The red colour of the tomato, the sharp taste of horseradish or the aroma of peppermint oil are only a few examples of the so-called phytochemicals. According to their definition, phytochemicals are natural substances contained in fruits, vegetables, legumes and grains. As the name suggests, they are formed only in plants, and are found even there in only very slight concentrations.

Phytochemicals serve plants as antibodies against pests and diseases; they give them colour, smell and taste; they protect them from sunlight or perform other important functions for vegetation. We have learned only recently that phytochemicals are also beneficial to us human beings in manifold ways.

The total number of phytochemicals found in nature is not fully known to date. It is estimated to be between 60,000 and 100,000. At present, only a very small portion of these are even known and have been chemically identified.

Phytochemicals differ greatly from one another in their chemical composition.

Contrary to vitamins and minerals, phytochemicals are not considered to be among the vitally necessary nutrients (essential nutrients).

With a varied diet we take in a total of approximately 1.5 g of phytochemicals daily. Phytochemicals can be divided into nine main groups:


  • Carotenoids
  • Glucosinolates
  • Protease inhibitors (enzyme inhibitors)
  • Phytooestrogens
  • Phytosterines
  • Polyphenols
  • Saponins
  • Sulphides
  • Monoterpenes

Possible effects of the phytochemicals on the human organism

Because science only recently recognized the importance of the phytochemicals for human health, only little is known to date about the precise effects and mechanisms of action of the numerous different substances. Much is presumed, other theories are so far speculative at best.

However, we already know that it is not so much the individual substance that is good for our health but rather the mixture of many different plant substances that interact with one another and only then are really effective.

The most important effects attributed to the phytochemicals, according to our current state of knowledge, are:

  • Protection from cancer (anticarcinogenic)
  • Antimicrobial
  • Antioxidant
  • Influence on the immune system
  • Anti-inflammatory
  • Influence on coagulation
  • Anti-hypertensive
  • Hypolipidemic
  • Hypoglycaemic
  • Digestive


Carotenoids are plant pigments; their colour spectrum ranges from yellow to red. Approximately 40 different carotenoids have been identified in plant foods to date; there are presumably about 600.

The carotenoids are divided into two groups, according to their chemical composition: In the first group the molecule consists exclusively of carbon and hydrogen. These "hydrocarbon carotenoids" are found mainly in yellow-red fruits and vegetables (e.g. the beta-carotene in carrots and the lycopene in tomatoes).

In the second group, the carotenoid molecule contains oxygen in addition. These oxygenic carotenoids are called xanthophylls. They also produce a yellow colour - examples are the zeaxanthin in maize and the lutein in kale and spinach. That the latter are not yellow but green lies in the fact that the yellow is masked by the green chlorophyll.

Some carotenoids are precursors of vitamin A and can be converted into vitamin A in the body; they are called provitamin A. The provitamin with the greatest vitamin A activity is beta-carotene.

Carotenoids are fat soluble. This means that they can be better used by the body when the food in question is eaten with some added fat. Moreover, the availability is dependent upon the manner of preparation. Thus beta-carotene from cooked carrots is resorbed better than that from raw carrots, and better still when some fat is added. The lycopene of tomatoes is assimilated better from tomato puree than from fresh tomatoes.

Nevertheless: A raw carrot nibbled without fat and a fresh tomato are also valuable and healthy foods which you should not avoid. After all, in addition to beta-carotene or lycopene, they contain numerous other useful substances that are better absorbed in a fresh, unprocessed state. It's thus a question of the right mixture.

In contrast to the other carotenoids, the group of xanthophylls are sensitive to heat. Therefore, we benefit most from dark-green leafy vegetables - as far as the carotenoids are concerned - when we eat them raw.

Foods rich in carotenoids are above all carrots, tomatoes, pumpkin / squash, peppers, green leafy vegetables (such as leaf lettuce, spinach, kale, broccoli, parsley, cress, dill), as well as the fruits apricots, peaches, pink or ruby-red grapefruit, honeydew melon.

Many carotenoids have a strong antioxidant effect and protect cells from damage by free radicals. Others have a favourable influence on the immune system. A diet that is rich in carotenoids presumably also protects against cardiovascular disease and cancer.


Description and occurrence
Approximately 80 different glucosinolates are found in nature, and they are compounds with a sulphate group.

