High or Low Cholesterol: Causes, LDL, and HDL

On a checkup, cholesterol is easier to make sense of when you read total cholesterol, LDL, and HDL together rather than one at a time. When LDL is high, things like how you eat fats, your weight, your age, how your thyroid is working, and inherited traits may be involved. When LDL is low, eating too little or the state of your thyroid or liver may be.

What cholesterol reflects

Cholesterol is a type of fat (lipid). Your body uses it to build cell membranes, hormones, and bile acids, which help you digest fat. What matters is how much there is, and whether the way it’s carried in the blood is out of balance.

Most of the cholesterol in your blood is made inside your body, with the liver at the center. When more comes in from food, the body adjusts by making less, but how far it can adjust varies from person to person.

LDL and HDL

Cholesterol doesn’t dissolve in water, so it travels through the blood inside particles called lipoproteins, which combine fats with proteins.

• LDL cholesterol: The cholesterol in particles that travel from the liver to tissues throughout the body. Genetic studies and clinical trials have confirmed that when it stays high for a long time, it builds up in the walls of blood vessels and is one of the causes of atherosclerosis (hardening of the arteries). It’s often called “bad” cholesterol.
• HDL cholesterol: The cholesterol in particles that carry extra cholesterol from the tissues back to the liver. It’s often called “good” cholesterol.
• Total cholesterol: LDL and HDL, plus the cholesterol in the particles that mainly carry triglycerides, and so on.

Total cholesterol minus HDL is called non-HDL cholesterol, and it serves as a guide to all the particles involved in atherosclerosis. LDL can either be measured directly (the direct method) or calculated from fasting values (total cholesterol − HDL − triglycerides ÷ 5), and which one is used depends on the facility. After a meal, or when triglycerides are 400 mg/dL or higher, the calculated value is no longer accurate, so non-HDL or a directly measured LDL is used instead.

Reference ranges, and the values checkups use to rate results

The values printed on your results aren’t all the same kind of number. Two kinds are the most common.

One is Japan’s shared reference intervals (JCCLS). They’re the ranges that hold the middle 95% of values measured in healthy people, made for use nationwide, though whether to adopt them is up to each facility. For lipids, the guide itself notes that many facilities hesitate to adopt these intervals because their upper limits differ from the society’s values. The other is the screening thresholds for dyslipidemia set out in the Japan Atherosclerosis Society’s guidelines (2022 edition). These values were chosen as a cue to act early to prevent atherosclerotic disease.

The right-hand column shows the precision nutrition optimal ranges I use in my counseling. These are precision nutrition guides, not medical reference ranges. Falling outside them doesn’t by itself mean you have an illness.

Both the shared reference intervals and the society’s “fasting” values assume a blood draw after at least 10 hours without food. Triglycerides come out higher after a meal, so a non-fasting result can’t be compared with them as is.

Checkups may also rate results using the cutoffs of Japan’s Specific Health Checkups (the national checkup for adults aged 40 to 74) or of a comprehensive health screening (what Japan calls a ningen dock), and the dividing lines differ a little. Taking LDL as an example, the upper limit of the shared reference interval is 163, but by the society’s thresholds, 120 is already borderline. Even when a result is marked “within the reference range,” checking which standard it was judged by helps you avoid misreading it.

What high values can mean

High LDL or total cholesterol

These are some of the known factors behind a high LDL:

• Diet: Too much saturated fat, found in fatty cuts of meat, butter, heavy cream, and Western-style sweets, or too much trans fat. Eating a lot of cholesterol also tends to push LDL up.
• Weight gain, or moving less
• Age and sex: For women, female hormones decline around menopause, and LDL tends to rise.
• Other illnesses or medications: Low thyroid function, kidney or bile duct disease, diabetes, and some medications can raise it.
• Inherited traits: Some people are born with a weaker system for taking LDL up into the liver. This is called familial hypercholesterolemia, and it’s estimated to affect about 1 in 300 people in Japan. LDL is quite high from a young age and hard to bring down by changing your diet, so it’s important to talk with a doctor.

Low HDL or high triglycerides

When HDL is low, triglycerides are often high, and smoking, too little exercise, more visceral fat, and too many sweets often overlap. Drinking too much alcohol raises triglycerides, but it actually tends to push HDL up. If your triglycerides are high, look back on how much you drink, even if your HDL is on the high side.

High HDL

It’s easy to assume that the higher HDL is, the better, but observational studies have found more deaths among people with extremely high levels. In a Danish study, the death rate was higher above roughly 100 mg/dL for men and 135 mg/dL for women. In a Japanese study, people at 90 mg/dL or higher (about 1.5% of participants) had more deaths from atherosclerotic disease, especially among those who drank. Still, it isn’t known whether HDL itself is the cause, and the society says it will wait for more evidence before deciding whether to count high HDL as a risk factor. I look at it together with the background, such as exercise habits, how much you drink, and inherited traits.

What low values can mean

The low side isn’t often flagged at checkups, but the JCCLS guide gives the example of LDL dropping with poor nutrition and says the lower limit matters clinically, too.

These are the main factors behind low results:

• Eating too little, or too few calories: Extreme dieting, rapid weight loss, or a diet with very little fat.
• An overactive thyroid: The liver takes up LDL faster and turns more cholesterol into bile acids, which can lower it.
• A drop in the liver’s ability to make cholesterol
• Problems with digestion and absorption, or illnesses that wear the body down
• Taking cholesterol-lowering medication: In that case, it’s a value your doctor sets a target for and manages.
• Inherited traits: Rarely, people are born with low LDL.

