How to Cook Vegetables to Preserve Their Nutrients

Katrin Peo
3 hours ago
8 min read

We often hear that we should eat more vegetables, fruits and berries. But what happens to their nutrients once we bring them home, wash them, chop them and cook them?

The nutritional value of food is influenced long before it reaches our kitchen. The variety of the plant, soil and growing conditions, ripeness at harvest, transportation, storage time and temperature can all influence its nutrient content. By the time we buy a carrot, head of broccoli or bag of spinach from the supermarket, we have relatively little control over how much of its original vitamin content remains.

If we grow vegetables, fruits and berries ourselves, buy them locally in season, or receive freshly harvested produce from family or a local farmer, we may have more control over freshness and storage.

But wherever our food comes from, there is one part of the journey we can influence: what happens to it in our own kitchen.

Hod different cooking methods affect nutrients in foods?

Vitamins are more fragile than minerals

Vitamins and minerals behave differently during food preparation.

Vitamins can be affected by heat, water, oxygen, light, pH, storage time and other components within the food. Water-soluble vitamins, particularly vitamin C and some B vitamins such as folate and thiamine, tend to be among the most vulnerable during cooking.

Minerals such as calcium, iron, magnesium and potassium are different. Heat does not destroy minerals in the way it can degrade some vitamins. However, minerals can move from the food into cooking water. This means that when vegetables are boiled and the water is discarded, some minerals—and water-soluble vitamins—may be lost with it.

Research comparing different home cooking methods generally finds that prolonged cooking in larger amounts of water tends to cause greater losses of water-soluble nutrients. Steaming and microwaving, which usually involve less water and shorter cooking times, often preserve them better.

This doesn't mean that boiling is a “bad” cooking method. If you are making a soup, stew, curry or sauce and consume the cooking liquid, many of the nutrients that have moved into the water are still part of your meal.

Raw isn't always more nutritious

It is tempting to assume that raw vegetables are always nutritionally superior because cooking can destroy vitamins.

But nutrition is more complicated than simply measuring how much of a nutrient is present in a food.

A nutrient first has to be released from the food's structure during digestion before our body can absorb and use it. This is sometimes referred to as its bioaccessibility and bioavailability.

Plants have strong cellular structures. Cooking, chopping, crushing and puréeing can soften or break down these structures and help release nutrients that would otherwise remain partly trapped inside the plant matrix.

Therefore, cooking may reduce the amount of one nutrient while simultaneously making another more accessible to us.

A good example is the carrot.

Carrots: cooking can improve access to beta-carotene

Carrots are rich in beta-carotene, a carotenoid that our body can convert into vitamin A.

Human research has shown that beta-carotene can be absorbed more effectively from cooked and puréed carrots than from raw chopped carrots. Cooking softens the plant tissue, while puréeing further disrupts the cellular structure, making the carotenoids easier to release during digestion.

Because carotenoids are fat-soluble, eating carrots with some fat—such as extra-virgin olive oil, nuts, seeds or avocado—can also help their absorption.

So a raw carrot is certainly nutritious, but so are roasted carrots drizzled with olive oil or carrots cooked into a vegetable soup.

Tomatoes: cooking helps us access lycopene

Tomatoes provide another excellent example.

Their red colour comes largely from lycopene, a carotenoid found in tomatoes, watermelon, pink grapefruit and several other red or pink fruits.

The lycopene in raw tomatoes is relatively tightly held within the plant matrix. Chopping, crushing, puréeing and heating tomatoes can disrupt that structure and make lycopene more available for absorption.

Human studies have found greater lycopene uptake from processed or heat-treated tomatoes than from raw tomatoes. Adding a source of fat can help further: research has shown increased blood lycopene concentrations after people consumed tomatoes cooked with olive oil.

This makes tomato sauce cooked with extra-virgin olive oil a good example of a traditional preparation method that can actually increase the bioavailability of certain beneficial plant compounds.

Dark green vegetables: preparation changes the food matrix

Dark green vegetables such as spinach and kale provide carotenoids including beta-carotene and lutein.

Breaking down the plant structure through chopping, blending or cooking can influence how accessible these compounds are during digestion. However, the effect varies according to the vegetable, carotenoid and cooking method, and longer heating is not necessarily better.

This is another reason why variety is useful: eat leafy greens in different forms—raw in salads, blended into pesto or smoothies, and lightly cooked in soups, omelettes and stir-fries.

Broccoli and other cruciferous vegetables

Broccoli, cabbage, Brussels sprouts, kale and cauliflower contain vitamin C and compounds called glucosinolates, from which biologically active compounds such as isothiocyanates can be formed.

These vegetables are a good example of why cooking time matters.

Research comparing cooking methods has found that steaming broccoli can preserve vitamin C and glucosinolates better than prolonged boiling or stir-frying followed by boiling. In one study comparing five cooking methods, steaming resulted in the smallest overall losses of several health-promoting compounds in broccoli.

That doesn't mean broccoli always needs to be steamed. Rather, avoid cooking it until it is extremely soft and colourless. Lightly steamed, quickly stir-fried, microwaved or eaten raw are all useful ways of including cruciferous vegetables in the diet.

Sometimes losing something from a vegetable can be beneficial

Interestingly, cooking doesn't only change nutrients. It can also reduce certain naturally occurring plant compounds.

For example, spinach, beet greens and rhubarb contain oxalates, which can bind minerals such as calcium. Research has shown that boiling can substantially reduce soluble oxalate levels because oxalates move into the cooking water.

This illustrates an important point: there is no single cooking method that is nutritionally superior in every situation.

Sometimes we want to minimise nutrient loss. At other times cooking helps us digest a food, access particular nutrients, improve flavour and texture, or reduce compounds we may not want in large amounts.

