Tens-of-thousands of runners are expected to take part in the gruelling 26-mile London Marathon this weekend. 

Here, experts reveal the extraordinary toll undertaking a marathon has on the body, before, during and after the race. 

They’ve also detailed some of the surprising health impacts that can leave runners shorter, more prone to infection and even at risk of life-threatening complications. 

Hour Zero: Runners hype themselves up at the starting line

Such is the impact of a marathon that people’s bodies actually being to exhibit changes even before the race itself. 

Physiotherapist Sammy Margo from muscle and joint care range Deep Heat and Deep Freeze explained that the anticipation of the race can have a physical effect. 

‘From a hormone perspective, adrenaline and cortisol start surging,’ she said. 

Adrenaline is part of the body’s ‘fight or flight’ response prepping muscles for action.

On the other hand, cortisol, also known as the ‘stress hormone’, helps the body unlock energy reserves. 

Ms Margo said the release of this due of hormones then triggers a number of other biological responses.  

‘Your heart rate increases in preparation as does your blood flow and your breathing rate starts going to get your oxygen levels up,’ she said. 

‘People have been building themselves up for this event which for some is a major life milestone for months if not years. It’s a big deal.’ 

Hour One:..and now they’re off

Now, with the race underway the real impact on the body starts. 

Firstly, to fuel the first hour of running the body starts to burn all of the readily available carbohydrates, called glycogen, which is stored in the muscles. 

Ms Morgan added that, after an immediate spike from the starting line, a runner’s heart rate will begin to settle to a still elevated but sustainable 60 to 80 per cent of its maximum.

All this activity will lead to a rise in body temperature, which in turn will lead to sweating in response.

Ms Morgan said, while the exact increase varies by person and the weather on the day, runners can see their body temperature rise by a couple of degrees. 

Personal Trainer Elyn Marwick, also of Deep Heat and Deep Freeze, added that within the first hour is when runners’ muscles first start to become damaged. 

But she added this wouldn’t cause any issues for most people, at least not at this stage. 

‘These are just microscopic tears in the muscles fibres, very normal from exercising,’ she said. 

‘They’re not going to impact you in that first hour, they’ll just start accumulating.’

Hour two: the race begins to take its toll 

For many runners this is when a bit of pain and soreness starts to kick in. 

Ms Marwick said it was inevitable that waste products from the muscles burning energy, like lactic acid, start to accumulate and may even cause cramps.

While the ‘when’ varies depending on the runner, this can lead to pain in the legs which are naturally the muscles working the hardest during the event.

Ms Marwick also said this is when a runner’s energy stores of glycogen start to ebb. 

‘These usually last 60 to 90 minutes when you’re running at a moderate heart rate,’ she said.

‘If you don’t start to fuel up a little bit with carbs, from sweets, gels or energy drinks, you’re going to run out of that glycogen and then out of energy.’ 

Sweat continues to build with Ms Marwick saying a runner may lose between two to four litres of water by the end of the race. 

Ms Margo added that, as glycogen reserves begin to dry up runner starts burning fat to keep going. 

She added that all this strain leads to start ramping up production of stress hormones to unlock more energy to keep a runner going. 

Hour 3: ‘The Wall’ 

Ms Margo said while some runners will now be approaching the end, this is when many participants hit ‘the wall’.

This references a sudden collapse in energy levels as the body starts primarily using fat for energy a more taxing process than using glycogen.

‘Your fat becomes your main fuel source, which is less efficient,’ she said.

All the other processes that started in hours one in two now take on a new dimension, with dehydration leading to electrolyte imbalances in the body and muscles damage becoming increasingly significant.  

Joint pain now joins the cascading issues a body has to contend with, as ankles and knees start to feel the impact of hours of continuous use. 

This is also when some of the more surprising impacts of a marathon start to take effect. 

A runner’s immune system can now become weakened, leaving them vulnerable to infection from viruses and bacteria, as the body contends with dehydration, stress hormones and lowered energy levels.    

Hour 4: Zombie-runners and health risks in the final stretch

Muscles damage has now reached a point where injury becomes increasingly likely and stress hormone levels begin to rise,

Ms Margo said this was when runners need to be aware of a particular danger which can occur during marathons.

Dehydration, and subsequent depletion of electrolytes like sodium, can leave runners at risk of hyponatremia.

Drinking two litres of water per day — around eight cups — is vital for keeping the body hydrated (left). But rapidly getting through this amount can be deadly, experts warn. Centers for Disease Control and Prevention bosses advise no more than 1.4 litres per hour, around six cups (right)

Also known as water intoxication, this where they drink too much, too quickly leading to brain tissue swelling to a dangerous size with potentially fatal consequences.

Sadly, fatally have occurred from this in the past with hyponatremia killing London Marathon runner David Rogers, 22, who died after consuming too much water during the 2007 event.

Now in the final stretch the body can be forced to call upon energy levels in the brain, which Ms Margo said could leave some people in zombie-like state as they near the finish line. 

‘People can look like zombies, and someone can come and pick them up to help them finish,’ she said. 

Post race: the surprising aftermath 

While the race is over the impact on the body can last for days.

Ms Margo said a runner’s immune system, their defence against infections, can remain weakened for days after the event. 

‘After finishing your immune system might be compromised for 24 to 72 hours,’ she said.

Similarly, she added that a runner’s glycogen stores may take up to two days to recover. 

This is also when, for many the pain from muscle damage from the race now peaks in the aftermath as the body goes out of fight or flight mode. 

To help combat this, Ms Marwick advised people to try and keep moving a little after the marathon and not just collapse on the ground. 

And in a final surprising quirk, runners may find themselves shorter than when they started. 

Scientists have found that the course of a marathon the vertebrae in the spine compress leaving athletes about 1cm shorter.

This naturally reverses about 24 hours after the event.