Carotenoids are a group of pigments, mostly of plant origin, responsible for the yellow, orange and red colors in fruits and vegetables. All have antioxidant activity, and some are precursors of vitamin A. Moreover, carotenoids have a role in intercellular communication, immune system activation and disease prevention [3,7], and hence they promote human health.

Carotenoids comprise eight repetitive units of isoprene, with cyclic or linear structures at both ends of the carbon chains, resulting in multiple cis and trans isomers, with the latter being more abundant in nature [8,9]. Carotenoids are classified into carotenes and xanthophylls. Carotenes, such as α-carotene, β-carotene, γ-carotene, and lycopene [8], are highly soluble in organic solvents and insoluble in polar solvents. In contrast, xanthophylls are soluble in polar solvents (e.g., alcohols) and organic solvents (e.g., ether and hexane). They are oxygenated derivatives of carotenes, forming alcohols, aldehydes, ketones, and acids. Examples of xanthophylls are fucoxanthin, lutein and violaxanthin [8,9].

Carotenoids are stored in plant tissues (plastids), such as chromoplasts (colored plastids), amyloplasts (starch storage plastids) and elaioplasts (lipid storage plastids). In fruits, flowers and roots, carotenoids are located in the chromoplast, whereas in grains and oilseeds they are located in amyloplasts and elaioplasts, respectively [10]. In contrast, xanthophylls are freely found in green plant tissues, whereas in fruits and flowers they are found as esters of fatty acids [11].

The biosynthesis of carotenoids (Figure 1) takes place in the chloroplasts. Two molecules of geranylgeranyl diphosphate (GGPP) are produced from isopentenyl pyrophosphate (IPP) and dimethylallyl diphosphate (DMAPP), catalyzed by geranylgeranyl pyrophosphate synthase (GGPS). After this, two molecules of GGPP are condensed into phytoene by phytoene synthase (PSY). Subsequently, phytoene is desaturated into lycopene by ζ-carotene desaturase (ZDS) and phytoene desaturase (PDS). Lycopene is cyclized into α-carotene (α pathway) and β-carotene (β pathway), in reactions catalyzed by lycopene-ε (LYC-ε) and β-cyclase (LYC-β), respectively. Xanthophylls are synthesized from carotenes; lutein is formed by the action α-carotene ring-ε hydroxylase (CHY-ε) via the α pathway; β-carotene is converted into β-cryptoxanthin via the β pathway, in a reaction catalyzed by β-carotene hydroxylase (CHY-β), which also catalyzes its conversion into zeaxanthin. In turn, zeaxanthin can be converted into violaxanthin by zeaxanthin epoxidase (ZEP); conversely, violaxanthin can be converted into zeaxanthin by violaxanthin de-epoxidase (VDE). Finally, violaxanthin is converted into neoxanthin by neoxanthin synthase (NXS) [5,10,12].

More than 700 carotenoids have been identified, yet only 40 are included in the human diet, with β-carotene, α-carotene, lycopene, β-cryptoxanthin, lutein, and zeaxanthin the most common [13]. Carotenoid intake comes primarily from foods of plant origin (fruits and vegetables). α- and β-carotene are usually found in carrot, orange, pumpkin, pepper, sweet potato, mango, and vegetable leaves, varying in color from yellow, orange, and red. Lycopene imparts a bright red color to food, and is the main carotenoid in tomatoes, although it is also present in watermelon, guava, and papaya. β-cryptoxanthin is found in citrus fruit, melon, potato, guava and apple, giving them yellow and orange colors. The isomers lutein and zeaxanthin are found together naturally in yellow corn and marigold flower, although they are also found in broccoli, pumpkin, pepper, vegetable leaves, seeds and legumes. The xanthophylls violaxanthin (yellow), capsanthin, and capsorubin (orange to red) are frequently found in paprika and pepper. Neoxanthin, characterized by its yellow color, is a natural component of vegetable leaves. Bixin is the main component of annatto and is responsible for its characteristic red-brown color. Crocin is responsible for the yellow coloration of saffron [3,8,9,10].

Some carotenoids are found only in algae and seafood. Astaxanthin is responsible for the pink-red color of shrimp, salmon, and flamingo feathers, as it is naturally found in krill and microalgae haematococcus pluvialis, which are consumed by small crustaceans (shrimp and crawfish), fish (salmon), and birds (flamingo) [14,15], in that order. Fucoxanthin has a characteristic brown color and is only found in microalgae and the chloroplasts of brown algae [16].

The most abundant carotenoids in the Western diet include β-carotene and α-carotene from carrot, pumpkin, apricot, pepper, mango, coriander and spinach [3,13,17,18]; lutein from broccoli, pumpkin, spinach, corn, mango and papaya [3,13]; and lycopene from tomato, guava, papaya and watermelon [3,13,17,18,19,20].

Britton and Khachik (2009) proposed a ranking of fruits and vegetables based on their carotenoid levels, grouping the foods into the following categories: low level (0–1 µg/g), moderate level (1–5 µg/g), high level (5–20 µg/g), and very high level (>20 µg/g). Table S1 lists the carotenoid levels of some common fruits and vegetables consumed in America.

Carotenoid composition and content in food are affected by many factors, such as those inherent to the plant (variety, genotype and ripening stage), external to the plant (harvest season, growth conditions, post-harvest treatment, handling, storage conditions, plant diseases, and climatic conditions) [3,13].

Carotenoid bioavailability is defined as the fraction of carotenoid released from food that is absorbed in the intestine and made available for physiological processes or storage [13].

The nature of the food matrix containing carotenoids strongly affects their bioavailability. Due to their hydrophobic nature and location in plant tissues (plastids), carotenoid bioavailability in raw fruits and vegetables is limited. Therefore, carotenoids must be released from the cellular matrix and incorporated into a lipid fraction (micelles) during digestion to be absorbed [2].

Carotenoids are released from food mechanically by chewing, and chemically by the action of digestive enzymes (amylases, lipases, pepsin) and hydrochloric acid in the stomach [21]. These processes contribute to particle size reduction, thus increasing contact area for pancreatic lipases, bile salts, and enzymes, such as pancreatic amylases, nucleosidases, trypsinogen, chymotrypsinogen, carboxypeptidase, elastases, phospholipases, and carboxyl lipase ester, and the release of carotenoids and their incorporation into micelles [3,22]. Bile salts act as emulsifiers assisting with micelles formation and stabilization, whereas lipases break down lipids into fatty acids and monoglycerides, encouraging emulsification [22]. Micelles are absorbed into the enterocytes through passive diffusion or by binding to receptor proteins in the apical membrane for easy diffusion [2].

After absorption, carotenoids are enclosed in chylomicrons and transported to the bloodstream through the lymphatic system [7,17]. Once they have entered the bloodstream, the carotenoids are transported to the liver, where they are either stored or metabolized into vitamin A (only provitamin A carotenoids) by β-carotene binding oxygenases (BCO1 and BCO2). The rest are released back into the circulation and integrated into very low density (VLDL), low density (LDL), and high-density (HDL) lipoproteins to be distributed to other tissues, such as adipose tissue (vitamin A storage), skin and subcutaneous tissues (carotenes and xanthophylls reserve), macula lutea in the retina (lutein, zeaxanthin, and mesozeaxanthin), pancreas and vascular endothelium [22].

Carotenoid bioavailability is influenced by dietary factors, such as content and nature of carotenoids, fat content in the diet, and the interaction between carotenoids and other food components; and physiological factors, such as the rate of carotenoid absorption, nutritional state, genetic factors, and metabolism [2,17]. For instance, dietary fiber, especially soluble fiber, has been found to limit carotenoid availability as it affects the viscosity of the gastrointestinal content, size of lipid droplets, availability of bile salts, and enzymatic lipolysis of triglycerides [21]. Furthermore, Gul et al. [17] and Saini et al. [3] reported that the bioavailability of β-carotene in plants is low because carotenoids are bound to protein complexes, fibers, and cell walls, rendering them resistant to digestion and degradation, thus limiting their release. In contrast, several researchers have demonstrated the effect of minerals on the bioavailability of carotenoids. Borel et al. [23] found that the bioavailability of lycopene found in tomato paste is limited when ingested with 500 mg of calcium, although the mechanism is not completely understood, and micelle formation may be the limiting factor. Corte-Real et al. [24] found that the bioavailability of spinach carotenoids is not affected by 500 mg and 1000 mg of calcium.

Adding fat to the food improves the bioavailability of carotenoids as lipids favor micelle formation through the release of bile salts [2]. In this sense, Marriage et al. [25] showed that plasma concentrations of lycopene and zeaxanthin are higher when carotenoids are ingested with mono-and diacylglycerides (safflower oil) than when fat is not consumed. Similarly, White et al. [26] studied the effect of soybean oil on the absorption and bioavailability of carotenoids from spinach, lettuce, carrot, and tomato. The plasma concentrations of α-carotene, β-carotene, lutein, and lycopene increased as the concentration of soybean oil increased.

Thermal treatment affects both the cell wall and carotenoid content of plants, in turn altering their bioavailability. Aschoff et al. [27] demonstrated that the bioavailability of β-cryptoxanthin, zeinoxanthin and lutein in pasteurized orange juice is higher than in fresh orange juice. In contrast, Vimala et al. [28] evaluated carotenoid content in sweet potato undergoing different treatments (cooking, frying, oven-drying, and sun-drying). Oven-drying (50–60 °C) maintained 90% of β-carotene in sweet potato compared to the fresh product, whereas all other treatments decreased carotenoid content between 15% and 30%. Odriozola-Serrano et al. [29] examined the effect of pasteurization and electrical pulses on the carotenoid content of tomato juice. They found that tomato juice treated with electrical pulses had a higher carotenoid content. Thus, pulse treatment is the most efficient method of preserving carotenoid content and increasing their bioavailability compared to the traditional treatment. In all previously cited examples, there is a decrease in total carotenoid content; nevertheless, the bioavailability of carotenoids improves by reducing dietary fiber, releasing cellular content, softening plant material, and reducing the interactions between carotenoids and other food components. Thus, promoting both the release of carotenoids and formation of micelles helps increase their absorption.