Tony K. McGhie, Harry Martin, Rosheila Vather, and Laura E. Barnett, Healthful Fruit Team, The Horticulture and Food Research Institute of New Zealand, Private Bag 11 030, Palmerston North, New Zealand. E-mail: tmcghie@hortresearch.co.nz
Anthocyanins are a subclass of flavonoids with widespread occurrence in plant-derived foods. They are responsible for the red and blue colours in many foods and are present in high concentrations in red fruit, red wine and some vegetables. Dietary consumption is estimated at up to 200 mg/day (1) and often exceeds that of other flavonoid groups. Anthocyanins are bioactive molecules that are reported to have anti-oxidant, chemopreventative, and anti-inflammatory activities and have been widely used as treatments for eyesight and circulatory ailments. We (2), and others (3;4), have shown that anthocyanins are absorbed from the intestinal tract into the circulatory system and a portion excreted in the urine of humans in an intact molecular form. Thus, bioavailability of anthocyanins appears to differ from other flavonoids studied such as the flavonols and flavanols. A unique feature of anthocyanins is that their molecular structure is pH dependent. At any given pH, an equilibrium containing up to four different molecular structures occurs. The red flavylium cation is the dominant form at low pH, whereas other forms such as quinonoids, hemiketals and chalcones dominate at higher pH. The equilibrium present in any given situation is likely to have a major effect on anthocyanin bioavailability and bioactivity. However, the bioavailability and bioactivity is still considered mostly from the perspective of the flavylium cation, which is unlikely to be the predominant form present, a biological pH.
We have used are variety of experiments to investigate the bioavailability and bioactivity of anthocyanins. These include animal and human studies combined with LC-MS-MS analysis to confirm the structure of excreted anthocyanins and determine the nature of anthocyanin metabolites also excreted in the urine. The absorption characteristics of anthocyanins were investigated further using Ussing chambers loaded with mouse intestinal tissue. We have also investigated the bioactivity of anthocyanins included in the diet in animal and human studies
The studies with animals and humans confirmed that exactly the same anthocyanins are excreted in urine as provided in the oral dose. However, less than 0.1% of the dose is recovered in the urine, suggesting poor bioabsorption but not ruling out other routes of metabolism. In addition urinary metabolites of anthocyanins were identified by LS-MS-MS as glucuronyl and methyl conjugates (4;5). In Ussing chamber experiments the stability of anthocyanin is very dependent on experimental pH, but absorption of anthocyanin into mouse intestinal tissue has been demonstrated. Absorption of anthocyanin appears to be greater in the jejunum compared with other regions of the GI tract suggesting the presence of an active transport mechanism in the jejunum (6).
We conducted a series of rat studies with extracts of Boysenberry and more recently a human study with Boysenberry and blackcurrant drink in elderly (>65 years) people with below average memory ability. In the rat studies the Boysenberry extract functioned as an in vivo antioxidant and raised the antioxidant status of plasma while decreasing some biomarkers of oxidative damage, but the effect was highly modified by basal diet. In the human study we measured cognitive performance, oxidative stress, immune function and inflammation. Although the data has still to be fully analysed, there appears to be little effect on cognitive function (simple reaction time; choice reaction time; one-back memory; monitoring; matching; and learning). Small effects were observed in the markers of oxidative stress.
References
(1) Kuhnau, J. The flavonoids. A class of semi-essential food components: Their role in human nutrition. World Review of Nutrition and Diet 1976, 24, 117-191.
(2) McGhie, T. K.; Ainge, G. D.; Barnett, L. E.; Cooney, J. M.; Jensen, D. J. Anthocyanin glycosides from berry fruit are absorbed and excreted unmetabolised by both humans and rats. Journal of Agricultural and Food Chemistry 2003, 51, 4539-4548.
(3) Matsumoto, H.; Inaba, H.; Kishi, M.; Tominaga, S.; Hirayama, M.; Tsuda, T. Orally administered delphinidin 3-rutinoside and cyanidin 3-rutinoside are directly absorbed in rats and humans and appear in the blood as the intact forms. Journal of Agricultural and Food Chemistry 2001, 49, 1546-1551.
(4) Talavera, S.; Felgines, C.; Texier, O.; Besson, C.; Gil-Izquiedo, A.; Lamaison, J.-L.; Remesy, C. Anthocyanin metabolism in rats and their distribution to digestive area, kidney, and brain. Journal of Agricultural and Food Chemistry 2005, 53, 3902-3908.
(5) Cooney, J. M.; Jensen, D. J.; McGhie, T. K. LC-MS identification of anthocyanins in boysenberry extract and anthocyanin metabolites in human urine following dosing. Journal of the Science of Food and Agriculture 2004, 84, 237-245.
(6 Matuschek, M. C.; Hendriks, W. H.; McGhie, T. K.; Reynolds, G. W. The jejunum is the main site of absorption for anthocyanins in mice. Journal of Nutritional Biochemistry 2005, in press.