The antioxidant capacity of milk--the application of different methods in vitro and in vivo

Milk contains a wide array of compounds with established or putative pro- or anti-oxidant function. The functions of these compounds have been intensively studied. This review focusses on some important aspects in this wide field namely the methodology for measurement of the total antioxidant capacity (TAC), the content of TAC and some related compounds in human and animal milks and infant formulas, and the effect of milk intake on antioxidant status in the body and on the activity of dietary flavonoids as studied in vitro and in vivo. Regarding methodology TAC in milk can be measured by spectrophotometric and electrochemical methods and some of their characteristics are reviewed. Milk, whey, high-molecular-weight and low-molecular-weight (LMW) fractions of whey have all been found to have antioxidant capacity using these techniques. The major antioxidant in the LMW fraction has been identified as urate. An extensive literature survey was made regarding data on the antioxidant capacity and related variables of milk obtained from different sources (human milk, infant formulas and animal milk) and subjected to different treatments. Differences in TAC between milks from different sources have been observed but due to the variety of techniques used no clear pattern is evident at present. Another important aspect is the putative effects of the intake of milk products on the antioxidant status of the consumer. A few studies performed in adults and premature infants are reviewed and it is stated that too little information is available to make any firm conclusions in this regard. Finally, a high interest has been devoted to the possible interference of milk with the antioxidant properties of flavonoid-rich food like tea. Most in vitro studies show an inhibition by milk on tea flavonoid activity whereas the results from the corresponding in vivo studies are equivocal. Our general conclusion is that several compounds in various milk fractions contribute to the antioxidant capacity of milk and that much further work is needed to unravel the complex interactions among the pro- and antioxidants, and their putative health effects on the consumer.