Abstract
Minerals and trace elements (MTEs) are micronutrients involved in hundreds of biological processes. De ciency in MTEs can negatively affect athletic performance. Approximately 50% of athletes have reported consuming some form of micronutrient supplement; however, there is limited data con rming their ef cacy for improving performance. The aim of this study was to systematically review the role of MTEs in exercise and athletic performance. Six electronic databases and grey literature sources (MEDLINE; EMBASE; CINAHL and SportDISCUS; Web of Science and clinicaltrials.gov) were searched, in accordance with PRISMA guidelines. Results: 17,433 articles were identi ed and 130 experiments from 128 studies were included. Retrieved articles included Iron (n= 29), Calcium (n= 11), Magnesium, (n= 22), Phosphate (n= 17), Zinc (n= 9), Sodium (n= 15), Boron (n= 4), Selenium (n= 5), Chromium (n= 12) and multi-mineral articles (n= 5). No relevant articles were identi ed for Copper, Manganese, Iodine, Nickel, Fluoride or Cobalt. Only Iron and Magnesium included articles of suf cient quality to be assigned as `strong'. Currently, there is little evidence to support the use of MTE supplementation to improve physiological markers of athletic performance, with the possible exception of Iron (in particular, biological situations) and Magnesium as these currently have the strongest quality evidence. Regardless, some MTEs may possess the potential to improve athletic performance, but more high quality research is required before support for these MTEs can be given. PROSPERO preregistered (CRD42018090502). Keywords: ergogenic aids; nutritional supplements; physical performance; exercise and sport nutrition; muscle function 1. Introduction Minerals and trace elements (MTEs) are inorganic micronutrients found in
currently have the strongest quality evidence. Regardless, some MTEs may possess the potential to improve athletic performance, but more high quality research is required before support for these MTEs can be given. PROSPERO preregistered (CRD42018090502). Keywords: ergogenic aids; nutritional supplements; physical performance; exercise and sport nutrition; muscle function 1. Introduction Minerals and trace elements (MTEs) are inorganic micronutrients found in a variety of plant and animal foods [13]. Inadequate MTE intake has been linked to a number of health conditions, such as diabetes, cardiovascular and kidney disease, aging and fracture risk [413]. These micronutrients are involved in hundreds of biological processes relevant to exercise and athletic performance, such as energy storage/utilization, protein metabolism, in ammation, oxygen transport, cardiac rhythms, bone metabolism and immune function [1418]. However, despite the biological importance of MTEs, population data suggests that current RDAs are not being achieved, with Selenium, Magnesium, Calcium, Iron and Zinc of particular concern (60%, 50%, 51%, 30% and 17% reported de ciencies, respectively; [1921]). While not adhering to RDAs does not strictly result in biological de ciencies, certain dietary and lifestyle choices may introduce additional challenges to RDA adherence and lead to de ciencies. The Western-Type diet (high in animal protein, saturated fats and re ned Nutrients2019,11, 696; doi:10.3390/nu11030696
Nutrients2019,11, 696 2 of 32 carbohydrates) is the most adopted diet in rst-world adult populations and shows de ciencies in Phosphorus (supplemented as Phosphate) and Magnesium [22]. The Atkins for Life diet, South Beach Diet, and DASH diet result in Chromium, Iodine and Molybdenum de ciencies [23]; Eat to Live-Vegan, Aggressive Weight Loss diet in Calcium, Selenium and Zinc; Fast Metabolism Diet in Calcium, Magnesium and Potassium; Eat, Drink and Be Healthy diets in Calcium and Potassium [24]; and a strict Vegan diet shows de ciencies in Iodine and Selenium [25]. However, a Mediterranean-style diet has been suggested to mitigate some of these de ciencies and may be superior to other diets for micronutrient intake [26]. The exposure of many exercisers and athletes to commercially available diets, via the wider and social media, can lead to the adoption of food choice in-line with these diets [27] and result in associated