You work hard. You eat well. You try to take care of yourself. So when you do finally come down with a cold or flu, could it be because you worked too hard? Or have all those tough workouts made you stronger, so you're only on your first cold of the season instead of your third?

Exercise can either be a friend or foe to your immune system. The truth is that exercise seems to have a "J-curve" effect on immunity. When a sedentary person starts exercising, their risk of getting sick drops. Moderate exercise is the most protective, but the more and harder you train, the more your risk of getting sick rises. Beyond a certain point, you're actually more likely to get sick than your buddy who sits on the couch all day.

Where is that certain point? Bad news: there's no clear way to find out. Researchers are still struggling to find it, and they suspect their troubles are because everybody is different (duh) and there are a ton of factors that can affect your immune system, including stress from other areas of your life. It's also a hard subject to study because not many athletes want to be infected with an experimental cold during their hardest training times. So a lot of research measures indirect effects, like measuring blood chemicals rather than counting the number of full-blown colds. Sometimes researchers skip people entirely and study mice instead.

What we do know is this: exercise temporarily lowers your immune defenses, but 24 hours after a workout, athletes' immune systems return to normal. The tougher the workout, the stronger the effect.

Is it overtraining?

Getting flu-like symptoms after lots of exercise is sometimes chalked up to overtraining, which can cause fatigue and decreased performance, among other things. Some sports scientists draw a line between true overtraining, a serious problem that can take months to recover from, and overreaching, its milder and more common cousin. Informally, people use "overtraining" to refer to both. Endurance athletes talk a lot about overtraining, but it's definitely possible in strength sports, too.

The connection between overtraining and the immune system is murky. One study in swimmers found that those who were overreaching were actually less likely to get sick. Sounds like a good thing? Not so fast. Even the regular hard-working athletes were already getting sick pretty often, and those that had the most colds spent less time in the pool, thereby protecting themselves against overreaching.

This may explain why endurance athletes who trained the most hours reported taking the fewest sick days. Rather than exercise keeping them healthy, it's likely that the athletes that got the fewest colds, for whatever reason, were able to get in the most training time.

"Most athletes should be able to train with high loads provided their program includes strategies devised to control the overall strain and stress," write a team of sports scientists on behalf of the International Society of Exercise Immunology.

How to avoid overtraining? They recommend the things you probably already know you should be doing:

● increase intensity gradually
● add variety to limit monotony and stress
● avoid excessively heavy training schedules
● keep a close watch for signs of overtraining (but elevated RHR is not one of them; lowered max HR is a better measure)
● pay extra attention to recovery and nutrition during the times you're most susceptible to overtraining, including heavy weeks and competitions.

How to not catch a cold

So you're being careful not to overtrain, but your schedule is still putting you at a higher risk of catching a cold than, say, your jazzercising aunt. How can you boost your immunity, especially at this time of year when the germs are flying?

Nutrition is important, but back away from the jugs of supplements: this mostly means eating some carbs during exercise (which helps immune function), and otherwise making sure to get plenty of calories, protein, vitamins and minerals--in other words, eating a balanced diet.

Bad news: Few supplements have any proven effects, no matter how big the hype.

Skip the Echinacea and megadose vitamin C (eat the occasional orange and you're covered in that department). If you really want to take something, quercetin may help if you're over 40, and probiotics containing Lactobacillus species have shown promise in several studies.

The boring truth about preventing colds is that the most effective strategies aren't fancy supplements or anything; it's just the same advice you skim over every time you hear it. Wash your hands, don't drink from your gym buddy's water bottle, take care of yourself in general with good sleep and food, and get your flu shot.

You may have heard that the shot isn't a good match for the flu this year, but it still cuts your chance of getting the flu in half. Imagine somebody sneezes on you and you can flip a coin: Heads and you'll spend a week in bed, tails and you get off cough-free. That's what this year's flu shot buys you: a chance to flip that coin instead of just going home with the germs. (And yes, you can still go get the shot: better late than never.)

Bottom line: Yes, hard workouts may be setting you up for more colds. But if your training program is ramping up, take care of yourself as best you can, and consider the occasional cough the price of success.