Elevated blood cholesterol is a risk factor for cardiovascular disease. Moderate cholesterol, however, is required for proper body function. Whether consumed through foods or made in the body, cholesterol must be moved through the bloodstream and delivered to tissues for use. Like oil in a river, cholesterol resists mixing and gliding through the watery bloodstream. Consequently, the body uses a collection of specialized vehicles and mechanisms to distribute cholesterol. This process is called cholesterol transport.
Dietary Cholesterol Transport
Dietary cholesterol enters the digestive tract during the consumption of animal foods. Cholesterol, accompanied by dietary fats, absorb from the intestines into transport vehicles called chylomicrons. To chauffeur hydrophobic (water-fearing) compounds within the watery bloodstream, chylomicrons are structurally arranged with water-loving outer shells and fat-loving inner cores. Once filled with fatty compounds, chylomicrons enter circulation. Fat distribution to tissues occurs first, while residual compounds, including cholesterol, absorb into the liver hours later.
Liver Transport Preparation
The liver produces the majority of cholesterol needed by a body. Much of this cholesterol combines with a portion of dietary cholesterol anticipating transfer to body tissues. Like the intestines, the liver generates transport carriers called very low-density lipoproteins (VLDL). VLDL resemble chylomicrons both structurally and functionally. Fats and cholesterol saturate the fatty core of VLDL, which promptly pass into circulation.
Cholesterol Transport in the Bloodstream
Fat removal from VLDL precedes cholesterol delivery to tissues. Depleted of its fats, the cholesterol-laden remnant particles, known as low-density lipoproteins (LDL), distribute cholesterol to all body cells. Cells store or use cholesterol to enhance their membrane fluidity or make sterol-based hormones such as testosterone and vitamin D.
Reverse Cholesterol Transport
Poor dietary choices or genetic abnormalities in cholesterol production may cause cholesterol accumulation in the tissues. To manage this, the liver generates high-density lipoproteins (HDL), which remove cholesterol from body tissues and recycle it in the liver. The operation of HDL is often referred to as "reverse cholesterol transport."
Cholesterol Transport from the Body
The liver rids the body of cholesterol by integrating it into bile, a substance required for fat digestion. Bile passes through a series of ducts to the small intestine when food is present. Post-digestion, bile binds to indigestible food waste (such as fiber) and is excreted from the body during defecation.
Cholesterol Transport and Health
All cholesterol transport vehicles are essential to health. However, skewed concentrations of the particles may contribute to heart disease risk. Though guidelines vary depending on existing health conditions, the Mayo clinic recommends that most people keep LDL below 130mg per deciliter and HDL above 60mg per deciliter to minimize cardiovascular risk.
References
- Mayo Clinic
- "Understand Normal and Clinical Nutrition"; Sharon Rady Rolfes et al; 2009
- "Advanced Nutrition and Human Metabolism"; Sareen Gropper et al; 2009
