Understanding Dietary Cholesterol

Cardiovascular disease (CVD) is the #1 killer in developed and developing countries. Early research suggested that dietary cholesterol raises total cholesterol, and higher levels of total cholesterol has a poor relationship with cardiovascular disease risk. This led to the recommendations to eat low amounts of cholesterol. More research has suggested otherwise, which is why the 2015 Dietary Guidelines of America committee stated that there is a lack of evidence suggesting the need to limit cholesterol intake.

 

Introduction

Cholesterol has many functions. It’s a precursor for steroid hormones, vitamin D, bile acids, and is an important structural component on cell membranes.

The amount of cholesterol in the body is a total between de novo synthesis, dietary intake, absorption efficiency, metabolism, enterohepatic recirculation and excretion.

Dietary cholesterol is only found in animal foods, specifically eggs, dairy, and meat. People in the U.S. get most of their dietary cholesterol from eggs, chicken, and beef. Other sources of cholesterol are cheese, sausage, bacon, fish, pork, and shrimp.

Some studies have found that dietary cholesterol increases the risk of cardiovascular disease, while others have found that it actually decreases risk, or that it has no effect on risk. (R)

 

Lipoproteins

Lipoproteins are a type of molecule that carry cholesterol in the blood. The two main types of lipoproteins are low-density lipoprotein (LDL) and high-density lipoprotein (HDL). LDL is generally considered the bad one, because having high levels of LDL cholesterol, inflammation, and oxidative stress can lead to heart disease. HDL is generally considered the good one, because it helps remove cholesterol from peripheral tissues and brings it back to the liver, where cholesterol can be recycled or removed from the body. Having higher levels of HDL may help reduce the risk of heart disease. (R)

Dyslipidemia, which is characterized by having high LDL-cholesterol, high triglycerides, and low HDL-cholesterol, is associated with increased risk of cardiovascular disease. (R)

A large meta-analysis from 1946-2013, included 40 studies, and found that higher intakes of dietary cholesterol were not associated with an increased risk of cardiovascular disease. However, they did find that having higher LDL cholesterol and triglycerides, as well as lower HDL cholesterol are risk factors for cardiovascular disease. (R)

 

How Atherosclerosis Develops (Inflammation and LDL oxidation)

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It is thought that atherosclerosis develops from high levels of LDL-cholesterol in the blood, and when LDL is lowered, it reverses or slows the development of atherosclerosis and cardiovascular disease. (R)

Atherosclerosis is characterized by hardening of the arteries, and a buildup of plaque. Plaque is made up of LDL cholesterol, macrophages, smooth muscle cells, platelets, and debris. As the plaque builds up in blood vessels, it narrows the lumen, which increases blood pressure. The increased blood pressure may cause the plaque to rupture, and can result in a blood clot, stroke, or heart attack.

The development of plaque starts when the blood vessel is damaged, which allows LDL to move from the blood to the intima, where it can be oxidized to ox-LDL. Macrophages ingest ox-LDL, and become foam cells, and secrete chemokines to attract more macrophages. HDL also enters the intima and is oxidized, but is not ingested by macrophages. The oxidation of HDL helps to take up some of the free radicals that would otherwise oxidize LDL. (R)

HDL is important for reducing the oxidation of LDL, and HDL also has anti-apoptotic and anti-thrombotic effects. (R)

VLDL is converted to IDL, which is then converted to LDL. The remnant particles contribute to atherosclerosis because they are not eliminated by the liver, and can be taken up in the arterial walls. The macrophages then take them up and become foam cells, leading to atherosclerosis. (R)

Oxidative Stress

Many people consume large amounts of oxidized fatty acids, because the food they eat contains a lot of fat, cholesterol, and is often cooked at high temperatures. (R)

Free radicals, oxidative stress, and inflammation are important factors to consider when assessing cardiovascular disease risk. (R)

Dietary cholesterol increases LDL susceptibility to oxidation by 37%-39%. (R)

After eating a high fat and high cholesterol meal, endothelial function is impaired for about 4 hours. When antioxidant supplements (vitamin C and E) are given during this time, endothelial function increases, which means the decline in endothelial function is likely due to oxidative stress. (R)

Dietary cholesterol (generally) doesn’t affect plasma LDL cholesterol very much, unless you eat a lot of it. However, dietary cholesterol increases the oxidation of LDL and postprandial lipemia, which may increase the risk of cardiovascular disease. (R)

This is why it’s a good idea to always consume a lot of antioxidants with each meal, to help blunt the inflammation and oxidative stress.

 

People Respond Differently to Dietary Cholesterol

One of the issues with cholesterol is that people respond differently to the amount of the cholesterol they consume, and are so called hyperresponders or hyporesponders.

