SEB Bulletin January 2008
Dietary Antioxidants
There is compelling evidence that a diet rich in fruit and vegetables can help maintain a healthy lifestyle (1). Current recommendations are that everyone should eat at least five portions of a variety of fruit and vegetables each day to reduce the risks of chronic illnesses such as cancer and cardiovascular diseases (1,2). However, the average fruit and vegetable consumption in England is less than three portions a day, with consumption often lower in children. The national (UK) diet and nutrition survey on adults aged 19 to 64 years, published by the WHO in 2001, highlighted less than 2 in 10 individuals meeting their required five daily portions. The survey also estimated that the total worldwide attributable mortality due to an insufficient intake in fruit and vegetable was around 2.7 million deaths (3) (Fig. 1).
The human body relies on reactive oxygen / nitrogen species (ROS / RNS), which are involved in mechanisms such as energy supply, detoxification and signalling (4). Endogenous antioxidant mechanisms, including enzymatic defences, contribute to their efficient removal. However, when such defence mechanisms fail or are insufficient to cope, these free radicals trigger chain reactions resulting in the rapid oxidation of target molecules. The increased exposure to free radicals, known as oxidative / nitrosative stress, can lead to DNA, lipid and protein damage and potentially increase the risk of cardiovascular illnesses and cancer (5).
Many components in foodstuff can contribute to their health-promoting effects, including micronutrients such as vitamins C and E and minerals like selenium, carotenoids, glucosinolates, indoles, isothiocyanates, fibre, folic acid etc. The antioxidant capacity of several natural compounds has been thought to be a key factor in the negative correlation between a fruit- and vegetable-rich diet and chronic illnesses, potentially through their ability to supplement cellular defence systems. Major sources of antioxidants in the human diet are varied and include cereals, fruits, vegetables, chocolate, oils and beverages such as tea, coffee, wine and fruit juices. The antioxidant profile of plants differ according to the species and is also influenced by farming (organic vs. traditional), packaging, transport, storage as well as cooking and commercial processing (6, 7).
Dietary antioxidants include the micronutrients vitamin C (ascorbate), vitamin E, beta-carotene and the polyphenols. Beta-carotene and vitamins E and C have been well studied, possess strong antioxidant activities and are well absorbed, with a relatively high bioavailability. On the other hand, the polyphenols are a complex family of molecules, ubiquitous in plants, and include the flavonoids and phenolic acids. Little is known about their fate in the gastro-intestinal tract; however they have attracted a lot of research interest due to their complex structure and potent in vitro antioxidant activity. Commonly occurring flavonoids are quercetin derivatives (onion, apple), catechins (tea), anthocyanins (berries) and hesperitin derivatives (citrus fruits), while commonly occurring phenolic acids are caffeic acid, occurring in many fruit and vegetable and chlorogenic acid (coffee) (8).
Some of the work being carried out at the University of Glasgow, under the supervision of Professor Alan Crozier, is investigating which fruit and vegetables have the highest antioxidant levels. Bill Mullen, who took part in the study, describes how these analyses may help the public make an educated choice in their food in order to optimise the antioxidants content of their plate: “Knowing the hectic pace of life most people lead today, it is not always possible for them to get their daily five portions. Fruit and vegetable drinks are becoming more popular as a way of adding to or supplementing our fruit and vegetable intake. Recent research we carried out into the most popular juice drinks in the UK showed that Purple grape juice and Cloudy apple juice both had high levels of antioxidants, although the antioxidants they contained were different types. Looking at four different types of lettuce showed that Lollo Rosso contained many times the antioxidants than the other types of lettuce (Fig. 2). It is always worth remembering that no one fruit or vegetable, even if it is has been called a super food, is better for you than a balanced intake of a range of fruit and vegetables.”
