Secondary Plant Compounds

Secondary Plant Compounds: Definition, Importance, and Sources

Plants produce secondary plant compounds not for their primary energy metabolism but as part of what’s known as the secondary metabolism—hence the name. These compounds are essential for the plant, as they serve various protective functions against environmental influences such as sunlight, pests, or bacteria. 

For humans, secondary plant compounds may also have protective effects and play different roles in the body. Moreover, they are likely responsible for the effects of many traditional medicinal plants. However, research into individual compounds is still in its early stages. 

Secondary plant compounds are widely found in vegetables, fruits, herbs, and spices – the more colorful the diet, the healthier it is. These compounds are primarily concentrated in the outer layers of plants, where protection is most critical. For example, an apple with its peel contains more secondary plant compounds than one without it.

Groups and Examples of Secondary Plant Compounds

There are countless secondary plant compounds, and an estimated 5,000 to 10,000 may be relevant to human nutrition. 

Based on their chemical structure, they can be classified into polyphenols (such as flavonoids and anthocyanins), phenolic acids, phytoestrogens, carotenoids, and glucosinolates (mustard oil glycosides). 

Well-known examples of secondary plant compounds include: 

• Epigallocatechin gallate (EGCG) from green tea 
• Curcumin from turmeric 
• Oligomeric proanthocyanidins (OPC) and resveratrol from grapes 
• Soy isoflavones