Your training has gone well. You have built up your base and gradually added intensity to your training regimen. You have completed key workouts in record times.

But with only a couple of hard workouts to go before the big race of your season, you notice a tickle in the back of your throat. The next morning you have a sore throat, headache, and cough and think that you might have a fever. A tempo run is on your agenda. What should you do?

Take Care of Yourself

If you think that you have a simple cold without systemic symptoms, exercise at a low heart rate and easy level of perceived exertion (long-slow-distance pace or easier) for a short period of time. If you are any sicker than stuffy-nose-sick, you should skip the run altogether.

Adhering to the following recommendations may help you recover faster and could even save your life.

1. Never exercise with a fever, lower respiratory tract infection, or symptoms of systemic illness (muscle aches, chills, malaise, etc.).

2. Do not ignore symptoms that could be attributable to the heart, including shortness of breath, irregular heartbeat, fainting, chest pain or lightheadedness. See a doctor immediately if you have any question about the health of your heart.

3. Remember to have reasonable and lowered performance expectations after you've been ill.

4. Listen to your body and practice restraint. Wait to resume training until your motivation and health have completely returned. You have only one body, take care of it.

Now lets look at the "whys" behind exercising caution in training when you're ill.

Metabolic Response to Infection

Acute infection results in a biological response that could be equated with calling out the National Guard. This acute phase response involves chemicals being produced throughout the body (white blood cells, the cells lining blood vessels and the airways, etc.). Evolutionarily, this process increases a sick person's chances of survival.

Nutrients are mobilized (e.g., amino acids, the building blocks of protein), which the sick person uses to make infection-fighting substances. The acute phase response is stereotypical, or essentially the same regardless of the cause of the infection. Like any good military operation, the magnitude of the response is generally well-regulated and determined by the intensity and duration of the threat.

Protein Catabolism

During an infection, the body becomes catabolic (the opposite of anabolic) and breaks down muscle protein. The degree of muscle catabolism and protein loss is related to the height and duration of the fever caused by the infection.

Unfortunately, there is no evidence that controlling a fever with acetaminophen or ibuprofen decreases this loss of hard-earned muscle.

The amino acids that are liberated from muscle are scavenged by the liver and used as an emergency energy source (glucose production via gluconeogenesis) and as the building blocks for acute phase proteins, which the body employs to fight infection.

Your muscles have many good reasons to ache when you have an infection. Skeletal muscle is the main source of catabolized protein, but heart muscle contributes as well. Skeletal muscle biopsies done in feverish people and laboratory animals during acute infections demonstrate microscopic evidence of muscle damage.

The same chemicals that initiate muscle breakdown during infection also inhibit effective muscle building and repair, making it virtually impossible to build muscle during any infection more serious than a cold.

Unfortunately, you can't even lose body fat while you are sick, as fat metabolism is impaired during infections. This causes the sick person to rely more heavily on muscle as an energy source than it normally would during times of physical stress (e.g., starvation or heavy training). So any weight loss during a febrile illness is generally lean muscle mass.

During a febrile or systemic illness, athletes should be wary of stressing muscles that are actively being broken down to fuel the body's defenses. Normal recovery after a workout can be greatly prolonged and the risk of injury may be heightened.

Studies have shown a 25 percent decrease in isometric muscle strength after a simple febrile illness such as the flu. Replenishing muscle mass lost during a three-day febrile illness may take up to two weeks.