Understanding Blood Cholesterol: Lipoproteins

You may have heard of the abbreviations LDL and HDL with respect to heart health. These abbreviations refer to low-density lipoprotein (LDL) and high-density lipoprotein (HDL), respectively. Lipoproteins are characterized by size, density, and composition. As the size of the lipoprotein increases, the density decreases. This means that HDL is smaller than LDL. Why are they referred to as “good” and “bad” cholesterol? What should you know about these lipoproteins?

Major Lipoproteins

Recall that chylomicrons are transporters of fats throughout the watery environment within the body. After about ten hours of circulating throughout the body, chylomicrons gradually release their triglycerides until all that is left of their composition is cholesterol-rich remnants. These remnants are used as raw materials by the liver to formulate specific lipoproteins. Following is a list of the various lipoproteins and their functions:

1. VLDLs: Very low-density lipoproteins are made in the liver from remnants of chylomicrons and they transport triglycerides from the liver to various tissues in the body. As the VLDLs travel through the circulatory system, the lipoprotein lipase strips the VLDL of triglycerides. As triglyceride removal persists, the VLDLs become intermediate-density lipoproteins.
2. IDLs: Intermediate-density lipoproteins transport a variety of fats and cholesterol in the bloodstream and are a little under half triglyceride in composition. While travelling in the bloodstream, cholesterol is gained from other lipoproteins while circulating enzymes strip its phospholipid component. When IDLs return to the liver, they are transformed into low-density lipoproteins.
3. LDLs: As low-density lipoproteins are commonly known as the “bad cholesterol,” it is imperative that we understand their function in the body so we can make healthy dietary and lifestyle choices. LDLs carry cholesterol and other lipids from the liver to our tissues throughout the body. LDLs are comprised of very small amounts of triglycerides and house over 50 percent cholesterol and cholesterol esters. How does the body receive the lipids contained therein? As the LDLs deliver cholesterol and other lipids to the cells, each cell’s surface has receptor systems specifically designed to bind with LDLs. Circulating LDLs in the bloodstream bind to these LDL receptors and are consumed. Once inside the cell, the LDL is taken apart and its cholesterol is released. In liver cells, these receptor systems aid in controlling blood cholesterol levels as they bind the LDLs. A deficiency of these LDL binding mechanisms will leave a high quantity of cholesterol travelling in the bloodstream, which can lead to heart disease or atherosclerosis. Diets rich in saturated fats may prohibit the LDL receptors which are critical for regulating cholesterol levels.
4. HDLs: High-density lipoproteins are responsible for carrying cholesterol out of the bloodstream and into the liver, where it is either reused or removed from the body with bile. HDLs have a very large protein composition coupled with low cholesterol content (20 to 30 percent) compared to the other lipoproteins. Hence, these high-density lipoproteins are commonly called “good cholesterol.”

Figure 5.15 displays the classification of the major types of lipoproteins based on their densities. Density range is shown as well as lipid (red) and protein (blue) content.

Health: Saturated Fats, Cholesterol and Trans Fats

Because heart disease, cancer, and stroke are the three leading causes of death in Canada, it is critical to address dietary and lifestyle choices that will ultimately decrease risk factors for these diseases. In light of that, we present the following informational tips to help you define, evaluate, and implement healthy dietary choices to last a lifetime. The amount and the type of fat that composes a person’s dietary profile will have a profound effect on the way fat and cholesterol are metabolized in the body.

The Saturated Fat Debate

How saturated is the fat in your diet? In Canada and other developed countries, populations acquire their saturated fat content mostly from meat, seafood, poultry (with skin consumed), and whole-milk dairy products (cheese, milk, and ice cream). Some plant foods are also high in saturated fats, including coconut oil, palm oil, and palm kernel oil.

There is conflicting evidence relating to the consumption of saturated fat and health. Some meta-analyses find no evidence that a reduction in saturated fat consumption may reduce the incidence of cardiovascular disease or mortality.^{[1] [2] [3] [4]}, whereas others report a significant beneficial effect ^{[5] [6] [7]}

Until further research is conducted and given the conflicting evidence, it could be beneficial to follow the current dietary recommendations to replace saturated fats with polyunsaturated fats.

Cholesterol

A buildup of cholesterol in the blood can lead to a blockage in blood vessels and affect blood flow to the affected area. For healthy total blood cholesterol, the desired range you would want to maintain is under 200 mg/dL. More specifically, when looking at individual lipid profiles, a low amount of LDL and a high amount of HDL prevents an excess buildup of cholesterol in the arteries, and wards off potential health hazards. An LDL level of less than 100 milligrams per deciliter is ideal while an LDL level above 160 mg/dL would be considered high. In contrast, a low value of HDL is a telltale sign that a person is living with major risks for disease. Values of less than 40 mg/dL for men and 50 mg/dL for women mark a risk factor for developing heart disease. In short, elevated LDL blood lipid profiles indicate an increased risk of heart attack, while elevated HDL blood lipid profiles indicate a reduced risk.