Glucosinolates occur mainly in the plant family Cruciferae. In this family we find all varieties of cabbage (such as white cabbage, red cabbage, savoy cabbage, brussels sprouts, broccoli and cauliflower), as well as in swedes (rutabaga turnips), radishes, cress and mustard. Kohlrabi and cress contain the largest amounts of glucosinolates.

The glucosinolates contribute to the sharp taste of these vegetables, although, strictly speaking, it is not the glucosinolates themselves that have a sharp taste but their decomposition products (compounds called isothiocyanate, thiocyanate and indole), which are formed when the plant cells are damaged by mechanical action, for example by being cut up during food preparation.

Glucosinolates are very sensitive to heat and are partially destroyed by cooking. With cabbage, for instance, the content is reduced during cooking by 30-60%.

The effects on our health are also not brought about by the glucosinolates themselves, but by their decomposition products. These act as antimicrobials, particularly in urinary infections. In addition, an anticarcinogenic effect is attributed to them.

Protease inhibitors, or enzyme inhibitors

Description and occurrence
Enzymes that break down food protein into its components, the amino acids, are called proteases. Only the individual amino acids (not the entire protein molecule) can be resorbed from the intestine via the intestinal cells into the blood. Therefore, proteases are imperative for the digestion of protein.

Protease inhibitors are compounds that hinder the activity of the enzymes that split the protein. It could be inferred from their name that they interfere with protein digestion and might thus be unhealthy for human beings. In fact, however, the activity of the protein-splitting enzymes in the human digestive tract in particular are only minimally influenced by the protease inhibitors, so that protein digestion can proceed unhampered even when protease inhibitors are contained in the ingested food.

Protease inhibitors are found above all in unheated legumes such as soybeans, mung beans, peas, but also in potatoes and some types of grain (rice, maize, oats, wheat). When heated, they lose their function as inhibitors of proteases.

Today various health-promoting effects are attributed to the protease inhibitors. For instance, they are thought to be anticarcinogenic, particularly in the digestive tract, as well as antioxidant and anti-inflammatory.


Description and occurrence
Phytooestrogens are substances that resemble the (female sex hormone) oestrogen in their chemical structure and also function in a similar way, although in a much weaker form.

The isoflavonoids and lignins belong to the phytooestrogens.

Isoflavonoids occur almost exclusively in soybeans, which are especially rich in genistein, the best known isoflavonoid. In countries where large amounts of soy products are eaten, e.g. Japan, many isoflavonoids are absorbed; here the intake is generally only scant.

In contrast, we take in considerably more lignins, the second group of phytooestrogens. They are found mainly in grain products, further in sunflower seeds and pumpkin seeds. The lignins are found above all in the outer layers of the grain. Therefore, products that are made from white flour have only a low lignin content, while whole-grain products are one of the best sources of lignin.

Phytooestrogens are presumed to protect from cancer, particularly hormone-related types such as breast, uterine and prostate cancer. Moreover, they are supposed to work as antioxidants.


Description and occurrence
Phytosterines belong - as does cholesterol - to the chemical group of sterines. While cholesterol is found in appreciable amounts only in animal organisms, however, phytosterines occur exclusively in plants. The most important representatives of this group are beta-sitosterine, campesterine and stigmasterine.

The sterines are fatlike substances; consequently, they occur above all in plants that are rich in fat, while fruits and vegetables contain only very small amounts. Mainly seeds such as sunflower and sesame, soybeans, nuts, and cold-pressed, unrefined plant oils are rich in phytosterines.

The phytosterines are hypolipidemic. This is due mainly to the fact that they interfere with the uptake of food cholesterol in the intestine, whereby less cholesterol from food reaches the blood.

In animal experiments a protective effect against cancer was also observed. This has not yet been proven for human beings, however.


Description and occurrence
In the group of polyphenols are numerous different substances which, chemically speaking, all trace back to the basic structure of phenol. Many colour, taste and odorous substances, as well as substances with effects similar to those of hormones, belong to this non-uniform group (e.g. genistein).

Two main groups of polyphenols are the phenic acids and the flavonoids. Caffeic acid and ferula acid belong to the phenic acids and occur in many plants. Caffeic acid, as its name implies, is found in high concentration in coffee, but also in many other plants, where it is chiefly in the outer layers, as is ferula acid.

