Vitamin C (VC, also known as ascorbic acid) is a water-soluble nutrient that cannot be synthesized in humans. Therefore, dietary supplementation and subsequent distribution of this physiologically important bioactive compound are necessary for the maintenance of human health. Since VC can hardly traverse cellular membranes passively owing to its hydrophilicity, active transport plays a pivotal role in VC handling, suggesting that VC transporters are of physiological significance. Until recently, only two VC transporters had been identified in mammals: sodium-dependent vitamin C transporter 1 (SVCT1) and SVCT2, which act as importers. Furthermore, the molecular mechanism of VC secretion from cells―a key step in transcellular VC transport―has been a longstanding mystery. In this connection, a VC exporter regulating VC supply from blood to the brain was identified in 2022 and named vitamin C efflux protein (VCEP). As well as overviewing the metabolism and handling of VC in the human body, this review details recent advances in VC transporter research, including the discovery of VCEP. To understand the whole picture of VC handling systems, further research on latent VC transporters and their physiological roles is vital, as this will also have implications in the field of nutrition.

Vitamin K, a fat-soluble vitamin that plays important roles in blood coagulation and bone formation, has homologues with different side chain structures. Humans mainly consume vitamin K₁ (phylloquinone, PK) in green vegetables and menaquinones (MK-n, n = 6－14) in fermented foods. However, the most abundant vitamin K in the body is vitamin K₂ (menaquinone-4, MK-4), due to the fact that ingested PK and MK-n are converted to MK-4 in the body. The enzyme responsible for this conversion reaction is UbiA prenyltransferase domain containing protein 1 (UBIAD1), and the conversion to MK-4 occurs in tissues throughout the body. This article describes the mechanism responsible for conversion of vitamin K homologues to MK-4 discovered by the author, the important role of UBIAD1, and newly clarified biological functions of vitamin K revealed by the use of UBIAD1-deficient mice.

Zinc is an essential micronutrient, and recent studies of zinc transporters and other molecules involved in zinc metabolism have revealed that this mineral has a wide range of important physiological functions. Concerningly, a number of epidemiological studies have reported that Japanese people tend to be deficient in zinc. Therefore, it is important to understand the physiological functions of this micronutrient to maximize its effectiveness for the maintenance and improvement of health. This review summarizes the importance of zinc by briefly describing the symptoms of zinc deficiency, the roles of zinc transporters―which are closely related to the physiological functions of zinc―and their relationship with diseases. Furthermore, we review and discuss the mechanisms of zinc absorption in the small intestine and the activation of secretory, membrane-bound, and organelle-resident zinc enzymes from the perspective of zinc transporters, based on our research results.

Phosphorus is one of the essential elements of the human body and is required for a variety of processes, including ATP synthesis, signal transduction, and bone mineralization. Phosphorus is found in various foods, such as those with animal protein sources like meat and fish, as well as in dairy products and seeds, and therefore is not deficient or absent in a normal diet. Blood phosphate (Pi) concentrations are controlled by intestinal absorption, bone formation, bone resorption, and renal excretion and reabsorption, which maintain a balance in response to various factors. Intestinal and renal Pi (re)absorption are mediated by Pi transporters, which are strictly mediated by the phosphorus-regulating hormones parathyroid hormone, active vitamin D, and fibroblast growth factor 23. We review the basic mechanisms responsible for regulation of phosphorus homeostasis, as well as novel regulators of phosphorus metabolism that we have recently identified.

Sake lees, a byproduct of the sake brewing process generated after pressing, is mostly disposed of as industrial waste. However, sake lees not only contains rice-derived components but also yeast and koji mold components and their metabolites. These are highly nutritious and have been reported to have various physiological functions. In this study, we examined the effects of mixing freeze-dried sake lees into the diet of model mice with type 2 diabetes. Mice fed with the sake lees-mixed diet for 40 days showed lower serum glucose levels than the control group. The results of the oral glucose tolerance test performed on the sake lees-fed mice showed a decrease in the diabetic phenotype characterized by a decrease in serum insulin levels and a significant increase in serum adiponectin levels. Furthermore, the expression of glucose transporter 4 mRNA in skeletal muscles was increased significantly in the sake lees-mixed diet group. These results suggest that consumption of freeze-dried sake lees may improve impaired glucose tolerance in patients with type 2 diabetes.