«Wild Edible Plant Consumption and Age-Related Cataracts in a Rural Lebanese Elderly Population: A Case control Study By Joelle Zeitouny School of ...»
The people who eat wild edible plants do not usually mention them in nutritional surveys (Kabuye & Ngugi, 2001) but the use of these foods, which has evolved over the decades, has served to provide food and maintain general health among populations. In fact many of the food plants are used for both nutrition and medicine (Kabuye & Ngugi, 2001).In Jordan, Tukan et al (1998) showed different uses of common edible wild plants such as sumac (Rhus coriara), chicory (Cichorium pumilum Jacq.), Spanish thistle (Centaurea iberica Trev. Ex. Sprengel.), wild lettuce (Lactuca tuberosa Jacq.), viper’s grass (Scorzonera papposa DC.), goat’s beard (Tragopogon coelesyriaca Boiss.) and gundelia (Gundelia tournefortii L.).
Interestingly, over half of these plants were consumed raw without any preliminary preparation other than cleaning and trimming. Many were also consumed as snacks thus providing important sources of nutrients as compared to some modem emptycalorie foods. Tukan et al. (1998) also highlighted the numerous ways of consuming such plants as part of salads, stews, spices or seasoning or even as hot drinks.
However, the dietary intake pattern of people worldwide is changing from a traditional diet (i.e. one containing plant and animal foods harvested from the local environment) to one containing many manufactured, processed, and otherwise nontraditional foods (Kuhnlein & Receveur, 1996). The rapid urbanization in the region (particularly in Lebanon) might suggest a decrease in the consumption of wild 4 edible plants and a break in the transmission of indigenous knowledge. The risks of the transition from a primarily traditional diet to one containing more market (i.e.
store-bought) foods include an increase in the prevalence of chronic diseases and a decrease in the dietary intakes of some key micronutrients that are present (often in abundance) in wild edible plants (Whiting & MacKenzie, 1998). In a recent study by Batal and Hunter (2007), nutrient and food composition analyses on Lebanese wild edible plant-based dishes revealed that the latter offered a healthier alternative to increasingly common processed dishes. In fact, wild edible plants’ nutritional content is superior in vitamin and mineral content to widely raised domesticated field crops (Calloway et al., 1974; Grivetti & Ogle, 2000; Farhat, 2006). In a study conducted by Humphry et al. (1993) in eastern Niger, more than eighty edible wild species were regularly used by 93% of households and contributed substantial amounts of Cu, Fe, Mg, and Zn to the diet. In Gambia, edible wild plants, especially leaf sauces prepared from edible species, are important during pregnancy and lactation (Villard & Bates, 1987). In Bangladesh, dark green leaves are major sources of pro-vitamin A (Zeitlin et al., 1992). In fact, dark leafy greens are an excellent source of carotenoids such as vitamin A precursors (such as α- and β-carotene) and xanthophylls (such as lutein and zeaxanthin) (Krinsky & Johnson, 2005).
2.3 CAROTENOIDS AND HEALTH
Carotenoids are a family of natural pigments that are widely distributed in nature and contribute to the color in plants and their fruits (Krinsky & Johnson, 2005). β-carotene is probably the best studied carotenoid because of its importance as a vitamin A precursor; however, it is only one of the approximately 600 naturallyoccurring carotenoids (Krinsky & Johnson, 2005). In addition to β-carotene, αcarotene, lycopene and lutein (Figure 2.1) are important carotenoid components of the human diet (Micozzi et al., 1990).
5 Figure 2.1: The structures of the predominant carotenoids found in human plasma (Krinsky & Johnson, 2005).
Carotenoids are mainly known for their antioxidant properties but they have also been shown to inhibit the growth of tumor cell lines, prevent bacterial mutagenesis, and modulate genotoxicity (Krinsky & Johnson, 2005). The structure of the carotenoids (especially their conjugated double bond system) gives rise to many of their fundamental properties (including their antioxidant properties) and also affects how they are incorporated into biological membranes (Figure 2.2). This, in turn, alters the way they interact with reactive oxygen species, so that the in vivo behaviour may be quite different from that seen in solution (Young & Lowe, 2001).
Figure 2.2: How the structure of the carotenoids affects their incorporation into biological membranes.
β-carotene (and other carotenes such as lycopene) lies parallel with the membrane surface, deep within the hydrophobic core. In contrast, the dihydroxy carotenoid zeaxanthin entirely spans the membrane and therefore reactions with its conjugated C=C bonds are possible throughout the depth of the membrane (Young & Lowe, 2001).
