Cardiorespiratory fitness may soon be considered as a vital sign on par with blood pressure and heart rate.
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Three decades’ worth of solid evidence linking a sedentary lifestyle to higher risk of cardiovascular disease, cancer, and early death has silenced even the most ardent exercise deniers. Nonetheless, a metric quantifying a person’s level of aerobic fitness has yet to make it into the set of tools commonly used to predict heart disease.
But that may soon change. The American Heart Association recently released a statement calling for cardiorespiratory fitness (CRF) to be considered a clinical vital sign, similar to how blood pressure, heart rate, and tobacco use are treated now. Assessing fitness level at your annual visit would help your doctor get a better picture of your underlying heart health and risk of disease down the road.
“As a practicing physician, I find this really exciting,” says Dr. Malissa Wood, co-director of the Corrigan Women’s Heart Health Program at Harvard-affiliated Massachusetts General Hospital. “If you determine that a person’s fitness level is less than ideal, you can give concrete advice on how to adjust their amount of physical activity to lower their cardiovascular risk.”
How do you measure aerobic fitness?
One of the challenges in making fitness measurement part of regular clinical practice is finding a consistent way to define and quantify activity level. The gold standard for gauging how well your heart, lungs, and muscles function is called VO2 max, which is the peak amount of oxygen your body uses during vigorous exercise. However, directly calculating VO2 max is generally not feasible during a regular office visit because it involves breathing into an oxygen mask while exercising on a treadmill for a specified time.
Instead, clinicians and exercise experts often use a surrogate measure of exercise capacity called the metabolic equivalent, or MET, to quickly assess a person’s aerobic fitness. One MET is defined as the energy it takes to sit quietly. For the average adult, this translates to about one calorie of energy per every 2.2 pounds of body weight per hour. So a person who weighs 160 pounds would burn approximately 70 calories an hour while sitting or sleeping.
Getting a reading of your baseline fitness level using METs is generally a straightforward process. A routine exercise stress test conducted on a treadmill or stationary cycle will document your peak aerobic capacity. Alternatively, your doctor can get a snapshot of your fitness level by asking you a set of standard questions about your activity level. In her practice, Dr. Wood uses a research-validated questionnaire called the Duke Activity Status Index. The tool consists of 12 yes-or-no questions. They range from whether you can walk a block or two on level ground (2.75 METs), climb a flight of stairs or walk up a hill (5.5 METs), or participate in vigorous sports such as swimming, singles tennis, or skiing (7.5 METs).
The score, which is based on your answers, provides an estimate of your peak work capacity in terms of METs. A “yes” answer to all questions will net you a score of close to 10, which is ideal. A score below 7 METs is a cause for some concern, and people with a CRF of 5 METs or lower are at particularly high risk.
Managing your MET minutes
The standard exercise recommendation is to get a minimum of 30 minutes of moderate exercise at least five days a week (for a total of 150 minutes). Alternately, you can engage in at least 25 minutes of intense activity three times a week (for a total of 75 minutes). Moderate-intensity activities are those that clock in at 3 to 6 METs. Vigorous-intensity activities score more than 6 METs.
If you like to mix up your exercise routine, you can keep track of your quota by computing your MET minutes for the week. To do this, multiply the number of minutes you spend doing either a moderate or vigorous activity by the number of METs it requires (see below). Then, tally the MET minutes from each exercise session to get your weekly total. You should aim for 500 to 1,000 MET minutes per week.
Improving your CRF
However, each 1-MET increase in your CRF can drop your risk of heart disease and death by 10% to 20%. The first step in improving your CRF is to make sure you meet the recommended quota of moderate to intense exercise each week (see “Managing your MET minutes”). Be sure to check with your doctor before starting an exercise program if you haven’t been active for a while.
In addition to structured exercise, you can boost your fitness by increasing the amount of activity you get in the course of a regular day. An easy way to do this is to use a fitness tracker, pedometer, or smartphone app to record the number of steps you take each day. A good target is 10,000 daily steps, which equals about 4.5 miles, says Dr. Wood.