Introductory Chapter: Dietary Carotenoids – Sources, Properties, and Role in Human Health

1. Introduction

Global dietary guidelines recommend increased consumption of plant-based foods as a good source of essential nutrients, such as vitamins and minerals. However, recent research has shown that plants also contain a wide variety of biologically active compounds other than the essential nutrients that may play an important role in human nutrition and health. Over the past few decades, extensive research has been directed at identifying these biologically active compounds, their chemical properties, and biological activities and mechanisms of action. It is now recognized that consumption of these compounds is an important complimentary approach in the prevention, treatment, and management of several of human diseases. Although, technically they are not recognized as nutrients but are referred to as “phytonutrients,” “nutraceuticals,” “functional ingredients,” and “beneficial bioactive compounds” [1]. The scope of these compounds has now expanded beyond foods to include therapeutics, pharmaceuticals, and cosmeceuticals. Unlike essential nutrients, there are no recommended levels of daily intake for these compounds as of now. Several thousand phytonutrients have been identified as being present in plants. Although not all of them have been studied in detail, two classes of the phytonutrients that have been studied extensively include the water-soluble and the fat-soluble carotenoids [2]. The focus of this book is in the area of carotenoids and their role in human nutrition and health.

Carotenoids are organic pigments produced mainly by plants. However, some algae, bacteria, and fish have also been known to contain carotenoids. They are responsible for a full spectrum of colors all the way from yellow to red. The recent recommendation by health professionals to consume multicolored foods reflects the importance of carotenoids as an important part of a healthy diet [3]. Over 1000 carotenoids have so far been identified and classified into xanthophylls containing oxygen in their molecule and carotenes contain no oxygen. Carotenoids, amongst which β-carotene is an important and well-known carotene. Other important carotenes include α-carotene, lycopene, β-cryptoxanthin, and lutein. They contain eight isoprene molecules and 40 carbon atoms. One of the characteristic features of carotenes is the presence of multiple, ranging from 9 to 11, conjugated double bonds [2, 4].

The typical chemical structure of β-carotene is shown in Figure 1.

Figure 1.

Basic structure of a β-carotene showing 9 conjugated double bonds.

Humans cannot synthesize carotenoids in their bodies and must get them from their diet. Many studies have shown that low intake of carotenoids, low concentration of serum carotenoids, and low accumulation of carotenoids in various organs, such as skin and eyes, are risk factors for developing the abovementioned disorders. Taking carotenoid-rich foods and supplements has been shown to reduce these risks [5].

The antioxidant property of carotenoids is associated with the presence of these conjugated double bonds. Oxidative damage is closely associated with inflammatory damage of tissues, and in turn, with initiation and progression of several human chronic diseases [6, 7, 8, 9]. To understand the role of carotenoids in human health, several in vitro, animal, preclinical, and clinical studies have been undertaken over the past decade. Prevention of cardiovascular diseases was the focus of most of the initial studies directed at carotenoids [2]. This was later followed by studies that were directed at understanding their role in the initiation, management, and possible treatment of cancer, age-related macular degeneration, diabetes, and male infertility. More recently there is great interest in studying the role of carotenoids in immune deficiency-related diseases and neurodegenerative disorders. Diseases related to bone, such as osteoporosis are also areas of recent research.

Common carotenoids, their food sources, and amounts are shown in Table 1. The amounts shown in the table an approximation and vary depending on several factors.

Carotenoid	Food source	Amount (μgrams/100 grams)
Β-Carotene	Carrots, cooked	9771
	Spinach, cooked	5300
	Green collard	5400
	Cantaloupe	3000
Β-Carotene	Beet greens	2500
	Apricot, dried	2192
	Broccoli, cooked	1300
	Tomato, raw	520
Α-Carotene	Carrots, cooked	3723
Lycopene	Tomatoes, raw	3100
	Tomato juice	10,000
	Tomato pasta	36,500
	Tomato ketchup	12,390
	Tomato sauce	13,060
Β-Cryptoxanthin	Tangerine	1060
	Papaya	470
Lutein	Spinach, cooked	12,475
	Green collard	16,300
	Beet green	7700
	Broccoli, cooked	1839
	Green peas, cooked	1690

Table 1.

Examples of some food sources of major carotenoids in the North American Diet.

Source: Rao and Rao [6].

2. Challenges and future research

Since the discovery of β-carotene in carrot juice almost two centuries ago, several thousand more carotenoids have now been identified. Over the past couple of decades, a great deal of advancement has been made in terms of our understanding the role of carotenoids in human health. Studies have produced information in terms of the chemical and biological properties of carotenoids, their dietary sources, and mechanisms of action. Martini et al. [10] in 2022 provided insight into the future directions in research related to carotenoids. Similarly, Melendez-Martinez et al. [11] and Rocha et al. [12] also pointed out to the need for future research on carotenoids and human health.

In recent years, a few of the carotenoids have been subjected to basic, preclinical, and clinical investigations to understand their role in human nutrition and health. More new knowledge about carotenoids is now being generated relating to their chemistry, dietary and supplemental sources, biological properties, mechanisms of action, and their role in human health. However, there is still a need for clear understanding about the results, particularly from limited number of interventional clinical studies.

Recognizing the need for more recent information, this book is being published to present the readers up to date knowledge on carotenoids, including techniques of isolation; characterization of their chemical, physical, and biochemical properties; sources; mechanisms of action; and basic in vitro, animal, and observational. Preclinical and clinical studies are currently being undertaken to gain information relating to their role in human health and diseases. This book will be of interest not only to the scientific community undertaking research but also to the consumers who are concerned more and more about their diet and health. It will also be of interest to government regulating agencies in developing regulatory guidelines for the safety of carotenoids and to validate their claimed benefits in human health, disease prevention and management, and even in the possibility of treatment of the diseases in the future.

Some of the biological mechanisms and disease prevention of carotenoids are shown in Table 2.

Biological actions of carotenoids	Examples of disease prevention
Antioxidant function	Cancer
Anti-inflammatory function	Diabetes
Provitamin A activity	Cardiovascular diseases
Immune response	Cataract and other ophthalmic disorders
Gap junction communication	Human immunodeficiency viruses (HIV)
Drug metabolism	Age-related macular degeneration (AMD)
UV Protection	Neurogenerative diseases
Antimicrobial	Respiratory disorders

Table 2.

Some examples of the biological properties and disease prevention of carotenoids.