An Update on the Health Effects of Tomato Lycopene

INTRODUCTION

Epidemiological research indicates that diets rich in fruits and vegetables are associated with a decreased risk of chronic diseases such as cardiovascular disease (CVD) (Ignarro et al. 2007) and cancer (Block et al. 1992). It is estimated that approximately 50% of cancer cases and 35% of cancer deaths in the United States can be attributed to poor diet (Williams et al. 1999). Epidemiological studies have associated tomato consumption with a decreased risk of prostate cancer ( Jain et al. 1999, Giovannucci et al. 2002) and CVD (Arab & Steck 2000). Despite this evidence, it is not clear which individual compounds present in tomato, such as lycopene, impart these potential benefits or whether other constituents of tomatoes and tomato products produce beneficial effects.

Lycopene, a naturally occurring red carotenoid pigment found in tomatoes, pink grapefruit, watermelon, papaya, guava, and other fruits, has been extensively studied for more than 70 years, with more than 2000 articles published in peer-reviewed journals and 4000 other publications (scientific and otherwise) written on the subject. Most of these articles have focused on lycopene derived from tomatoes. Given the vast amount of information already published about lycopene, we focus on the recent literature (1999–2009) related to the health effects of tomato lycopene in humans. Although in vitro and animal studies are vitally important to understanding the mechanisms behind potential health effects, human studies, specifically clinical trials, are ultimately used to determine the effect of dietary constituents on health, as well as to set nutrition and food labeling policy. For more information on in vitro and animal studies related to the potential health effects of lycopene, please see the reviews by Rao et al. (2006) and Cohen (2002). Over the past decade, lycopene-containing foods (primarily tomato products) and lycopene supplements have been reported to affect diseases ranging from cancer to heart disease to asthma (Dahan et al. 2008, Riccioni et al. 2008, Wood et al. 2008). Most recently, researchers have begun to investigate lycopenoids, oxidative metabolites of lycopene, based on the possibility that these lycopenoids may produce biological effects (Erdman et al. 2009).

Given the scope of literature published on the potential health benefits of this carotenoid in the diet, herein we review publications related to lycopene chemistry, sources, intake, and bioavailability. In addition, we summarize the literature on the presence and formation of lycopenoids, and discuss the most promising directions for future lycopene research.

LYCOPENE CHEMISTRY

Lycopene is one pigment in a large family of plant pigments known as carotenoids. Carotenoids produce colors ranging from the yellow color of squash, to the orange color of pumpkins, to the red color of tomatoes. Carotenoids also contribute to some plant food aromas (Rodriguez- Bustamante & Sanchez 2007). Some carotenoids also possess provitamin A activity and have shown potent antioxidant activity. To date, more than 700 carotenoids have been identified (Britton et al. 2004). There are two primary types of carotenoids: hydrocarbon carotenoids and xanthophylls. Hydrocarbon carotenoids, such as lycopene, are composed entirely of hydrogen and carbon. In contrast, xanthophylls, such as lutein, contain oxygen in addition to carbon and hydrogen (Furr & Clark 1997). Some hydrocarbon carotenoids, such as β-carotene and α-carotene, can be enzymatically cleaved to form vitamin A. Lycopene lacks provitamin A activity because of the absence of a terminal beta ionone ring (Rao & Rao 2007). Carotenoids typically contain 40 carbons. Apo-carotenoids contain less than 40 carbons. The prefix apo is used to identify carotenoids that have been shortened in length by one or more carbons (Britton et al. 2004). Regardless of the number of carbons, all carotenoids possess an isoprenoid backbone (Britton et al. 2004).

