Secondary plant compounds – and their multiple gifts to our health

What are secondary plant compounds?

Dark purple pigments in red cabbage, cherries or aubergines (anthocyanins), pungent sulphides like allicin from garlic and onions, or the aromatic essences of peppermint, caraway or citrus fruits – all these, and many others, are classified as secondary plant compounds. These are bioactive compounds that are found in plants.

The many roles of secondary plant compounds

Unlike compounds such as carbohydrates, proteins and fats that are essential for a plant’s growth and development, secondary plant compounds serve other functions: They help plants by attracting insects, for example, or protecting them from pests, pathogens or UV radiation. In most cases, they are present in only trace amounts in plants.

How many secondary plant substances are there?

The diversity is astounding: Estimates suggest there are up to 100,000 different secondary plant compounds. Between 5,000 and 10,000 of these can be found in the foods we eat, although only a small fraction has been fully identified. Among the best-known groups are flavonoids, carotenoids, phytosterols, polyphenols, glucosinolates and saponins.

Good for plants – and for us

Scientists have long recognised the importance of secondary plant compounds for human health. The potential health benefits are almost as diverse as the compounds themselves. One of their most significant functions is their antioxidant effect. Antioxidants protect our cells and genetic material from damage caused by free radicals. Free radicals can harm cellular structures and are implicated in the development of many diseases. They are generated by metabolic processes or environmental factors such as UV radiation or pollutants. Carotenoids, for instance, act as antioxidants that neutralise free radicals. The best-known carotenoid is lycopene, which gives tomatoes their red colour.

Reducing chronic inflammation

Many secondary plant compounds, such as flavonoids, have anti-inflammatory effects. Chronic inflammation is considered a major cause of conditions like cardiovascular disease and type 2 diabetes. The anti-inflammatory properties of these compounds are especially relevant for age-related illnesses such as arthritis, inflammatory bowel diseases and heart diseases.

Supporting the immune system

Some secondary plant compounds have immunomodulatory properties, meaning they can support the immune system. By boosting immune cell activity and promoting antibody production, they protect the body against infections. Glucosinolates are compounds that are known for their immunomodulatory effects. They also have antiviral, antibacterial and anti-inflammatory properties.

Regulating blood pressure

Certain secondary plant compounds, such as polyphenols, can help regulate blood pressure by improving blood flow and increasing the flexibility of blood vessels.

Benefiting the gut

Many secondary plant compounds improve the diversity of the gut microbiome. Together with the microbiome, secondary plant compounds promote the production of short-chain fatty acids and other bioactive compounds that have anti-inflammatory effects and strengthen the protective function of the gut lining.

What does the science say?

In recent years, numerous studies have examined the health benefits of secondary plant compounds. However, as many of these findings are based on laboratory experiments using cell cultures or animals, they aren't always directly applicable to humans. Or they are observational studies, which often examine whole foods rather than individual substances, meaning that the positive effects can also result from a combination of different ingredients. This is why further research is needed and there are currently no specific recommendations for a person’s daily intake.

Not always healthy – sometimes even harmful

While most secondary plant compounds are beneficial to our health, some can be harmful because they’re toxic. The glycoalkaloid solanine is a well-known example and is found in the green parts and sprouts of potatoes. Also toxic is hydrocyanic acid, or its precursor, to be precise. This is present in bitter almonds, the seeds of stone fruits and citrus fruits, and flaxseed.

Secondary plant compounds in foods

We can make the most of the health benefits of secondary plant compounds by eating a varied diet and enjoying a wide range of vegetables, fruits and legumes.

Eat a healthy and balanced diet

A good guide is: think colour. Include all the colours of the rainbow in your diet, both raw and cooked.