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  Latest update: 4/12/2022

Infections, Immunity, and Cycling.

Remaining physically active is considered synonymous with staying healthy but too much bicycling may have negative health impacts. Generally these are musculoskeletal complaints but may include impacts on the immune system with an increased susceptibility to infection as well.

An incidental risk is increasing your contact with uncommon infections as you travel through varied ecosystems around the world.


There is no question that 20 to 30 minutes of aerobic exercise four times a week improves cardiovascular health and decreases cardiac disease. A recent study of 1000 middle aged men followed for 11 years showed 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).

In addition to CV disease, there are many additional health benefits of regular exercise. As one example, a number of long term studies have revealed a decrease in cancer rates. One analysis of breast cancer rates suggested recreational level physical activity decreased a woman's chance of developing the disease. And many studies show an improvement in mental health and a decrease in depression in regular exercisers.


The "immune system" is a complex, interaction of cells, hormones, and molecular modulators that defend the body from the invasion of foreign cells - infectious organisms such as viruses or bacteria as well as abnormal cancerous cells.

Moderate aerobic activity has a beneficial effect on the immune system while overtraining and exercise to exhaustion (running a marathon as an example) are thought to be detrimental. The relationship between infection risk and exercise volumes is a J shaped curve. The increased risk is mainly respiratory and a particular problem in the fall and winter months when respiratory infections are more common as people congregate in enclosed areasat work and in school.

A decrease 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 an "open window" of increased susceptibility to infection.

This increased susceptibility to infection is more pronounced and longer lasting (up to 2 weeks) after extreme endurance events such as a marathon. A study of upper respiratory tract infections in athletes nicely supports this contention. While Matthews et al demonstrated a 20% reduction in upper respiratory infections (URIs) in a group of regular, moderate exercisers, the group of marathoners mentioned above demonstrated a 13% incidence in the week following their event compared to a baseline rate of 2% in a comparison group that was enrolled but did not compete.

Why does extreme exercise have a negative effect? Speculation includes an elevation of stress hormones (cortisol and epinephrine), dehydration, and a change in the white blood cell count (which 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 directly linking these changes to the higher incidence of infection and illness in the 1 to 2 weeks post event.

What can you do to mitigate the potential negative effects of a vigorous training program? Probably very little. In 1999 it was speculated that carbohydrate ingestion during exercise, might be beneficial. But a more recent study measuring actual immune response rates has refuted that assumption.

Maybe supplements? The benefits of zinc, vitamin C, and glutamine on the immune response to prolonged exercise have been measured in endurance athletes, but support for protection is inconclusive. Antioxidant supplements as well as ibuprofen have likewise produced inconclusive results. Zinc (in the form of commercial lozenges containing zinc gluconate) ,remain an "after the fact" option for early treatment of (not protection from) upper respiratory illnesses. In a controlled study, the lozenges did reduce the duration of symptoms from 7.6 to 4.4 days.

Decreased salivary IgA levels are associated with increased URI rates. And moderate exercise increases IgA levels. It is easy to assume that this is the reason mild activity decreases infection (the J shaped curve phenomena).

Could other modifiers of IgA such as Chlorella (Study 1 and Study 2) moderate infection rates in athletes? No clinical infection rate data using Chlorella is available. Or another natural option, nutritional yeast? Although studies have shown an increase in the level of IgA antibody in the respiratory tract (IgA) with both Chlorella and nutritional yeast, there are no clinical trials documenting an actual decrease in infection rates.

What would I suggest?

A nice set of references on the effects of exercise (and over exercise) on immunity.


This interesting study on exercise and immunity found that "..125 long-distance cyclists, some now in their 80s, .....had the immune systems of 20-year-olds."

As nicely summarized by Dr. Mirkin "T-cells recognize foreign proteins on the surface of invading germs and cancers to tell your immunity to attack and kill these cells. They then stimulate your immune system to make antibodies to attach to and kill invading germs and cancer cells, and produce chemicals called cytokines that activate other T-cells to remove germs and cancer cells from your body. Other regulatory T-cells dampen down your immunity so that your immunity does not attack and destroy your own healthy cells."

The study suggests that this weakening of the aging immune system is from a significant reduction in thymic activity and the production of T cells. This senescence is blunted in regular cyclists, and to quote: "....many features of immunesenescence may be driven by reduced physical activity with age."

The COVID pandemic of 2020 - 2022 provided solid evidence supporting the immune strengthening benefits of regular exercise. This study of 65,000 adults with confirmed COVID showed that 150 minutes of vigorous exercise a week provided significant protection from severe disease with a:


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 a protective benefit if you are exposed.

But if you are already feeling the symptoms of a respiratory of flu illness - fever, achy muscles - it is better to take a short break.You won't lose significant conditioning with a short break. Then once the aches and fever have passed, you can readily return to your program.


In an active COVID infection, too little immune protection allows the virus to get a head start, duplicating relatively unrestrained. Then, once the immune system does kicks in, there is an over response with a cytokine storm in week two of the infection with a cytokine storm.

Although speculative, it appears obesity (a known risk factor for severe COVID) and a lack of physical conditioning share a common pathway leading to a more difficult clinical course - a state of chronic inflammation which hinders a robust immune response.

In this COVID specific study 150 minutes of vigorous exercise a week provided significant protection from severe disease with a:

And a Kaiser study where inactivity (less than 10 minutes of activity per week) was associated with a 91% increase in hospitalization and a 291% increase in mortality.

As Dr. Mirkin notes, a lack of physical conditioning also increases the odds of chronic inflammation.

If chronic inflammation is a risk factor, you should also consider the impact of your diet. There are multiple articles connecting a state of chronic inflammation with diet. Take aways?

With this pandemic, what are the risks of riding outdoors alone or with friends? This article suggests they are quite low. Only 2 out of 1200 cases.

Risk is related to prolonged exposure to a high viral load. Bars where loud talking increases aerosol formation and the air recirculates in a closed environment. Other indoor venues with prolonged close contact - homes especially where a single member brings it home and everyone gets it. Unless you are talking face to face to someone who is less than 6 feet away for an extended time (I'd say 15 minutes or more) the outdoor risks must be quite minimal, especially in a pace line where air circulation clearing any virus is ongoing and at a high level. This is supported by this study from early in the pandemic with only 2 of 1200 cases felt to be from outdoor transmission. Stopping for a break and talking in a group or congregating in the parking lot before or after the ride are higher risk. That post ride beer? If your group is sitting on the patio discussing the high points of the day, less than being indoors, but still above that of a pace line. So I see minimal risk riding alone and very little increase if you opt to ride in a group as long as you minimize the group camaraderie before and after the ride.

What if you get COVID? How do you return to your training and riding? With over 6 million cases recorded in Australia & NZ in the first 4 months of 2022, and few reports of serious adverse outcomes with exercise, this review suggests a return to exercise is a personal decision with no data one way or the other. For individuals with few or no symptoms, it seems reasonable to return to the prior level of exercise in a graduated fashion over 7 - 14 days.

RIDING IN THE WOODS - 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.

All questions and suggestions are appreciated and will be answered.

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