Citation: Turska, M.; Paluszkiewicz, P.; Turski, W.A.; Parada-Turska, J. A Review of the Health Benefits of Food Enriched with Kynurenic Acid. Nutrients 2022, 14, 4182. https:// doi.org/10.3390/nu14194182 Academic Editor: Laura Domínguez Díaz Received: 14 September 2022 Accepted: 4 October 2022 Published: 8 October 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). nutrients Review A Review of the Health Benefits of Food Enriched with Kynurenic Acid Monika Turska 1, * , Piotr Paluszkiewicz 2 , Waldemar A. Turski 3 and Jolanta Parada-Turska 4 1 Department of Molecular Biology, The John Paul II Catholic University of Lublin, 20-708 Lublin, Poland 2 Department of General, Oncological and Metabolic Surgery, Institute of Hematology and Transfusion Medicine, 02-778 Warsaw, Poland 3 Department of Experimental and Clinical Pharmacology, Medical University of Lublin, 20-090 Lublin, Poland 4 Department of Rheumatology and Connective Tissue Diseases, Medical University of Lublin, 20-090 Lublin, Poland * Correspondence: turskamk@gmail.com Abstract: Kynurenic acid (KYNA), a metabolite of tryptophan, is an endogenous substance produced intracellularly by various human cells. In addition, KYNA can be synthesized by the gut microbiome and delivered in food. However, its content in food is very low and the total alimentary supply with food accounts for only 1–3% of daily KYNA excretion. The only known exception is chestnut honey, which has a higher KYNA content than other foods by at least two orders of magnitude. KYNA is readily absorbed from the gastrointestinal tract; it is not metabolized and is excreted mainly in urine. It possesses well-defined molecular targets, which allows the study and elucidation of KYNA’s role in various pathological conditions. Following a period of fascination with KYNA’s importance for the central nervous system, research into its role in the peripheral system has been expanding rapidly in recent years, bringing some exciting discoveries. KYNA does not penetrate from the peripheral circulation into the brain; hence, the following review summarizes knowledge on the peripheral consequences of KYNA administration, presents data on KYNA content in food products, in the context of its daily supply in diets, and systematizes the available pharmacokinetic data. Finally, it provides an analysis of the rationale behind enriching foods with KYNA for health-promoting effects. Keywords: food; food analysis; food ingredients; infant formula; kynurenic acid; nutrition 1. Introduction In 2013, we published a review paper describing the potential role of kynurenic acid (KYNA) in the digestive system. It summarized the presence of KYNA in the lumen of the gastrointestinal tract, and its beneficial health effects in digestive diseases, as well as its presence in food, were summarized. At that time, we pointed out many gaps in the knowl- edge and understanding of KYNA’s effects outside the central nervous system. However, even then, we assumed that KYNA administration might have some therapeutic potential and, despite many knowledge gaps, we envisaged the relevance of its supplementation [1]. KYNA is a metabolite of tryptophan. It is synthesized endogenously in human and an- imal bodies and/or absorbed from the digestive system. Although KYNA’s deficiency symptoms have not been described to date, it is reasonable to investigate the benefits of its supplementation. Nowadays, after 10 years of unprecedented progress in the research on KYNA, we can deliberate whether it is legitimate to supplement our diet with KYNA. The current review focuses on the presence of KYNA in food, including estimations of the daily dietary intake of KYNA, its absorption, distribution, and excretion, and it summarizes the scientifically approved health benefits of KYNA administered via the alimentary route, which are not limited to the digestive system. Nutrients 2022, 14, 4182. https://doi.org/10.3390/nu14194182 https://www.mdpi.com/journal/nutrients