What is High Cholesterol?

Cholesterol is a fat-like substance, naturally found in humans and animals. It has many important functions that are essential to the body. Cholesterol forms part of the basic structure of many hormones, all cell membranes and the insulation layer around nerves.

Although some cholesterol is found on its own in the bloodstream, most cholesterol molecules are bonded to specialised proteins called lipoproteins. The better-known ones are low-density lipoprotein (LDL), and high-density lipoprotein (HDL).

What is the difference between HDL and LDL cholesterol?

Because of certain chemical properties, fat cannot be dissolved in a watery medium unless it undergoes a chemical dispersion process. Called ‘emulsification’, this process allows fatty substances to disperse into a range of micro-droplets varying in size that allows them to remain in suspension.

Besides the portion of cholesterol found on its own in the bloodstream, the majority of cholesterol is contained within micro lipoprotein droplets. These are distinguished by their size, density and the proteins they are composed of. Within its core, ‘Low Density Lipoprotein’ or LDL contains the highest number of cholesterol molecules (estimated at 1500). Its main function is to transport cholesterol from the liver to the peripheral tissues via the bloodstream. Cholesterol bonded to LDL tends to accumulate inside arteries and is therefore ‘bad’ for our bodies. Cholesterol bonded to high-density lipoproteinor HDL, on the other hand, gets transported to the liver, where it is naturally expelled from the system as a component of bile. It is therefore beneficial to the body to have high levels of HDL.

How is blood cholesterol levels controlled?

Blood cholesterol levels are influenced by several metabolic pathways which involve cholesterol production, absorption, transport and reabsorption.

In our diet, cholesterol is mainly derived from food sources that contain animal products. This is because cholesterol is predominantly manufactured by cells of animal origin. Although plants produce miniscule amounts of cholesterol, they produce large amounts of substances that are structurally very similar to cholesterol, called plant- or ‘phytosterols’. The relevance of phytosterols is that they compete with cholesterol for absorption within the intestinal tract, thereby offering a natural protective mechanism that helps to reduce cholesterol absorption.

Although diet plays a crucial role in the development of cardio-vascular disease, the actual cholesterol content within the average diet only influences blood cholesterol levels to a small degree. This is because the human body is highly efficient at manufacturing cholesterol itself, using a molecule called ‘Acetyl coenzyme A’ (acetyl-CoA) as a building block. It is estimated that on average, a man weighing about 70 kg will typically produce around 1000 mg of cholesterol per day, a large amount in relative terms.

Although all kinds of cells can produce cholesterol, liver cells are responsible for the majority of production. This is the site of action where the statins, a group of cholesterol-lowering drugs, play a dominant role. Other sites with higher cholesterol production rates are the intestines, adrenal glands and reproductive organs.

Cholesterol recycling – nowhere to go

Besides cholesterol production, an important factor that influences blood cholesterol levels is the ability of the body to recycle its own cholesterol continually. As a consequence of normal physiology, large amounts of cholesterol are excreted by the liver via bile as a natural waste product. Once this cholesterol arrives in the intestines, up to 50% of it is automatically re-absorbed and therefore recycled back into the system. Since this mechanism contributes to elevated cholesterol blood levels, blocking this pathway has become a new therapeutic target to help control blood cholesterol levels.

What is hardening of the arteries?

Hardening of arteries called ‘atherosclerosis’ in medical terms is a complex process whereby a layer or crust of cholesterol accumulates within an artery. As this layer increases in thickness, the inner passage of the artery, called the lumen, becomes progressively obstructed. This causes a decrease in the amount of blood flowing through the artery. A clot may also easily form inside this diseased and narrowed blood vessel, sealing it off entirely. This event causes heart attacks and strokes.

What role does cholesterol play in predicting cardiovascular disease (CVD)?

Cardiovascular disease, or CVD, is a broad term that refers to many different conditions that affect the heart and blood vessels. The causes of CVD are numerous, but hardening of the arteries (atherosclerosis) and high blood pressure are the most common. Since various risk factors other than cholesterol are involved in the development of CVD, cholesterol cannot really be used to accurately determine CVD risk single-handedly.

However, when it comes to the development of hardening of the arteries, numerous studies have indicated that cholesterol plays a dominant role. Various health authorities have based their predictions on either determining CVD risk, or providing treatment guidelines, on cholesterol, HDL, LDL or HDL/LDL ratios, or other lesser known lipoproteins.  Over the years, these have regularly been adjusted in accordance with the publication of new studies.  A recent new meta-analysis published in the Journal of the American College of Cardiology (July 2014), suggests that lowering LDL-cholesterol levels to very minor levels does equate to a significant reduction in cardiovascular events. [1,2]

What are South African guidelines?

