Attempts have been made to use non-compositional parameters, such as Total Antioxidant Capacity (TAC), determined by assays such as oxygen radical absorbance capacity, ferric-reducing ability of plasma, and trolox-equivalent antioxidant capacity, as surrogate markers for food quality and for monitoring food-related changes in human plasma in dietary intervention studies. Increased TAC of plasma is often indiscriminately, and therefore incorrectly, interpreted as being favorable to human health. Whether or not dietary compounds may indeed exert health effects depends on factors other than their mere presence in food or body fluids. Many phytochemicals, for example, are poorly absorbed and rapidly metabolized into molecules with altered physicochemical and therefore biological properties. Consequently, the use of TAC assays for the in vitro assessment of antioxidant quality of food, which is often employed as a marketing argument or for the assessment of the “wholesomeness” of a food is to be discouraged.