US biohacker Bryan Johnson has been roasted after claiming Australians have older-looking skin because of the sun and that it had aged his by five per cent.

Mr Johnson, 48, travelled from Los Angeles to Brisbane last month to meet the family of his partner, Kate Tolo, 30, for the first time.

The California tech entrepreneur, who spends millions of dollars each year trying to slow the ageing process, provided commentary on how the trip had forced him to abandon his strict regime and how it played havoc on his health markers.

He noted changes in his diet greatly affected blood glucose levels, the time zone shift 'shocked' his circadian rhythm, and a lack of sleep 'weakened' his 'prefrontal cortex's ability to override the urge' to ignore 'emergency hunger signals' in his brain.

Mr Johnson continued to provide updates on how his health appeared to change, this time taking aim at the strength of the sun Down Under, saying it had aged his skin despite only spending one week over here.

'Australians have older looking skin,' he wrote on X.

'The Australian sun increased my skin ageing by 5 per cent (via UV damage and spots).'

Mr Johnson referred to a 2017 Monash University study that claimed Australian women aged 10 to 20 years quicker than US women. 

'Australians get more intense UV radiation,' he wrote.

'For example, during summer, the Earth's orbit puts the Southern Hemisphere closer to the sun, making the sun stronger than in most of the US.

'Even outside of Southern Hemisphere summer, Australia is almost a "perfect storm" for UV exposure: thinner ozone overhead makes it stronger, cleaner air allows UV to penetrate deeper.'

Mr Johnson concluded the sun 'is great' and that it was important to strike a balance.

'You want the right amount. Not too much and not too little.'

Several Aussies were quick to roast Mr Johnson, saying they preferred the sunlight in Australia compared to America.

'Dude, when I go to America you all look translucent and unhealthy, some weird gooiness,' one wrote.

'We are used to it down here, Australia easily has the best looking adult males on earth,' another said.

'And we are happier and healthier and have better work/life balance than almost anyone. Wouldn't swap it for translucent baby skin,' a third added.

Others agreed with Mr Johnson, saying the sun could be very damaging to the skin if people were not careful.

'I recently had white skin cancer removed from my ear after growing up in NZ. I remember sometimes getting badly burnt as a kid. Thanks for sharing the message,' one wrote.

'My Australian boss looks like a 70-year-old. He is in his forties,' another said.

Mr Johnson previously shared a photo of himself cowering under an umbrella from the Australian sun. 

The biohacker and Ms Tolo, who have an 18-year age gap, have been together for more than three years after meeting when Ms Tolo joined Mr Johnson's Blueprint anti-ageing venture as chief marketing officer. 

Mr Johnson's rigorous biohacking regimen has radically transformed his appearance.

The trip came weeks after Mr Johnson posted a photo of Ms Tolo undergoing an internal ultrasound while he stood at the foot of the hospital bed watching the examination.

The photo showed Ms Tolo being examined for suspected endometriosis – an incurable chronic condition Mr Johnson has vowed to conduct more research into.

Mr Johnson has previously revealed Ms Tolo has one of the 'top one per cent of vaginas' after sharing details of her vaginal microbiome report online.

'The lab found nothing bad to report (no gardnerella, Candida, STIs, opportunistic pathogens, aerobic vaginitis markers). This is linked to lower risk of (bacterial vaginosis), UTIs, yeast infections, HPV persistence ... preterm birth and improved IVF outcomes,' he said.

'A vaginal microbiome is downstream of everything: sleep, glucose control, stress, gut health, sexual health, immune function, what you eat, and what you put in it.'

The couple publicly confirmed their relationship in December through a lengthy social media post and YouTube video.

Mr Johnson said they had known each other for five years but chose to keep their romance private while determining whether the relationship would last.

'We needed time to stabilise, mature, and assess whether this was short or long term. It took time to bridge our worlds,' he said.

Mr Johnson is worth an estimated $300million to $400million.

He amassed most of his fortune after founding the mobile and digital payments company Braintree, which was sold to PayPal for $800million in 2013.