Cooking

When cooking, the method that we use can enhance or leach the nutrients in foods. For the cooking of vegetables, light steaming is advised. This makes the food easier to digest than if eaten raw while minimising nutrient loss.

The cooking method you choose can have a great impact on the nutritional value of your food. Boiling results in water soluble nutrients, like the B vitamins, magnesium and potassium, leaching out in to the cooking water, as well as heat sensitive vitamins (like vitamin C) being destroyed. Over a third of the potassium and a quarter of the magnesium content can be lost this way; however, minerals are not destroyed by heat so some of the nutritional value can be salvaged by reusing cooking water in sauces or drinks.  

But as well as depleting nutrients, did you know that cooking can sometimes enhance nutritional value? Phytonutrients such as lycopene (in tomatoes) and beta carotene (in carrots) become more bioavailable when these foods are cooked. Steaming for a short period of time is a good choice as the nutrients are retained within the food while the steam gently helps to break down cellulose, making the foods easier to digest.

Cooking also affects the Glycaemic Index (GI) of many foods, and this is important for anyone trying to balance their blood sugar by following a low GI diet. For example, the GI of raw carrot is around 19, while boiled carrots can be up to 50. Baking potatoes in the oven for long periods of time increases the GI so much they become almost equivalent of sugar. This is because the cooking process enables the naturally present sugars to be release more quickly when the food is eaten.

Below are the main advantages and disadvantages of cooking:

Reasons for cooking

• Some foods must be cooked before eating e.g. pulses, grains
• Cooking kills bacteria – e.g. in meat
• Improves colour, flavour, smell and texture
• Smell of cooked food increases digestive secretions
• Cooking breaks down proteins and fats for easier digestion
• Fat content of meat decreases during roasting and grilling (fat drips out)
• Cooking softens rough fibre that can irritate the digestive tract
• Greater quantities of cooked veg can be eaten than raw – this may counteract any nutrient losses
• Cooking increases the availability of some phytonutrients (e.g. carotenoids and lycopene)

Reasons against cooking

• Heat damages proteins making them harder to digest
• Natural enzymes in foods (which aid digestion) are destroyed by heat
• Heating fats in the presence of oxygen forms damaging trans fats
• Cooking increases the glycaemic index of some carbohydrate foods
• Cooking meat at high temperatures (frying, grilling, BBQ) produces substances called HCAs which have been linked to cancer
• Nutrients are lost from vegetables into cooking water – particularly Magnesium, Potassium and the water soluble vitamins
• Heat destroys vitamin C – the greatest losses occur between 65-85ºC

Effect of cookware

The choice of cookware can also make a difference - copper and iron pots can add much needed additional minerals to a meal. But long term and frequent use could potentially lead to excess consumption of these minerals causing an imbalance in the ratio of nutrients in the body. For long term use, stainless steel pots are recommended.

Nutrient losses in peeling

We know that the peel of fruit and vegetable contains a high proportion of the food’s nutritional value, so removing it before cooking will have a significant effect. The table below shows nutrient losses from removing the peel of apples and potatoes: