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  Last updated: 1/20/2020

Infections, Immunity, and Cycling

Being in shape and remaining physically active are usually considered synonymous with staying healthy but too much bicycling has the potential for negative health impacts as well. Generally these are cycling specific musculoskeletal complaints, but as you will read below, can impact the immune system and your susceptibility to infection as well. A third risk is contact with uncommon (at least in the US) infections as you travel through varied ecosystems around the world.


There is little question that aerobic exercise for 20 to 30 minutes, four times a week improves cardiovascular health and decreases cardiac disease. How much is enough? One recent study of 1000 middle aged men followed for 11 years demonstrated that those who burned fewer than 800 Calories a week in leisure activity (equal to 20 miles of cycling) were 3 1/2 times more likely to have died of a heart problem than those men who expended 2100 Calories per week in aerobic exercise (approximately 50 cycling miles).

Although cardiovascular disease is mentioned most often, there are many other health benefits of regular exercise. A number of long term studies have shown a decrease in cancer rates - one analysis of exercise and breast cancer rates suggested that recreational level physical activity decreased a woman's chance of developing the disease. And finally there are many studies showing an improvement in mental health and a decrease in depression in regular exercisers.


The "immune system" refers a complex, interactive grouping of cells, hormones, and molecular modulators that defend the body from the invasion of foreign cells - whether it be infectious organisms such as viruses or bacteria, or abnormal cells such as cancer cells. Moderate aerobic activity is felt to have a beneficial effect (compared to a sedentary lifestyle) while overtraining and exercise to exhaustion (a marathon for example) are felt to be detrimental. The relationship between infection risk and exercise volumes is expressed as a J shaped curve. This risk is mainly respiratory and can be a particular problem in the fall and winter months when respiratory infections are more common as people congregate in enclosed areas at work and in school.

Decreases in the white blood cell count (and presumably immunity) can be detected after 2 hours of endurance exercise at 90% maximum exertion and will last for up to 8 hours. This interval is referred to as the "open window" (of increased susceptibility to infection) in the medical literature. The increased susceptibility to infection is even more pronounced and longer lasting (up to 2 weeks) after extreme endurance events such as a marathon. A study of upper respiratory tract infections (URTI) in athletes nicely supports this contention. Matthews et al demonstrated a 20% reduction in URIs in a group of regular, moderate exercisers. The group of marathoners mentioned above demonstrated a significant increase, a 13% incidence of URIs in the week following the event compared to 2% in the comparison group that enrolled but did not compete.

Why does extreme exercise have this negative effect? Speculation has focused on an elevation of stress hormones (cortisol and epinephrine), dehydration, or a change in the white blood cell count (these are the blood cells that are active in protecting from infection). Although significant changes in the level and functional activity of immune system parameters have been demonstrated in athletes, investigators have had little success in linking these changes directly to a higher incidence of infection and illness, have yet to demonstrate that athletes showing the most extreme post-exercise changes in immunity (immunosuppression) are those that are at increased risk for infections during the subsequent 1 to 2 weeks.

What can you do to avoid the negative effect of a vigorous (and perhaps overly enthusiastic) training program? Probably very little. In 1999 it was speculated that carbohydrate ingestion during exercise, which has been associated with an attenuated cortisol and growth hormone response, fewer perturbations in blood immune cell counts, lower granulocyte and a diminished pro- and anti-inflammatory cytokine response, might be beneficial. But a more recent study measured actual immune response rates and has refuted that assumption.

How about supplements? The benefits of zinc, vitamin C, and glutamine on the acute immune response to prolonged exercise have been measured in endurance athletes, but support for protection is inconclusive. Antioxidant supplements and ibuprofen, have likewise produced disappointing results. Zinc remains an "after the fact" option for early treatment (versus protection) of an upper respiratory illness or "cold" - the commercial lozenges contain zinc gluconate. The lozenges have been shown in a controlled study to reduce the duration of symptoms from 7.6 to 4.4 days.

It has been shown that decreased IgA levels in saliva are associated with increased URI rates. And that moderate exercise will increase IgA levels and decrease URI episodes. It is speculated that this is the reason mild activity decreases infection (the J shaped curve phenomena). The question is whether other modifiers of IgA such as Chlorella (Study 1 and Study 2) might moderate infection rates in athletes. The assumption is that anything that might attenuate the IgA antibody decrease would be beneficial. However, no clinical infection rate data when using Chlorella is available and this is all conjecture. Another natural option is nutritional yeast. Although there have been studies showing an increase in the level of the type of antibody that is found in the respiratory tract (IgA) with Chlorella and nutritional yeast, I could not find any clinical trials that documented an actual decrease in infection rates.

What would I recommend?


An interesting study on exercise and immunity. It found that "..125 long-distance cyclists, some now in their 80s, .....had the immune systems of 20-year-olds." And a commentary in the lay press.


With travel, to new areas within the United States as well as overseas, comes the potential for exposure to infectious diseases not normally encountered in your home state or town. And the incubation time (time from exposure to illness) may be long enough that you will not become ill until after you return home.

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.


Should you be concerned about continuing a training program during the fall and winter when viral infections are more common? Or you kids are coming home from school with coughs and runny noses?

There isn't much in the medical literature, at least human studies. But these two articles (article 1, article 2) with studies in animal models, suggest there a protective benefit if you happen to be exposed. Fewer symptoms on both a chronic exercise program, and acutely (if you happen to be exposed while doing your intervals on the spin bike at the gym).

But if you are already feeling the symptoms of a respiratory of flu illness - fever or achy muscles - it is better to take a short break. As Dr. Mirkin notes in his short blog on the topic, you won't lose significant conditioning with a short break.

And once the aches and fever have passed, you can feel comfortable getting back on your program.

RIDING IN THE WOODS - a few funny bugs

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 physicians see enough of this infection to be keyed in to the early and progressive 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.


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.

The bottom line? The overall health benefits clearly outweigh the risks of regular cycling.

Questions on content or suggestions to improve this page are appreciated.

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