Applied nutritional investigation Melatonin in walnuts: Influence on levels of melatonin and total antioxidant capacity of blood Russel J. Reiter, Ph.D.*, L. C. Manchester, Ph.D., and Dun-xian Tan, M.D., Ph.D. Department of Cellular and Structural Biology, The University of Texas Health Science Center, San Antonio, Texas, USA Manuscript received November 15, 2004; accepted February 4, 2005. Abstract Objective: We investigated whether melatonin is present in walnuts (Juglans regia L.) and, if so, tested whether eating walnuts influences melatonin levels and the total antioxidant status of the blood. Methods: Melatonin was extracted from walnuts and quantified by high-performance liquid chromatography. After feeding walnuts to rats, serum melatonin concentrations were measured using a radioimmunoassay and the “total antioxidant power” of the serum was estimated by using the trolox equivalent antioxidant capacity and ferric-reducing ability of serum methods. Results: Mean  standard error melatonin concentrations were 3.5  1.0 ng/g of walnut. After food restriction of rats and then feeding them regular chow or walnuts, blood melatonin concen- trations in the animals that ate walnuts were increased over those in the rats fed the control diet. Increases in blood melatonin were also accompanied by increases in trolox equivalent antioxidant capacity and ferric-reducing ability of serum values. Conclusions: Melatonin is present in walnuts and, when eaten, increase blood melatonin concen- trations. The increase in blood melatonin levels correlates with an increased antioxidative capacity of this fluid as reflected by augmentation of trolox equivalent antioxidant capacity and ferric- reducing ability of serum values. © 2005 Elsevier Inc. All rights reserved. Keywords: Walnuts; Melatonin; Cardiovascular system; Antioxidant; Trolox equivalent antioxidant capacity; Ferric-reduc- ing ability of serum Introduction Walnuts (Juglans regia L.) are receiving increasing in- terest as a healthy foodstuff because their regular consump- tion has been reported to decrease the risk of heart disease [1–3]. These findings are sufficiently compelling to the point where the U.S. Food and Drug Administration has affirmed the following health claim for walnuts: “Support- ive but not conclusive research shows that eating 1.5 ounces per day of walnuts as part of a diet low in saturated fat and cholesterol may reduce the risk of heart disease” (July 14, 2003). The published basic and clinical research on the benefits of walnut consumption on cardiovascular physiol- ogy was used to support this claim. The health benefits of walnuts are usually attributed to their high content of -3 fatty acids [3–5] and, to a lesser degree, to the amount of vitamin E they contain [6], in particular -tocopherol, which promotes the cellular uptake of -tocopherol. In addition, walnuts have other compo- nents that may be beneficial for health including a low lysine:arginine ratio and high levels of arginine, folate, fiber, tannins, and polyphenols [1,7]. Importantly, walnut consumption does not cause a net gain in body weight when eaten as a replacement food [8]. Several reports that have appeared in the past decade have also identified melatonin, a molecule with beneficial effects on the cardiovascular system [9,10], in edible foods and seeds [11,12]. Melatonin (N-acetyl-5-methoxytryptamine) is synthe- sized and widely distributed in the animal kingdom [13,14], and its presence in plants has prompted investigations into the potential health benefits of consumption of foodstuffs contain- ing this indole. Given the numerous observations regarding the positive effects of walnut consumption on cardiovascular This study was supported by a grant from the California Walnut Commission. * Corresponding author. Tel.: +210-567-3859; fax: +210-567-6948. E-mail address: reiter@uthscsa.edu (R.J. Reiter). Nutrition 21 (2005) 920 –924 www.elsevier.com/locate/nut 0899-9007/05/$ – see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.nut.2005.02.005