A study of upper respiratory tract infections (URTI) in athletes nicely supports this contention. A recent study by Matthews et al demonstrated a 20% reduction in URTIs in a group of regular, moderate exercisers. In contrast, a study in a group of marathoners (an example of exercising to exhaustion) did demonstrate a significant increase, a 13% incidence of URTIs in the week following the event compared to 2% in the comparison group that enrolled but did not compete.
Is it possible to identify why extreme exercise has this neative effect? Although significant changes in the level and functional activity of immune system parameters have been observed in athletes, investigators have had little success in linking these to a higher incidence of infection and illness. And investigators have yet to demonstrate that athletes showing the most extreme post-exercise changes in immunity (immunosuppression) are those that contract an infection during the subsequent 1 to 2 weeks.
What can you do to avoid this negative effect of a vigorous (and perhaps overly enthusiastic) training program? Probably very little. Reference 1: Although carbohydrate beverage ingestion has been associated with higher plasma glucose levels, an attenuated cortisol and growth hormone response, fewer perturbations in blood immune cell counts, lower granulocyte and monocyte phagocytosis and oxidative burst activity, and a diminished pro- and anti-inflammatory cytokine response, it remains to be proven whether carbohydrate supplementation diminishes the frequency of infections in the recovery period after strenuous exercise. Reference 2: Likewise the influence of nutritional supplements (primarily zinc, vitamin C, and glutamine) on the acute immune response to prolonged exercise have been measured in endurance athletes, but the support for protection is inconclusive.
But there is an option for treatment (versus protection) of an upper respiratory illness or "cold" - the commercial lozenges which contain zinc gluconate. Zinc is thought to be the active ingredient and the lozenges have been shown in a careful scientific study to reduce the duration of symptoms from 7.6 to 4.4 days.
What is the best advice?
Traveler's diarrhea (gastroenteritis,tourista, Montezuma's Revenge) can be devastating when you are trying to make your daily miles and maintain hydration. The first step in prevention is being aware of the potential for infection in the area being visited. In many countries, particularly in the third world, all water coming out of a tap is suspect. But even in the southwestern US, water coming from a spigot may be meant for irrigation purposes and is not drinkable. So be aware of any particular precautions for the area you are visiting, and don't assume that water coming out of a pipe or faucet is always safe (even in the US). Food can also be a source of infectious gastroenteritis - either from contamination from the use of human waste as fertilizer as well as the hands of the people preparing it.
What can you do? As far as the water itself, drink only bottled or filtered water if there is any question about potability (drinkability) of the water. Consider taking your own filter or iodine to prepare you water for the day's ride. And consider a camelback type bladder so you can prepare and carry enough water for the entire days ride.
If you do get sick, work to maintain your hydration, and consider taking a day off the bicycle if it seems appropriate. Peptobismol is often a great help for controlling minor symptoms and shortening the duration of the diarrhea. If you are going outside the US, consider asking your physician for an antibiotic - Ciproflaxin or doxycycline - to use if you experience prolonged (greater than 3 days) or bloody diarrhea.
Before you set off on that tour outside the US, check the status of your immunizations (tetanus, polio, hepatitis A) as well as any infectious disease risks specific for the area you will be visiting (malaria, rabies, yellow fever to name a few) - the CDC has a great site. And of course, check your personal health insurance coverage to be certain that you collect the necessary information to be reimbursed should a major illness or accident occur on your trip.
OFF ROAD HEALTH HAZARDS
With mountain biking taking more people off road, there are the unusual (until recently)
infections of Lyme disease
and other tick borne illnesses to consider.
Lyme disease has been described in many states throughout the US - although the original descriptions focused on the northeast.The initial infection mimics the flu, but long term it can cause a debilitating arthritis. It is caused by a bacterial infection (Borellia burgdorfer) which is spread by the deer tick. The tick is found on bushes and in tall grass, and is more active in May and June, although they are active into the Fall as well. They are dormant in the winter (the safest time to ride in areas where the disease is common). There is some protection from wearing long sleeved tops and long pants, and DEET repellents may be of some help. But a post ride tick inspection by a buddy is a good idea as it generally takes 12 to 24 hours for the tick to attach itself. The ticks are small (pin head size) so look carefully. The first sign of the disease is a skin rash at the site where the tick was attached. Fortunately early treatment with antibiotics (tetracyclines) will cure the disease, so if you think you may have been exposed, see your doctor as there is a blood test available to determine if you have Lyme disease.
A vaccine for Lyme disease was released in 1999. It is NOT the ultimate answer as it gives only an estimated 80% protection after a three shot series given over 12 months (after the first two shots, given a month apart, there is 50% protection). Antibody (and thus protection) levels drop off quickly, so a booster will probably be needed every few years. Although there were concerns that the vaccine might make those who had already acquired Lyme disease worse, there was no evidence that this was the case during the vaccine's trial period. The most important thing to remember is that even if vaccinated with the full three shot series, you can still develop Lyme disease (as well as all the other tick borne diseases). So pay attention to any rash that might indicate infection and remember to use an insect repellent and check yourself for ticks after you've been out.
Two other relatively unknown diseases are also carried by the deer tick. Babesosis which has flu like symptoms has been reported in New England, Wisconsin, and Minnesota. Ehrlichiosis cause chills and fever (without a rash) and has been reported in the central and south Atlantic states. Both are cured by antibiotics and preventive measures are similar to Lyme disease - including the tick inspection post ride.
Rocky Mountain Spotted Fever (a bacterial infection) is characterized by both a fever and rash, as indicated by the name. It, too, is tick born and antibiotic (tetracyclines/erythromycin) sensitive. Originally described in the western US (Rocky Mountains), it is now known to be common on the east coast of the US as well, and North Carolina is one of the hotbeds of this bacterial infection in the US. But NC physcians see enough of this infection to be keyed in to the early and progessive symptoms. If the infection goes undiagnosed and untreated it can be fatal.
And the newest disease on the scene is West Nile Virus. Protection should be focused on using good mosquito inspection when in high risk areas.
Wildlife presents another personal risk that has increased as we invade more deeply into their territory. Mountain lion attacks on mountain bikers have been reported. Being aware of this danger is the first step in minimizing your risk, but also try not to ride alone and always stay on trails.
HEAT STRESS
Heat stress during exercise may cuse sudden death as late as the next day. In military
recruits there is an increase in exercise related deaths the morning after intense exercise
before the temperature rises. Heat, humidity, and direct sunlight seem to correlate with
the problem so be careful to stay hydrated and wear the correct clothing.
DENTAL DISEASE
A recent British study suggested that the use of sports drinks, which are both acidic and
high in carbohydrates, could increase the incidence of dental caries (cavities). This is
a particular problem as they are usually ingested slowly over a long period of time and
thus are similar to sucking on a piece of candy.
It makes sense to consider brushing after a ride just as it is suggested that we brush after meals. And carrying two water bottles, chasing your energy drink with a drink of plain water makes sense as well.
Although cardiovascular disease is mentioned most often, there are many other health benefits of regular exercise. Various long term studies have indicated a decrease in cancer rates - one analysis of exercise and breast cancer suggests that recreational physical activity alone decreases a woman's chance of developing the disease - as well as an improvement in mental health and a decrease in depression.
The bottom line? The overall health benefits clearly outweigh the risks of regular cycling.