The chemical formula for lycopene is C₄₀H₅₆. The 11 conjugated and 2 unconjugated double bonds present in lycopene allow for extensive isomerization, resulting in 1056 theoretical cis-trans configurations (Zechmeister 1944). Only a few isomers are actually found in nature, however, (Figure 1) with the all-trans configuration of lycopene being the most common isomer found in foods (Nguyen & Schwartz 2000). The thermodynamic stabilities of the common lycopene isomers have been determined relative to the all-trans isomer. The 5-cis isomer was found to be the most stable followed by all-trans, 9-cis, 13-cis, 15-cis, 7-cis, and 11-cis (Chasse et al. 2001). The lycopene isomers found in human blood plasma, breastmilk, and human tissues are mainly of the cis-isomer type (Hadley et al. 2003, Allen et al. 2002, Ferruzzi et al. 2001, Boileau et al. 2002). The color of lycopene is directly related to its isomeric form. The all-trans isomer and most other isomers of lycopene are red, whereas tetra-cis-lycopene possesses an orange hue (Nguyen & Schwartz 2000, Zechmeister 1944).

Reactive oxygen species (ROS) are oxygen-containing molecules that either are or have the potential to generate free radicals. Overproduction of ROS results in a condition known as oxidative stress, which has been linked to both cancer and cardiovascular disease (Halliwell 1994). Carotenoids, including lycopene, can be potent antioxidant molecules and are especially effective at scavenging the ROS singlet oxygen. Of the carotenoids, lycopene is the most effective singlet oxygen scavenger in vitro (Sies & Stahl 1995). This antioxidant activity has been proposed as a mechanism for the potential health benefits of carotenoids (Sies & Stahl 1995, Agarwal & Rao 2000). Recently, this antioxidant mechanism has been called into question given the low concentration of lycopene in the body relative to other antioxidant molecules, such as vitamin E (Erdman et al. 2009). This has led to speculation that observed health benefits may be due to the effect of lycopene or oxidative metabolites on gene expression (Erdman et al. 2009).

LYCOPENE SOURCES, INTAKE, AND BIOAVAILABILITY

More than 80% of dietary lycopene intake in the U.S. is derived from processed tomato products such as ketchup, tomato juice, spaghetti sauce, and pizza sauce (Clinton 1998). The amount of lycopene present in processed foods is often much higher than that found in fresh foods given that processing often involves concentration via water loss. For example, ketchup contains 9.9–13.44 mg lycopene/100 g, whereas fresh tomatoes contain anywhere from 0.88–7.74 mg lycopene/100 g wet weight (Rao et al. 1998, Nguyen & Schwartz 1998).

Dietary intake of lycopene varies greatly depending upon the studied population. The average Italian consumes 14.3 mg/day of total carotenoids (Lucarini et al. 2006). Lycopene makes up the largest proportion of carotenoids in the Italian diet, with an average intake of 7.4 mg/day (Lucarini et al. 2006). The average daily intake of lycopene in the United States ranges from 6.6–10.5 mg/day for men and from 5.7–10.4 mg/day for women (Porrini & Riso 2005). The average reported daily lycopene intake is 1.1 mg/day in the United Kingdom, 1.6 mg/day in Spain, 3.8 mg/day in Australia, 4.8 mg/day in France, and 4.9 mg/day in the Netherlands (Porrini & Riso 2005).

Lycopene bioavailability can be affected by a number of factors, including food processing and dietary composition. Lycopene can occur in several forms in fresh plant foods, including carotenoid-protein complexes in chloroplasts or in crystalline form inside chromoplasts (Parada & Aguilera 2007). The effects of processing and storage on lycopene structure and stability are of interest for a number of reasons. Improper processing and storage (i.e., exposure to light and oxygen) may alter the ratio of lycopene isomers or degrade lycopene entirely, making these food products less desirable to the consumer (Xianquan et al. 2005). Traditional commercial processing methods do not have a significant effect on lycopene levels or on cis/trans isomerization (Nguyen & Schwartz 1998). In fact, thermal processing generally improves lycopene bioavailability by disrupting cellular membranes, which allows lycopene to be released from the tissue matrix (Nguyen et al. 2001). Multiple studies have shown that lycopene from thermally processed tomato products is more bioavailable than lycopene from fresh tomatoes (Gärnter et al. 1997, Stahl & Sies 1992, Allen et al. 2002).

