packaged as triglyceride, shipped out in chylomicrons and VLDL, and either burned by muscle or stored. Cholesterol is a building material: every cell membrane contains it, and it is the raw material for cortisol, testosterone, estrogen, vitamin D and the bile acids that digest fat. The body makes all the cholesterol it needs, mostly in the liver, which is why dietary cholesterol has a smaller effect than people expect and saturated fat — which raises the liver's output of LDL — has a larger one.

Triglycerides respond to the energy side of the diet: sugary drinks, juice, refined starch, alcohol and excess calories drive the liver to make VLDL, and insulin resistance slows its clearance — so triglycerides track weight, waist, glucose and alcohol closely and move within weeks when those change. Cholesterol responds to the fat composition of the diet and to genetics: saturated and trans fats reduce the liver's LDL receptors, raising LDL by 10–20%; inherited variants (familial hypercholesterolemia) raise it far more and do not respond to diet. An underactive thyroid raises both; so do some medications, in different directions.

LDL cholesterol is causal: lowering it with statins and other drugs reduces heart attacks in proportion to the drop, in trial after trial. Triglycerides are more complicated — a high level marks the presence of cholesterol-carrying remnant particles and of insulin resistance, both of which raise risk, and lowering triglycerides with icosapent ethyl reduced events in REDUCE-IT, but triglyceride-lowering with fibrates or niacin on top of a statin did not. In practice the LDL (or ApoB) is treated first, and a persistently high triglyceride is treated when overall risk is high, or urgently above 500–1,000 mg/dL to prevent pancreatitis.

The combination that matters most is high triglycerides with a low HDL and a normal-looking LDL — the insulin-resistance pattern, in which the LDL understates the number of plaque-forming particles. Non-HDL cholesterol (total minus HDL) or ApoB corrects for that and is the better single number when triglycerides are above about 150–200 mg/dL. High triglycerides also make the calculated LDL unreliable above 400 mg/dL, which is why a fasting repeat or a direct LDL is asked for.