 

Cholesterol Regulation

Most of the cholesterol is made in our bodies, and our bodies have a way of reducing cholesterol synthesis when we consume more dietary cholesterol. Intervention trials have shown that consuming 500-900 mg/day of cholesterol significantly raises both LDL and HDL cholesterol. However, there is a plateau in rising serum cholesterol levels when one consumes more than 900 mg/day of dietary cholesterol. (R)

 

LDL-cholesterol Receptors

The liver contains LDL receptors that recognize and bind apoprotein (Apo) B100 and Apo E on LDL particles, which allows LDL particles to be taken in to the liver.  The LDL particles are broken down, and free cholesterol is released. (R)

 

HMG CoA Reductase

The release of free cholesterol inhibits HMG CoA reductase activity, which is the rate limiting enzyme for synthesizing cholesterol (and is a target for treatment to reduce cholesterol in the body). (R)

 

Plant Sterols (Phytosterols)

Plant sterols (phytosterols) can be used to decrease total cholesterol. Plant sterols are similar in structure to cholesterol, and can block cholesterol absorption in the intestine, by which more cholesterol will be excreted, decreasing total body cholesterol levels. The plant sterols are also excreted. (R)

 

Bile Acids

A lot of the bile acids are recycled and reused. Bile acids are made from cholesterol, so if the body does not recycle bile acids, more will be excreted, which forces the body to use more cholesterol in the body. This in turn lowers total cholesterol. Cholestyramine is a drug that binds with bile acids, and causes them to be excreted instead of recycled. This lowers total cholesterol in the body, because the body has to use more cholesterol to make more bile acids.

 

Exercise

Endurance exercise significantly increases HDL, and decreases Apo B concentrations and LDL, even when persons are consuming 12 eggs per week. (R)

 

Reducing LDL Cholesterol

Reducing LDL cholesterol is an effective way to prevent and treat coronary heart disease. A 1% decrease in LDL cholesterol will reduce the likelihood of a cardiac event by 1%. (R)

Reducing LDL cholesterol can reduce the risk of coronary heart disease and stroke. (R)

 

Eggs

One egg per day is not associated with coronary heart disease or stroke. (R)

Egg consumption may increase LDL and HDL particle size, which may protect against atherosclerosis. (R)

Eggs are high in cholesterol, but low in saturated fat. (R)

This study found that insulin sensitive individuals have increased total cholesterol, LDL-C, HDL-C, and Apo B concentrations when they consume 4 eggs per day, but not 2 eggs per day. (R)

 

Carbohydrates

One study found that high-carbohydrate diets increase triacylglycerol levels and smaller, denser, more atherogenic LDL particles. (R)

It likely depends on the type of carbohydrate though, such as refined flour or high fructose corn syrup vs. brown rice.

 

Saturated Fat

A diet low in cholesterol but high in saturated fat increases LDL cholesterol by only 0-6 mg/dL, so LDL cholesterol is hardly affected when eating this way. When a diet is high in both cholesterol (600 mg) and saturated fat, LDL increases substantially by 16-31 mg/dL. When a diet is high in cholesterol (600 mg) and polyunsaturated fats, LDL cholesterol increases by about 16 mg/dL. (R)

 

Coconut Oil

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The saturated fatty acid lauric acid increases cholesterol the most. Lauric acid is a major saturated fatty acid found in coconut and palm oil. Although it raises cholesterol significantly, a lot of the increase in cholesterol is HDL cholesterol, which is favorable for decreasing the risk of cardiovascular disease. (R)

 

Other Factors to Consider

There are other factors to consider which may affect study results. Saturated fat and percentage of calories from fat are both associated with increased risk of cardiovascular disease. Also, vegetable intake can reduce the risk of cardiovascular disease, and people who consume more vegetables and fiber tend to consume less cholesterol. (R) This means that the reduction of cardiovascular disease risk may not entirely be due to the lower intake of cholesterol, or maybe not at all.

 

Particle Size

Larger LDL and HDL particles are thought to be less atherogenic than smaller LDL and HDL particles. (R)

 

Genetics Play a Role in Cholesterol Metabolism

Familial hypercholesterolemia is characterized by really high total cholesterol, especially LDL cholesterol. Heterozygous is somewhat common, but homozygous is not. Homozygotes can have total cholesterol greater than 600 mg/dL, and if not treated, usually die of a heart attack before 20 years old. (R)

Familial defective Apo B100 increases LDL cholesterol, and increases risk of coronary heart disease. (R)

 

Other causes of hyperlipidemia

Dietary cholesterol can increase total cholesterol levels, but there are many other causes of increased cholesterol.

 

Diet (R)

Saturated and trans fats

Excess calories

Alcohol

Red Meat

Whole milk

High sugar beverages and foods

 

Drugs (R)

Thiazide diuretics

Beta-blockers

Glucocorticoids

Sex hormones

Retinoic Acid derivatives

Antipsychotics

Antiretrovirals

Immunosuppressants

 

Diseases (R)

Hypothyroidism

Obesity

Type 2 diabetes

Metabolic syndrome

Renal disease

HIV

PCOS

 

Conclusion

To determine if you should be concerned about dietary cholesterol, you should assess your risk of cardiovascular disease. It’s a good idea to check your cholesterol and triglyceride levels, as well as blood pressure. Testing CRP levels can also be a good indication of inflammation. Also, remember that dietary cholesterol is just one piece of the puzzle of a diet.