The absolute amount of lycopene absorbed does not appear to vary greatly with dose. A study performed in men observed lycopene absorption after consumption of one serving of tomato juice (Diwadkar-Navsariwala et al. 2003). Different volumes of tomato juice, with a constant percent fat, were administered to deliver 10 mg to 120 mg of lycopene. The range of lycopene absorbed, independent of dose, was between 1.8 mg and 14.3 mg, with an average of 4.7 mg. The amount of lycopene absorbed by the men consuming juice containing 120 mg lycopene was not significantly different from that absorbed by the men consuming juice containing 10 mg lycopene. These authors suggested that inter-individual differences, not dose, has the greatest impact on the amount of lycopene absorbed (Diwadkar-Navsariwala et al. 2003).

Lycopene bioavailability is greatly affected by dietary composition. Given that lycopene is a lipid-soluble compound, consuming it with fat increases its bioavailability. For example, consuming salads with full-fat dressing results in higher blood carotenoid levels than eating salads with reduced fat dressing. When salads were consumed without fat in the same study, no measurable lycopene uptake occurred (Brown et al. 2004). A study by Unlu et al. (2005) showed a similar result, whereby the consumption of tomato salsa with avocado (as lipid source) led to a 4.4-fold increase in lycopene absorption as compared with salsa without avocado.

A schematic of lycopene digestion and absorption is shown in Figure 2. Once ingested, lycopene must first be released from the food matrix before it is incorporated into mixed micelles. Micelles contain bile salts, cholesterol, and fatty acids from the meal, and the amphiphilic nature of the micelle structure helps to keep the lipophilic nutrients soluble in the aqueous digesta (During & Harrison 2004). The micelles approach the unstirred water layer of the apical side of the intestinal cells (enterocytes), and lycopene passively diffuses across the apical membrane (During & Harrison 2004). Historically, researchers believed that lycopene was absorbed by the same route as dietary lipids, i.e., passive diffusion (Furr & Clark 1997), and this is still believed to be at least partially true. However, in the past five years, investigators have discovered that lycopene absorption can be facilitated by a cholesterol membrane transporter known as scavenger receptor class B type I (SR-BI) (During et al. 2005, Moussa et al. 2008). Research has also suggested that lycopene absorption may be facilitated by other transporters, but this has not yet been confirmed (During et al. 2005, Moussa et al. 2008). Once inside the enterocyte, lycopene is packaged with other dietary lipids into chylomicrons (During & Harrison 2004). Chylomicrons are then transported across the basolateral membrane and make their way into the lymphatic system, which eventually releases chylomicrons into the blood.

Competition by other carotenoids or cholesterol may also influence lycopene absorption. One post-prandial human study demonstrated that co-consumption of tomato puree (30 mg lycopene) + spinach lutein (30 mg) or encapsulated lutein (30 mg) reduced chylomicron lycopene levels by 70% and 61% respectively, as compared with lycopene levels observed after consumption of tomato puree alone (Tyssandier et al. 2002). However, when subjects consumed these foods daily for three weeks at half of the previous dose (15 mg of lycopene + 15 mg lutein), no difference was observed in steady-state plasma levels of lycopene (Tyssandier et al. 2002). A human study by Johnson et al. (1997) observed that co-administration of β-carotene crystals in oil (60 mg) with lycopene crystals in oil (60 mg) increased the 24-hour postprandial serum area under the curve (AUC) of lycopene by four-fold as compared with the administration of lycopene alone. It is unclear whether absorption of lycopene as part of a food product would increase if co-consumed with a β-carotene–containing food product. Considering the recent research demonstrating the transport of lycopene by cholesterol transporter SR-BI, it is likely that genetic differences in SR-BI expression may also affect the absorption of lycopene. However, no studies have yet been published on this topic.

