It was during a routine trip to the supermarket in the immediate months after he suffered a serious brain injury in 2013 that David Jackson realised something wasn’t right.

That year, then aged just 31, he was forced to call time on his rugby career as Nottingham’s top try scorer and record appearance maker after suffering a severe head injury in training which led to a worrying seizure and a significant bleed on his brain. 

It was the end of his playing days but only the start of a number of serious side effects.

‘When your brain is trying to recover, it can’t deal with normal levels of stimulation,’ Jackson told Daily Mail Sport. 

‘I remember shopping for yoghurt after my injury, looking at the shelves, and being totally overwhelmed by the number of options. I just broke down in tears. I knew I shouldn’t be crying over yoghurt! But I just couldn’t control it. I’d just got married to my wife, Catherine. We were navigating my recovery process together. 

'The statistics show 48 per cent of couples break up when one of them has had a serious brain injury. I remember thinking to myself: “Am I ever going to have a normal life again?” When I left hospital, I was given a leaflet and told if my symptoms got worse, to come back. Catherine took me back and said: “This isn’t the guy I married.” Thankfully, we’re still together.’

More than a decade later, Jackson’s world is now light and not dark. 

He hopes his story can help other former players and any members of the public who are, or have been, in a similar situation. 

He is currently in the midst of a world record challenge, attempting to run an ultramarathon (a minimum of 50 kilometres) each day for 100 consecutive days to raise money and awareness for head injury charities Headway and Head For Change.

‘I want to use the recovery I’ve made to help others,’ Jackson said. ‘Every 90 seconds, someone goes to hospital in the United Kingdom with a brain injury.

‘I didn’t know about the help that was out there when I had mine. I don’t want anyone else to be in the same boat.’

The brain injury that ended Jackson’s career was suffered in innocuous fashion. 

He collided with a team-mate during a touch rugby training session with Nottingham and a few months later, an MRI confirmed a brain bleed. 

The expert medical advice was to stop playing immediately.

‘I just thought I’d be OK and I’d get back to normal like I had before,’ he said. ‘But this one (head injury) was different. The days turned into weeks and then months.

‘At the time, I wasn’t totally aware of what was going on, but I knew I was struggling. I had chronic fatigue. If I did anything physical, even walking for two minutes, I’d feel like I’d have to go to bed for a few hours. 

'If someone turned a light on in a room, I’d squint and get a headache. I couldn’t read a book. Regulating emotions and thinking rationally over decision making was super challenging because of my poor cognitive function.’

Jackson is far from being alone in reporting such symptoms. 

Hundreds of former players from rugby union, as well as league, remain locked in legal action with their game’s authorities over the effects of brain injuries they say are as a result of their careers.

The case action shows no sign of coming to any conclusion six years after it was launched, with claimants now seeking new legal representation.

Initially, Jackson – who had suffered multiple concussions prior to his training incident – had hoped to play on. It never happened.

He is one of the more fortunate ex-players however in that after time, his symptoms began to improve – even if his memory is still sometimes hazy today.

‘Running became a pivotal part of my recovery process,’ he said. ‘Eventually, my outlook started to focus on what I was going to do next.’

Slowly but surely, running allowed Jackson to get back to normal. 

He began to focus on breathing techniques to improve both health and performance, the subject becoming the focus of a book he authored. 

Jackson now works with elite athletes – including England and Saracens lock George Martin and Gloucester wing Ollie Thorley – to do exactly that. 

When we speak, he has just completed a 20-kilometre morning run in Cornwall on day 76 of his challenge which has taken him all over the United Kingdom. 

Television presenter Ben Shephard has been among the famous faces to join Jackson during his journey. 

‘I want to directly try and make a difference to those who have brain injuries,’ he said. ‘My body is getting used to doing this, but it’s not particularly happy! I’ve had over 700 people run with me now.

‘Most don’t know support is out there. Anyone can get a brain injury. It’s not just rugby players or sports stars. I went down a bit of a rabbit hole looking at how breathing can impact the nervous system, reduce inflammation in the brain and restore blood flow.

‘Improving my own breathing ironically helped me improve my own running. Rather than hating it – which I did at the start – I started to enjoy it.

‘On day 14 of the challenge I suffered a leg injury, but I’ve kept on limping round and lots of the people who have run with me have shared their own stories of overcoming challenges in life. They’ve been amazing to hear. 

'There was one guy this week who 23 years ago was run over in a motorbike crash. He went under a lorry. They thought he’d died.

‘They saved his life, but he was told he'd never walk again. He was in hospital for seven months but overcame that. 

'It’s those sorts of crazy stories that put things into perspective.’

Click here to support Dave Jackson’s fundraising for Headway and Head For Change.