Improving public health?: The role of antioxidant-rich fruit and vegetable beverages

Abstract

Recent evaluations of diet quality in the UK reveal that two thirds of people still do not consume the recommended five portions of fruit and vegetables per day. Fruit and vegetables contain high concentrations of bioactive compounds including antioxidants which may be beneficial to health. The precise nature of the health benefits which are to be gained from the consumption of antioxidant-rich foods is yet to be fully elucidated. There has been a visually evident increase in the number of beverages which use their antioxidant capacity as a marketing tool in recent years. This poses the question as to whether or not this is an entirely positive change. The aims of this review are to 1) summarise the current evidence supporting the health benefits of antioxidant-rich foods in CVD, cancer, neurological decline and diabetes, 2) summarise the antioxidant components of commonly consumed beverages, 3) investigate the emergence of new and novel beverage ingredients within the context of functional foods and 4) establish the potential role of antioxidant-rich beverages in human health. In conclusion, the evidence for specific health benefits which can be attributed to the antioxidant content of foods is still limited. However, antioxidant rich foods are most commonly from plant sources which contain a number of healthy components which make them a positive addition to the diet. Many compounds which display antioxidant activity (such as polyphenolic compounds) also have many other positive effects in the human system and therefore if consuming them on the basis of their antioxidant content increases consumption, then this can be viewed as a positive change in diet. Furthermore, the synergistic and interactive effects of antioxidant combinations from a mixed diet has not been adequately simulated in a research setting and the true nature of these interactions remains largely unknown.

Introduction

The Food Standards Agency (FSA) has established its message that a minimum of five portions of fruit and vegetables a day contributes towards a healthy diet (FSA, 2010). However, in the most recent report from their national diet and nutrition survey (NDNS), it was revealed that two thirds of the UK population are still not consuming the recommendation for fruit and vegetables (Bates, 2010). Some of the responsibility for addressing this disparity obviously lies with the consumer, but there is also an inherent challenge to the food industry to develop new, exciting and convenient food products to help inspire people to make a positive change to their diet. Beverages have been used habitually to deliver high concentrations of functional ingredients (i.e. sports and performance beverages, ready to drink teas, vitamin enhanced water, soy beverages and energy beverages). This is due, in part, to their ease of delivery but also to the innate human requirement for fluid. Beverages represent an appropriate medium for the dissolution of functional components, but also a convenient and widely accepted method of consumption. The influx of functional beverages and novel ingredients has prompted improved legislation (Regulation No. 258/97) from the European Commission (EC) which now requires an extensive safety document to be submitted before novel ingredients can be incorporated into food. The EC has been willing to grant approval and support the industry, provided that their development is based on evidential science and has been validated in human populations (Wilkinson & Hall, 2008). Within the emerging functional food paradigm, functional beverages may contribute toward increasing the consumption of fruit and vegetables to re-equate the balance between recommendations and actual intake, although juice intake does not equate to intake of whole fruits or vegetables. This review aims to summarise data pertaining to the relationship between antioxidant rich beverage consumption and public health and provide a framework to answer the question; is the promotion of antioxidant-rich fruit and vegetable beverages good for public health?

Research aimed at fully elucidating the mechanisms by which antioxidants contribute to good health is a relatively young discipline which has intensified over the last three decades. The term ‘antioxidant’ encompasses a vast array of substances which are divided into endogenous and exogenous antioxidants (Fig. 1). Fruit and vegetable juices are predominately rich sources of polyphenols, carotenoids, vitamins and minerals. Huang, Ou, and Prior (2005) defined dietary antioxidants as “substances which can (sacrificially) scavenge reactive oxygen/nitrogen (ROS/RNS) to stop radical chain reactions, or can inhibit the reactive oxidants from being formed in the first place”. Some antioxidants perform this function by being oxidised themselves, thus performing a rate limiting role in initiation, propagation and termination of radical chain reactions where the resulting ‘antioxidant radical’ is less reactive. Antioxidants differ in their efficacy against differing substrates; some are potent free radical scavengers whilst others have stronger metal chelation effects, for example, carotenoids are particularly effective at inhibiting the oxidation caused by singlet oxygen (Niki & Noguchi, 2000). Direct antioxidant action of dietary compounds is now considered unlikely in most conditions due to low bioavailability. It is however widely accepted that many compounds traditionally thought of as antioxidants have important non-antioxidant functions including maintenance of redox balance (Forman, Torres, & Fukuto, 2002), stimulation of the antioxidant response element (ARE) and various signalling roles (Hensley, Robinson, Gabbita, Salsman, & Floyd, 2000) as well as some impact on the oxidation of lipids, proteins and DNA (Valko et al., 2007). It has been suggested that some molecules which are traditionally thought of as antioxidants (such as polyphenols) may also have pro-oxidant effects in certain tissues (Azam, Hadi, Khan, & Hadi, 2004). This position raises the question as to whether dietary antioxidants act most beneficially as anti-oxidant or pro-oxidant molecules in maintaining an optimum level of ROS for health.

Within the biological system the primary ROS in question is the superoxide radical (O₂⁻) which is created by a premature electron ‘leak’ to oxygen in the electron transport phase of aerobic metabolism. The unpaired electron in the valence shell of the superoxide radical makes it reactive and it subsequently reacts with other molecules to form secondary radicals such as the hydroxyl radical (OH), peroxynitrate (ONOO), hydrogen peroxide H₂O₂ and the peroxyl radical (LOO) it can also be split to form singlet oxygen (O). Under normal conditions the removal of ROS is somewhat regulated by antioxidant enzymes such as superoxide dismutase (SOD), glutathione peroxidase (GTP) and catalase (CT) (Fig. 1), rendering most additional antioxidant consumption less important. However, in periods of excess ROS production, or down-regulation of antioxidant enzymes induced by CVD (Van Berkel et al., 2003) for example it is proposed that increased intake of antioxidants may be of benefit to maintain an optimum balance of ROS.