A Review of the Impact of Exercise on Cholesterol Levels

A Review of the Impact of Exercise on Cholesterol Levels
WHY HIGH-DENSITY LIPOPROTEIN, OR HDL, CHOLESTEROL LEVELS ARE ASSOCIATED WITH A REDUCED RISK

R

OF HEART DISEASE–AND HOW EXERCISE CAN HELP.

ecently, the National Institutes of Health issued new criteria for the detection, categorization and treatment of unhealthy cholesterol levels (American Medical Association 2001). As a result of these new criteria, the number of Americans in the high-cholesterol category rose substantially. The revised guidelines recommend that individuals with elevated cholesterol levels undertake “therapeutic lifestyle changes” and, when needed, drug therapy. Consequently, fitness professionals can expect to see more and more clients wanting to lower their cholesterol levels through exercise and lifestyle modifications. The link between cholesterol and coronary heart disease (CHD) has been well established through numerous long-term studies. What isn’t as well established or understood is how to achieve and maintain optimal cholesterol levels. While dietary changes and certain medications can lower cholesterol, exercise is effective as well. But how much and what type of exercise is needed to positively change cholesterol levels? And do cholesterol levels in men and women respond differently to exercise? This article gives an overview of what cholesterol is; explains how it functions in the body; and presents the latest research on how exercise impacts cholesterol levels.
By Chantal A. Vella, MS; Len Kravitz, PhD; and Jeffrey M. Janot, PhD

November-December 2001 IDEA HEALTH & FITNESS SOURCE

UNDERSTANDING CHOLESTEROL
Cholesterol is a waxy, fat-like substance found in all animal products (i.e., meats, dairy products and eggs). The human body makes cholesterol in the liver and absorbs cholesterol through the diet. Cholesterol is essential to the body and is used to build cell membranes, produce sex hormones and form bile acids, which are necessary for the digestion of fats; however, when cholesterol levels in the blood are too high, some of the excess is deposited in the artery walls, increasing the risk for CHD. (See “Glossary” on this page for a review of the terms used in this article.) Cholesterol travels through the bloodstream as part of larger particles known as lipoproteins. Lipoproteins are special transporters that carry lipids–fatty substances such as cholesterol– in the bloodstream to the liver, the intestine and peripheral tissues. Lipoproteins are classified based on the thickness of the protein shell that surrounds the cholesterol. Low-density lipoproteins (LDL) are the primary transporters of cholesterol; they deposit

excess cholesterol in the artery walls. High-density lipoproteins (HDL) are involved in the reverse transport of cholesterol to the liver (Durstine & Haskell 1994); they remove cholesterol from the blood and arteries (the reverse transport process). When LDL levels get too high or HDL levels get too low, cholesterol builds up in the blood, forming artery-clogging deposits.
LDL CHOLESTEROL

The role of LDL cholesterol (LDL-C), sometimes called the “bad,” “lousy” or “unhealthy” cholesterol, is to transport cholesterol to various body cells and deposit any excess cholesterol in the artery walls, which increases the risk of heart disease. (For a more detailed discussion of LDL-C, see “A Closer Look at LDL Cholesterol” on the next page.) A desirable level of LDL-C is below 130 milligrams per deciliter (mg/dl), with 100 mg/dl or less being optimal.
HDL CHOLESTEROL

Glossary
Cholesterol

is a waxy, fat-like substance found in all animal

products (i.e., meats, dairy products and eggs). The body absorbs cholesterol from the diet and makes cholesterol in the liver. Cholesterol is essential for building cell membranes, producing sex hormones and forming bile acids, which are necessary for the digestion of fats.
Coronary heart disease

results from a decreased flow of blood

to the heart, usually caused by atherosclerosis, an accumulation of fatty deposits in the arteries.
Dose-response relationship

is a linear relationship between

exercise volume (total kilocalories expended) and a particular health outcome.
HDL 2

is an HDL3 molecule that has increased its cholesterol is a molecule produced in the liver and put into circula-

content, becoming less dense, and is then classified as HDL2.
HDL 3

tion to collect cholesterol. As an HDL3 molecule increases its cholesterol content, it becomes less dense and is then classified as HDL2.
High-density lipoproteins

are involved in the reverse trans-

port of cholesterol to the liver.
Lipids

are fatty substances found in the diet and made in the are special transporters that carry cholesterol and are the primary transporters of

