Ceramides are abundantly present in the stratum corneum of the epidermis on the body surface and play a crucial role in the formation of the skin barrier, which protects against infection and prevents water loss. Human stratum corneum contains a variety of ceramides, with 23 classes and over 1,500 species. Ceramides are classified into free ceramides and protein-bound ceramides, which are components of the multilayered lipid structures (lipid lamellae) present between corneocytes and the corneocyte lipid envelope, respectively. Free ceramides are further classified into non-acylated ceramides and acylceramides. Protein-bound ceramides and acylceramides are especially important for skin barrier formation, and mutations in genes involved in their synthesis cause congenital ichthyosis. This review presents the latest findings on the molecular mechanisms underlying ceramide diversity, the roles of individual ceramide classes in skin barrier formation, and their relationships to skin diseases.

Sensitive skin is a syndrome defined by the occurrence of unpleasant sensations in response to external factors, and impaired skin barrier function and/or hypersensitivity of the cutaneous nerve have been hypothesized to be the main culprits. However, the self-perceived sensitive skin individuals can potentially include various types of sensitive skin individuals, from those who have normal skin condition to pathological skin condition, and thus, there are still few systematic views on skin physiology of sensitive skin. This has been an important issue in the field of cosmetic science. Recently, we have focused on the barrier function of the stratum corneum, and have investigated the relationship between ceramides, which play an essential role in the permeability barrier function, and skin physiological properties specific to sensitive skin, mainly skin hypersensitivity. In this manuscript, we review several previous studies regarding the relationship between sensitive skin and ceramides, categorizing the study subjects according to whether they have skin diseases or not, and then present our latest study.

Sphingolipids are components of eukaryotic cell membranes and are known to function as signaling molecules such as cell differentiation, proliferation, and apoptosis. However, knowledge of their role as food components is limited, and their nutritional significance has not been widely recognized. In recent years, the health benefits of sphingolipids in food have been increasingly recognized, with reports on their effects on lifestyle-related diseases through regulation of lipid metabolism, their effects on cancers and inflammation in the lower digestive tract, and their effects on cognitive and motor functions. Currently, sphingolipids derived from food sources are gaining attention as “food ceramides,” particularly for their potential benefits for the skin, and are being applied in foods with functional claims and food for specified health uses. On the other hand, the chemical structure and composition of sphingolipids vary significantly among biological species, meaning that we consume sphingolipids with various chemical structures from food daily. Therefore, to further understand sphingolipids as food components, it is necessary to comprehensively understand the detailed molecular structure of food sphingolipids, their content and composition, the details of their intestinal absorption, and their food functionality, including mechanism of action.

The glycosylinositol phosphoceramide (GIPC) is the most abundant lipid class among plant sphingolipids and is one of the main constituents of membrane lipids of plant cells. Although 60 years have passed since the structure of GIPCs was determined, metabolic pathways and physiological roles of the GIPC remain unclear, and it is not used industrially. The difficulty in studying GIPC is that it is water-soluble, which means that the extraction methods commonly used by lipid researchers cannot be applied. In addition, there is no commercially available GIPC standard, making isolation difficult. In this review, I reviewed the isolation methods of GIPCs and the phospholipase D which degrades GIPC at its D position.

Reducing the oxidation of oils and fats is an important issue, especially in the food industry, which handles foods containing oils and fats. Oxidation of oils and fats basically follows the oxidation stability of the fatty acid composition, but is also influenced by the triacylglycerol molecular species and lipid class. Oxidation of oils and fats is initiated by the withdrawn of hydrogen from unsaturated fatty acids, but the subsequent process is complex, and a variety of radical species are generated, which themselves are involved in promoting oxidation. A variety of antioxidants exist to efficiently inhibit reactions in these processes. The effectiveness of these antioxidants depends on whether they are used in bulk oils or emulsions. This paper introduces these basic issues with some recent findings.