Is the in vitro antioxidant potential of whole-grain cereals and cereal products well reflected in vivo?

Abstract

There is strong epidemiological evidence that whole-grain cereals protect the body against age-related diseases such as diabetes, cardiovascular diseases and some cancers. This may be due to the fibre and micronutrients in the outer layer and germ fractions of the grain acting together to combat oxidative stress, inflammation, hyperglycaemia and carcinogenesis. Oxidative stress is associated with these metabolic diseases. Whole-grain cereals are a good source of vitamin E, folates, phenolic acids, zinc, iron, selenium, copper, manganese, carotenoids, phytic acid, lignins, lignans, and alkylresorcinols, all of which have significant antioxidant potential in vitro. Phenolic acids such as ferulic acid are characteristic of cereals. They may scavenge free-radical oxygen species both in vitro and in vivo. Phenolics may also act in vivo by triggering gene induction/repression via cell signalling through transcription factors. Whole-grain cereals are also a good source of betaine, choline and sulphur amino acids that can improve antioxidant status. Betaine, which accounts for about 1% (w/w) of the bran fraction in wheat, acts as a methyl donor that may decrease hyperhomocysteinaemia, a cardiovascular risk factor, but it has been neglected. Cereals and cereal products are antioxidative in vitro, as are most fruits and vegetables. The in vitro antioxidant capacity of cereals and their constituent fractions is significantly correlated with their polyphenol content, except for maize. However, the in vitro antioxidant capacity of cereals is only an approximate reflection of their in vivo antioxidant effect due to differences in antioxidant solubility/bioavailability within the digestive tract and the metabolism/conjugation of compounds such as polyphenols. During digestion, the antioxidant capacity of cereals is increased and is likely to provide a favourable antioxidative environment for the epithelium tract, notably in the large intestine. Most of the studies performed on animals have been concerned with the antioxidant property of coloured rice, especially black rice and its anthocyanin fraction, showing a positive effect on some antioxidant biomarkers. Those very few studies that have been done on humans have shown that wheat bran and rye product supplements have no effect on antioxidant status, while a black rice pigment fraction and an avenanthramide-enriched mixture extracted from hulled oats have a positive effect. In vivo studies are therefore needed to further explore the real antioxidant potential of cereals.

Introduction

Epidemiological studies have clearly demonstrated that a diet containing whole-grain cereals can protect against metabolic disorders such as cardiovascular diseases (Anderson, 2003, Truswell, 2002), cancer (Chatenoud et al., 1998, Chatenoud et al., 1999) and diabetes (Venn and Mann, 2004). The effect is mainly attributed to the fibre and micronutrients in the outer layer of the grain and in the germ fraction (Slavin et al., 1999, Thompson, 1994). However, the bran fraction plays the more important role due to the germ released during cereal processing. The protective effects of cereal fibres depend on their solubility. Soluble fibre (soluble arabinoxylans and β-glucans) can lower blood cholesterol (Braaten et al., 1994, Kahlon et al., 1993) and reduce the post-prandial glycaemic response (Casiraghi et al., 2006). Insoluble fibres (cellulose and insoluble arabinoxylans) increase the speed of transit time and the faeces volume (ballast effect), decreasing the contact between carcinogens and the colon epithelial cells, and their fermentation may also produce significant quantities of butyrate, which itself protects epithelial cells against carcinogenesis (Olmo et al., 2007). Whole-grain cereals also contain micronutrients such as vitamin E, folates, phenolic acids, zinc, iron, selenium, copper, manganese, carotenoids, betaine, choline, sulphur amino acids, phytic acid, lignins, lignans, and alkylresorcinols, which all may have potential antioxidant effects. They may act as such in vivo to improve the redox state of various tissues (Slavin et al., 1999). The in vivo bioavailability of each of these micronutrients varies, and some, such as the polyphenols may be metabolised. Thus, some micronutrients may lose their antioxidant potential or their concentration in the plasma and tissues may be too low for them to act as significant free radical scavengers. Other mechanisms are probably also involved. The in vivo reality is therefore much more complex than it appears in a first view. Obesity and metabolic diseases have been all associated with increased oxidative stress (Aruoma, 1998, Furukawa et al., 2004, Hwang and Kim, 2007). Antioxidant phytomicronutrients from whole-grain cereals could therefore protect the body from this oxidative stress. Exactly how they exert their protective effect remain to be elucidated.

This review discusses the potential antioxidant effect of whole-grain cereals, their various fractions, and cereal products, relating their in vitro activities to their antioxidant effects in vivo. We first discuss the antioxidant properties of cereal micronutrients and their significance, followed by the in vitro antioxidant capacity of cereals and their fractions. Lastly, we review the main in vivo studies on the antioxidant effect of cereals, a brief comparison being made with studies carried out with fruits and vegetables.