HDL

What is High-density Lipoprotein (HDL)?

High-density lipoprotein (HDL) is responsible for transporting fats around the body in the bloodstream. It is commonly referred to as ‘good’ cholesterol because it has a protective role against heart disease.
HDL consists of a high amount of protein and very little fat, which makes it very dense, hence its name. It is one of five types of lipoproteins and carries around a quarter of the cholesterol in your blood.

HDL has three main functions in the body:

1. Removes excess cholesterol from the blood, transporting it back to the liver where it is either recycled or excreted from the body.
2. Protects the artery walls from LDL cholesterol deposits because of its anti-inflammatory effects.
3. Protects cells and other chemicals from being broken down through anti-oxidant benefits^[1][2].

Low-density lipoprotein (LDL), on the other hand, is ‘bad’ cholesterol. High levels of LDL circulating in the blood are indicative of an increased risk of heart disease and cardiovascular events such as heart attacks and stroke. Unlike HDL which works to remove cholesterol from the blood, LDL deposits cholesterol on blood vessel walls, contributing to the formation of plaques and the development of atherosclerosis^[3].

What Should HDL Levels Be?

According to the NHS, healthy levels of HDL should be over 1.0 mmol/L for men and 1.2 mmol/L for women^[4].

Dr Thom Phillips says: “HDL reaches optimal ‘protective’ effect between 1.5-1.6 mmol/L. Levels below 1/1.2 are independent risk factors for atherosclerotic events, such as strokes.”

Our data shows that the average HDL levels in the UK for men are 1.4 mmol/L and 1.7 mmol/L for women, indicating healthy ranges.

Research indicates that analysing the LDL:HDL ratio is more indicative of cardiovascular risk than measuring LDL or HDL levels alone^[5].

HDL is defined as a particle size less than 17nm across. Non-HDL cholesterol is essentially then all cholesterol molecules that are bigger than that! In the lab it’s calculated by subtracting your HDL cholesterol levels from your total cholesterol.. A healthy ratio is between 0 and 6.

Data collected from Forth customers shows the average HDL ratio is 4.1 for men and 3.3 for women. The data can also be broken down by age.

Age	Average HDL ratio  (mmol/L)
18-29	1.4
30-39	1.4
40-49	1.5
50-59	1.6
60+	1.7

What Causes High and Low HDL Levels?

Although having very high HDL cholesterol levels may seem like a good thing, recent research shows that they may be harmful.

A study published in 2022 found that individuals with coronary heart disease who had high HDL cholesterol levels had an increased risk of mortality^[6].

Researchers are not sure why high levels of HDL have a negative effect on health, but in some individuals, very high HDL cholesterol can cause the development or progression of cardiovascular disease^[7].

There are several causes of high HDL cholesterol, some can be reversed while others can’t. They include

• Consuming a diet high in saturated fat
• High alcohol intake
• Medicines including the oral contraceptive pill, oestrogen-based hormone replacement therapy and anti-convulsant therapy
• Genetics, it is possible to inherit high HDL levels
• Menopause
• Japanese ancestry
• Underactive thyroid (hypothyroidism)
• Infections and inflammation^[8]

Symptoms of High HDL

There are usually no symptoms associated with high cholesterol levels, so it is important to regularly test your cholesterol. However, there are some very recognisable signs that your cholesterol may be high, including a condition called Xanthelasma Palpebrarum. This is where your body deposits small amounts of cholesterol under the skin, usually around the eyes causing distinctive yellowy/cream patches.

Another common sign that is similar to this is something called a tendon Xanthoma, where your body deposits cholesterol into your tendons. This is commonly seen in the achilles tendon although they can appear in any tendon in the body including hands, wrist and elbows.

There are also other causes of very high HDL levels including thyroid dysfunction, alcohol consumption and high levels of exercise. If high HDL is caused by an underlying condition, you should look out for related signs and symptoms.

For example, symptoms of hypothyroidism include:

• Weight gain
• Fatigue
• Joint and muscle pain
• Dry skin or dry, thinning hair
• Slow heart rate
• Depression
• Difficulty tolerating cold temperatures^[9]

Equally, there are several factors which may contribute to low HDL levels which can also have a negative effect on health, particularly as healthy levels have cardioprotective benefits.

Several inherited conditions can cause low HDL levels in some people, including:

• Tangier disease
• Familial hypoalphalipoproteinemia
• Familial HDL deficiency
• Familial combined hypolipidaemia^[10]

Metabolic syndrome is associated with low levels of HDL and elevated levels of LDL cholesterol, increasing the risk of cardiovascular disease^[11].

Other causes include:

• Smoking
• Medicines, such as beta-blockers, androgens, progestogens, and anabolic steroids
• Insulin resistance

If your HDL levels are above 2.3mmol/L we know that they behave more like LDL cholesterol and are bad for your health. If your levels are persistently and/or significantly above this level you should speak to your GP as you may require specialist investigations from your local lipid clinic as well as possibly being offered genetic screening for you and your family members.

"You may be surprised to find out that this condition is virtually absent from most medical school curricula. It wasn’t until I developed strange lumps on the back of my fingers that I had scanned and found to be cholesterol deposits that I really got to grips with this condition. A quick check with Forth showed that my cholesterol was pathologically high and I’m now on appropriate treatment."

Dr Thom Phillips

Symptoms of Low HDL

It’s likely you won’t experience any symptoms of low HDL levels, so it is important to regularly check your cholesterol levels. Measuring your HDL levels will help to routinely monitor for the silent risk factors associated with high LDL and low HDL cholesterol.

If low HDL is caused by an inherited condition, you may experience symptoms associated with this. For example, conditions that are associated with high LDL and triglycerides often present with xanthomas, fatty lumps that appear over pressure-prone areas such as the knees, elbows, and heels, and xanthelasmas, small, yellow growths that appear in the corners of the eyelids.

Age	% of Forth Customers with Low HDL Levels
18-29	11.43%
30-39	13.18%
40-49	11.15%
50-59	8.41%
60+	5.28%

How to Improve Your HDL Levels

Maintaining HDL levels within a healthy range is important to protect your cardiovascular health.

For many of the lifestyle causes of low HDL cholesterol, there are several things you can do to help improve the levels of HDL circulating in your bloodstream, such as:

• Improving your diet. Reducing or limiting your intake of saturated fat can help to lower circulating LDL and improve HDL levels. Research has shown that a Mediterranean diet improves circulating HDL levels^[12].
• Participating in regular aerobic exercise can improve HDL cholesterol by 5% in two months and improve circulating levels of triglycerides and LDL^[13]. Aim to complete at least 30 minutes of exercise five times per week.
• If you smoke, quitting will help to improve HDL and remove high levels of LDL from the blood, lowering the risk of heart disease.
• Some research suggests that a moderate intake of alcohol can improve HDL levels and lower the risk of developing heart disease^[14]. However, excessive alcohol intake can have the opposite effect and increase LDL levels instead. Therefore, it is important to monitor your alcohol consumption and always aim to keep within the guidelines of no more than 14 units per week with at least 3 alcohol-free days per week.

What Other Biomarkers Should I Test Alongside HDL?

• High-sensitivity C-reactive protein (hs-CRP) can signal the level of low-grade inflammation in the blood vessel walls. High levels of hs-CRP can indicate high blood pressure, diabetes, and cardiovascular disease^[15]. So, measuring your hs-CRP levels alongside cholesterol biomarkers can help predict your risk of developing cardiovascular problems, including heart attacks.
• Omega 6: Omega 3 Ratio. This ratio can give a good indication of your risk for developing many diseases. Typically, the Western Diet is associated with a high intake of omega-6 fatty acids and a high omega-6: omega-3 ratio and could be linked to the development of heart disease^[16].
• Thyroid biomarkers. An underactive or overactive thyroid can be an underlying cause of dysregulated HDL cholesterol. Therefore, it is important to rule out hypo- or hyperthyroidism as a potential cause of high or low HDL levels.
• Uric acid. Recent research has shown that high levels of uric acid, a by-product of purine breakdown, are linked to an increased risk of an irregular heart rate, called atrial fibrillation (AF)^[17].

Written by Leanne Edermaniger

Based in the UK, Leanne specialises in writing about health, medicine, nutrition, and fitness.

She has over 5 years of experience in writing about health and lifestyle and has a BSc (hons) Biomedical Science and an MSc Science, Communication and Society.