Toro Rosso's Carlos Sainz fears he and some other Formula One drivers have had to lose so much weight before the season starts, to compensate for heavier cars, that they risk compromising their health.

"How do you tell a driver to perform at his highest level while taking into account that he needs to lose three or four kilos?," the 21-year-old Spaniard told Reuters at the first test of the year.

"It's one thing against the other. You turn up to a test wanting to be as strong as possible but you cannot be as strong as possible because of weight (loss)," added the driver who did a massive total of 161 laps on Wednesday.

Drivers from three of the 11 teams have said they needed to lose weight before the opening race in Australia on March 20.

Australia's Daniel Ricciardo from Red Bull has shed three kilos, and says more must go, while Renault's Kevin Magnussen and Sainz's 18-year-old team-mate Max Verstappen have been told to lose a few.

Toro Rosso and Renault have changed engine supplier since last year, with the decisions coming very late in the day and causing design headaches, but Sainz saw other reasons too.

"There are new parts, more aerodynamic parts in the car which means more carbon fibre. A longer wheelbase so the car is a bit longer which makes it a bit heavier," he said.

Cars have more cockpit protection and also an extra exhaust pipe this year.

All teams aim for the lightest car and driver combination, with ballast used to bring it up to the minimum weight. This can be moved around, improving performance.

The heavier the driver, the less ballast available.

Sainz said his athletic weight was around 70kg and he had been asked to lose three or four kilos.

"It's a bit drastic," he said. "Maybe we (drivers) have to discuss it. I heard really drastic things from Jean-Eric Vergne, what he had to do.

"It's not safe and not the right way to go. We like to be fit and thin, that's our job, but it's not our job to be extremely skinny."

Former Toro Rosso driver Vergne was so weak he was taken to hospital after the 2014 Australian Grand Prix after dieting to reduce the advantage enjoyed by much lighter Russian team-mate Daniil Kvyat.

Minimum car weights were increased for 2015.

Kvyat, now with Ricciardo at Red Bull, told Reuters he did not have to lose any weight. Other teams, such as Williams, said it was not an issue.

Verstappen said he had lost two kilos.

"I'm still growing in terms of strength because I'm only 18. Then you do the training and at the end muscle is weight. So it all gets a bit more tricky," the Dutch driver said.

"I am 68 or 69kg now but it's quite hard if you are 1.80 metres. And in general the whole family is quite strongly built."

Reuters