Several studies have demonstrated that fruits and vegetables exert a protective effect against the development of human diseases such as cardiovascular disease, diabetes and cancer [1] -[4] . It has been hypothesized that the protective role could be due to nutrients contained in the vegetables such as fiber, vitamins and phytochemicals. Among phytochemicals, the protective role of phenolics has been mainly investigated. Phenolics are secondary plant metabolites characterized by having at least one aromatic ring with one or more hydroxyl groups attached. The nature and the distribution of phenolics differ by plant tissue, with many of the phenolics synthesized from carbohydrates via the shikimate and phenyl propanoid pathways. Phenolics range from simple, low molecular weight, single-aromatic ring compounds to the large complex tannins. Polyphenols generally occur as glycosylated derivatives in plants, although conjugation with inorganic acid and malonylation is also known [5] [6] .

Apples are among the most widely consumed fruits in various countries. They are widely consumed fresh or in processed forms, such as juices and dried apple. Apples contain several nutrient as well as non-nutrient components, including dietary fiber, minerals, and vitamins (Table 1). Moreover, apple is one of the main natural sources of phytochemicals most of which express relevant antioxidant capacities in vitro [7] -[9] . The biological activities of apple polyphenols have often been evaluated in vitro on cultured cells and in animal models [7] [10] [11] . Previous studies have also investigated the effect of apples derivatives. Among these, apple pomace, a waste material from apple juice processing, which contains significant amounts of dietary fiber and phytochemicals has been studied [12] [13] . However, it has to be stressed that it is not always clear whether the effects in animals can be extrapolated to humans. Furthermore, studies in cell culture have been conducted before it is known that phytochemicals are processed in vivo, how they are absorbed and metabolized in the body. For

a. in red apples.

example, some phytochemicals are fermented by colonic bacteria and the original phytochemical may not even be detectable in the blood [14] .

This review focuses on the nutrient and phytochemical contents of apples. An overview on the bioavailability and metabolism of the most abundant apple phytochemicals after consumption is also presented, and the currently hypothesized health benefits related to apple consumption in humans are reviewed, with a particular attention given to recent evidence on the impact of apple on cardiovascular health and the biological effects related to the protective effect of this fruit.

The chemical composition and the nutritional properties of different cultivar of apple and their derivatives (juice, dried apple) have been previously studied [15] [16] . According to its nutrient profile (Table 1), apple represents a healthy food choice.

Bioavailability is defined as the proportion of a phytochemical that is digested, absorbed, and utilized in normal metabolism; however, measurement of bioavailability relies heavily upon estimates of amounts of antioxidant absorbed. Positive health effects of apple-derived polyphenols in vivo depend on their absorption, metabolism, distribution, and elimination from the body after consumption. Prerequisites for these compounds to have any in vivo effects are that they must be absorbed from the gastrointestinal tract after food consumption and subsequently reach sufficiently high plasma concentrations in the systemic circulation to induce biological activity. Bioavailability of polyphenols may be influenced by food matrix and dose ingested. A major part of the polyphenols ingested (75% - 99%) is not found in urine. This implies they have either not been absorbed through the gut barrier, absorbed and excreted in the bile or metabolized by the colonic microflora or our own tissues. Only very rare measurements of the intestinal absorption of polyphenols in humans are available [14] [27] [28] . Large uncertainties remain due to the lack of comprehensive data on the content of some of the main polyphenol classes in food. The maximum concentration in plasma rarely exceeds 1 microM after the consumption of 10 - 100 mg of a single phenolic compound. However, the total plasma phenol concentration is probably higher due to the presence of metabolites formed in the body’s tissues or by the colonic microflora. The bioavailability of some apple phytonutrients has been recently investigated in human studies and the results are summarized.

