A 19-year-old fitness fanatic died suddenly in his sleep after travelling to Scotland on business with his father, an inquest heard.

Rugby player Joshua Merrick, who had just completed an intense fitness medical to join the Royal Navy, had been boosting his workouts using a high caffeine energy drink. An inquest was told the apparently healthy teenager had used the drinks to help "improve workouts and optimise focus".

But although tests revealed he had an enlarged heart that could have been caused by excessive exercise, only a moderate dose of caffeine was found in his system and it is not thought the drink played a part in his death. His cause of death remains "unascertained".

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Mr Merrick was working with his father, Andrew, an electrical engineer, when they travelled to Glasgow and booked into the Holiday Inn in January last year.

He and his father took part in a weight-lifting session and the teenager showed no signs of unease or distress. After he said goodnight to his father and went to bed, he failed to appear for breakfast the following morning.

When he failed to answer his father's phone calls at 7.10am it was assumed he had slept in, but a colleague knocked on his father's door and told him he had found Mr Merrick dead in his bed.

Andrew Merrick, from Didsbury, Manchester told the inquest: "He saw fitness as a lifestyle. He had no general health problems."

Mr Merrick told the inquest in Manchester the pair tended to "push each other" in the gym, but his son, a flanker for the senior colts team at Burnage Rugby Club in Manchester, did not say he was struggling.

Mr Merrick added: "I was concerned about him drinking high-caffeine drinks as stimulants. There is very little, if any, regulation of these products from my investigations.

"I am not saying it is a cause, I'm saying it is contributory to people who have a disposition. People have extreme lows and get palpitations when they crash.

"It is new on the market and available to everybody and rarely gets a check."

Dr Gwen Ayers, who conducted the toxicology tests, found Mr Merrick had 5.8mg of caffeine per litre of blood in his system. Normally, people are found with between one and 10mg in their blood - with a lethal amount being in excess of 15mg.

She did not rule out the possibility that high-caffeine drinks could have contributed to Mr Merrick's death, but noted that the likelihood of their effect would depend on complications arising from his large heart.

The Scottish doctor who conducted an original post-mortem examination weighed Mr Merrick's heart at 490g. Anyone with a heart weight over 500g is at risk of 'dropping down dead', said Dr Stephen McGrath, a consultant histopathologist at Manchester Royal Infirmary.

He said Mr Merrick's heart was 420g when weighed for the second time after his body was repatriated from Glasgow.

He compared the weight to the heart of someone in their 50s or 60sm, or if someone had high blood pressure.

He added: "There are several reasons Josh could have had a big heart. Some people just develop it and nobody knows why.

"Some people have a disease that makes the heart work very hard, so it tends to get big. Also, sometimes when people do something to exercise their heart it responds excessively to that."

Manchester Coroner Nigel Meadows described the death as "one of those strange things that happens on occasion with no rhyme or reason to it".

Recording a verdict of death by natural causes, he said: "The doctors have not been able to explain an identifiable cause of death so the medical cause is unascertained.

"I agree that one explanation is a malfunction of the heart. The rhythm may have been disturbed. It is likely there has been some sort of cardiac arrhythmia. There will be conditions we do not know about at this time.

"On the issue of stimulant drinking, I agree that in some circumstances there may be something in it, but in this particular case I can't see it has played a part in his death.

"This could be described as a parent's worst nightmare. To have a fit young man suddenly die is awful. It is just one of those strange things that happens on occasion with no rhyme or reason to it. Sometimes it just happens."