So which cooking methods preserve nutrients best?

As a general rule, think about three things:

water + temperature + time.

The longer a vegetable is exposed to high temperatures and large amounts of water, the greater the opportunity for heat-sensitive nutrients to degrade or water-soluble nutrients to leach into the cooking liquid.

Steaming is one of the best all-round methods for vegetables. Because the food isn't submerged in water, there is less opportunity for water-soluble vitamins and minerals to leach away.

Microwaving can also preserve nutrients very well. Despite its sometimes poor reputation, microwave cooking usually requires little water and relatively short cooking times. Research has frequently found good retention of vitamin C and other nutrients with microwaving.

Boiling can cause greater losses of vitamin C, folate and other water-soluble nutrients, particularly when vegetables are cut into small pieces, boiled for a long time and the cooking water is discarded. If you boil vegetables, use only as much water as needed, avoid overcooking them and consider using the cooking liquid in soups or sauces.

Stir-frying and sautéing expose foods to higher temperatures but usually for a relatively short time. A little oil can also improve the absorption of fat-soluble compounds such as carotenoids.

Roasting and baking use no cooking water, so nutrients aren't lost through leaching. However, longer exposure to high temperatures can reduce some heat-sensitive vitamins. Roasting remains a nutritious way to prepare vegetables—especially if it encourages you to eat more of them.

Pressure cooking combines higher temperatures with short cooking times. Nutrient retention varies depending on the food and nutrient, but the shorter cooking time can be advantageous.

Deep-frying uses very high temperatures, which can reduce heat-sensitive nutrients such as vitamin C and some B vitamins. Minerals are generally retained, while the presence of fat may improve the absorption of fat-soluble compounds such as carotenoids. However, foods absorb oil during frying, increasing their fat and energy content, and repeatedly heated oil can form undesirable oxidation products. Starchy foods can also develop acrylamide when fried until very brown, so deep-frying is best used occasionally and foods should be cooked to a golden rather than dark-brown colour.

Raw foods avoid heat-related vitamin losses altogether. Raw peppers, cabbage, carrots, tomatoes, leafy greens, berries and fruits can therefore make valuable contributions to the diet, particularly as sources of vitamin C and other heat-sensitive compounds.

What about cutting vegetables?

How we prepare vegetables before cooking matters too.

Once a fruit or vegetable is cut, a much larger surface area is exposed to oxygen and light. Cutting also damages plant cells and exposes their contents to enzymes and oxygen.

As a practical rule:

wash produce before cutting rather than soaking chopped vegetables for long periods;
keep edible skins where appropriate, as many nutrients and phytochemicals are concentrated in or close to the skin;
cut vegetables into reasonably large pieces when cooking rather than finely chopping everything;
prepare produce relatively close to when you plan to eat or cook it;
don't leave cut fruit and vegetables sitting uncovered for unnecessarily long periods.

At the same time, don't become afraid of chopping food. Chopping, crushing and blending can also break down plant structures and improve the accessibility of certain phytochemicals. Again, there is a balance.

Don't forget the cooking liquid

One of the easiest ways to reduce nutrient waste is simply to eat the liquid your vegetables were cooked in.

When you prepare vegetable soup, lentil soup, curry, stew or tomato sauce, nutrients that move from the vegetables into the liquid haven't necessarily disappeared—they have moved into another part of the dish.

This is quite different from boiling vegetables in a large saucepan of water and pouring the water down the sink.

The same principle applies when preparing legumes. Beans, lentils, chickpeas and peas provide plant protein, fibre, folate, potassium, magnesium, iron and many other nutrients. Their preparation also influences digestibility and the presence of naturally occurring compounds such as phytates.

How different cooking methods affect nutrients?

My practical tips for preserving nutrients at home

You don't need to measure cooking temperatures or worry about every milligram of vitamin C. A few simple habits can make a meaningful difference:

Eat vegetables and fruits in a variety of forms—raw and cooked.
Avoid routinely overcooking vegetables. Aim for cooked but still colourful and with some texture.
Steam or microwave vegetables when you want to minimise contact with water.
Use shorter cooking times where possible.
When boiling, use only the amount of water you need.
Use cooking liquids in soups, stews and sauces rather than throwing them away.
Don't cut vegetables into unnecessarily small pieces before cooking.
Keep edible skins when practical.
Include a little healthy fat with carotenoid-rich vegetables. Extra-virgin olive oil, nuts, seeds and avocado can help with the absorption of fat-soluble carotenoids.
Cook some tomatoes and carrots. Raw is good, but cooked can be good too.
Eat foods soon after preparing them rather than leaving chopped or cooked vegetables exposed to air and light for long periods.
Most importantly, eat plenty of plants. The small differences between steaming a carrot for six or eight minutes matter much less than whether vegetables, fruits, berries, legumes, whole grains, nuts and seeds regularly appear on your plate.

Cooking changes food—it doesn't simply make it less nutritious

When we cook vegetables, several things can happen simultaneously.

Some heat-sensitive vitamins may be lost. Water-soluble nutrients may move into the cooking water. Plant cell walls soften. Some phytochemicals become easier to extract. Certain compounds become more bioavailable. And the food may become easier to chew, digest and—equally importantly—more enjoyable to eat.

So instead of asking:

“Is raw or cooked healthier?”

a better question might be:

“How can I prepare this food so I enjoy eating it while retaining—or improving access to—as many of its beneficial nutrients as possible?”

For most of us, the answer is beautifully simple: eat a variety of colourful whole foods, prepare them in different ways, don't routinely overcook them, and enjoy both raw and cooked vegetables.