Functional foods

A functional food contains bioactive components which demonstrate health benefits beyond their basic nutritional function (Arai, 1996). The term ‘functional food’ was introduced in Japan during the 1980s when the Ministry of Education, Science and Culture, recognising the impending burden of chronic disease within the aging population, stimulated research into whether tertiary benefits (on immune, endocrine, nervous, circulatory and digestive systems) could be gained from certain foods consumed on a regular basis (Arai, 1996). In 1991, the Japanese Ministry of Health and Welfare established a policy whereby functional foods with tertiary benefits were approved as ‘foods for specified health uses’ and foods identified for their anti-oxidative effects included rice, soybean, and ginger. Research since has identified a wide variety of plant foods including fruits, vegetables,

61

whole grains, juices, plant extracts, tea, wine and chocolate, as naturally rich in polyphenols and these represent major sources of antioxidants in the diet (Miller & Shukitt-Hale, 2012; Zamora-Ros et al., 2013).

Initially, the benefits of functional foods were largely attributed to their antioxidant ability to

protect against oxidative damage, a major contributor to CVD (Scalbert, Manach, Morand, Remesy, & Jimenez, 2005); however, it is now recognised that benefits extend beyond antioxidant activities. More complex modes of action, such as the pro-oxidant effects of polyphenols, contribute to: anti- inflammatory, anti-carcinogenic, anti-proliferative, anti-tumorigenic and anti-viral properties (Miller & Shukitt-Hale, 2012), and improve cell survival by inducing apoptosis and preventing tumour growth (Scalbert, A. et al., 2005). Cell protection can also occur through direct interactions with cellular signal transduction pathways in such a way that controls pathogenic processes relevant to the progression of chronic disease (Scalbert, Johnson, & Saltmarsh, 2005; Vauzour, Rodriguez- Mateos, Corona, Oruna-Concha, & Spencer, 2010).

Previous studies in adults have also shown that increasing intakes of functional foods (fruits, vegetables, and wholegrains) in the diet, naturally increases fibre intakes (Tovar, Johansson, & Björck, 2015; Tovar et al., 2012). Consuming a diet rich in fibre is known to protect against risk factors for developing CVD, including being overweight or obese (Slavin, 2008).

1.10.1.1 Polyphenols

Polyphenols are plant secondary metabolites which exist as part of natural defence mechanisms to protect the plant against pathogens, toxins and ultraviolet radiation (Kennedy, 2014). They are sensitive to heat, light and air oxidation and are highly soluble in water, which can result in major losses during cooking practices (Spencer, Abd El Mohsen, Minihane, & Mathers, 2008). The

composition and content of polyphenols can also vary widely within dietary sources due to differences in agricultural, processing and storage practices (Miller & Shukitt-Hale, 2012).

Phenols comprise around 8000 naturally occurring compounds which all possess an aromatic ring with at least one hydroxyl group attached (Leopoldini, Russo, & Toscano, 2011). Phenols are split into polyphenols and simple phenols depending on the number of phenol subunits. Simple phenols have one phenol subunit and include the phenolic acids. Polyphenols with at least two phenol subunits include flavonoids and stilbenes whereas those with three or more subunits comprise the tannins.

Flavonoids make up around two-thirds of polyphenol intake whilst phenolic acids account for much of the rest (Miller & Shukitt-Hale, 2012). Flavonoids share a common underlying structure of two aromatic rings (A and B) linked by three carbon atoms forming ring C, which is an oxygen- containing heterocycle (Vauzour, Vafeiadou, Rodriguez-Mateos, Rendeiro, & Spencer, 2008). According to the degree of oxidation of the ring as well as the hydroxylation pattern of the nucleus and the substituent at carbon three, flavonoids are categorised into flavones, flavonols, flavanols (catechins), flavanones, isoflavones, anthocyanins and proanthocyanidins (Table 6) (Vauzour et al., 2008).

Anthocyanins differ from other flavonoids as they possess a positively charged oxygen atom in the C-ring which gives them potent antioxidant as well as anti-inflammatory capabilities (Leopoldini et al., 2011). Findings from in-vitro studies question the bioavailability of anthocyanin components during digestion with other food sources (McDougall et al., 2005). In animal models anthocyanins have been shown to be neuro-available as evidenced by their residing in tissues longer than plasma (Pribis & Shukitt-Hale, 2014), although more studies are needed to elucidate specific mechanisms in humans (Miller & Shukitt-Hale, 2012).

63

Overall, polyphenol intakes from the diet are estimated to be around 1g per person per day, but intakes and sources vary between countries (Scalbert, Augustin et al., 2005). Higher intakes have been reported in Japan, where coffee and green tea are major contributors (Fukushima et al., 2009), whereas across Europe intakes ranged from around 0.25g in Greek individuals to around 0.5g in UK individuals (Zamora-Ros et al., 2013). As yet, due to a limited understanding of the actions of polyphenols in-vivo and their bioavailability, there are no definitive recommendations for optimal intakes of polyphenols (Chiva-Blanch & Visioli, 2012; Scalbert, Augustin et al., 2005).

1.10.1.2 Fibre

The physiological effects of dietary fibre predominantly relate to its potential to increase satiety and prolong satiation (Figure 2). Fibre-rich foods increase viscosity, which influences satiety by gastric distension and can prolong nutrient digestion and absorption (Dikeman & Fahey, 2006). The bulking properties of fibre require additional work during mastication that can improve satiation, as well as reducing the overall energy density of the diet (Slavin & Green, 2007). Satiety factors are affected due to increases in circulating gut hormones including cholecystokinin, which stimulates the digestion of fat and protein and is important in enhancing satiety (Burton-Freeman, Davis, & Schneeman, 2002). It is suggested that the type of fibre (soluble or insoluble) may alter satiety and hunger cues using different mechanisms (Slavin & Green, 2007). Insoluble fibre is thought to influence satiety in the small and large intestines, which may be linked to changes in gut hormones or transit time through the intestine (Slavin & Green, 2007). Furthermore, the addition of insoluble fibre to white bread was shown to increase insulin sensitivity by altering the patterns of insulin secretion (Weickert et al., 2006). Soluble fibre (of which oat and barley provide a good source) has been shown beneficial for altering the degree of starch breakdown in food and for managing blood glucose and insulin levels (Cavallero, Empilli, Brighenti, & Stanca, 2002; Tappy, Gugolz, & Wursch, 1996; Thondre & Henry, 2009).