In addition, lycopene absorption may be affected by other factors such as probiotics and single nucleotide polymorphisms (SNPs). Consumption of a probiotic-containing yogurt versus a regular yogurt lowered blood lycopene levels in a group of 17 women who consumed probiotics for a total of 4 weeks (Fabian & Emaldfa 2007). This study suggests that probiotics may affect lycopene bioavailability or metabolism (Fabian & Emaldfa 2007). Borel et al. (2007) reported that human blood carotenoid levels are influenced by SNPs in apolipoproteins A-IV and B, which are associated with lipid transport.

Greater than 90% of the lycopene found in processed tomato products is in the all-trans conformation (Nguyen & Schwartz 1998, Nguyen et al. 2001, Boileau et al. 2002). In vivo studies demonstrate that the cis-isomers of lycopene appear to be more bioavailable than the all-trans isomer (Stahl & Sies 1992, Unlu et al. 2007). In vitro experiments support the conclusion that increased bioavailability of lycopene cis-isomers is at least partially due to increased micellarization and increased uptake by the enterocyte relative to all-trans lycopene (Failla et al. 2008).

Human organs store lycopene to varying degrees. Lycopene is found in the highest concentrations in the liver, testes, adrenal glands, and adipose tissues (Kun et al. 2006). It is found in lower concentrations in the kidney, ovary, lung, and prostate (Kun et al. 2006).

POTENTIAL HEALTH BENEFITS OF LYCOPENE

Over the past decade, the effects of lycopene have been studied with respect to a wide range of diseases (various cancers, CVD) (Dahan et al. 2008, Riccioni et al. 2008). Here we summarize data from recent human studies examining lycopene’s biological effects on these disease processes.

LYCOPENOIDS: POTENTIAL OXIDATIVE METABOLITES OF LYCOPENE

Researchers have recently used the term lycopenoids to refer to oxidative products of lycopene that contain at least one oxygen atom (Lindshield et al. 2007). Historically, lycopenoids have been reported in tomatoes and tomato products, but only recently have they garnered attention from individuals interested in the potential health benefits of lycopene or its metabolites. This newfound interest is largely stimulated by the recent discovery of a second carotenoid cleavage enzyme in mammals (β-carotene oxygenase 2, or BCO2) and the possibility that it may cleave lycopene to produce metabolites (Kiefer et al. 2001). Previous to this report, only one carotenoid cleavage enzyme, the enzyme responsible for converting provitamin A carotenoids into vitamin A, was known to exist in mammals.

A handful of lycopenoids have been identified in food products. Apo-6′-lycopenal and apo-8′-lycopenal were reported in tomato paste (Winterstein et al. 1960). Later in 1973, Ben-Aziz et al. confirmed the presence of apo-6′-lycopenal and apo-8′-lycopenal in raw tomatoes, indicating that these products are present in the unprocessed fruit as well. Recently, we observed apo-10′-, -12′-, and -14′-lycopenal (in addition to the previously reported apo-6′- and apo-8′-lycopenals) in raw tomatoes, watermelon, grapefruit, and tomato products (Kopec et al. 2008, 2009). Epoxide derivatives have also been reported in tomatoes. Lycopene-1,2-epoxide and lycopene-5,6-epoxide were identified in tomatoes and tomato products by multiple groups (Britton & Goodwin 1969, 1975; Ben-Aziz et al. 1973; Kamber & Pfander 1984; Khachik et al. 1992). Later, nuclear magnetic resonance (NMR) research revealed that lycopene-5,6-epoxide had been misidentified and was actually 2,6-cyclolycopene-1,5-epoxide (Khachik et al. 1998b). An alcohol derivative, 2,6-cyclolycopene- 1,5-diol, was also identified in tomato products and later found to be present in human blood plasma and breast milk (Khachik et al. 1992, 1997, 1998a). These researchers hypothesized that 2,6-cyclolycopene-1,5-epoxide was converted to 2,6-cyclolycopene-1,5-diol in humans, presumably during digestion, given that none of the epoxide was identified in human samples, and the alcohol is present in small quantities in the food (Khachik et al. 1998b).

