Scientists found "boiled before cut" carrots contained 25% more of the anti-cancer compound falcarinol than those chopped up first.
Lead researcher Dr Kirsten Brandt, from Newcastle University's School of Agriculture, Food and Rural Development, said: "Chopping up your carrots increases the surface area so more of the nutrients leach out into the water while they are cooked.
"By keeping them whole and chopping them up afterwards you are locking in nutrients and the taste."
The Newcastle scientist, along with colleagues at the University of Denmark, discovered the health benefits of falcarinol in carrots four years ago.
Rats fed on a diet containing carrots or falcarinol were found to be one-third less likely to develop full-scale tumours than those in the control group.
Since then the scientists in Newcastle have been studying what happens when carrots are chopped and cooked.
The latest findings show that when carrots are heated, the heat also softens the cell walls, allowing water-soluble compounds such as sugar and vitamin C to be lost , leading to the leaching out of other compounds such as falcarinol.
If the carrot is cut before being boiled, the surface area becomes much greater--and so the loss of nutrients is increased.
Dr Brandt added that in blind taste studies the whole carrots also tasted much better.
Eight of ten people favoured the whole vegetables over those that were pre-chopped.
Dr Brandt said: "The great thing about this is it's a simple way for people to increase their uptake of a compound we know is good for you. All you need is a bigger saucepan."