Dietary cholesterol does have a small impact on overall blood cholesterol levels, but not as much as some people may think. The average Canadian female consumes 237 milligrams of dietary cholesterol per day and for males, the figure is slightly higher—about 358 milligrams. Most people display little response to normal dietary cholesterol intake as the body responds by halting its own synthesis of the substance in favour of using the cholesterol obtained through food. Genetic factors may also influence the way a person’s body modifies cholesterol. Evidence from randomized controlled trials shows that replacing saturated fat in the diet with linoleic acid lowers cholesterol.^[8]

The University of Maryland Medical Center reports that omega-3 fatty acids promote lower total cholesterol and lower triglycerides in people with high cholesterol.^[9] It is suggested that people consume omega-3 fatty acids such as alpha-linolenic acid in their diets regularly. Polyunsaturated fatty acids are especially beneficial to consume because they both lower LDL and elevate HDL, thus contributing to healthy blood cholesterol levels. The study also reveals that saturated and trans fatty acids serve as catalysts for the increase of LDL cholesterol levels. Additionally, trans fatty acids decrease HDL levels, which can impact negatively total blood cholesterol levels.

Trans Fats

What is the first thing that comes to mind when you read ingredients such as “partially hydrogenated oil” and “hydrogenated oil” on a food label? Do you think of heart disease, heart health, or atherosclerosis? Most people probably do not. As we uncover what hydrogenation is and why manufacturers use it, you will be better equipped to adhere to healthier dietary choices and promote your heart health.

According to the Harvard School of Public Health, for every extra 2 percent of calories from trans fat consumed per day—about the amount found in a midsize order of French fries at a fast-food establishment—the risk of coronary heart disease increases by 23 percent.^[10]

Hydrogenation: The Good Gone Bad?

Food manufacturers are aware that fatty acids are susceptible to attack by oxygen molecules because their points of unsaturation render them vulnerable in this regard. When oxygen molecules attack these points of unsaturation, the modified fatty acid becomes oxidized. The oxidation of fatty acids makes the oil rancid and gives the food prepared with it an unappetizing taste. Because oils can undergo oxidation when stored in open containers, they must be stored in airtight containers and possibly refrigerated to minimize damage from oxidation. Hydrogenation poses a solution that food manufacturers prefer.

When lipids are subjected to hydrogenation, the molecular structure of the fat is altered. Hydrogenation is the process of adding hydrogen to unsaturated fatty-acid chains so that the hydrogen atoms are connected to the points of saturation, resulting in a more saturated fatty acid. Liquid oils that once contained more unsaturated fatty acids become semisolid or solid (upon complete hydrogenation) and behave like saturated fats. Oils initially contain polyunsaturated fatty acids. When the process of hydrogenation is not complete, for example, not all carbon double bonds have been saturated, the end result is a partially hydrogenated oil. The resulting oil is not fully solid. Total hydrogenation makes the oil very hard and virtually unusable. Some newer products are now using fully hydrogenated oil combined with nonhydrogenated vegetable oils to create a usable fat.

Manufacturers favour hydrogenation as a way to prevent the oxidation of oils and to ensure longer shelf life. Partially hydrogenated vegetable oils are used in the fast food and processed food industries because they impart the desired texture and crispness to baked and fried foods. Partially hydrogenated vegetable oils are more resistant to breakdown from extremely hot cooking temperatures. Because hydrogenated oils have a high smoking point, they are very well suited for frying. In addition, processed vegetable oils are cheaper than fats obtained from animal sources, making them a popular choice for the food industry.

Trans fatty acids occur in small amounts in nature, mostly in dairy products. However, the trans fats used by the food industry are produced from the hydrogenation process. Trans fats are a result of the partial hydrogenation of unsaturated fatty acids, which cause them to have a trans configuration, rather than the naturally occurring cis configuration.

Health Implications of Trans Fats

“No trans fats!” “Zero trans fats!” We see these advertisements regularly. So widespread is the concern over the issue that restaurants, food manufacturers, and even fast-food establishments proudly tout either the absence or the reduction of these fats within their products. Amid the growing awareness that trans fats may not be good for you, let’s get right to the heart of the matter. Why are trans fats so bad?

Processing naturally occurring fats to modify their texture from liquid to semisolid and solid forms results in the development of trans fats, which have been linked to an increased risk of heart disease. Trans fats are used in many processed foods such as cookies, cakes, chips, doughnuts, and snack foods to give them their crispy texture and increased shelf life. However, because trans fats can behave like saturated fats, the body processes them as if they were saturated fats. Despite the debate that exists related to the consumption of saturated fats, there is no debate when it comes to trans fat consumption. Industrial (not naturally occurring) trans fat intake was associated with mortality and the risk of coronary heart disease.^[11] Consuming large amounts of trans fats has been associated with tissue inflammation throughout the body, insulin resistance in some people, weight gain, and digestive troubles. In addition to this, the hydrogenation process robs the person of the benefits of consuming the original oil because hydrogenation destroys omega-3 and omega-6 fatty acids. Like saturated fats, trans fats raise LDL “bad cholesterol,” but unlike saturated fats, trans fats lower HDL “good cholesterol.” You should limit trans-fat consumption to less than 1 percent.