Carotenoids were implicated as protective agents, first against lung cancer and then against a variety of other chronic diseases. The results of 10 to 17 case-control studies show that a high intake of fruit and vegetables that are rich in carotenoids has been associated with decreased risk of cancer (Kinsky & Johnson, 2005). However, intervention trials employing β-carotene either have shown no preventive effect or indeed, in two cases, have enhanced the incidence of lung cancer in middle-aged male smokers and asbestos workers (Olson, 1999).On the other hand, lycopene seems to be protective against prostate cancer and plasma levels of α and β-carotene, lycopene, and lutein seem to be inversely related to ischemic stroke and myocardial infarctions (Kinsky & Johnson, 2005). In addition, lutein and zeaxanthin have been suggested to be protective against age-related macular degeneration and cataracts (Krinsky & Johnson, 2005).
2.4 LUTEIN AND ZEAXANTHIN 2.4.1 About lutein and zeaxanthin Lutein and zeaxanthin are oxygenated carotenoids (xanthophylls) that consist of 40-carbon hydroxylated compounds (Pfander, 1992). Lutein was discovered in 1869 by a chemist at St. Thomas’s Hospital in London called Johann Ludwig Wilhelm Thudichum who found, in parts of plants and animals, a yellow crystallizable substance, that he named ‘luteine’. Zeaxanthin, on the other hand, was isolated from maize and characterized in 1929 by the Swiss biochemist Paul Karrer (Karrer et al., 1929).
7 Lutein and zeaxanthin cannot be synthesized by humans and must be obtained through diet. Foods that are rich in lutein and zeaxanthin include egg yolk, corn, orange juice, honeydew melon, orange pepper, and dark green leafy vegetables such as kale, spinach, collards, turnip greens, broccoli, and all kinds of wild leafy plants (Sommerburg et al., 1998; Holden et al., 1998). Recently, in 2005, Calvo compiled from the literature a huge database on the lutein composition of fresh fruits and vegetables or of fruits and vegetables submitted to different treatments. Adults on average consume around 1-2 mg of lutein per day but levels of around 3 mg per day can be easily achieved with a high fruit and vegetable diet (O’Neill et al., 2001;
Mares-Perlman et al., 2001).
Of the 40 to 50 carotenoids typically consumed in the human diet, lutein and zeaxanthin are deposited at an up to 5-fold higher content in the macular region (Figure 2.3) of the retina as compared to the peripheral retina (Handelman et al., 1988). Zeaxanthin is preferentially accumulated in the foveal region whereas lutein is abundant in the perifoveal region (Bone et al., 1988).
Figure 2.3: The human eye shown in a 3D structure.
Lutein and zeaxanthin are the only carotenoids present in the macula lutea, located in the mid portion of the retina, and the lens, two ocular tissues required for vision (Alves-Rodrigues & Shao, 2004).
Figure 2.4: The structures of the three major components of the macular pigment (Bone et al.
Lutein, zeaxanthin and meso-zeaxanthin represent about 36%, 18%, and 18% of the total carotenoid content of the retina but 100% of the total carotenoid content in the macula (Landrum & Bone, 2001). In addition, lutein and zeaxanthin are the only carotenoids reported to be present in eye lens (Yeum et al., 1995). What is more, there seems to be an inverse relation between macular pigment density and lens density, suggesting that the macular pigment may serve as a marker for xanthophylls in the lens. As a matter of fact, Hammond et al. (1997) have shown that higher levels of retinal lutein and zeaxanthin in the elderly were related to more transparent lenses.
One interpretation of these results is that higher retinal lutein and zeaxanthin may predict higher lenticular lutein and zeaxanthin, which could lead to a more protected lens resulting in increased transparency.
92.4.2 Absorption, transport, and bioavailability
During the digestion process (Figure 2.5), dietary lutein or zeaxanthin esters are hydrolyzed in the lumen of the small intestine prior to uptake by the mucosa (Wingerath et al., 1995). Their bioavailability is important since they are not synthesized by humans and since most of the studies performed reported that they had a low bioavailability in general (Calvo, 2005).
Figure 2.5: Steps of carotenoid absorption and dietary factors that affect carotenoid absorption (Van het Hof et al.