inadequate MTE intakes and de ciencies [2832]. For example, some athletes (n= 25 male Polish) may be up to 60% de cient in the dietary intake of particular micronutrients [Magnesium (Mg); 33]. Similar to the available population data (for example, 15% de ciency of Mg; [2225]), there are variabilities in the level of micronutrient de ciency in the diet, depending on the geographical location [31,3335]. In an Australian population of elite female athletes (n= 72), Calcium (22%), Iron (19%) and Magnesium (15%) intakes were identi ed as de cient when assessed by a food frequency questionnaire [34]. Conversely, in a large Dutch population of sub-elite athletes (n= 553, femalen= 226), the only de ciencies identi ed were Selenium (11%) in the whole group and Iron in females (38%; [35]), assessed by 24 h recall. These may be due to the mineral constituents of the dietary choices within each population [1,2,36] and the soil environments in different geographical locations [37]. There is also considerable potential for error in validity and reliability at all stages of dietary intake assessment, regardless of the method used [38]. Biomarkers can provide an objective assessment of nutrient status. However, among other limitations, few nutrients have reference
mineral constituents of the dietary choices within each population [1,2,36] and the soil environments in different geographical locations [37]. There is also considerable potential for error in validity and reliability at all stages of dietary intake assessment, regardless of the method used [38]. Biomarkers can provide an objective assessment of nutrient status. However, among other limitations, few nutrients have reference ranges for well-trained athletes [38]. In situations of high metabolic demand, such as exercise or athletic training, inadequate circulating and cellular MTEs may impair optimal physiological performance [14,30] and may require supplementation [17]. The exact impact of these de ciencies or supplementation on athletic performance remains generally unclear [39]. However, there may be ergogenic properties of MTEs in achieving or possibly surpassing the RDA, that are speci cally designed for the general population health. While there is currently no consensus as to the ef cacy of MTE supplementation for exercise and any physiological measure of athletic performance, recent studies show that ~50% of athletes consume some form of micronutrient supplement and between 527% are MTEs (Iron, Calcium, Zinc, Selenium or Chromium; [31,40]). Synthesis of some MTE research, in the context of athletic performance, can be found throughout the literature [4148]; however, there are some MTEs and more recent studies that have yet to be systematically reviewed. Collating the current evidence on the ef cacy of MTE supplementation for athletic performance in a single article is warranted and may provide a useful tool for improving the knowledge base of athletes and sport/exercise practitioners; speci cally, the effects of MTEs on those phenotypes that bene t general markers of performance, for example, a lower body power output, maximal endurance capacity, maximal and relative muscle mass and strength, and fatigue recovery capacity. Therefore, the present aim was to systematically review the literature and critically synthesise the available evidence on MTE supplementation for enhancing exercise and physiological aspects of athletic performance. In addition, this review aimed to make recommendations about the ef cacy of MTE supplementation for optimising athletic performance, based on the quality of the retrieved research. 2. Methods The review was conducted
the present aim was to systematically review the literature and critically synthesise the available evidence on MTE supplementation for enhancing exercise and physiological aspects of athletic performance. In addition, this review aimed to make recommendations about the ef cacy of MTE supplementation for optimising athletic performance, based on the quality of the retrieved research. 2. Methods The review was conducted in accordance with the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA, checklist in Appendix) statement [ 49] and preregistered with PROSPERO (CRD42018090502).