The presence of apo-lycopenoids in tomato tissues is not surprising. Analysis of the Arabidopsis genome has revealed nine potential carotenoid cleavage enzymes (Schwartz et al. 2001). Carotenoid cleavage dioxygenase 1 (CCD1) in the annatto plant has been shown to cleave lycopene to produce bixin aldehyde, a precursor of the carotenoids bixin and norbixin (Bouvier et al. 2003). CCD1 from tomato has also been shown to cleave lycopene at the 5′-6′ and 9′-10′ double bonds when transfected into an Escherichia coli model that accumulates lycopene (Vogel et al. 2008). The nonvolatile fragments of these reactions would be apo-6′-lycopenal and apo-10′-lycopenal, respectively (see Figure 3), although they were not investigated in this study (Vogel et al. 2008). Carotenoid cleavage dioxygenase 7 (CCD7) has been shown to cleave the 9′-10′ bond of lycopene (Schwartz et al. 2004). However, it is not known if the other identified CCDs cleave lycopene. Alternatively, lycopene could be broken down nonspecifically by reaction with reactive oxygen species or co-oxidation with other lipids in the plant tissue.

More recently, lycopenoids have been identified in the tissues of animals consuming lycopene. In one study, ferrets consumed a lycopene supplement (LycoVit® powder, 10% lycopene) daily for nine weeks (Hu et al. 2006). Following sacrifice, apo-10′-lycopenol was found in the lung tissue of these animals (Hu et al. 2006). In a separate study by Gajic et al. (2006), rats were fed lycopene as part of their daily diet for 50 days, followed by a radioactive dose of lycopene on day 51. Rat livers were then extracted and apo-8′-lycopenal and putative apo-12′-lycopenal were identified (Gajic et al. 2006). In addition, we identified apo-6′, -8′-, -10′-, -12′-, and -14′-lycopenal in the blood plasma of humans who had consumed 300 mL of a tomato juice based beverage daily for eight weeks (Kopec et al. 2008, 2009).

The presence of lycopenoids in animals has been hypothesized to be the result of enzymatic cleavage. In fact, lycopene has been shown to be cleaved enzymatically by BCO2 in vitro (Keifer et al. 2001, Hu et al. 2006). Other researchers have incubated lycopene with various tissue homogenates ex vivo and produced oxidative products, but it is unclear whether these are products of a BCO reaction or whether they could be produced in vivo (dos Anjos Ferreira et al. 2004).

There are very limited data on the biological effects of lycopenoids. Apo-10′-lycopenoic acid has been shown to inhibit the growth of BEAS-2B human bronchial epithelial cells in vitro (Lian & Wang 2008). The feeding of apo-10′-lycopenoic acid has also been shown to reduce lung tumor multiplicity in an A/J mouse model, an effect that was dose dependent (Lian et al. 2007).

CONCLUSIONS

In this paper, we summarized lycopene chemistry, sources, intake, bioavailability, and recent data regarding the potential health effects of lycopene. Many questions remain, however. First, and most importantly, does lycopene produce health effects in humans? A review of the epidemiological literature by Giovannucci (2005) suggests that it is still too early to determine whether tomatoes or lycopene have health benefits.

Further, it is unclear whether apparent reductions in disease risk observed in epidemiological and short-term prospective studies result from the whole tomato or from lycopene alone (Giovannucci 2005, Clinton 2005). Lycopene studies have been performed with tomato products, tomato-based supplements, and synthetic lycopene. These treatments are not interchangeable and should not be considered equivalent. In fact, the distinction between lycopene and tomatoes was made by the Food and Drug Administration (FDA) in their 2005 review of the literature to evaluate a proposed health claim on tomatoes, lycopene, and cancer (Kavanaugh et al. 2007). The FDA concluded that there is “no credible evidence to support an association between lycopene intake and a reduced risk of prostate, lung, colorectal, gastric, breast, ovarian, endometrial or pancreatic cancer,” but the FDA found “very limited evidence to support an association between tomato consumption and reduced risks of prostate, ovarian, gastric, and pancreatic cancers” (Kavanaugh et al. 2007). This conclusion was based on the limited high-quality human clinical trial data necessary to meet FDA standards and could change if more rigorously designed studies with positive results are reported.

