It’s a good question and the answer is: it depends on the vegetable.
The way you cook your foods can certainly have a significant effect on the final nutrient content of your meal. Overcooking can lead to a loss of many vitamins but at the same time there are some nutrients which are improved by cooking. For example, cooking carrots has been found to increase the availability of beta-carotene, the antioxidant that gives them their orange colour.
Steaming is one of the best ways to cook most vegetables as it requires a short cooking time and little water. Longer cooking times, boiling in a lot of water and higher temperatures can all lead to loss of nutrients, particularly vitamins B and C.
And while you might think it’s better not to cook your vegetables in fat, this isn’t always the case. For example, research has shown that the lycopene in tomatoes (an important antioxidant that gives them their red colour and which may protect against diseases such as heart disease and cancer) is best absorbed into our bodies when cooked together with a little fat, such as olive oil. This means that all your favourite Italian dishes like tomato-based pasta and ratatouille are a great way to enjoy the health benefits of tomatoes.
To optimise nutrition, include a variety of raw (such as salads) and lightly cooked (such as steamed or stir-fried) vegetables. Vegetable or herb-based sauces and marinades (such as salsa and pesto) are also a great way to build more nutrition into your meals.
Finally, don’t forget to choose a variety of different vegetables and salads each week, aiming to make your plate as colourful as possible.