"The weight difference between myself and my teammate was making me lose four tenths (per lap)," Vergne, who was given a rare third consecutive season with the Red Bull junior team this year, told French media.
"I did a diet this winter but you get to certain limits that the body can no longer take.
"Actually, I was in hospital between the grands prix in Australia and Malaysia because of a lack of water and a little bit of lack of everything. I was very weak," the 23-year-old revealed.
Vergne said that while the minimum car-plus weight limit was raised by the FIA for the new turbo V6 rules, the 692kg figure is still far too low.
"Frankly, this (situation) is stupid," he insisted.
"Formula one cars are very difficult to drive and we need all of our skills. Being forced to lose weight is not good."
Vergne said the matter has been discussed by the drivers several times already in 2014, "but we have not reached a solution.
"Some lighter drivers want to keep their edge," he said.