As with other natural products with potential health benefits, lycopene continues to be researched because of promising epidemiological data as well as encouraging results in cell culture and animal studies. Discrepancies between animal data, which are generally positive, and human data, which are less clear, may be due to differences in carotenoid absorption and metabolism in humans relative to other species as well as inter-individual differences in humans. Animal studies are typically conducted using inbred animals, reducing genetic variability and producing clearer results. Lycopene’s effects may vary from person to person based on dietary lycopene and fat intake, probiotics, genetic differences in metabolism, and other factors. One further limitation of human studies is the lack of easily accessible, specific disease biomarkers to estimate the effects of a treatment on a disease process. Although PSA is frequently used to monitor prostate-cancer risk, and LDL is widely accepted as a marker for cardiovascular disease, many other diseases lack specific biomarkers. For this reason, more general markers, such as IGF-I or 8-OHdG, are often used to assess the effects of lycopene or tomato products on biochemical disease processes.

When considering the effects of a dietary component on health, it is difficult to separate the effect of a single compound from that of multiple compounds found in whole foods and whole diets. If lycopene in tomatoes does affect health, is it the major active component, or does it act synergistically with other bioactive compounds in tomatoes (provitamin A, flavonoids, vitamin C, fiber, etc.)? In fact, tomato flavonoids, including rutin, quercetin, naringenin, have been reported to have potential health effects. Quercetin and rutin have been shown to reduce IGF-1-induced prostate cell proliferation in vitro (Wang et al. 2003). Quercetin has been shown to reduce neutrophil-induced LDL oxidation (Loke et al. 2008). Naringenin chalcone from tomato skin has also been shown to produce anti-inflammatory effects in mice (Yamamoto et al. 2004). These studies provide evidence that flavonoids may play a role in the health effects of tomatoes and tomato products. Other studies have demonstrated that glycoalkaloids present in tomato also produce multiple biological effects (see review by Friedman 2002). In addition, recent research has suggested that water soluble components of tomatoes may reduce platelet aggregation, a risk factor for cardiovascular disease (O’Kennedy et al. 2006), although the components present in this fraction have not been clearly identified. Likewise, some authors have suggested that blood levels of lycopene and other carotenoids are simply indicative of a diet that includes fruits and vegetables. A growing body of evidence indicates that whole foods may be more effective than individual compounds for lowering disease risk (Clinton 2005, Gómez-Romero et al. 2007).

If lycopene does produce positive health effects, what are the mechanisms involved? Does lycopene act by itself, or is it metabolized into a biologically active compound in humans? Are the oxidative products of lycopene found in foods (lycopenoids) absorbed? Lycopene can be a potent antioxidant under the appropriate chemical conditions, but it is present at such low levels in the human body some researchers have speculated that any health effects observed may be due to the effects of lycopene or its metabolites on gene regulation (Erdman et al. 2009). However, the accumulation of lycopene in some organs may allow for multiple mechanisms of action, including antioxidant activity. Evidence from animal studies suggests that lycopene could be metabolized in the human body (Hu et al. 2006, Gajic et al. 2006), whereas data from human studies suggest that oxidative products of lycopene can be absorbed and further metabolized in humans (Khachik et al. 1992, 1997, 1998a, 1998b). It is possible that there are other bioactive lycopene metabolites that are yet to be identified. Further work must be done to understand how lycopene is metabolized in fruits and vegetables and whether these products are absorbed into the bloodstream and distributed and stored in tissues. In addition, more research is needed to understand how lycopene is metabolized in humans and whether these metabolites have biological effects.