One reason for the high concentration of phenic acids in the outer layers of vegetables and seeds is that they protect the plant tissues lying beneath from the influence of oxygen, thus acting as antioxidants in the plant. In the potato, for example, 50% of the caffeic acid is in the skin, while cereal brans contain large amounts of ferula acid.

Another important phenic acid is ellagic acid, which is found, however, only in certain nuts and fruits (above all in walnuts, pecans, blackberries, raspberries and strawberries).

Generally speaking, freshly harvested fruits and vegetables have the highest phenic acid content. Being sensitive to oxidation, a portion of the phenic acid is degraded during storage.

The group of flavonoids includes more than 4000 different substances that are very widespread in the plant kingdom and thus also an important component of our diet. A subgroup of flavonoids (the flavonols) are yellow and gave the entire group its name (Latin flavus = yellow). Also among the flavonoids are the anthocyans, which are responsible for the red, blue or purple colour of fruits and vegetables, e.g. cherries, plums, berries, red cabbage and aubergines. The most common flavonoid is quercetin. This is found in very high amounts in onions, kale, broccoli, apples and cherries.

The flavonoids, like the phenic acids, are found mainly in the outer layers of plants. This means that a considerable amount of flavonoids are lost when, for instance, apples are pared or tomatoes are peeled. Their concentration is also reduced during storage.

As different polyphenols occur simultaneously in most plants, it is exceptionally difficult to examine the effects of individual phenic acids or flavonoids on human health. It is also not exactly known to date whether the various polyphenols can be utilized by the human organism at all, and if so, how well.

However, it is already safe to say that the polyphenols are very powerful aids to our health. Especially significant are the excellent antioxidant capabilities of the flavonoids, which are presumably able to lower the risk of cardiovascular diseases, to protect against cancer and to retard the ageing process.

Moreover, flavonoids are anti-inflammatory, antimicrobial, and anticoagulant and have an influence on the immune system. These effects are also attributed in part to the phenic acids.


Description and occurrence
The term saponins is derived from the property of these substances that causes them to foam in watery solutions, similar to soap. They are very bitter in taste.

Saponins are present in small amounts in a great many plant foods. The legumes in particular are rich in saponins.

The saponins were long thought to be detrimental to health, because it was assumed that they would damage the cells of the intestinal mucosa. However, recent studies have shown that this happens only on a small scale and has no serious consequences for human beings. Thus, legumes can be eaten without hesitation in spite of their saponin content.

Only a small portion of the saponins from food are resorbed, i.e. absorbed into the blood from the intestines. Their effect is exhibited mainly in the gastrointestinal tract. There they form insoluble compounds with various other substances, e.g. with cholesterol. Similar to the phytosterines, therefore, they should act - on a small scale - to lower cholesterol levels. In addition, saponins are presumed to have antimicrobial and anti-inflammatory effects, especially in the intestines.


Description and occurrence
Sulphides are sulphuric compounds that occur above all in bulbous plants. Garlic has the highest content, but they are also abundant in onions, shallots, leeks and chives.

When the plant cells are damaged, for instance by mincing or cutting during preparation, the sulphides are transformed into their active form and begin to develop their characteristically pungent sharp smell and taste. For the same reason, only the cut onion causes the eyes to tear.

Owing mainly to its content of various sulphides, garlic has been an important component of folk medicine for thousands of years. It is believed to be helpful for heart troubles, headaches, insect bites or stings, and tumours. Some of these effects have also been scientifically proven in the meantime.

Sulphides have a strong antimocrobial action; this is due above all to the substance allicin in garlic. In the First World War it was therefore used as an antiseptic, i.e. as a germicide. A further sulphide found in garlic is ajoene, which works as an anticoagulant.

Sulphides also act as antioxidants and intercept free radicals. In addition, they protect against cancer. Furthermore, they stimulate the flow of saliva and the secretion of gastric juices and in this way promote digestion.


Description and occurrence
Monoterpenes are a group of substances that are especially important as aromatics. They are highly volatile and occur mainly in citrus fruits, herbs and spices. Among them are menthol from the peppermint plant. Citrus oil consists largely of the monoterpene limonene, and caraway oil contains carvon.

Monoterpenes are also supposed to have an anticarcinogenic effect. To date, however, this has been substantiated only in animal experiments.