Nutrients 2023, 15, 538. https://doi.org/10.3390/nu15030538 www.mdpi.com/journal/nutrients Article The Effects of Amorphous Calcium Carbonate (ACC) Supplementation on Resistance Exercise Performance in Women Yitzhak Weinstein 1, *, Yarden Ovadia 1 , Bar Weinstein 2 and Ayelet Weinstein 1,3 1 Department of Nutritional Sciences, Tel-Hai Academic College, Kiryat Shmone 1220800, Israel 2 Department of Statistics and Operations Research, Tel Aviv University, Tel Aviv 6997801, Israel 3 Department of Sports Medicine and Research, Wingate Institute, Netanya 4290200, Israel * Correspondence: itzw@wincol.ac.il Abstract: The effects of 9 weeks of amorphous calcium carbonate (ACC) supplementation (1000 mg/day) and resistance exercise training (RT) on one repetition maximum (1-RM) values were tested. Thirty-one women (33.1 ± 7.3 y) were randomly assigned into a supplement (ACC, n = 14) or a placebo (PL, n = 17) group. On day 1 and following 9 weeks of intervention, the participants un- derwent anthropometric measurements and filled out a food frequency questionnaire (FFQ) and sports injuries questionnaires. 1-RM values were measured for the back squat and bench press ex- ercises. All the participants significantly (p = 0.01) improved their mean back squat and bench press 1-RM values (time effect). While no between-group difference was observed in the bench press 1- RM values, the ACC groups’ mean post-pre bench press 1-RM differences (∆1-RM) were signifi- cantly higher than in the PL group, expressed in kg (p = 0.049), per body mass (p = 0.042), or per lean body mass (p = 0.035). No significant interaction was observed for time X group effect (p = 0.421). No differences (within- or between-groups) were observed in the anthropometric values or in the questionnaires’ results. ACC supplementation revealed an ergogenic effect by augmenting the im- provement of maximum amount generated force, which can possibly be attributed to the calcium and/or the carbonate components. Keywords: performance; 1-RM; exercise; amorphous calcium carbonate; supplementation 1. Introduction Calcium supplementation is often used to attain the recommended calcium intake. However, the effectiveness of calcium supplementation in increasing bone strength is still questionable [1–3]. Calcium is a micronutrient that has an important role in both bone health and skeletal muscle function [4]; in addition, it plays an integral role in bone struc- ture and is involved in skeletal muscle function regulation (contraction and relaxation) [5]. A few reviews (i.e., meta-analyses) have concluded that long-term calcium supple- mentation (over 12 months) is effective in enhancing bone strength; it also has been sug- gested that this supplementation is more effective when combined with vitamin D [6,7]. Most of the calcium carbonate supplementation studies were carried out on an older pop- ulation and often combined with vitamin D. Except for Hoffman et al.’s [8] study using amorphous calcium carbonate (ACC) supplementation, there is little research which stud- ies the effectiveness of calcium carbonate supplementation on muscle performance or re- covery from exercise in physically active individuals and athletes. Calcium supplements are available in either organic or inorganic form, which differ- entiates their chelating capability [9]. While the organic calcium supplements consist of negatively charged organic molecules (e.g., malate, citrate, and gluconate), the inorganic form includes negatively charged inorganic molecules (e.g., carbonate, phosphates, and Citation: Weinstein, Y.; Ovadia, Y.; Weinstein, B.; Weinstein, A. The Effects of Amorphous Calcium Carbonate (ACC) Supplementation on Resistance Exercise Performance in Women. Nutrients 2023, 15, 538. https://doi.org/10.3390/nu15030538 Academic Editors: Tara M. Henagan, Natalie Lenard and Rosa Casas Received: 30 November 2022 Revised: 11 January 2023 Accepted: 17 January 2023 Published: 19 January 2023 Copyright: © 2023 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/license s/by/4.0/).