Vegetables cooked in the microwave may lose ingredients that could help fight cancer.
Don't overcook your broccoli
Broccoli "zapped" in the microwave with a little water lost up to 97% of some of the antioxidant chemicals it contains, say Portuguese researchers.
By comparison, steamed broccoli lost 11% or fewer of its antioxidants.
Writing in the Journal of the Science of Food and Agriculture, experts said that many nutrients simply dissolved away during the cooking process.
The researchers examined the levels of three major antioxidant groups in broccoli after cooking.
These chemicals are thought to protect cells against damage which could in theory increase the change of cancerous changes.
Microwaved broccoli lost between 97% and 74% of the three compounds.
One antioxidant was not removed at all during steaming .
Dr Cristina Garcia-Viguera, from the University of Porto in Portugal, said: "Most of the bioactive compounds are water soluble - during heating they leach in a high percentage to the cooking water, reducing their nutritional benefits in the foodstuff.
"Because of this it is recommended to cook vegetables in the minimum amount of water in order to retain their nutritional benefits."
Separate research from Finnish researchers suggested many antioxidants are removed before the produce even left the supermarket shelf.
Blanching of vegetables - momentarily dipping them in boiling liquid - prior to freezing caused losses of up to a third of their antioxidant content.
Dr Anne Nugent, a nutrition scientist at the British Nutrition Foundation, said it was the presence of water rather than the cooking process in itself which was the problem.
She said: "It is not the microwaving per se that is causing the antioxidant loss but the presence of water, and boiling.
"In other words, the antioxidants would also be lost upon boiling rather than steaming.
"I think the important thing to take from this article is that when boiling or microwaving broccoli, it is important not to over-cook or over-boil it as this will result in excess antioxidant loss. "
Steaming is a good option - however, there will always be losses of vitamins and antioxidants during the storage and processing of all fruits and vegetables, so it is also important to store the broccoli in a dark airy cupboard rather than in direct sunlight as this will help protect the antioxidants."