Nothing brings your training progress to a halt more than getting a flu or cold. As your training volume or intensity increases, your immune system”s ability to fight off viruses decreases. A little bit of exercise and fitness is terrific for strengthening your immune system, but either too little or too much both have the effect of weakening your immunity and increasing the risk of contracting minor infections like colds and the flu. Because of this, athletes like cyclists and triathletes need to be vary careful of this fine line. There’s a lot of old wives tales out there on what you should and shouldn’t do. Sweat it out? Overdose with vitamin C? etc. Well Amy Hallihan just emailed me an old saying "if it’s above the neck - it’s okay to train, if it’s below the neck – rest up ". After speaking with a doctor and doing some research there seems to be some truth to this.
There are basically two different types of illnesses that we are commonly affected by. The flu is caused by viruses known as Influenza A or B, and the common cold which is caused by viruses called coronaviruses and rhinoviruses. If one of them hits you, your immune system builds a lifelong immunity to it
The flu is much more severe than a common cold as it is usually accompanied by body aches and a fever. Therefore, your body’s immune system is taxed much more by the flu than by the common cold. At this time, training would not only be detrimental to recovery and adaptation, but it would also to your health as well. While training can help us gain muscle, lose fat and feel good, it is a catabolic activity . The body needs to be in good health in order to go from the catabolic state caused by the exercise to an anabolic state of recuperation and muscle growth. So if you have the flu, your body is already fighting a catabolic state caused by the Influenza virus. Training on the bike would only add more catabolism, which in turn would negatively affect immune system against the virus, causing you to get sicker. Absolutely no training if you have the flu. Once the flu runs its course, slowly start up back on your training program not pushing yourself too hard. Go on how you feel in subsequent weeks.
If the common cold is bringing you down and the virus is mild (runny nose, slight coughing, sneezing, etc), you may get away with training. It would definitely be wise to keep the intensity to a minimum. Again, if the cold virus is causing you to feel run down, achy, with a sore throat and headaches, it would be best to stop training all together until the symptoms subside. You don’t want to make it any harder for the immune system to fight the virus by introducing more catabolic activity, so keep the intensity down.
Once you get back on the bike you need to resist the temptation to immediately hammer out hard intervals to make up for lost time. Your body can take up to a week to recover from even a minor cold, and it was the intensity and volume that helped to get you sick in the first place. Remember, it’s always better to be under-trained and healthy than over-trained but sick.