Health authorities in South Africa recommend that in the absence of other risk factors, cholesterol levels should ideally be no higher than 4.9 mmol per litre for total cholesterol and 2.9 mmol/l for LDL cholesterol. If there are other known cardiovascular risk factors present, for example a previous heart attack or diabetes, then the target for LDL should be reduced to 1.8mmol/L, as per the LASSA treatment guidelines.

As cholesterol-related deaths continue to rise, South Africans need to start managing their cholesterol levels far more proactively if they wish to extend their lifespan. The first step is to check your cholesterol levels. Women are particularly bad at doing this, in part because they tend to be under the erroneous impression that heart disease is predominantly a ‘male’ condition. Althoughcardiovascular disease in children is rare, the pathologic processes underlying hardening of the arteries actually begins in childhood. In a surprise discovery, a recent study conducted at primary-care paediatric clinics in Texas found that one out of three children between the ages of 9 and 11 either had borderline or high cholesterol. It has also been found that the blood cholesterol levels in childhood are the most significant predictor of high cholesterol levels in adults.

How do ‘Saturated’ and ‘Unsaturated’ fats differ from each other?

Several different compounds naturally found in food and in the body are chemically classified as ‘fats’ or ‘lipids’. All fats are made up from smaller components such as ‘triglycerides’. Within a typical Western diet, triglycerides form the dominant source of fat (90%).

Within its structure, each triglyceride contains three fatty acids. Individually, fatty acids are made up of a central chain of carbon atoms, with hydrogen and oxygen atoms attached to this chain. The term ‘saturated’ is used when a fatty acid has no more space for hydrogen atoms and is therefore ‘saturated’ with hydrogen.  ‘Monounsaturated’ fatty acids (MUFAs) have one space left, whilst ‘polyunsaturated’ fatty acids (PUFAs) have more than one space available.Due to the fact that these ‘empty spaces’ allow a fatty acid to behave differently in a biochemical manner, they are considered either ‘beneficial’ or ‘detrimental’ to health.

The chemical differences between fatty acids also contribute to taste. ‘Butyric acid’, for example, is one of the saturated fatty acids responsible for the characteristic flavour of butter.

What role does ‘saturated fat’ play in cardiovascular disease (CVD)?

Currently, all leading medical, heart-health and governmental authorities advise that saturated fat is a risk factor for cardiovascular disease (CVD). This includes the World Health Organization, the American as well as British Dietetic Associations, the World Heart Federation, the Food and Drug Administration and the European Food Safety Authority.

However, some recent questions have been raised in medical literature querying this accepted norm. A meta-analysis published in the American Journal of Clinical Nutrition (2010), for example, did not find significant evidence for concluding that dietary saturated fat is necessarily associated with an increased risk of CVD.[3] In another review published in the Annals of Internal Medicine (2014), the authors concluded that their findings did not necessarily support cardiovascular guidelines that encourage high consumption of polyunsaturated fatty acids and low consumption of total saturated fat. [4]

Other experts have warned that these conclusions could potentially be misleading due to certain omissions based on available data. A central question is what replaces saturated fat if someone reduces the amount of saturated fat in their diet. If it is replaced with refined starch or sugar, which are the largest sources of calories in the typical Western diet, then the risk of heart disease remains the same. However, if saturated fat is replaced with polyunsaturated fat or monounsaturated fat in the form of olive oil, nuts and probably other plant oils, there is still significant scientific evidence that CVD risk will be reduced.

Why have opinions become slightly contentious?

Because the numerous factors that contribute towards health and the burden of disease are multifactorial. Besides consuming fat, various other factors help to determine cardiovascular health. This includes the process of fat digestion and the intake of other nutritional agents that offer proven health benefits.

Large fatty chains in food cannot be absorbed into the system until they are reduced in size. This happens naturally through the action of enzymes during digestion, a process that leads to the separation of chemical bonds that join individual fatty acids. In the human body, this process is controlled by several digestive enzymes called lipases. Fat digestion, called ‘lipolysis’, starts in the stomach and continues in the small intestine by means of the combined action of various fat digestive enzymes. The rate at which fat digestion takes place and how many free fatty acids it supplies can be influenced by many factors. These include the levels of fat digestive enzymes, as well as the presence of agents that reduce the biological efficacy of fat-digestive enzymes, referred to as ‘lipase inhibitors’ in medical terms.

Besides blocking fat digestion, other agents naturally present in food form insoluble bonds with fat in the intestines, thereby allowing fat to leave the system as waste matter. This is where fibre plays a significant role. The plant-fats or phytosterols, on the other hand, directly compete with cholesterol for absorption in the intestines. Besides blocking the uptake of new dietary cholesterol by the system, a more important task is blocking the body’s tendency to reabsorb its own cholesterol excreted through the liver, thereby breaking the cholesterol recycling pathway.

