Holistic Analysis Enhances the Description of Metabolic Complexity in Dietary Natural Products

Abstract

In the field of food and nutrition, complex natural products (NPs) are typically obtained from cells/tissues of diverse organisms such as plants, mushrooms, and animals. Among them, edible fruits, grains, and vegetables represent most of the human diet. Because of an important dietary dependence, the comprehensive metabolomic analysis of dietary NPs, performed holistically via the assessment of as many metabolites as possible, constitutes a fundamental building block for understanding the human diet. Both mass spectrometry (MS) and nuclear magnetic resonance (NMR) are important complementary analytic techniques, covering a wide range of metabolites at different concentrations. Particularly, 1-dimensional 1H-NMR offers an unbiased overview of all metabolites present in a sample without prior knowledge of its composition, thereby leading to an untargeted analysis. In the past decade, NMR-based metabolomics in plant and food analyses has evolved considerably. The scope of the present review, covering literature of the past 5 y, is to address the relevance of 1H-NMR–based metabolomics in food plant studies, including a comparison with MS-based techniques. Major applications of NMR-based metabolomics for the quality control of dietary NPs and assessment of their nutritional values are presented.

FIGURE 1

Dietary dependence (A) and analytic challenges (B) of the plant metabolome.

FIGURE 2

Key steps for the NMR identification of metabolites in complex mixtures. (A) The interpretation of the 1D ¹H resonances in terms of chemical shifts (δ in ppm), multiplicity, coupling constants (J in Hz), and intensities leads to the determination of the relative molar concentrations (A1) and structural classes (A2) of the most abundant metabolites. (B) The acquisition of 2D NMR data enables the determination of structural connections. (C) The consultation of databases guides the analysts in the identification of known metabolites. The entire process is illustrated with metabolomic fingerprinting of licorice extract (41), leading to the simultaneous identification/quantification of major nutrients (sucrose [1], proline [2]) and phytochemicals (glycyrrhizin [3] and liquiritin [4]). COSY, correlation spectroscopy; HMBC, heteronuclear multiple-bond correlation spectroscopy; HSQC, heteronuclear single quantum coherence spectroscopy; 1D, 1-dimensional; 2D, 2-dimensional.

FIGURE 3

¹H NMR as a first-pass metabolomic screening. The 4 steps of the metabolomic workflow show the analytic progression, starting with a sample of unknown composition and leading to its metabolomic description. In step 2, results obtained from NMR-based metabolomics assist in the generation of chemical hypotheses, enabling the design of LC-based analyses. In step 3, the use of specific detection methods, hyphenated with chromatographic systems, enhances the detection specificity for certain types of metabolites according to their physical properties (11). CAD, charged aerosol detector; ELSD, evaporating light-scattering detection; HR-MAS, high-resolution magic angle spinning; RI, refractive index; UHPLC, ultra-HPLC.

FIGURE 4

Applications of NMR-based metabolomics for food plant analysis. ¹H NMR-based metabolomics can evaluate the effects of crop breeding, cultivation, geographical origins, and industrial processing on nutritional composition and organoleptic properties of food plants. By offering an unbiased metabolomic overview NMR has a fundamental place in food biosafety and for the determination of the metabolomic equivalence.