Do fruits and vegetables lose their nutrients when cooked? – Debunking a myth

Fruit and vegetables are healthy and should be part of a balanced diet. Most people know that. But is it also important how we eat our vegetables? Is raw food really always the healthier choice? And do fruits and vegetables really lose their health benefits when they are “cooked to death”?

What happens to nutrients when cooking fruits and vegetables?

Fruits and vegetables contain numerous compounds that are responsible for their health benefits, including vitamins, minerals, fiber and phytochemicals. To assess how the health benefits of a food change when cooked, it is first necessary to understand how these compounds can be changed and how they can be lost in the first place.

Minerals, for example, are elementary compounds in the context of nutrition that we must ingest through food. Elementary means that it is a pure substance that cannot be chemically broken down into other substances (1). Minerals cannot therefore be destroyed during cooking. However, they can pass from food into the cooking water, for example (2).

The situation is different with vitamins and secondary plant substances: These are more complex molecules that can break down under the influence of heat (1). This means that they can not only be lost from the vegetables, but can also break down into potentially inactive components, which of course reduces their health benefits.

Finally, there are the so-called macronutrients, i.e. carbohydrates (and fiber), fats and proteins. These components of food provide us with energy. What they have in common is that they are usually larger molecules that are broken down by the body into small building blocks such as glucose, amino acids and fatty acids. These are then used to actually generate energy (1).

This process can also take place to a certain extent during cooking: Large starch molecules can be broken down into sugar molecules under the influence of heat (3). However, these smallest building blocks do not break down further during cooking, which is why the overall energy content of the food does not change: Sugar can turn into liquid caramel under the influence of heat. However, as long as it is not completely burned, the amount of glucose molecules in it remains the same.

Dietary fiber plays a special role in this context: these are carbohydrate molecules that we do not digest at all, or at least incompletely. As a result, we get less energy from these carbohydrates, but they have a positive effect on intestinal health, for example. Cooking can convert some indigestible carbohydrate compounds into digestible starch. For this reason, for example, potatoes are only eaten boiled (4). Conversely, starch molecules can also reassemble when cooled to form so-called resistant starch, which is then counted as dietary fiber (4,5).

It can therefore be said that a loss of nutrients in the cooking water can be a problem. This affects minerals, vitamins and secondary plant substances. In addition, larger molecules such as vitamins and secondary plant substances can break down under the influence of heat. The actual composition of fruit and vegetables, consisting of carbohydrates, fats, proteins and fiber, is only affected relatively little by cooking. These components can be changed, but since they are hardly "destroyed" completely, but only broken down into smaller components that the body would also produce from them, the relevance for health benefits is rather negligible.

How many nutrients are lost during cooking?

It is difficult to say with any certainty how many nutrients are lost during cooking. It depends on the food, the cooking method and the nutrient in question. However, some general assumptions can be made and examples given:

Vitamins can be divided into water-soluble and fat-soluble vitamins (1), which are lost in cooking water with varying degrees of ease: while the fat-soluble vitamins A, D, E and K are unlikely to pass into the water, this can certainly happen with vitamin C and B vitamins. The same applies to secondary plant substances, many of which are also water-soluble. For example, up to 60% of the vitamin C content can be lost through long cooking (9). Losses of up to 95% have even been observed for the B vitamin folate, with folate being broken down under the influence of heat (10).

In the case of minerals, the way they are incorporated into the food can play a role: iron, for example, which is complexed in the protein heme in red meat, is much more difficult to dissolve than other minerals. However, if the mineral is not bound in a protein, there is a higher chance that it will be “washed out” during cooking (6).

In terms of the cooking method, it is particularly important whether the food is cooked in water and what heat is applied to it and for how long. To put it simply: Long cooking in water will lead to a high loss of nutrients through release into the cooking water. Long frying at high heat can contribute to the breakdown of vitamins and plant substances.

Which cooking methods preserve the most nutrients?

If high temperatures, long cooking times and contact with water reduce the nutrient content, this also results in the most gentle cooking methods: steam cooking and cooking in the microwave, for example, minimize the factors mentioned. But short blanching and searing are also possible without any problems, as long as you don't do it for too long (7,8).

It can also be helpful to cook vegetables and fruit in water that is then consumed: vegetables in soups, stews and even many casseroles are essentially cooked. However, because the cooking water becomes part of the dish, the nutrients that are lost in the water are still eaten (2). Only heat-unstable compounds are lost.

Why can some nutrients become more available through cooking?

Just because a nutrient is contained in a food does not necessarily mean that it will reach the body. During digestion, the food must first be broken down mechanically and then chemically. Only then can individual components be absorbed into the blood via the intestinal mucosa.

However, vegetables and fruit in particular have a relatively firm, often fibrous structure. These fibers consist of dietary fiber, which is part of the health benefits. However, they also make it more difficult for the body to break down food and extract nutrients from it. This can mean that raw vegetables, for example, contain a lot of nutrients, but these are simply excreted unused.

Especially with tomatoes, but also with carotenoid-rich vegetables such as carrots and pumpkin, large differences were observed depending on the state of cooking: certain secondary plant substances could be absorbed much better from the cooked vegetables than from the raw version (11,12).

Why is it important not to just focus on raw food?

Although raw vegetables and fruit undoubtedly have the highest nutrient content, it is not advisable to rely exclusively on raw food. There are some types of vegetables that should only be eaten cooked, as cooking breaks down harmful substances. Examples of these are potatoes and eggplants. But many vegetables that could in principle be eaten raw are also better tolerated by many people when they are cooked. Typical examples of this are onions and brassicas. The reasons for this individual difficulty in digesting them can vary. However, the fact is that many people benefit when vegetables are "pre-digested" a little by the heat treatment during cooking.

With some vegetables, it is also possible to reduce the content of potentially harmful substances by cooking them. Spinach and beetroot, for example, can be eaten raw without any problems. However, in this state they contain large amounts of oxalic acid, which increases the risk of kidney stones. However, by cooking these vegetables, a large part of the oxalic acid is lost in the cooking water. This is very similar to the valuable ingredients mentioned above. Beetroot and spinach are so rich in nutrients that a small loss of nutrients and a significant loss of oxalic acid is probably a good trade-off (13).

How to ensure a balanced diet with raw and cooked vegetables?

If you consider that raw and cooked fruit and vegetables each have their own advantages and disadvantages, it should be clear that this actually makes it easier to incorporate them into your daily diet: Basically, you just have to ask yourself which foods you prefer cooked and which you prefer raw. Apart from a few exceptions such as eggplant and potatoes, you are completely free to decide how you want to eat most types of fruit and vegetables. Ideally, you decide for yourself which foods you like best in which form. Then you ask yourself whether you tolerate these foods well with the respective method of preparation and in this way create your own portfolio of healthy foods from which you can choose flexibly and without worry every day.

Conclusion

There is little reason for dogmatism when it comes to the question of whether to eat vegetables and fruit raw or cooked. Both have advantages and disadvantages: raw food contains the most nutrients, but some are harder to absorb and many people find raw vegetables difficult to digest. Cooking can also reduce the content of some harmful substances. In the end, it seems to make the most sense to combine raw food and various types of cooked vegetables. Even vegetables that have been cooked for longer are still rich in fiber and if you eat them as a soup or stew, the loss of nutrients is moderate even after longer cooking. Add a fresh salad now and then and you don't really have to worry at all.

Sources

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