Are some foods more nutritious when cooked?

Eating more plant food is a nutritionally sound idea but there is growing trend for taking this one step further by including more raw food in the diet.

But is it healthy to eat too much raw plant food? Scientists have shed a light on this tricky issue and identified nine common foods that are more nutritious cooked than raw.

What does cooking do to vegetables?

Vegetables are made up of cells and fibrous cell walls. In vegetables, vital nutrients are sometimes trapped within the walls of these cells. Cooking helps to break down cell walls, deactivate enzymes and release nutrients.

Long cooking times in boiling water will cause water soluble vitamins (Vitamin C and B group vitamins) to be lost from vegetables so it is best to avoid soaking vegetables in water and use a minimum amount of water when cooking.

Steaming or roasting vegetables can help to preserve some nutrients. If you have cooking water left over, use it in soup, stock, or gravy as it holds the leached nutrients.

Here are some examples of foods that are more nutritious cooked than raw

1. Tomatoes

Tomatoes contain a red pigment called lycopene. Lycopene is an antioxidant which helps to prevent cell damage and is associated with a lower risk of a range of chronic diseases including heart disease and cancer. Cooking tomatoes breaks down their thick cell walls and increases the lycopene content by 50% within 30 minutes of cooking.

2. Spinach

Spinach is packed with nutrients, including folic acid, iron, magnesium, calcium and zinc. However, these nutrients are not easy for the body to use if spinach is eaten raw. This is because the green leaves of spinach contain oxalic acid which blocks the absorption of iron and calcium. When spinach is cooked oxalic acid is deactivated and the iron and calcium in spinach is released.

Steaming spinach also helps to preserve its folic acid content. Folic acid helps to prevent heart disease and is vital during pregnancy and foetal development.

3. Kale

Kale is a good source of iodine needed for a healthy thyroid gland which helps to regulate metabolism. However raw kale also contains enzymes that prevent the body from using the iodine. When kale is cooked, the enzymes are deactivated making the iodine more available to the body.

4. Mushrooms

Mushrooms are the major source of an antioxidant ergothioneine, which is released during cooking. Oats also contain smaller amounts of ergothioneine. Antioxidants help break down “free radicals”, chemicals that can damage our cells, causing illness and ageing.

5. Carrots

Carrots contain an orange pigment called beta-carotene which the body converts into vitamin A. This fat-soluble vitamin supports bone growth, eyesight and the immune system. Cooked carrots contain more beta-carotene than raw carrots. Also the beta carotene is more easily absorbed from cooked rather than raw carrots. A useful tip is to cook carrots with the skins on as this can double their overall antioxidant power. Carrots are more nutritious if boiled whole before slicing as the skin stops nutrients escaping into the cooking water.

6. Asparagus

Cooking asparagus breaks down its cell walls, making vitamins A, C, E and Folic acid more available to be absorbed.

7. Red peppers

Red peppers are a great source of immune-system-boosting antioxidants, especially the carotenoids, beta-carotene, beta-cryptoxanthin and lutein. Heat breaks down the cell walls, making the carotenoids easier for your body to absorb.

8. Broccoli, cauliflower and brussels sprouts

These vegetables are high in glucosinolates which the body can convert into a range of cancer-fighting compounds. Steaming these vegetables preserves both the vitamin C and makes the cancer fighting glucosinolates available for the body to use. Chopping broccoli and letting it sit for at least 40 minutes before cooking helps to maximise the availability of the glucosinolates.

9. Green beans

Green beans have higher levels of antioxidants when they are microwaved or baked in dishes compared to being boiled. Water is not a cook's best friend when it comes to preparing vegetables and maintaining their nutritional value.