Fibroblast Growth Factor 21 Levels in Young Healthy Females Display Day and Night Variations and Are Increased in Response to Short-Term Energy Deprivation Through a Leptin- Independent Pathway JOO-PIN FOO, MD 1 KONSTANTINOS N. ARONIS, MD 1,2 JOHN P. CHAMBERLAND, BSC 1,2 JASON PARUTHI, BSC 1 HYUN-SEUK MOON, PHD 1 CHRISTOS S. MANTZOROS, MD 1,2 OBJECTIVEdFibroblast growth factor (FGF)-21 is an endocrine factor with potent metabolic effects. Its day–night patterns of secretion and/or its physiological response to energy deprivation and relationship to free fatty acids (FFA) and/or leptin remain to be fully elucidated. We aim to elucidate day–night pattern of FGF-21 levels and its relationship to FFA, to assess whether energy deprivation alters its circulating patterns, and to examine whether leptin may mediate these changes. RESEARCH DESIGN AND METHODSdSix healthy lean females were studied for 72 h in a cross-over interventional study under three different conditions: on isocaloric diet and in a fasting state with administration of either placebo or metreleptin in physiological replacement doses. Blood samples were obtained hourly from 8:00 A.M. on day 4 until 8:00 A.M. on day 5. RESULTSdFGF-21 exhibited day–night variation pattern during the isocaloric fed state. Fasting significantly increased FGF-21 levels (P , 0.01) via a leptin-independent pathway. Day–night variation pattern in the fed state was lost on fasting. Leptin replacement in the hypo- leptinemic state restored approximate entropy of FGF-21 time series but did not alter circulating levels. FGF-21 levels were closely cross-correlated with FFA levels in all three states. CONCLUSIONSdA day–night variation in the levels of FGF-21 exists in young lean females in the fed state. Energy deprivation increases FGF-21 levels via a leptin-independent pathway. The interaction between FGF-21 and starvation induced lipolysis as indicated by its close cross- correlations with FFA in both fed state and energy deprivation needs to be studied further. F ibroblast growth factor (FGF)-21 is an endocrine factor predominantly expressed in the liver (1) that acts as a potent regulator of glucose and lipid metabolism (2). FGF receptors are ex- pressed in pancreatic b cells of adult mice, and dominant-negative mutations of the FGF receptors lead to decreased number of b cells and development of di- abetes (3). Administration of FGF-21 in rodents reduces plasma glucose and tri- glycerides to near-normal levels and im- proves insulin sensitivity independent of reduction in body weight and adiposity (2,4). In humans, FGF-21 is positively correlated with glycemia, adiposity, fasting insulin, and triglycerides, and is significantly higher in obese than in lean subjects (5,6). The higher FGF-21 levels suggest the possibility of an FGF-21- resistant state in obesity (7). Recently, high plasma FGF-21 levels were found to be an independent predictor of diabe- tes (8), highlighting its metabolic role in humans. Despite significant roles in metabolic regulation and energy homeostasis, the physiology of FGF-21 in humans, includ- ing its biological rhythm in states of energy deprivation, remains unclear. A previous study reported the absence of any diurnal variation in FGF-21 in healthy subjects (9). Other studies in con- trast reported the presence of a circadian rhythm, with varying response to fasting (10–13). Furthermore, although free fatty acids (FFA) have been shown to be a positive regulator of FGF-21 production through the activation of peroxisome proliferator-activated receptor-a (14), the relationship between FFA and FGF-21 in both physiological conditions of energy- repleted and energy-deficient states in humans remain unclear. The lack of con- sistent data for its biological characteristics and its potential interaction with lipolysis hampers a clearer understanding of its bi- ological role in humans. Importantly, de- spite the substantial interest in FGF-21 as a therapeutic target in diabetes, there is lack of knowledge of potential day–night variation pattern and responses thereof in the energy-repleted and energy-deprived states, and it remains unknown whether any effects of the energy-deprived con- dition could be mediated by energy deprivation–induced changes of leptin levels. Such findings could have signifi- cant ramifications on how various clinical studies of FGF-21, each utilizing varying sampling time and conditions, can be robustly interpreted. ccccccccccccccccccccccccccccccccccccccccccccccccc From the 1 Division of Endocrinology, Diabetes, and Metabolism, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts; and the 2 Section of Endocrinology, Boston VA Healthcare System, Harvard Medical School, Boston, Massachusetts. Corresponding author: Christos S. Mantzoros, cmantzor@bidmc.harvard.edu. Received 14 March 2012 and accepted 4 September 2012. DOI: 10.2337/dc12-0497. Clinical trial reg. no. NCT00140231, clinicaltrials.gov This article contains Supplementary Data online at http://care.diabetesjournals.org/lookup/suppl/doi:10 .2337/dc12-0497/-/DC1. © 2013 by the American Diabetes Association. Readers may use this article as long as the work is properly cited, the use is educational and not for profit, and the work is not altered. See http://creativecommons.org/ licenses/by-nc-nd/3.0/ for details. care.diabetesjournals.org DIABETES CARE 1 Pathophysiology/Complications O R I G I N A L A R T I C L E Diabetes Care Publish Ahead of Print, published online November 27, 2012