How can you benefit from this information? Keep industrial trans fat (e.g., that found in cakes cookies, etc.) intake to a minimum. When selecting your foods, steer clear of anything that says “hydrogenated,” “fractionally hydrogenated,” or “partially hydrogenated,” and read food labels in the following categories carefully:

1. cookies, crackers, cakes, muffins, pie crusts, pizza dough, and bread
2. stick margarine and vegetable shortening
3. premixed cake mixes, pancake mixes, and drink mixes
4. fried foods and hard taco shells
5. snack foods (such as chips), candy, and frozen dinners

Choose brands that don’t use trans fats.

A Prelude to Disease

If left unchecked, improper dietary fat consumption can lead you down a path of severe health problems. An increased level of lipids, triglycerides, and cholesterol in the blood is called hyperlipidemia. Hyperlipidemia is inclusive of several conditions but more commonly refers to high cholesterol and triglyceride levels. When blood lipid levels are high, any number of adverse health problems may ensue. Consider the following:

1. Cardiovascular disease: According to the Heart and Stroke Foundation, cardiovascular disease encompasses a variety of problems, many of which are related to the process of atherosclerosis. Over time, the arteries thicken and harden with plaque buildup, causing restricted or at times, low or no blood flow to selected areas of the body.
2. Heart attack: A heart attack happens when blood flow to a section of the heart is cut off due to a blood clot. Many have survived heart attacks and go on to return to their lives and enjoy many more years of life on this earth. However, dietary and lifestyle changes must be implemented to prevent further attacks.
3. Ischemic stroke: The most common type of stroke in Canada, ischemic stroke, occurs when a blood vessel in the brain or leading to the brain, becomes blocked, again usually from a blood clot. If part of the brain suffers a lack of blood flow and/or oxygen for three minutes or longer, brain cells will start to die.
4. Congestive heart failure: Sometimes referred to as heart failure, this condition indicates that the heart is not pumping blood as well as it should. The heart is still working but it is not meeting the body’s demand for blood and oxygen. If left unchecked, it can progress to further levels of malfunction.
5. Arrhythmia: This is an abnormal rhythm of the heart. The heart may beat above one hundred beats per minute (known as tachycardia) or below sixty beats per minute (known as bradycardia), or the beats are irregular. The heart may not be able to pump enough volume of blood to meet the body’s needs.
6. Heart valve problems: Stenosis is a condition wherein the heart valves become compromised in their ability to open wide enough to allow for proper blood flow. When the heart valves do not close tightly and blood begins to leak between chambers, this is called regurgitation. When valves bulge or prolapse back into the upper chamber, this condition is called mitral valve prolapse.
7. Obesity: Obesity is defined as the excessive accumulation of body fat. Obesity has been linked to increased risks of developing diabetes and heart disease. To help combat this problem, important dietary changes are necessary. Reducing the type and amount of carbohydrates and sugar consumed daily is critical. Limiting the intake of saturated fats and trans fats, increasing physical activity, and eating fewer calories are all equally important in this fight against obesity.

What Can You Do?

Remember that saturated fats are found in large amounts in foods of animal origin. They should be limited within the diet. Polyunsaturated fats are generally obtained from non-animal sources. While they are beneficial for lowering bad cholesterol, they also lower good cholesterol. They are better for you than saturated fats but are not to be consumed in excess amounts. Monounsaturated fats are of plant origin and are found in most nuts, seeds, seed oils, olive oil, canola oil, and legumes. Monounsaturated fats are excellent because not only do they lower LDL levels, but they also elevate HDL levels. Replace current dietary fats with an increased intake of monounsaturated fats.

Choose whole-grain and high-fiber foods. Reduced risk of cardiovascular disease has been associated with diets high in whole grains and fibre. Fibre also slows down cholesterol absorption. The AHA recommends that at least half of daily grain intake should originate from whole grains. The Adequate Intake value for fibre is 14 grams per 1,000 kilocalories. These amounts are based on the amount of fibre shown to reduce cardiovascular risk.

Do not be sedentary. Get more exercise regularly. Increasing your energy expenditure by just twenty minutes of physical activity at least three times per week will improve your overall health. Physical exercise can help you manage or prevent high blood pressure and blood cholesterol levels. Regular activity raises HDL while at the same time decreasing triglycerides and plaque buildup in the arteries. Calories are burned consistently, making it easier to lose and manage weight. Circulation will improve, the body will be better oxygenated, and the heart and blood vessels will function more efficiently.