Sodium Aspartate as a Specific Enhancer of Salty Taste Perception—Sodium Aspartate Is a Possible Candidate to Decrease Excessive Intake of Dietary Salt

Chem. Senses 39: 781–786, 2014 doi:10.1093/chemse/bju051 Advance Access publication October 11, 2014 Sodium Aspartate as a Specific Enhancer of Salty Taste Perception— Sodium Aspartate Is a Possible Candidate to Decrease Excessive Intake of Dietary Salt Tomohiro Nakagawa1, Jun Kohori1, Shin Koike1, Yoshihisa Katsuragi1 and Takayuki Shoji2 1 Health Care Food Research Laboratories, Kao Corporation, Sumida-ku, Tokyo 131-8501, 2 Japan and Department of Marine Biology, Tokai University, Shizuoka-shi, Shizuoka, Japan Correspondence to be sent to: Tomohiro Nakagawa, Health Care Food Research Laboratories, Kao Corporation, Sumida-ku, Tokyo 131-8501, Japan. e-mail: Accepted August 18, 2014 Abstract The excessive intake of dietary salt is a global issue in health. Attempts have been made to address this issue, including the development of salt substitutes. Yet, none of these substances are currently in wide use, because of their weak saltiness. The purpose of this study was to assess the effects of sodium aspartate (Asp-Na) on salty taste perception using the bullfrog glossopharyngeal nerve response and human sensory tests. When added to the mixture of NaCl and KCl, Asp-Na significantly enhanced the glossopharyngeal nerve response to the mixture by 1.6-fold compared to control. Asp-Na did not enhance the response to NaCl, nor did Asp-Na enhance the response to sour, bitter, or umami stimuli. The optimal concentration for Asp-Na to enhance the salt mixture was 1.7 mM. The largest enhancement was induced when NaCl and KCl were mixed at equimolar concentrations. Asp-Na significantly suppressed the glossopharyngeal nerve response to quinine hydrochloride, which suggests that bitterness of KCl is suppressed by Asp-Na. The salty taste enhancing effect of Asp-Na was also confirmed with human sensory tests. The present results suggested that the mixture of NaCl and KCl containing Asp-Na can be used as a salt substitute. In addition to demonstrating that Asp-Na enhanced salt taste responses in an experimental animal and human, our findings provide clues to identify the elusive salty taste receptors. Key words: nerve response, potassium chloride, reducing salt intake, salt substitute, salt taste receptor, salty taste enhancing substance Introduction In many countries the amount of dietary salt intake currently exceed 6.0 g/day, which is a standard set forth by the World Health Organization (WHO) (Elliott and Brown 2007). For example, dietary salt intake in Japan, China, the United States, and the United Kingdom are 12.3, 14.3, 10.7, and 9.4 g/day, respectively. Such salt intake higher than the standard may poses a serious problem in health, as recent meta-analyses have shown that excessive dietary salt intake will lead to hypertension, stroke, and gastric cancer (Intersalt Cooperative Research Group 1988; He and MacGregor 2009; Strazzullo et al. 2009). If left unchecked, this could trigger a global health crisis. To address this growing concern, many countries have focused their efforts on developing strategies to reduce salt intake (Henney et al. 2010). Food intake can be reduced a number of ways, including the use of substitute products and/or augmenting taste. © Crown copyright 2014 For instance, sugar intake can be controlled using the sugar substitutes aspartame or acesulfame-K, which are 200-fold sweeter than sugar. One can also add small amounts of salt to sugar to enhance sweetness (Kumazawa and Kurihara 1990), a widely used cooking trick, thereby reducing sugar intake. Salt substitutes have not been as successful. In fact, while various candidate salt substitutes or replacements exist, none have really caught on. For example, potassium chloride (KCl), while a promising candidate, has a weaker salty taste than sodium chloride (NaCl). When used in large amounts, it is also associated with bitterness, which is one of the reasons why its use has not been widely pur sued. Other substitutes such as ammonium chloride, potassium sulfate, and sodium gluconate have also been suggested as candidate substances, but because they share the same problems with KCl, their use has been limited. Moreover, the lack of 782 T. Nakagawa et al. effective methods to increase salty taste is somewhat responsible for the lack of effective strategies to decrease salt intake. Taste is divided into 5 basic qualities: sweet, bitter, umami, sour, and salty tastes. In order to understand the characteristics of each basic taste and their mechanisms of transduction, it is important to elucidate the receptors for each taste in question. The receptors for sweet, bitter, and umami tastes have been identified, and current work in the field has focused on elucidating their mechanisms of transduction (Hoon et al. 1999; Adler et al. 2000; Chandrashekar et al. 2000; Bachmanov et al. 2001; Kitagawa et al. 2001; Max et al. 2001; Montmayeur et al. 2001; Nelson et al. 2001, 2002; Sainz et al. 2001). A likely candidate for the receptor of sourness has been recently discovered, and it is anticipated that future work will focus on elucidating its characteristics and transduction (Huang et al. 2006; Ishimaru et al. 2006). Studies on salt taste receptors have been carried out extensively, but those receptors have not been concretely identified. At present candidate receptors are the epithelial sodium channel (ENaC) (DeSimone et al. 1981; Heck et al. 1984; Brand et al. 1985; DeSimone and Ferrell 1985) and transient receptor potential vallinoid 1 (TRPV1; Lyall et al. 2004). In foods of reduced sodium content the salty taste can be maintained by adding potassium to the foods. This method has been widely practiced, for example, with light salt (i.e., table salt with NaCl partially replaced by KCl). Japan has historically focused on the umami taste in food products. If the umami taste in soup stock is strong enough, it is thought that the good taste of food can be maintained while keeping the dietary salt concentration low (Yamaguchi and Takahashi 1984). Konbu (kelp) is a source of umami taste, the most important taste in Japanese cuisine. It contains glutamic acid and aspartic acid as a source of umami taste (Mouritsen et al. 2012). In this context, we assessed the effects of potassium ions and these acidic amino acids coexisting in salty stimulant on salty taste perception by means of the bullfrog glossopharyngeal nerve recording and human sensory tests. We found Asp-Na a powerful enhancer of salty taste perception. The present results suggested that the mixture of NaCl and KCl containing Asp-Na can be used as a salt substitute. Materials and methods Test animals American bullfrogs (Lithobathes catesbeianus) (250–350 g) purchased from a biological supply company(Ouchi Kazuo Animals for Teaching Materials, Saitama, Japan) were used for animal experiments. Ten bullfrogs were kept alive in a plastic water tank holding a small amount of water at temperature of 25°C. All experiments were conducted with the approval of the A (...truncated)