Importance of Accurate Measurements in Nutrition Research: Dietary Flavonoids as a Case Study

Abstract

Accurate measurements of the secondary metabolites in natural products and plant foods are critical for establishing relations between diet and health. There are as many as 50,000 secondary metabolites that may influence human health. Their structural and chemical diversity presents a challenge to analytical chemistry. With respect to flavonoids, putative identification is accessible, but positive identification and quantification are limited by the lack of standards. Quantification has been tested with use of both nonspecific and specific methods. Nonspecific methods, which include antioxidant capacity methods, fail to provide information on the measured components, suffer from numerous interferences, are not equatable, and are unsuitable for health research. Specific methods, such as LC with diode array and mass spectrometric detection, require the use of internal standards and relative molar response factors. These methods are relatively expensive and require a high level of expertise and experimental verification; however, they represent the only suitable means of relating health outcomes to specific dietary components.

Keywords: antioxidants; dietary flavonoids; mass spectrometry; proanthocyanidins; secondary metabolites.

FIGURE 1

Hydroxycinnamic acid structure in flavonols and flavones. The trace numbers correspond to the groups of compounds listed in Tables 1 and 2. OH, hydroxyl; OMe, methoxy; Gly, glycoside. Reproduced from reference with permission.

FIGURE 2

Structures of common phenolic compounds. OH, hydroxyl: COOH, carboxyl

FIGURE 3

Absorbance profiles (190–420 nm) for flavonols and flavones. MRRF_P = best predicted molar relative response factor. Adapted with permission from reference .