High cholesterol: causes, risks and what your blood test results mean

Diet, saturated fat, and the liver's cholesterol factory

The liver produces around 80% of the cholesterol circulating in your blood, with the remaining 20% coming from diet. This is why dietary cholesterol (from eggs, for example) has a smaller impact on blood cholesterol than was once thought. What has a more significant effect on LDL production is saturated fat, found in fatty meats, full-fat dairy, butter, palm oil, and coconut oil. Saturated fat reduces the liver's ability to clear LDL particles from the bloodstream by downregulating LDL receptors. Trans fats, found in many ultra-processed foods, have an even more adverse effect on the LDL-to-HDL ratio. Replacing saturated and trans fats with unsaturated fats from olive oil, nuts, seeds, and oily fish consistently improves the lipid profile in intervention studies.

Genetics and familial hypercholesterolaemia

For a significant minority of people, high LDL is not primarily driven by diet or lifestyle but by inherited genetic variants that impair the liver's ability to clear LDL from the blood. Familial hypercholesterolaemia (FH) is the most important of these, affecting around 1 in 250 people in the UK, many of whom are undiagnosed. People with FH typically have total cholesterol above 7.5 mmol/L and LDL above 4.9 mmol/L regardless of diet, and they carry a significantly elevated lifetime risk of premature cardiovascular disease. A family history of heart attacks or strokes before age 55 in a first-degree male relative or before age 65 in a first-degree female relative, combined with high LDL, should prompt a GP discussion about FH screening. FH can be confirmed through a genetic test.

Insulin resistance, triglycerides, and the metabolic lipid pattern

Insulin resistance produces a characteristic lipid signature that a standard cholesterol test may partially obscure: elevated triglycerides, reduced HDL, and a shift in LDL particle composition toward smaller, denser particles. Small dense LDL particles are more atherogenic than large buoyant LDL particles because they are more likely to enter arterial walls and trigger the inflammatory process that leads to plaque formation. The triglyceride-to-HDL ratio (TG/HDL) is a useful indicator of this pattern: a ratio above 2.5 is associated with increased cardiovascular and metabolic risk, often indicating a predominance of small dense LDL even when total LDL appears moderate. This is one reason why measuring only total cholesterol gives an incomplete picture for people with metabolic syndrome, diabetes, or obesity.

Thyroid and other medical causes

Hypothyroidism (underactive thyroid) raises LDL cholesterol and total cholesterol as one of its metabolic effects, because thyroid hormone is required for normal LDL receptor activity in the liver. A person with untreated hypothyroidism may have cholesterol that responds poorly to dietary changes and comes down significantly once thyroid treatment is started. This is why a TSH test is often done alongside a lipid panel when high cholesterol is first identified, to check whether a treatable thyroid condition is driving the picture. Kidney disease, liver disease, and certain medications (including some blood pressure treatments and steroids) can also raise cholesterol independent of diet.

Age, sex, and hormonal transitions

Cholesterol levels tend to rise with age, and the lipid profile shifts around major hormonal transitions. In women, the menopause transition is particularly significant: oestrogen has a protective effect on lipid metabolism, and as oestrogen levels decline, LDL tends to rise and HDL may fall. This is why the cardiovascular risk gap between men and women narrows significantly after menopause, and why baseline lipid testing around this transition is clinically useful. In men, the metabolic effects of testosterone decline (reduced muscle mass, increased visceral fat, worsening insulin resistance) also tend to worsen the lipid profile over time.

A standard cholesterol blood test UK (also called a lipid panel or lipid profile) measures four components, and it is the relationship between them, not just the individual numbers, that matters most for cardiovascular risk assessment.

Total cholesterol is the sum of all cholesterol components. In the UK, a level below 5.0 mmol/L is the population target, but total cholesterol in isolation is a poor predictor of individual risk because it combines both the beneficial and harmful forms.

LDL cholesterol (low-density lipoprotein) is the primary target for cardiovascular risk reduction. LDL particles carry cholesterol from the liver to tissues, and when in excess they accumulate in arterial walls, initiating the atherosclerotic process. UK guidelines suggest LDL below 3.0 mmol/L for the general population, with lower targets for people with established cardiovascular disease or diabetes.

HDL cholesterol (high-density lipoprotein) carries cholesterol back from tissues to the liver for processing. Higher HDL is generally protective. A level below 1.0 mmol/L in men and below 1.2 mmol/L in women is considered a risk factor.

Triglycerides are the most diet-sensitive of the lipid markers, rising with refined carbohydrate and alcohol intake and falling with exercise and dietary improvement. A level above 1.7 mmol/L is elevated; above 5.6 mmol/L carries additional risks beyond cardiovascular disease.

The total cholesterol-to-HDL ratio is often reported alongside the individual values and gives a more useful single number than total cholesterol alone. A ratio below 4.0 is generally considered favourable.

Dietary fat quality and fibre intake

The most evidence-supported dietary changes for improving the lipid profile are replacing saturated fats with unsaturated fats (olive oil, avocado, nuts, oily fish) and increasing soluble fibre intake. Soluble fibre, found in oats, beans, lentils, apples, and psyllium husk, binds bile acids in the gut and reduces cholesterol reabsorption, which lowers LDL. A systematic review of dietary fibre trials found that increasing soluble fibre by 5-10 grams per day produces modest but consistent LDL reductions. Plant sterols and stanols (found in fortified foods and supplements) have a stronger effect, reducing LDL by approximately 10% when taken consistently.

Regular aerobic exercise and its effect on HDL

Sustained aerobic exercise is one of the most reliable ways to raise HDL and reduce triglycerides. Exercise increases the production of the enzymes that form HDL particles and improve lipid clearance from the bloodstream. The effect on LDL from exercise alone is more modest, though it compounds with dietary changes. Regular moderate-intensity aerobic exercise (150 minutes per week minimum) produces measurable improvements in triglycerides and HDL within 6-12 weeks, which is why retesting 3-6 months after initiating an exercise programme gives a meaningful signal of biological response.

Reducing refined carbohydrates and alcohol

Triglycerides respond more rapidly and dramatically to dietary changes than LDL or HDL. The primary dietary drivers of elevated triglycerides are refined carbohydrates (white bread, pasta, rice, sugar-sweetened drinks) and alcohol. Reducing these while increasing fibre and protein typically produces a visible improvement in triglycerides within weeks. Since elevated triglycerides (particularly with low HDL) are associated with small dense LDL even when the LDL number looks moderate, reducing triglycerides through diet is an important part of overall cardiovascular risk reduction beyond what the LDL number alone captures.

Stress, cortisol, and the lipid profile

Chronic stress raises cortisol, which promotes the release of free fatty acids from fat tissue and drives hepatic LDL synthesis. Consistently elevated cortisol also worsens insulin resistance, which compounds the unfavourable lipid pattern associated with metabolic syndrome. Managing chronic stress is not simply a general wellness recommendation but has a specific, measurable effect on both cortisol and the lipid markers that contribute to cardiovascular risk.