A recent study on participants with known high cholesterol, for example, examined the effects of a reduced saturated fat diet, against a diet containing foods rich in phytosterols and fibre. After six months, this diet resulted in a significantly greater LDL-cholesterol reduction when compared to the low-saturated fat diet, and produced results that almost equalled the reduction in cholesterol levels that were observed in the earlier trials on statins.

‘An apple a day keeps the doctor away’ has been a health message delivered since the 19th century. Apples, a rich source of phytochemicals such as polyphenols, are widely consumed and many epidemiological studies have linked the consumption of apples to a reduced risk of some cancers, cardiovascular disease, asthma and diabetes. In the laboratory, apples have been found to inhibit cancer cell proliferation, decrease the oxidation of fats and lower the production of cholesterol in intestinal cell cultures.

To determine whether a daily apple could potentially have a similar protective effect against CVD mortality as statins, adults over 50 years old in the UK were prescribed either a statin or an apple a day. In a recent study (December 2013) released by the British Medical Journal (BMJ), researchers from the University of Oxford concluded that apples managed to rival statins in preventative medicine when it comes to lowering the burden of CVD.

From these findings, it becomes clear that the numerous factors that contribute towards health and the burden of disease are complex. Ideally, they should not be reduced into a one-size-fits-all problem or solution.

Which lifestyle changes will help you reduce your CVD risk?

The following lifestyle modifications will help you to reduce your risks significantly:

  • Avoid any secondary exposure to smoke, and get help to QUIT SMOKING if you are a smoker.
  • Increase your level of physical activity by any means, even if just moderately.
  • Try to maintain your body weight as close to your ideal level. Even slight reductions can make a statistically-measurable difference.
  • Reduce your intake of saturated fats, trans-fats and cholesterol. An easy way to achieve this goal is to avoid processed food and take-aways. (See below)
  • Increase your intake of heart-healthy foods. This will automatically result in an increase of fibre, unsaturated fats and phytosterols.
  • Significantly reduce or avoid the intake of refined carbohydrates and sugar.
  • Consume alcohol in moderation.
  • Avoid adding salt to your food.
  • Monitor your cholesterol levels on a regular basis and start doing this from early adulthood.

Which foods are high in ‘saturated fat?’

Various foods contain different proportions of saturated and unsaturated fat. Examples of foods containing a high proportion of saturated fat include animal fat products such as cream, cheese, butter, ghee, lard, and fatty meats. Certain vegetable-based products also have a high saturated fat content. Examples are coconut oil, cottonseed oil, palm kernel oil and chocolate.

Foods to Avoid:

  • Fatty meats
  • Processed meats like salami and sausages
  • Snack foods like chips, crisps and samoosas
  • Most takeaway foods, especially deep-fried foods
  • Cakes, biscuits, doughnuts and pastries
  • Chocolate

Which foods are ‘heart healthy’?

These are foods rich in phytosterols and sticky fibres.  Since both these come from plants, a diet rich in plant-based products will ensure that this objective is easily achieved.   The richest naturally-occurring sources of phytosterols are vegetable oils and nuts. Cereal-based products, vegetables, fruit and berries, which are not as rich in phytosterols as nuts and vegetable oil, may also be significant sources of phytosterols if consumed in higher quantities. Sources of fibre are all fruits, all vegetables, legumes (peas and beans) and grains (oats, rye and barley).

References

  1. Boekholdt SM, Hovingh GK, Mora S, et al. Very low levels of atherogenic lipoprotein and the risk for cardiovascular events. J Am Coll Cardiol 2014; DOI:10.1016.j.jacc.2014.02.615.
  2. Ben-Yehuda O, DeMaria AN. LDL-cholesterol after the ACC/AHA 2013 guidelines. J Am Coll Cardiol 2014; DOI:10.1016.j.jacc.2014.05.020.
  3. Siri-Tarino PW, Sun Q, Hu FB, Krauss RM (2010). “Meta-analysis of prospective cohort studies evaluating the association of saturated fat with cardiovascular disease”. American Journal of Clinical Nutrition 91 (3): 535–546.
  4. Chowdhury R, Warnakula S, Kunutsor S, et al. Association of dietary, circulating, and supplement fatty acids with coronary risk. Ann Intern Med 2014; 160(6):398-406.
  5. Oh K, Hu FB, Manson JE, Stampfer MJ, Willett WC. Dietary fat intake and risk of coronary heart disease in women: 20 years of follow-up of the Nurses’ Health Study. Am J Epidemiol 2005;161:672-9.
  6. American Heart Association, Cholesterol Guidelines
  7. South African Dyslipidaemia Guideline Consensus Statement Vol 102, No 3, (2012)
  8. Health Society of South Africa, Cholesterol Guide, 2014
  9. World Health Organisation, Cardiovascular Disease Program, Avoiding Heart Attacks and Strokes, 2005