On April 24, thousands will take to the streets of the capital to run the London Marathon. And whether you’re a professional athlete, a seasoned long-distance runner or a first-timer (good luck), running 26.2 miles is a lot for anyone’s body to go through.

But what impact is running that sort of distance actually having? We spoke to GP Dr Paul Stillman to get the lowdown.

The heart and circulatory system

3D reconstruction of the heart
(Bill Johnson/AP)

“During a long-distance run a subconscious part of the brain opens up the arteries that fill your muscles and closes down other parts of the blood stream that aren’t being used as much at the time,” he says.

“For some runners anaemia happens. Repetitive pounding of the feet on the ground and an increase in muscle tone to the legs, arms and hands can cause damage to red blood cells to to point where toxins can get into the blood, or red blood cell count is reduced.

“When you look at the heart after a marathon, both for professional athletes and anyone doing it for the first time, there can cardiac arrhythmias – when the heart beats too fast. Of course the heart is a muscle, and if you exercise the heart muscle, starting slowly and building it up, it will get stronger.”

The digestive system

“What tends to happen is to do with glycogen. Sugars are an essential part of the human body’s metabolism, but during a marathon sugar levels will be depleted. Most people anticipate this with training and taking on carbohydrates before and during the run. But having carbs while running does not appear to be as effective – as it’s difficult to metabolise carbs. The blood is too busy being diverted to elsewhere while you’re running.

“Taking on sugar – like a sugary drink – while you’re running, may not be as effective as it world if you’re at rest. So it’s worth taking low soluble sugars specifically for athletes.”

The skeletal system

Joint pain
(Thinkstock)

“If you run 26 miles, I read that it’s somewhere between 30,000 – 50,000 steps and it’s all hard running (i.e. on roads). There’s a reaction force, so just standing on one leg puts stress on the leg, multiply that by inherent body weight and the fact you’re putting on leg in front of the other 30,000 + times – that’s an enormous strain through the skeletal system.

“Even if you’re prepared, it can have an effect in later years on runners. Amateurs who are less prepared are much more likely to do acute damage to the joints and the musculoskeletal system. The more prep you can do, the better. Warming down and stretching afterwards are also very important to help this part of the body recover,” Dr Stillman explains.

“Even with the best preparation, injuries can persist, or old ones reactivate. A massage can do wonders for aching muscles. If pains persist you should consider specialist treatments such as AposTherapy, proven to help problems of knees and other lower limb joints, and even arrest the progression of arthritis.”

Sodium levels

Salt
(Anthony Devlin/PA)

“Everyone recognises the need for hydration, but how much water people should take on varies from individual to individual and the distance you’re running. But dehydration can become a real problem, even with people who run a lot of marathons. At the end of the marathon runners may have very low blood level of a certain substance, sodium. There have been reports of sudden unexpected deaths because of this. It’s important to make sure you drink enough but not too much.”

Hormones

“The level of cortisol (a steroid hormone) increases during a long run. It can actually be a problem for professional athletes, where they might have been accused of taking artificial stimulants but they say it’s physiological, because their cortisol levels are just high.

“High levels of cortisol can cut down the blood supply to the kidneys too, because again, the blood is busy doing other things. So some runners suffer from kidney problems.”

The immune system

Sore muscles
(Thinkstock)

“What tends to happen is microscopic damage to the muscles as a result of stiffness. But there’s also a release of chemicals from injured areas to promote the immune system and respond with white blood cells. Not unlike what happens during an infection. So running as far as a marathon, can temporarily disable the immune system. Immediately after you may well be more susceptible to infection,” Dr Stillman says.