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Written by June 7, 2012

Steve Farrell, PhD

Science Officer
The Cooper Institute

Tags
cardiorespiratory fitness
cardiovascular disease
CVD
fitness
fitness level
metabolic syndrome
MetSyn
risk factor clustering
risk factors
Cardiorespiratory Fitness Level and the Metabolic Syndrome

Many people are aware that cardiovascular disease (CVD), also sometimes called heart disease, is the leading cause of death among U.S. men and women.  CVD does not choose its victims randomly; therefore risk factor identification is crucial to CVD prevention and treatment. A risk factor is either a behavior or a characteristic that is predictive of a higher risk for disease. There are many well-known major risk factors such as abnormal blood cholesterol level, hypertension (high blood pressure), tobacco use, prediabetes, family history, sedentary lifestyle, obesity and age.  Contributing risk factors include elevated blood triglyceride level and chronic stress.  Receiving regular medical exams is the most important way to identify and monitor these risk factors.

Risk Factor Clustering

You probably have heard about many of these CVD risk factors but what is much less commonly known is that certain risk factors often go hand in hand.  This is known as ‘risk factor clustering’, which was first observed more than a quarter century ago.  For Example:

  • many people with low HDL cholesterol also have high blood triglyceride levels
  • many people who are obese also have hypertension
  • many people who are sedentary are also obese and have prediabetes as well

Much of the work in the area of risk factor clustering was performed in the laboratory of Gerald Reaven M.D.  Dr. Reaven found that risk factor clustering was not coincidental.  In 1988, he coined the term ‘Syndrome X’ which was subsequently changed to ‘Metabolic Syndrome’ (MetSyn) 1. Individuals with MetSyn share a common characteristic in that they are all insulin resistant. Insulin is a hormone produced by the pancreas that allows glucose (sugar/carbohydrate) in the blood to be taken up by the liver, muscle and fat to either be used or stored. When someone is insulin resistant they are producing insulin, but the insulin is not having the desired effect of transporting enough glucose from the blood into the cells.  Consequently, the pancreas has to produce extra insulin to get the job done. This overproduction of insulin often causes increases in blood pressure and triglyceride levels, as well as decreases in HDL.  Consequently, individuals with MetSyn have substantial risk factor clustering and substantial risk of CVD.

How is Metabolic Syndrome Identified?

It is relatively easy for physicians to identify MetSyn.  Let’s take a look at the cut points:

  • fasting blood triglyceride level >150 mg/dl
  • HDL cholesterol <40 mg/dl in males or <50 mg/dl in females
  • resting blood pressure >130/85 mm Hg
  • waist circumference >40 inches in males or >35 inches in females
  • fasting blood glucose >100 mg/dl

If three or more of these five criteria are met, then the individual has MetSyn 2.  It is currently estimated that over 60 million Americans have MetSyn.  Unfortunately, many are unaware of its presence because they have not seen a doctor in years.  As an unwanted bonus, some with MetSyn will go on to develop type 2 diabetes, further increasing their risk of early CVD. So you can see why receiving a regular medical exam is so important.

Can Anything Be Done to Prevent Metabolic Syndrome?

In 2004, my colleagues and I were among the first to report on the prevalence of metabolic syndrome in 7104 Cooper Clinic women; all of whom had a maximal treadmill stress test to determine their cardiorespiratory fitness level 3. The women were divided into five fitness groups (quintiles) based on their age and treadmill performance.  We found a very strong trend for decreased prevalence of MetSyn across fitness quintiles. This is summarized below:

  • Quintile 1 (the least fit women):  19% had MetSyn
  • Quintile 2: 6.7% had MetSyn
  • Quintile 3:  6.0% had MetSyn
  • Quintile 4:  3.6% had MetSyn
  • Quintile 5 (the most fit women):  2.3% had MetSyn

What is striking about these results is that the largest decrease in the prevalence of MetSyn was seen when comparing quintile 1 with quintile 2.  This would suggest that just moving from the least fit 20% to the next fittest 20% is associated with a marked decrease in the prevalence of MetSyn.

The following year, an important step in MetSyn research was taken while again using the Cooper Clinic data base.  This time, Dr. Mike LaMonte and colleagues sought to determine if baseline cardiorespiratory fitness level could help predict the development of MetSyn in the future 4.  Using 9007 men and 1491 women who did not have MetSyn at the time of their baseline exam, the researchers tracked the subjects for an average of approximately 6 years. Moderately and highly fit men and women were substantially less likely to develop MetSyn during the follow-up period when compared to low fit men and women.  Thus, currently having a moderate or high level of cardiorespiratory fitness provides some protection against development of MetSyn in the future.

Can Having a Moderate to High Fitness Level Decrease my Risk of Death if I Have Metabolic Syndrome?

Around this same period of time, another important MetSyn paper was published from the Cooper Clinic database by Dr. Peter Katzmarzyk 5. He studied 15,466 men without MetSyn and 3757 men with MetSyn.  Men in the lowest quintile of cardiorespiratory fitness were characterized as unfit, while men in the remaining four quintiles were characterized as fit. During many years of follow-up after their initial Cooper Clinic exam, a total of 480 men died.  In the group without MetSyn at baseline as well as the in the group with MetSyn at baseline, fit men were substantially less likely to die during the follow-up when compared to unfit men.

Take—Home Message

These and other studies tell us many things.  First, individuals with moderate to high levels of cardiorespiratory fitness are less likely to have MetSyn than low fit individuals. Second, having a moderate to high level of cardiorespiratory fitness today provides some protection against developing MetSyn in the future.  Third, if you already have MetSyn, having a moderate to high level of cardiorespiratory fitness gives substantial protection against premature death.   So once again this shows the tremendous impact fitness has on our health.

How Do I Achieve a Moderate Level of Cardiorespiratory Fitness?

A good way for most people to achieve a moderate level of cardiorespiratory fitness is to follow the latest federal guidelines for physical activity:

  1. Accumulate a minimum of 150 minutes of moderate aerobic activity or 75 minutes of vigorous aerobic activity per week.
  2. For individuals wishing to achieve additional health benefits and/or a high level of cardiorespiratory fitness, as much as 300 minutes per week of moderate aerobic activity or 150 minutes per week of vigorous aerobic activity per week may be necessary.

1 Reaven GM. Role of Insulin Resistance in Human Disease.  Diabetes. 1988;37:1595-1607.

2 The Expert Panel.  Executive Summary of the Third Report of the National Cholesterol Education Program (NCEP) Expert panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults (Adult Treatment Panel III). JAMA. 2001;285:2486-2497.

3 Farrell SW, Cheng YJ, Blair SN.  Prevalence of the Metabolic Syndrome across Cardiorespiratory Fitness Levels in Women. 2004. Obesity Research. 2004;12:824-830.

4 LaMonte MJ, Barlow CE, Jurca R, Kampert JB, Church TS, Blair SN.  Cardiorespiratory Fitness is Inversely Associated with the Incidence of Metabolic Syndrome. Circulation. 2005;112:505-512.

5 Katzmarzyk PT, Church TS, Blair SN. Cardiorespiratory Fitness Attenuates the Effects of the Metabolic Syndrome on All-cause and Cardiovascular Disease Mortality in Men. Arch Intern Med. 2004;164:1092-1097.

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