'Tis the season to be marathon training. And falling ill. Like every other unathletic jogger building up their puny bodies for the rigours of a marathon, I was pretty chuffed when I trotted along the Thames and around Hyde Park for a whole two hours. Then I got sick.
After nearly two weeks out of action with flu, I returned to training. Building up with some decent runs, I hauled my way along the Norfolk coast for 18 miles into a punchy east wind. I was pretty pleased with myself, in an utterly knackered sort of way, but my smugness at being within touching distance of running a proper marathon soon vanished. Within three days, I had fallen ill again. Coughing appealingly down the phone line, I asked the experts whether exercise could really make you sick.
"It's a fascinating paradox - the fitter you are, the worse you are at fighting off illnesses," says Dr Ian Banks, president of the Men's Health Forum. "When you exercise very hard you increase the amount of steroids in your body. They are mainly steroid hormones and steroids are a very potent decreaser of the immune responses."
The role of these hormones, such as cortisol and adrenaline, in what is called post-exercise immune suppression is a well-documented phenomenon. Top athletes such as Paula Radcliffe have often been laid low by humble coughs, colds, flu and chest infections. Running seems to bring on these upper-respiratory-tract infections. As Banks puts it: "If you over-exercise, you create inflammation in the body, in the joints and the tendons. The body is reacting naturally - it produces its own steroids to reduce the amount of damage from excessive exercise."
Dr Mike Loosemore, a sports physician at the Olympic Medical Institute, says that research has discovered that a short window immediately after strenuous exercise is a particularly crucial time. That is when the body's weakened immune system makes it vulnerable to viral infections. "There does appear to be a reduction in your ability to resist infection for a couple of hours after a heavy training session," he says.
Look after your body in this period and you will stand a better chance of staying well. From now on, I'll be avoiding germ-rich environments such as buses or trains, and small children with runny noses during those crucial two hours of post-exercise high.
If you are training for a marathon and, like me, have spent two weeks not running because you have a nasty cough, you may need to rethink. Shorter, more intense runs, will still build up fitness without the strain on your immune system produced by epic runs (although every runner knows you still have to put in long stints at some point).
Logically enough, sports doctors counsel that you shouldn't train if you are ill. But how sick do they mean? Loosemore suggests that if you have a temperature, or if your resting heart rate is 10 beats per minute above your normal rate, you should not train. Running while fighting a virus risks worse conditions such as inflammation of the heart muscle caused by the spread of the virus. I always thought I mustn't run with any kind of cough but this need not be the case, according to Loosemore. "Listen to your body. If your body is saying stop, then stop."
Basic measures to prevent illness during your marathon training include getting a flu jab. You should also avoid training on busy roads or in the city where smoggy toxins in the atmosphere won't help.
Like every other form of simple exercise, running is ridiculously over-accessorised. I have always been sceptical about sports drinks and those showy energy sachets that one of my jogging friends swears by as a mid-run snack. They taste like over-sweetened jam, for a start. But even nutrition scientists such as Anna Denny, of the British Nutrition Foundation, recommends taking on extra rapidly absorbable carbs (30-60g per hour) during runs of more than one hour's duration.
In general, however, Denny says that ordinary people training for marathons shouldn't need fancy, expensive supplements. That old favourite, carbohydrate, has been shown in studies to be the best way to tackle post exercise immune suppression. Carbs break down to form glucose, stored as glycogen in the muscles and liver, which is the energy of choice for muscles. Denny suggests a diet full of starch - pasta, rice, bananas and bread - alongside high-protein, low-fat foods such as steamed fish or grilled chicken. But what about fruit and veg? I may have been nudging five-a-day but Denny isn't impressed. Five portions is recommended for an average adult; someone training for a marathon needs up to eight portions, she says.
When you eat is also crucial. If you are jogging for more than an hour, sports physicians say you should eat one to four grams of carbohydrate per kg of body weight during the six-hours before training, topped up with a high-carb snack 30 to 60 minutes before exercise (and those carbs during exercise). I tend not to feel hungry immediately after a long run but this is that crucial window again, so it's time to top up lost glycogen with more carbs.
Nutritionists say that vitamin A, vitamin C, zinc and selenium can be particularly important for boosting your immune system. Cashew nuts, brazils, shellfish - especially crab - and offal are good natural sources. Similarly, you can get extra zinc from red meat, fish and seeds such as sunflower or pumpkin. But you should not take more than 25mg of zinc in supplements, according to the Food Standards Agency, as it can hinder copper absorption, weakening the bones and leading to anaemia.
While sports scientists would never recommend overdosing on supplements, they do tend to advocate a more hi-tech approach. During periods of strenuous exercise, Loosemore says, it makes sense to take vitamin C and zinc. You can also buy a nasal spray from your chemist which acts as a chemical barrier to viruses in the nasal cavity. This, says Loosemore, could be just the thing to block out other people's sniffles during that key post-exercise window. If you are flying overseas to follow your dream of running in the New York or Boston marathons, a nasal spray could be handy for fending off all those nasty germs zipping around inside the aeroplane.