Nutrients2019,11, 696 3 of 32 2.1. Search Strategy A systematic search of six electronic databases and grey literature sources (MEDLINE; EMBASE; CINAHL and SportDISCUS; Web of Science and clinicaltrials.gov) was performed using prede ned search terms deduced from the eligibility criteria and PICO guidelines [50] between January and April 2018. The reference lists of identi ed reviews and included articles were hand searched for potentially relevant articles. Where appropriate, the search was conducted using Medical Subject Headings and Boolean operators of keywords relating to population (athlete etc.), intervention (Calcium etc.), and outcome (athletic performance etc.; Appendix). 2.2. Study Selection and Data Extraction Following the initial data search, a complete search of titles and abstracts was performed by two independent reviewers (SH and GC). Titles and abstracts were screened for eligibility according to prede ned inclusion and exclusion criteria. In the case of an inclusion discrepancy, the two independent reviewers discussed the merits of selection. If a consensus could not be reached, a third reviewer (KH) was consulted to resolve the issue and an agreement was achieved. Retrieved articles in ful lment of the inclusion criteria were accessed, in full, and critically appraised, and the article data was extracted using a customised form (content of data extraction was decided by two reviewers) and characterised into methodological (risk of bias etc.), participant characteristics (age etc.) and study characteristics (sample size etc.). In any cases where more than one distinct experiment was performed in a single article, the experimental data for each experiment was extracted and assessed separately (for example, in Shae et al. [51]). 2.3. Eligibility Criteria Studies of healthy adult and athletic populations (+17 years), English language, both sexes and supplement studies were included. Reports on animals, cells, children, diseased populations, dietary intake only and psychological phenotypes were excluded. Diseased populations where mineral de ciency was part of the diagnosis/prognosis were excluded; however, studies including individuals with de ciencies that were otherwise healthy were included. Interventional and control trials were the target designs for the present review to ensure the capture of all relevant articles. The following article types were excluded:
dietary intake only and psychological phenotypes were excluded. Diseased populations where mineral de ciency was part of the diagnosis/prognosis were excluded; however, studies including individuals with de ciencies that were otherwise healthy were included. Interventional and control trials were the target designs for the present review to ensure the capture of all relevant articles. The following article types were excluded: editorials, systematic reviews, letters to the editor, commentaries, duplicated publications and articles that combine minerals with other molecules such as the amino acid aspartic acid combined with Zinc and Magnesium in ZMA [52]. In additionally retrieved (not identi ed through database searches) articles, the references list(s) were scanned for appropriate original articles. 2.4. Quality Assessment and Risk of Bias Tool The Effective Public Health Practice Project Quality Assessment Tool (EHPP; [53]) was used to assess study quality and risk of bias independently by two authors (SH and GC). On the occasion of a discrepancy in the global quality, a third independent reviewer (KH) assessed the article(s) and a consensus was achieved. To ensure selection consistency and quality assurance, a random sub-sample of retrieved studies were cross-checked, post-hoc and independently, by KH (blinded to the original review decisions). For studies describing athletes as `well/highly/trained/elite', selection bias was graded as Somewhat Likely (this decision was made in consultation with the EPHPP licence holders at McMaster University). 3. Results A total of 17,459 articles were identi ed, and after removing duplicates, 14,144 articles remained. Of these, 13,816 were excluded after title and abstract screening. The remaining 328 articles were screened in full text and 128 studies met the eligibility criteria (Figure).
Nutrients2019,11, 696 4 of 32Nutrients 2019, 11, x FOR PEER REVIEW 4 of 33 Figure 1. PRISMA schematic summarising the search strategy and study selection. *One study assessed both Zn and Se separately and in combination (n = 3 groups). Thus, is counted in subsections Zn, Se and multi-minerals (also accounted for in all other cumulative study sample calculations). 3.1. Study Characteristics The 128 eligible studies consisted of 3643 participants (1387 Females), aged between 17–75 years, and included 24 studies of elite athletes (Supplementary Table S1). The eligible studies related to Iron (Fe; n = 29), Calcium (Ca; n = 11), Magnesium, (Mg; n = 22), Phosphate (P; n = 17), Zinc (Zn; n = 9), Sodium (Na; n = 15), Boron (Br; n = 5), Selenium (Se; n = 5), Chromium (Cr; n = 12) and Multi-mineral articles (n = 5). No relevant research articles were identified for Copper, Manganese, Iodine, Nickel, Fluoride or Cobalt. 