¹H NMR: A Powerful Tool for Lipid Digestion Research

¹H NMR spectroscopy has proved to be a valuable research tool in the field of food science and technology, especially in the case of food lipids. It is well known that degradation processes affecting food lipids during technological processing and storage can be a major cause of food deterioration, with negative implications from the economic and health point of view. Although classical methodologies based on long, multi-step, tedious and unspecific techniques are widely employed, a large number of studies have demonstrated the usefulness of ¹H NMR in not only characterizing qualitatively and quantitatively major and minor lipidic components of foods, oils and fats, but also when studying their degradation process under different oxidative conditions, helping to shed light on the underlying mechanisms by which lipid degradation occurs. Nevertheless, the nutritional quality and safety of lipids could also be modified during subsequent human gastrointestinal digestion. Since this physiological process is an inevitable step, it seems logical also to research the various chemical reactions that may affect food lipidic components under gastrointestinal digestive conditions, in order to better understand the effect of lipids on human health and also to be able to design healthier foods and diets. Recent studies along these lines have demonstrated the suitability of ¹H NMR for simple, fast and accurate global study of lipolysis, without chemical modification of the sample. This new methodology overcomes many of the limitations of the techniques currently used for this purpose. In addition, by using spectral data and applying different approaches, ¹H NMR is a valuable alternative for the quantification of the various kinds of glyceryl structures and fatty acids in a simple, fast and accurate way in complex lipid mixtures. Beside lipolysis, other chemical reactions affecting lipids, such as oxidation, might also take place in the gastrointestinal tract due to its highly reactive environment. However, given the small number of studies and the limitations of the methodologies usually employed (absorbance in the ultraviolet visible region for determining conjugated dienes, peroxide value, thiobarbituric acid reactive substances test), there is currently knowledge lacking about the extent of ongoing chemical reactions, especially of lipid oxidation, as well as about the specific nature of the oxidation products generated from lipids that could remain bioaccessible for intestinal absorption. It must be noted that 1H NMR allows, in the same run, a study not only of the above-mentioned lipolysis reaction, but also that of the occurrence and extent of oxidation reactions taking place during digestion. This technique provides valuable knowledge about oxidation products generated from polyunsaturated lipids under these conditions. It has been observed that both the amount and the nature of lipid oxidation products vary widely, depending on the unsaturation degree and the initial oxidation level of the digested lipids, as well as on the presence of other non-lipidic components that are usually present in food, like proteins or antioxidants. The information provided by ¹H NMR allows the simultaneous study of a broad variety of oxidation products, which is crucial for the selection of the most useful marker compounds of the occurrence and extent of lipid oxidation. Likewise, ¹H NMR facilitates the study of the bioaccessibility of certain minor lipidic compounds of interest, which is another important aspect of food digestion research that is difficult to tackle. All these matters will be dealt with in this chapter.

Keywords: Antioxidants, Bioaccessibility, Degradation process, Food science and technology, Lipid digestion research, Lipolysis, ¹H NMR spectroscopy.