Guide to Cholesterol
What is cholesterol?
Cholesterol is a waxy fat-like substance essential for proper body function. It is used to build cell membranes, produce hormones, and vitamin D. The liver produces all the cholesterol the body needs, but we also get it from food. Too high cholesterol levels can lead to atherosclerosis and heart disease.
Types of cholesterol
- LDL (low-density lipoprotein) - "bad" cholesterol, transports cholesterol to tissues, excess builds up in artery walls
- HDL (high-density lipoprotein) - "good" cholesterol, transports cholesterol from tissues to the liver
- Triglycerides - the most common form of fat in the body, high levels increase heart disease risk
Healthy cholesterol levels
- Total cholesterol: below 200 mg/dL
- LDL: below 100 mg/dL (optimal), 100-129 mg/dL (near optimal)
- HDL: above 40 mg/dL in men, above 50 mg/dL in women
- Triglycerides: below 150 mg/dL
How to lower cholesterol?
Lowering cholesterol requires lifestyle changes: limiting saturated and trans fats, increasing fiber intake, regular physical activity, maintaining healthy weight, and quitting smoking. In some cases, medication such as statins may be necessary. Regular cholesterol testing is crucial for cardiovascular disease prevention.
Two people with identical total cholesterol can have ratios twice as far apart
Total cholesterol on its own says remarkably little. Two results of 200 mg/dL — one with HDL 70 and triglycerides 100, the other with HDL 35 and triglycerides 175 — produce total-to-HDL ratios of 2.86 and 5.71. The same headline number, exactly twice the ratio, and a different clinical picture entirely.
How it works
- Estimates LDL from total cholesterol, HDL and triglycerides using the Friedewald equation.
- Computes the total-to-HDL ratio, which separates results that a single total cannot.
- Converts between mmol/L and mg/dL using the correct factor for each analyte.
Friedewald: mg/dL LDL = TC − HDL − TG ÷ 5 mmol/L LDL = TC − HDL − TG ÷ 2.2 ratio = TC ÷ HDL unit conversion differs by analyte: cholesterol 1 mmol/L = 38.67 mg/dL triglycerides 1 mmol/L = 88.57 mg/dL
Worked example
Two panels sharing a total cholesterol of 200 mg/dL.
- A: TC 200, HDL 70, TG 100 → LDL 110, ratio 2.86
- B: TC 200, HDL 35, TG 175 → LDL 130, ratio 5.71
- identical total, ratio differs by a factor of 2.00
- at TG 300 the equation subtracts 60 mg/dL for triglycerides alone
- above about TG 400 the Friedewald estimate is no longer considered valid
The total is the least informative number on the panel. What separates these two results is the split between HDL and everything else — and a calculated LDL that drifts further from reality as triglycerides climb.
Reading the result
- The unit conversion is the trap for anyone reading results across systems. Cholesterol converts at 38.67 mg/dL per mmol/L but triglycerides at 88.57 — applying the cholesterol factor to a triglyceride value is wrong by 129%, and the result still looks like a plausible number.
- LDL from Friedewald is calculated, not measured. It assumes a fixed relationship between triglycerides and VLDL cholesterol that breaks down as triglycerides rise, which is why laboratories switch to direct measurement above roughly 400 mg/dL and why a non-fasting sample can shift the estimate.
- Thresholds are not universal and depend on overall cardiovascular risk rather than the lipid panel alone. The same LDL is treated differently for someone with diabetes, existing heart disease or a strong family history, which is a judgement no calculator makes.
- This page does arithmetic on numbers you supply. It cannot interpret a panel, and decisions about treatment belong with a clinician who can see the rest of your risk profile.
Common questions
- Is my total cholesterol the number that matters?
- Less than almost any other figure on the panel. Two people at 200 mg/dL here differ twofold in total-to-HDL ratio, because the total lumps together fractions that behave very differently. The ratio and the individual components carry the information the total discards.
- Why does my result differ between two labs?
- Often units or method. Cholesterol and triglycerides convert between mmol/L and mg/dL at different factors, and LDL may be calculated by Friedewald in one place and measured directly in another — a difference that widens as triglycerides rise.