Introduction
Lipids are the building blocks of the fats and fatty substances found in animals and plants. They are microscopic layered spheres of oil, which, in animals, are composed mainly of cholesterol, triglycerides, proteins (called lipoproteins), and phospholipids (molecules made up of phosphoric acid, fatty acids, and nitrogen). Lipids do not dissolve in water and are stored in the body to serve as sources of energy.
Cholesterol
Cholesterol is a white, powdery substance that is found in all animal cells and in animal-based foods (not in plants). In spite of its bad press, cholesterol is an essential nutrient necessary for many functions, including:
- Repairing cell membranes
- Manufacturing vitamin D on the skin's surface
- Producing hormones, such as estrogen and testosterone
- Possibly helping cell connections in the brain that are important for learning and memory
Regardless of these benefits, when cholesterol levels rise in the blood, they can have dangerous consequences, depending on the type of cholesterol. Although the body acquires some cholesterol through diet, about two-thirds is manufactured in the liver, its production stimulated by saturated fat. Saturated fats are found in animal products, meat, and dairy products.
Triglycerides
Triglycerides are composed of fatty acid molecules. They are the basic chemicals contained in fats in both animals and plants.
Lipoproteins
Lipoproteins are protein spheres that transport cholesterol, triglyceride, or other lipid molecules through the bloodstream. Most of the information about the effects of cholesterol and triglyceride actually concerns lipoproteins.
Lipoproteins are categorized into five types according to size and density. They can be further defined by whether they carry cholesterol or triglycerides.
Cholesterol-Carrying Lipoproteins. These are the lipoproteins commonly referred to as cholesterol.
- Low density lipoproteins (LDL). (Often called the "bad" cholesterol.)
- High-density lipoproteins (HDL), the smallest and most dense. (Referred to as the "good" cholesterol.)
Triglyceride-Carrying Lipoproteins.
- Intermediate density lipoproteins (IDL). They tend to carry triglycerides.
- Very low density lipoproteins (VLDL). These tend to carry triglycerides.
- Chylomicrons (largest in size and lowest in density).
Lipoprotein(a). Lipoprotein(a), or lp(a) has a size and density somewhere between LDL and HDL. The molecules carry a protein that may interfere with the body's ability to dissolve blood clots. Lipoprotein(a) is being investigated as a possible marker or cause of heart disease.
Remnant Lipoproteins. Remnant lipoproteins are byproducts of chylomicrons, very low-density lipoproteins (VLDL), or both. Some research indicates that high levels may be an important risk factor for coronary artery disease, particularly in patients who have otherwise normal cholesterol levels.
Cholesterol Guidelines
Reducing LDL and total cholesterol levels, while at the same time boosting HDL levels, can prevent heart attacks and death in all people (with or without heart disease). Reducing LDL is the primary goal of most cholesterol therapy.
Blood tests can easily measure both HDL and overall cholesterol levels. It is very difficult to measure LDL levels by themselves, but LDL levels can be reliably calculated by subtracting HDL and triglyceride levels from total cholesterol. The exact formula is:
LDL = TOTAL CHOLESTEROL - HDL - TRIGLYCERIDES/5.
In 2004, the National Cholesterol Education Program updated its clinical practice guidelines. The new recommendations set lower treatment goals for LDL levels based on a patient's risk factors for heart disease.
The risk factors include:
- Having a first-degree female relative diagnosed with heart disease before age 65 or a first-degree male relative diagnosed before age 55
- Being male and over age 45 or female and over age 55
- Cigarette smoking
- Diabetes
- High blood pressure
- Metabolic syndrome (risk factors associated with obesity such as low HDL levels and high triglycerides)
Two or more of these risk factors increases by 20% the chance of having a heart attack within 10 years.
The LDL cholesterol level is one of the most important factors in determining whether a patient needs cholesterol therapy and whether the treatment is working properly. In particular, the new guidelines emphasize lower LDL levels and earlier treatment for people with coronary artery disease, or other forms of atherosclerosis, and diabetes.
LDL Goals
|
|
Risk Level
|
Goal (d/L)
|
Optimal
Goal
(d/L)
|
| Very High Risk |
70
|
70
|
| High Risk |
100
|
70
|
| Moderate Risk |
130
|
100
|
| Low Risk |
160
|
130
|
The following chart summarizes all goals.
Cholesterol Goals
|
|
Total Cholesterol Goals
|
LDL Goals
|
HDL Goals
|
Triglyceride Goals
|
|
Less than 200 mg/dL is desirable.
Between 200 and 239 is borderline.
Over 240 is high.
|
70 mg/dL is the new goal for very high-risk patients (recent heart attack; current active or unstable cardiovascular or cerebrovascular disease; or two multiple risk factors as defined above.)
Below 100 mg/dL is optimal for everyone. It should be the goal for high-risk people including those with existing heart disease, diabetes, or two or more risk factors for heart disease; 70 mg/dL is an optimal goal for these individuals.
130 mg/dL or below for people with two or more risk factors; 100 mg/dL is an optimal goal.
160 mg/dL or below for people at less risk (one or zero risk factors); 130 mg/dL is an optimal goal.
Anything above 160 is high, with levels above 190 being very high. LDL levels over 190 require medication even with no other cardiac risk factors present.
|
Levels above 40 mg/dL are desirable; levels above 60 mg/dL are optimal.
|
Below 150 mg/dL is normal.
150 - 199 is borderline high.
200 - 499 is high.
Over 500 is very high.
|
|
*Risk factors for heart disease include a family history of early heart problems before age 55 for men (before age 65 for women), smoking, high blood pressure, diabetes, being older (over 45 for men and 55 for women), and having HDL levels below 35 mg/dL. People with two or more of these risk factors may have a 10-year risk of heart attack that exceeds 20%, and may therefore need to aim for LDL levels of 100 mg/dL or below.
|
Although current guidelines as described in the table are extremely useful, they do have pitfalls. For example, the following cholesterol levels pose some dilemmas:
- Low LDL levels (protective) accompanied by low HDL, high triglycerides, or both (harmful)
- High total cholesterol (harmful) accompanied by high HDL (protective)
Would individuals with these cholesterol balances be at high risk or low risk for developing heart disease? To resolve this dilemma, experts have devised a calculation for a risk ratio by dividing the total cholesterol by either total HDL or LDL. It is not clear at this point which ratio is a better predictor of heart disease, although the HDL ratio may be superior. Using this ratio, the following results indicate better to worse outlook:
- The ideal ratio is 3.5 or below.
- A ratio of 4.5 carries an average risk.
- Ratios of 5 or higher are potentially dangerous.
For example, if a person has a high total cholesterol of 280 mg/dL but a high HDL level of 70 mg/dL, the risk ratio is 4, which actually carries a lower than average risk. The use of this ratio may predict coronary artery disease more accurately than using total cholesterol levels alone. Still, the primary goal of lipid-lowering therapy is reducing LDL levels. Evidence strongly suggests that the lower the LDL levels, the lower the risk for heart disease.