The idea that no single antioxidant could be deemed more essential than others was compounded by the EUROFEDA project (European Research on the Functional Effect of Dietary Antioxidants) set-up under the European Union fifth framework. The study reinforced the strong link already existing between a high-consumption of fruit and vegetables and a lower incidence of chronic illnesses and highlighted that the use of supplements could not be justified as yet, since more work needs to be carried out to understand whether antioxidants act via direct or indirect more complex pathways. One of the key tasks of the project was to investigate the factors influencing the antioxidant compound bioavailability (fraction of the dose administered reaching the systemic circulation or the target tissues) and understand the fate of the molecule from absorption to excretion (9)
“If diets rich in fruit and vegetables are good for health, then the benefit has to be associated with whatever reaches particular tissues, in whatever form it happens to be, and at the concentrations that are achieved under normal dietary conditions”, outlines Professor Mike Clifford, from the University of Surrey. While a comprehensive body of data has been gathered on the in vitro antioxidant activity of polyphenols, phenols and tannins, less has been discovered in vivo, and one of the greatest challenges is to understand how the body processes these chemicals, as well as copes with their metabolites. Phenolics undergo several metabolising steps in the body, including glucuronidation, methylation, sulphatation and dehydroxylation. All these reactions affect their overall antioxidant activity, and therefore potentially modulate their protective effect. “One of the main challenges for nutritional research in the years to come will be to identify the compounds responsible for the protective effects in vivo”, highlights Professor Crozier.
The antioxidant activity of dietary nutrients is also susceptible to its environment, the presence of proteins and other antioxidants. Certain polyphenols may readily pass the gastrointestinal barrier (e.g. caffeic acid) while other are poorly absorbed (e.g. rutin). This makes it very difficult to correlate the antioxidant capacity of a compound in vitro with its effect in vivo. Moreover, the composition of a meal affects the absorption and subsequent bioavailability of each polyphenol. What is known as the food matrix effect is currently being studied by Mullen, who adds that “comparing the bioavailability of the antioxidants in dark chocolate when eaten with and without milk revealed lower levels of antioxidants in the plasma when the dark chocolate was eaten with milk (10), while cream delayed the absorption of phenolics from strawberries (11; Fig. 3)” but also warns that this observation does not provide a complete answer to the problem, as many of the dietary antioxidants studied are rapidly metabolised and difficult to identify and quantify once absorbed. The role of these metabolites may be crucial. However little is known about them or their mode of action, which is potentially independent of their antioxidant activity.
The behaviour of a compound during its journey through the gastrointestinal tract is crucial in order to understand its impact on systemic health. Following ingestion of a polyphenol-rich meal, the plasma antioxidant capacity is enriched, suggesting that the antioxidant compounds may pass the blood barrier, reduce the oxidant pool derived from the meal (mainly iron) or spare the body’s antioxidant defences. Professor Clifford, who has a special interest in the absorption and metabolism of phenols, polyphenols and tannins (PPT), emphasises that PPT metabolites only occur in the plasma in trace amounts, even after massive doses, and will be barely detectable after one normal portion of fruit and vegetables (as defined in five-a-day): “The antioxidant effect in the tissues associated with these mammalian metabolites is negligible and unlikely to be health-promoting by such a mechanism. They do not accumulate following repeat doses as all the metabolism is designed to make them easily removed from the body. The native polyphenols can be powerful antioxidants, but normally they do not occur in the plasma and tissues. If they did, and especially if they accumulated (as when given intravenously), they would be dangerous because they can generate free radicals through redox cycling. This is why the body has developed such good defences against them”.
With such a vast degree of variation between the powerful, native dietary antioxidants tested in vitro, and their “watered-down” version occurring in vivo, it is difficult to establish a direct link between the antioxidant capacity of a molecule and its health-promoting effects. As Mullen points out, the requirement for ascorbate to prevent scurvy is independent from its antioxidant activity, as vitamin C is required for its role in collagen production. This point of view is shared by Professor Clifford, who highlights that dietary polyphenols are also likely to be beneficial through alternative routes, including their ability to delay glucose absorption from the duodenum, potentially protecting against the development of type-2 diabetes, or their prebiotic effect on the gut microflora and limitation of neurotoxin producing micro-organisms.
Thus, it is clear that the science underlying the recommended 5 portions of fruit and vegetables per day is more complex than thought when species, metabolism, environment and human factors are taken into consideration. We may therefore be able to refine this advice in the future but, in the meantime, the public will do well to heed the current basic recommendation for their health and well-being, bearing in mind that, in order to secure a wide range of antioxidants, 35 portions a week covering a wide variety of fruit and vegetables are better than the same five everyday.
Emilie Combet
University of Glasgow
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