Plants produce secondary metabolites to aid in self-defense. Secondary metabolites support plant survival and species propagation because they act as physical defenses against predators. For example, lactucin in chicory leaves is toxic to slugs. Its presence in leaves deters slugs from consuming and harming the species (M. Buanafina, PowerPoint Lecture, 2016). Some secondary metabolites act as warning signals to other plants of the same species to inform them that danger is present. Other metabolites attract pollinators with their aesthetic hue.
Secondary metabolites are a critical component to plant survival, however, they also play a powerful role in supporting human health. Humans can benefit from consuming secondary metabolites and therefore, a diet rich in plants provides marvelous benefits to health. The following are a few of the many secondary metabolites that we can consume to receive health benefits.
Carotenoids are a class of tetraterpenoids that occur naturally in plants, algae and photosynthetic bacteria. Fruits and vegetables are a major source of carotenoids in the human diet. Carotenoids are red, orange and yellow and are the source of the bright colors of pumpkins, sweet potatoes, cantaloupes, papayas and tomatoes. Leafy greens such as spinach are also a major source of carotenoids in the body. They do not appear red, orange, or yellow because the carotenoid pigments are masked by its high chlorophyll content. Two major carotenoids include lycopene and zeaxanthin. A high intake of zeaxanthin and lycopene poses implications for cancer prevention because they contain antioxidants that scavenge the body for reactive oxygen and nitrogen species (1).
Flavonoids are a group of phenols that occur naturally in plants. Tomatoes, chocolate, and red wine are a major source of flavonoids in in the human diet. Flavonoids consist of a wide range of colors from pale-yellow to blue. Like carotenoids, flavonoids have antioxidative properties. Free radicals injure endothelial walls of the vascular system and can contribute to atherosclerotic changes. By scavenging free radicals, flavonoids protect the walls of the vascular system and decrease the risk for heart disease. Flavonoids also prevent inflammation tumor growth, osteoporosis, and viral infections (2).
Glucosinolates are a group of compounds derived from amino acids. Dietary sources include cruciferous vegetables such as broccoli, collard greens, cabbage, and mustard. Glucosinolates can play a role in cancer prevention by inducing apoptosis. Therefore, glucosinolates can help to regulate the number of reproducing cells in an area of uncontrollable cell growth. They also have antioxidative properties and therefore, protect the body from oxidative stress (3).
Caffeine is an alkaloid produced in young leaves of plants such as tea leaves. Dietary sources of caffeine include tea from the Camellia sinensis plant and coffee produced from the seeds of the Coffea arabica plant. Caffeine acts as a stimulant in humans and increases mood and physical performance. Caffeine works by blocking adenosine receptors. Adenosine is responsible for slowing down nerve activity in the brain. Adenosine is also responsible for regulating neurotransmitters such as dopamine, leading to heightened brain activity. Although caffeine is a drug, it can produce positive effects in mood and efficiency in humans if consumed in moderate amounts (4).
Saponins are a group of terpenoids found in the allium species, such as onions and garlic. They are also abundant in legumes, tea and spinach. Saponins promote heart health by binding and removing cholesterol from cell membranes. LDL cholesterol intercalates cell membranes and decreases their flexibility. A rigid vascular system puts pressure on the heart and increases their risk for damage. Saponins can prevent that by binding to that cholesterol and removing it from arterial membranes. Therefore, consuming saponins is one way to promote heart health (5).
In all, although secondary metabolites play a role in plant defense, they also support human health. Secondary metabolites play a large role in disease prevention, health promotion, and aesthetic properties. New plant species have yet to be discovered and therefore, preserving our natural resources is crucial in making new discoveries about human health and plant compounds.
- Rao A, Rao L. Carotenoids and human health. Pharmacological Research.2007;55:207-216.
- Nijveldt R, Nood E, Hoorn D, Boelens P, Norren K, Leeuwen P. Flavanoids: a review of probable mechanisms of action and potential applications. American Society for Clinical Nutrition.2001;74:418-425.
- Traka M, Mitchen R.Glucosinolates, isothiocyanates and human health. The Phytochemical Society of Europe.2008;8:269-282.
- Shi J, Arunasalam K, Yeung D, Kakuda Y, Mittal G, Yueming J. Saponins from Edible Legumes: Chemistry, Processing, and Health Benefits. Journal of Medicinal Food.2004;7:67-68.
- Marangos P, Boulenger J, Patel. Effects of chronic caffeine on brain adenosine receptors: Regional and ontogenetic studies. Life Sciences.2014;89: 899-907.