Observational studies have also found more deaths from causes other than heart disease in groups with very low total cholesterol, but it has been pointed out that an illness itself may be what lowers cholesterol. Rather than trying to push the number up, the first step is finding out why it’s low.

How I read the numbers

What follows isn’t a medical standard. It’s the way I read blood data, from a precision nutrition perspective.

Where you sit within the reference range

For lipids, I read the values against the optimal ranges in the table above. The top of the HDL optimal range (100) goes past the upper limit of the shared reference interval for men (90) and overlaps with the “90 mg/dL or higher” group in the Japanese study mentioned earlier, so when HDL sits there, I also look at how much you drink and at inherited traits.

As an example of how I read the numbers, here are my own past results: total cholesterol 223, LDL 138, and HDL 59 mg/dL.

All three are within the shared reference intervals, but against the society’s thresholds, LDL at 138 fell in the borderline range (120–139), and so did non-HDL (223 − 59 = 164; borderline is 150–169). So the reading is: within range, but leaning toward the high side. Against the optimal ranges in the table above, total cholesterol and LDL were above the upper end, and HDL was below the lower end.

At the time, I’d been snacking on ice cream and chocolate day after day, and a way of eating that was high in saturated fat and sugar seemed to be one of the reasons. Rather than stopping at whether a value is within the range, I look at where in the range it sits, which way it has moved since last time, and whether the reason can be found in daily life.

The ratio of LDL to HDL

I also use LDL divided by HDL (the LDL/HDL ratio) as a clue to see which way things lean. For this ratio, the precision nutrition optimal range I use in my counseling is 0.9–1.5. This is a precision nutrition guide, not a medical reference range, and it isn’t the society’s standard either. The guidelines say the evidence for ratios like this is still insufficient, and that management should be based on each value itself. In the example above, 138 ÷ 59 is about 2.3: above 1.5, the top of the optimal range, and leaning toward LDL. A ratio alone doesn’t tell you which value moved, so I always look at it together with the original numbers.

Reading it alongside other markers

• Triglycerides, HDL, and HbA1c: A pattern of high triglycerides and low HDL, read together with HbA1c, is a clue for revisiting how much carbohydrate and alcohol you take in, and how much you move.
• AST, ALT, and GGT (γ-GTP): I consider these together for the possibility of fat building up in the liver, and for the effects of drinking.
• TSH and FT4: When cholesterol has suddenly gone up or down, I line up the thyroid markers too. If your thyroid values are outside the range, I recommend having a doctor check them.
• Total protein, albumin, and BUN (blood urea nitrogen): When cholesterol is low, I look at these together to see whether you’re eating enough overall, protein included.

What you can revisit in everyday life

These are general directions, in line with the society’s guidelines and the Dietary Reference Intakes for Japanese (2025 edition) from Japan’s Ministry of Health, Labour and Welfare.

When LDL is on the high side

• Cut back on fatty meat, processed meat, butter, heavy cream, and sweets, and gradually swap in fish, soy foods, and plant oils such as olive oil and canola (rapeseed) oil. Coconut oil and palm oil are a different story, since they’re high in saturated fat. The guidelines recommend eating less saturated fat, or replacing it with unsaturated fat, with the aim of bringing blood lipids into balance.
• Get fiber from vegetables, seaweed, mushrooms, soybeans, and less-refined grains.
• For cholesterol-rich foods such as eggs, the Dietary Reference Intakes don’t set a target (an upper limit), but they note that this doesn’t mean there’s no upper limit. The guidelines say it’s best to keep intake low even for people whose LDL isn’t high, and recommend less than 200 mg a day for people with high LDL.

When triglycerides are on the high side or HDL is on the low side

• Revisit how many sweet drinks and sweets you have, and how much alcohol you drink.
• Make aerobic exercise a habit, for example by walking more. There are reports of HDL going up and triglycerides going down.
• If you smoke, consider quitting.
• Eat fish more often. The guidelines recommend getting more of the fats found in fish (omega-3 fatty acids) with the aim of bringing triglycerides into balance. It isn’t meant for lowering LDL.

When cholesterol is on the low side

• Look back on whether you’re eating enough in a day, and whether your meals include a staple (such as rice or bread), a main dish, and side dishes.
• Check that you aren’t keeping up rapid weight loss or a diet with very little fat.

If you take medication

If you’ve been prescribed medication for your lipids, please don’t cut back or stop it on your own; keep following your doctor’s plan. Revisiting your diet and exercise isn’t a replacement for medication. It’s something to do alongside it. If you start a supplement, it’s also reassuring to let your doctor or pharmacist know.

If you’re concerned

In cases like these, please talk with an internist or your regular doctor:

• Your checkup rated a result as “further testing needed,” “medical care needed,” or something similar
• Your LDL is quite high (for example, 180 mg/dL or higher), or someone in your family had a heart attack or angina at a young age
• You have other risk factors for atherosclerosis, such as high blood pressure, diabetes, smoking, or chronic kidney disease (LDL management targets change depending on these)
• Your cholesterol has gone up or down sharply since last time
• Your cholesterol is low, and it comes with unintended weight loss or changes in how you feel, such as tiredness or a racing heart

When you go, bring your past checkup results along with this one. That makes it easier for your doctor to see how your numbers have changed over time.