Asked about playing at the highest altitude city in the United States, current Queensland Reds doctor Dr Macleod told Fairfax Media: "It's not what I would consider significantly high altitude and it is not the level of altitude where you would expect altitude sickness.
"I would say 1600 metres is above a level where you feel the effects in terms of performance, but Johannesburg is over 2000 metres and we have Test matches and Super Rugby games there, including travel, all the time.
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"I've spoken to medical staff in Denver – including paramedics who run the medical support at the stadium and work with the [NFL's Denver] Broncos – and one gentleman who has been there for 27 years as a paramedic said he's never seen a case of altitude-related illness from people playing at Mile High Stadium.
"This is NFL where guys are 350 pounds and 150 kilos and playing in full gear. I know it's a different sport, but it doesn't seem to happen for guys that have played there.
"You've got to remember Denver and Colorado has an Olympic training centre and people deliberately go there to train at altitude and deliberately go there to put themselves under physiological stress and challenge themselves to improve their performance. No one has any welfare concerns regarding that.
"I just can't see how 1600 metres is a concern because it's not proven [scientifically]. I've never seen anyone have health consequences because they've played at this sort of altitude."
Some NRL players, including the Dragons' Gareth Widdop and James Graham, would need to fulfil NRL commitments with St George Illawarra a little more than 48 hours after arriving back in Australia after the Test, which has been mooted for June 24.
I've never seen anyone have health consequences because they've played at this sort of altitude
Dr Greg Macleod
Both have given their backing for the match, which could be played in temperatures around the 30-degree mark in the northern hemisphere's early summer.
Dr Macleod argued NRL and Super Rugby pre-season and early regular season matches are often played in searing heat nudging the 40-degree mark, as evidenced when Manly brutalised Parramatta a little over a week ago at Lottoland.
That clash kicked off as the mercury nudged 39 degrees and didn't feature any mid-game drinks breaks.
"Denver has dry heat and a low humidity so that 30 degrees is a much more comfortable temperature than if it was combined with humidity," Dr Macleod said. "The heat thing is a non-issue and we have many examples of players playing in higher heat and more extreme conditions than that."
If the match gets the green light, New Zealand players will travel in business class to Denver to allow for better sleep patterns as well as using masks to help with hydration and reduce the chance of picking up viruses.
While conceding the heavy travel schedule could affect the performance of both New Zealand and England, Dr Macleod denied it would enhance the chances of any player picking up an injury – and would be no greater risk than if a NSW or Queensland representative turned out for his club 48 hours after a State of Origin match.
"The theory that you can't travel, but play a lot of games in a short space of time seems a bit odd," he said. "It is a long way and no one is denying that, but it's not the furthest a sports team has travelled.
"If you look at what the [Rugby] Sevens guys do – which is a far greater travel load than what we're looking at here – there was a study which followed players over a five-year period and it confirmed there is no increased significant injury risk for them as opposed to players who didn't travel that far.
"Travel does affect performance, but both teams are in the same boat."
Dr Macleod said he is yet to be consulted by anyone acting on behalf of the NRL about player welfare issues that may stem from the Denver match on June 24.
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Adam Pengilly is a Sports reporter for The Sydney Morning Herald.
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