10 Many factors influence lutein or zeaxanthin bioavailability (Figure 2.5). One important factor is solubility (Amar et al., 2003). Other factors include food source, dietary fat, food preparation and processing (Van het Hof, 1999), and interaction with other carotenoids (Tyssandier et al., 2002). The most likely mechanism for this interaction is competition between the carotenoids for incorporation into the mixed micelles during digestion. Another possible mechanism could be that there is competition between carotenoids for uptake and metabolism in the enterocytes or for incorporation into the chylomicrons.
Furthermore, Van het Hof et al. (1999b) found that the bioavailability of lutein varies substantially among different vegetables. Riso et al. (2003) for example, found that the ingestion of both spinach and broccoli raises the serum levels of lutein a few hours after eating but that after the ingestion of spinach, serum lutein concentration remains high until 80 hours after ingestion. The above studies have determined the plasma response of carotenoids after supplementation with vegetables or fruits and compared it with the response to supplementation with pure carotenoids. "Relative carotenoid bioavailability" is obtained by dividing the plasma responses that are induced by vegetables or fruit consumption and corrected for differences in intake by those induced by pure carotenoid supplementation (Van het Hof et al., 2000). The relative bioavailability of lutein from a diet supplemented with a variety of vegetables is much greater than that of ß-carotene (67 and 14%, respectively) (Van het Hof et al., 1999a). The same was found for the relative bioavailability of lutein and ß-carotene from spinach (45 and 5.1%, respectively) (Castenmiller et al., 1999).
Also, the amount of fat in the diet affects carotenoids’ bioavailability. For example, the amount of fat required for optimal intestinal uptake of lutein esters is higher than the amount of fat required for optimal uptake of vitamin E and α- and βcarotene (Roodenburg et al., 2000). On the other hand, sucrose polyester fat substitutes (Olean, olestra) can lower concentrations of serum lutein and other carotenoids (Westrate &Van het Hof, 1995) but apparently do not affect possible markers of disease risk in humans (Broekmans et al., 2003). Cholesterol-lowering 11 phytosterol and stanol products also reduce carotenoid concentrations, albeit to a lesser extent (Weststrate & Meijer, 1998; Plat &Mensik, 2001).
Once in the human blood stream, high-density lipoproteins (HDL) are the major carriers of lutein and zeaxanthin while carotenes are preferentially carried by low-density lipoproteins (LDL) (Clevidence &Bieri, 1993). Every 10% increase in estimated dietary intake of lutein and zeaxanthin was associated with a 2.4% increase in serum lutein concentration (Rock et al., 2002). Other determinants of serum lutein and zeaxanthin include cholesterol and lipoprotein status, metabolic status, body composition, smoking and BMI (Rock et al., 2002). Dietary lutein may be converted to mesozeaxanthin (Bone et al., 1993) or serve as a precursor for the very high concentrations of zeaxanthin found in the primate fovea (Bone et al., 1997).
Lutein and zeaxanthin-rich diets and serum lutein and zeaxanthin positively contribute to the macular pigment status in the retina (Burke et al., 2005). The macular pigment’s optical density increases with increasing dietary intake and serum levels of lutein and zeaxanthin (Figure 2.6).
Figure 2.6: Macular pigment optical density versus dietary (μg/d) and serum (μmol/L) lutein and zeaxanthin.
Subjects’macular pigment optical density was positively associated with both dietary (r =0.237, P =0.02, n =96) and serum (r =0.342, P =0.0006, n = 95) lutein and zeaxanthin (Burke et al., 2005).
2.4.3 Mechanism of action As a highly vascularized tissue possessing a high concentration of polyunsaturated fatty acids, the macula is particularly susceptible to oxidative damage (Beatty et al., 1999). Lutein and zeaxanthin are believed to protect the retina in two ways. Firstly, they are thought to play a role in the protection against light-dependent damage. By absorbing blue-light, they protect the underlying macular photoreceptor cell layer from light damage (possibly initiated by the formation of reactive oxygen species during a photosensitized reaction). The high absorptivity of lutein and zeaxanthin in the inner retina functions as an efficient filter for blue light. This filtering effect reduces chromatic aberration and short wavelength sensitivity (Reading & Weale, 1974). It is also believed to be responsible for the relative preservation of foveal short-wavelength cone sensitivity with age (HaegerstromPortnoy, 1988).
Blue light is the highest energy form of visible light, and is known to induce photo-oxidative damage by generating reactive oxygen species (Alves-Rodrigues & Shao, 2004). The absorbance spectrum of macular pigment peaks at 460 nm (Pease et al., 1987) but lutein has an absorption maximum of about 445nm (Figure 2.7) and zeaxanthin of about 451 nm (Schmidt-Erfurth, 2005).