body. Cholesterol is an example of a lipid.
Lipoproteins

HDL cholesterol (HDL-C), sometimes called the “good” or “healthy” cholesterol, is responsible for transporting cholesterol from the blood and artery walls to the liver, where it is converted to bile for digestion or disposed of by the body. This “reverse cholesterol transport process” is believed to be helpful in preventing or reversing heart disease. HDL molecules have two main subclasses: HDL 2 and HDL 3 (Durstine & Haskell 1994). HDL3 molecules are produced in the liver and put into circulation to collect cholesterol. As they increase their cholesterol content, they become less dense and are classified as HDL2. The HDL2 molecules are then recycled in the liver, where the cholesterol is extracted and either converted to bile for digestion or disposed of by the body. Then HDL3 molecules are again released into circulation (Durstine & Haskell 1994). Because HDL molecules collect and remove cholesterol from the blood and artery walls, high HDL-C levels are associated with a reduced risk of CHD (Neiman 1998). In fact, for every 1 mg/dl decrease in HDL-C, the risk of CHD increases by 2 to 3 percent (%) (Durstine & Haskell 1994). HDL-C levels equal to or above 60 mg/dl are considered desirable, while HDL-C levels below 35 mg/dl are considered undesirable. See “Measuring Cholesterol Levels” on the next page. While the current research clearly indicates that exercise does not independently affect LDL-C levels, some findings are promising on the connection between cardiovascular exercise and increases in HDL-C levels. Consequently, this article focuses on research into how exercise affects HDL-C levels, specifically.

other substances through the bloodstream.
Low-density lipoproteins

EXERCISE AND HDL CHOLESTEROL
It is well established that a sedentary lifestyle contributes significantly to the development of CHD and that physical activity

cholesterol; they deposit excess cholesterol in the artery walls.

November-December 2001 IDEA HEALTH & FITNESS SOURCE

plays an important role in decreasing CHD mortality. Some studies have found that regular physical activity and a single exercise session can both have a positive effect on cholesterol metabolism (Durstine & Haskell 1994). Exercise is partly responsible for increasing the production of several enzymes that enhance the reverse cholesterol transport system (Durstine & Haskell 1994). The precise mechanisms for this action are unclear, but evidence indicates that factors like diet, body fat, weight loss, hormones and enzyme activity interact with exercise to alter the rates at which cholesterol is synthesized, transported and cleared from the blood (Durstine & Haskell 1994). While exercise training has been associated with increased levels of HDL-C, the volume (amount) of exercise required to confer this benefit is still debatable. Research has provided inconsistent results, but it suggests that an exercise threshold must be met before significant changes in HDL-C are observed. Research also suggests that a dose-response relationship may exist between exercise volume and increases in HDL-C levels (Drygas et al. 2000). The following sections describe studies involving men and women engaged specifically in cardiovascular activity. (While resistance training may be beneficial in improving HDL-C levels, little research exists to support that finding. See “Resistance Training and HDL Cholesterol” on the next page.) The studies indicate that the impact of exercise on HDL-C levels is different for men and women, partly because female hormones influence HDL-C levels. In addition, most of the research on HDL-C and exercise has involved male subjects, so the results are clearer for men. For this reason, the research on men and women is presented separately.

Measuring Cholesterol Levels*
LDL Cholesterol (in milligrams/deciliter, or mg/dl)

< 100 100-129 130-159 160-189 optimal near or above optimal borderline high high HDL Cholesterol (mg/dl) < 35 35-59 60 low desirable optimal Total Cholesterol (mg/dl) < 200 200-239 240 desirable borderline high high *Adapted from the American Medical Association. 2001. Journal of the American Medical Association, 285 (19), 2486-97. A Closer Look at LDL Cholesterol Although both low-density lipoprotein (LDL) and high-density lipoprotein (HDL) levels affect risk for coronary heart disease, LDL levels get more attention because they are more clearly linked to this risk. LDL cholesterol (LDL-C) is often called the "bad," "lousy" or "unhealthy" cholesterol because its main function is to transport cholesterol to various cells throughout the body, including the artery walls. When LDL levels are elevated, cholesterol begins to accumulate in these vessel walls. LDL-C deposits can eventually reduce blood flow through the arteries (ACSM 1998). Of the 65 million Americans now considered to have high cholesterol, roughly half should be able to reach their LDL-C goals through lifestyle changes alone. The National Institutes of Health recommends the following dietary changes: