580
Proteins and Amino acids Although protein is a macronutrient that is easily
absorbed by the human body, its bioavailability is directly linked to digestibility
(absorption). The digestibility of proteins is influenced by total dietary energy intake
and the quality of protein in terms of meeting metabolic demand. Heat treatment
during processing leads to the formation of Maillard compounds and reduced lysine
availability. Oxidization of sulfur containing amino acids reduced the bioavailability of tryptophan and threonine. High pH induces racemization of L–amino acid
residues to D–isomers and formation of cross–linked amino acids, such as lysinoalanine, which also reduces bioavailability. Many foods contain other bioactive
substances that may inhibit amino acid bioavailability. These inhibitors may be
naturally occurring (e.g. tannins, phytates, trypsin inhibitors, glucosinolates, isothiocyanates), formed during processing (e.g. D–amino acids and lysinoalanine), or
formed during genetic modification of crops (e.g. lectins in lentils) (lectins suppress
growth at low levels and are toxic at high levels) (Lewis and Bayley 1995; Gibney
et al. 2009; Schönfeldt et al. 2016; Ribarova 2018).
Carotenoids Carotenoid absorption is a comparatively slow process (Afreen et al.
2018; Leopoldini et al. 2011). Only six carotenoids and their metabolites have been
reported to be metabolized in human tissues, particularly in the intestine. The bioavailability of carotenoids varies widely because of dependence on the structure of
the carotenoid itself in food matrix. For example, astaxanthin from Haematococcus
pluvialis (a green microalgae), owing to free carotenoid nature, has greater bioavailability than the polar β–carotene and lycopene from Spirulina platensis, and lutein
from Botryococcus braunii. Esters of xanthophylls however have lower bioavailability. It has been suggested that their hydrolyzation occurs in small intestine for
further absorption (Afreen et al. 2018; Dhankhar et al. 2012; Faraloni and Torzillo
2017). They are partially absorbed, which involves release from plant cells, and then
form micelles with the help of fatty food and bile acids. Evidence from the literature
indicates that thermal treatment and homogenization exerted positive effects on bioaccessibility of these compounds, however, dietary fiber produces a negative effect
on their bioavailability (Afreen et al. 2018; Fernández–García et al. 2012). To make
the carotenoids more bioavailable, the food industry has developed encapsulation of
carotenoids present in fresh vegetables and fruits, employing them in crystal form
or bound to protein complexes (Afreen et al. 2018; Ribeiro et al. 2010).
Phenolics, Phenolic Acids and Flavonoids Flavonoids are important dietary supplements found abundantly in green tea, red wine, and vegetables and fruits like
onions, tomatoes, and apples (Giuliani et al. 2014; Rakers et al. 2014; Wang et al.
2011, 2018). Bioavailability of flavonoids is generally poor as flavonoids exist as
glycosides. Flavonoid aglycones exhibit higher bioavailability and better absorption
than glycosides by virtue of better membrane interactions. In contrast to O–glycosides, C–glycosides are more resistant to hydrolysis. Since absorption and excretion
of O–methylated flavonoids are relatively slow, it facilitates better bioavailability
(Surichan et al. 2012; Wang et al. 2018; Wei et al. 2014). Some reports suggested that
isoflavone aglycones are more rapidly absorbed than their glucosides (Lim and Koffas
K. Sharma et al.
Proteins and Amino acids Although protein is a macronutrient that is easily
absorbed by the human body, its bioavailability is directly linked to digestibility
(absorption). The digestibility of proteins is influenced by total dietary energy intake
and the quality of protein in terms of meeting metabolic demand. Heat treatment
during processing leads to the formation of Maillard compounds and reduced lysine
availability. Oxidization of sulfur containing amino acids reduced the bioavailability of tryptophan and threonine. High pH induces racemization of L–amino acid
residues to D–isomers and formation of cross–linked amino acids, such as lysinoalanine, which also reduces bioavailability. Many foods contain other bioactive
substances that may inhibit amino acid bioavailability. These inhibitors may be
naturally occurring (e.g. tannins, phytates, trypsin inhibitors, glucosinolates, isothiocyanates), formed during processing (e.g. D–amino acids and lysinoalanine), or
formed during genetic modification of crops (e.g. lectins in lentils) (lectins suppress
growth at low levels and are toxic at high levels) (Lewis and Bayley 1995; Gibney
et al. 2009; Schönfeldt et al. 2016; Ribarova 2018).
Carotenoids Carotenoid absorption is a comparatively slow process (Afreen et al.
2018; Leopoldini et al. 2011). Only six carotenoids and their metabolites have been
reported to be metabolized in human tissues, particularly in the intestine. The bioavailability of carotenoids varies widely because of dependence on the structure of
the carotenoid itself in food matrix. For example, astaxanthin from Haematococcus
pluvialis (a green microalgae), owing to free carotenoid nature, has greater bioavailability than the polar β–carotene and lycopene from Spirulina platensis, and lutein
from Botryococcus braunii. Esters of xanthophylls however have lower bioavailability. It has been suggested that their hydrolyzation occurs in small intestine for
further absorption (Afreen et al. 2018; Dhankhar et al. 2012; Faraloni and Torzillo
2017). They are partially absorbed, which involves release from plant cells, and then
form micelles with the help of fatty food and bile acids. Evidence from the literature
indicates that thermal treatment and homogenization exerted positive effects on bioaccessibility of these compounds, however, dietary fiber produces a negative effect
on their bioavailability (Afreen et al. 2018; Fernández–García et al. 2012). To make
the carotenoids more bioavailable, the food industry has developed encapsulation of
carotenoids present in fresh vegetables and fruits, employing them in crystal form
or bound to protein complexes (Afreen et al. 2018; Ribeiro et al. 2010).
Phenolics, Phenolic Acids and Flavonoids Flavonoids are important dietary supplements found abundantly in green tea, red wine, and vegetables and fruits like
onions, tomatoes, and apples (Giuliani et al. 2014; Rakers et al. 2014; Wang et al.
2011, 2018). Bioavailability of flavonoids is generally poor as flavonoids exist as
glycosides. Flavonoid aglycones exhibit higher bioavailability and better absorption
than glycosides by virtue of better membrane interactions. In contrast to O–glycosides, C–glycosides are more resistant to hydrolysis. Since absorption and excretion
of O–methylated flavonoids are relatively slow, it facilitates better bioavailability
(Surichan et al. 2012; Wang et al. 2018; Wei et al. 2014). Some reports suggested that
isoflavone aglycones are more rapidly absorbed than their glucosides (Lim and Koffas
K. Sharma et al.