Dihydrochalcones. Phloretin (Phl) has been found exclusively in apples and in apple-derived products where is present as free and its glucosidic form, phloridzin (phloretin 2’-O-glucose). The bioavailability of dihydrochalcones following the consumption of apples and apple cider has been investigated in ileostomists and healthy subjects [29] [30] . After ingestion, the principal component in apples and cider, phloretin glucosides undergo cleavage in the small intestine with the released phloretin being subject to glucuronidation before appearing rapidly in the circulatory system as plorethin-2-O-glucuronide with a Tmax lower than 1 h. The short duration of the T_max values and the similar C_max of the glucuronide in healthy subjects and in ileostomist subjects after apple cider intake are indicative of absorption in the prossimal gastrointestinal tract. Glucuronide derivative has been detected in ileal fluid, urine, and plasma samples in addition to minor amounts of Phl conjugates (glucuronides and sulfates) and unconjugated Phl in the ileal samples [29] [30] .

Epicatechin. The bioavailability of the flavanol epicatechin has been studied using an apple extract (apple drink) and an apple puree containing different amounts of epicatechin (70 mg and 140 mg) [31] . The results of the randomized, placebo-controlled, crossover trial demonstrated that maximum plasma concentration, absorption and urinary excretion were all significantly higher after ingestion of both epicatechin drinks compared with apple puree (p < 0.05). Epicatechin bioavailability was >2-fold higher after ingestion of the 140 mg epicatechin drink compared to the 70 mg epicatechin drink (p < 0.05). The authors concluded that oral bioavailability of apple epicatechin increased at higher doses, but was reduced by whole apple matrix when compared with a epicatechin-rich apple extract incorporated in water-based drink [31] .

Quercetin. As far as it concerns the bioavailability of apple quercetin, previous studies have demonstrated that is lower when compared with onions [32] . DuPont et al. [33] have also determined the uptake and excretion of low doses of polyphenols in six subjects who each consumed 1.1 L of an alcoholic cider beverage. No quercetin was found in urine or plasma, but 3’-methyl quercetin was detected in plasma suggesting that low doses of quercetin are extensively methylated in humans [28] [33] .

Condensed tannin bioavailability. Very little is known about the metabolic fate and bioavailability of tannins. Proanthocyanidins (PA) absorption depends on their degree of polymerization [34] . In some in vitro experiments, only PA dimers and trimers, but not polymers with an average polymerization degree of 7, were absorbed through an intestinal epithelium cell monolayer [35] . Déprez et al. [36] have demonstrated that in vitro PA are catabolized by human colonic microflora into low-molecular-weight phenolic acids.

Other apple polyphenols. Two human intervention studies have investigated the bioavailability of other apple polyphenols in healthy men after ingestion of apples from different farming systems. The short-term intervention study included six men who consumed either organically or conventionally produced apples (randomized cross-over study). After intake of 1 kg apples, phloretin and coumaric acid plasma concentrations increased significantly (p < 0.0001) in both intervention groups, without differences between the two farming systems. In the long-term intervention study, 43 healthy volunteers consumed organically or conventionally produced apples (500 g/day; 4 weeks) or no apples in a double-blind, randomized intervention study. In the study, 24 h after the last dosing regime, the apple intake did not result in increasing polyphenol concentrations in plasma and urine compared to the control group suggesting no accumulation of apple polyphenols or degradation products in humans [37] .

In vitro apple polyphenols have been identified as potent radical scavenger, antioxidant and anti-inflammatory molecules. The effect of different varieties of apple has been recently investigated in human epidemiologic and

interventional studies. The results demonstrate that the consumption of the fresh and dried apple exerts a beneficial effect to human health. Different molecular mechanisms, as summarized in Table 4, can be suggested to explain the protective effect exerted by apple components.

The beneficial effects of whole apples on plasma lipid levels are probably related to synergistic interactions between apple components. Contrasting results have been reported in obese and hypercholesterolemic patients. Additional human studies are needed to confirm the hypothesized antioxidant, antinflammatory, and vascular protective effects of apples and derivatives in normal and pathological conditions.