I think he said he didn't drink enough... or something like this...
according to DM Vergne said:
' "I was in hospital between the Grand Prix in Australia and Malaysia because of a lack of water and a little bit of lack of everything. I was very weak."
If was trying to reduce his weight by reducing water in his body (Sauna before the race or qualifying

)... one can only speculate, but this diet is not a diet ,but attempted suicide (if you combine it with starving)...
If you drink enough water there is no problem with a little bit of starving. Makes you grumpy as f*ck, but won't get you in the hospital.... especially if you are F1 with enough money to hire an expert nutritionist (but maybe his was/is a charlatan) .
Finishing races is important, but racing is more important.