3.2. Study Quality Using the quality assessment tool, eight articles were identified as strong, 95 as moderate and 25 as weak. Of these, only Fe and Mg included articles of sufficient quality to be classified as strong (Fe = 6, Mg = 2; Figure 2). Overall, the majority of retrieved articles were assigned a moderate quality (77%, Figure 2 and Supplementary Table S1). Figure 1. PRISMA schematic summarising the search strategy and study selection.*One study assessed both Zn and Se separately and in combination (n= 3 groups). Thus, is counted in subsections Zn, Se and multi-minerals (also accounted for in all other cumulative study sample calculations). 3.1. Study Characteristics The 128 eligible studies consisted of 3643 participants (1387 Females), aged between 1775 years, and included 24 studies of elite athletes (Supplementary Table S1). The eligible studies related to Iron (Fe;n= 29), Calcium (Ca;n= 11), Magnesium, (Mg;n= 22), Phosphate (P;n= 17), Zinc (Zn;n= 9), Sodium (Na;n= 15), Boron (Br;n= 5), Selenium (Se;n= 5), Chromium (Cr;n= 12) and Multi-mineral articles (n= 5). No relevant research articles were identi ed for Copper, Manganese, Iodine, Nickel, Fluoride or Cobalt. 3.2. Study Quality Using the quality assessment tool, eight
eligible studies related to Iron (Fe;n= 29), Calcium (Ca;n= 11), Magnesium, (Mg;n= 22), Phosphate (P;n= 17), Zinc (Zn;n= 9), Sodium (Na;n= 15), Boron (Br;n= 5), Selenium (Se;n= 5), Chromium (Cr;n= 12) and Multi-mineral articles (n= 5). No relevant research articles were identi ed for Copper, Manganese, Iodine, Nickel, Fluoride or Cobalt. 3.2. Study Quality Using the quality assessment tool, eight articles were identi ed as strong, 95 as moderate and 25 as weak. Of these, only Fe and Mg included articles of suf cient quality to be classi ed as strong (Fe = 6, Mg = 2; Figure). Overall, the majority of retrieved articles were assigned a moderate quality (77%, Figure
Nutrients2019,11, 696 5 of 32Nutrients 2019, 11, x FOR PEER REVIEW 5 of 33 Figure 2. EPHPP global quality rating. Presented as percentage of articles rated as strong, moderate and weak for each mineral. 4. Discussion This systematic review aimed to synthesise the evidence relating to the effects of MTE supplementation on athletic performance and related physiological phenotypes in the adult population. Quality of evidence investigating MTEs and athletic performance is lacking, with only eight articles classified as strong. Nonetheless, there is limited but growing evidence for potential benefits of some MTEs in relation to athletic performance (Fe and Mg), although the majority of research quality remains moderate-weak (Figure 2). One article was identified that presented evidence for a possible benefit of a particular combination of minerals on athletic performance-related phenotypes (see Section 4.10). Furthermore, the present review retrieved recent, although still limited, evidence for a ‘natural’ mineral-rich compound Lithothamnion and its potential for athletic performance-related haematological phenotypes, although currently no evidence for functional performance. 4.1. Iron Twenty nine articles fulfilled the inclusion criteria for iron (Fe), including 946 participants (females, n = 776). These consisted of 19 randomised control trials (RCT’s), 10 of which referred to elite athletes. Iron is the most studied mineral in exercise and athletic performance, with the best quality research (Figure 2). Non-anaemic Fe deficiency (serum ferritin <20.0 µg·L −1 , Hb >115 µg·L −1 ) and anaemia (serum ferritin <12.0 µg·L −1 , Hb <115 µg·L −1 ) are common at all levels of athletic performance and are thought to affect physiological capacity. Fe deficiency with and without anaemia has been repeatedly shown to be attenuated following both oral and intravenous (IV) Fe supplementation in a variety of sports [54–59]. However, the potential benefits of Fe supplementation on physiological performance may be dependent of baseline ferritin level, Fe dose and route of administration. Baseline iron status or ferritin level is a major factor that could impact the efficacy of Fe supplementation on Fe status and performance-related outcomes. For example, in a group of elite endurance athletes (n = 178, 80 females) divided by baseline
of Fe supplementation on physiological performance may be dependent of baseline ferritin level, Fe dose and route of administration. Baseline iron status or ferritin level is a major factor that could impact the efficacy of Fe supplementation on Fe status and performance-related outcomes. For example, in a group of elite endurance athletes (n = 178, 80 females) divided by baseline ferritin levels prior to training at moderate altitude, those with high ferritin levels (>100 µg·L −1 ) were given no supplement, mid-ferritin levels (~76 µg·L −1 ) were supplemented with 105 mg Fe and low ferritin levels (~25 µg·L −1 ) were supplemented with 210 mg for two to four weeks [60]. Haemoglobin mass (HBmass) increased in those with low and mid-baseline ferritin levels supplemented with Fe, but there was no change in Figure 2. EPHPP global quality rating. Presented as percentage of articles rated as strong, moderate and weak for each mineral. 4. Discussion This systematic review aimed to synthesise the evidence relating to the effects of MTE supplementation on athletic performance and related physiological phenotypes in the adult population. Quality of evidence investigating MTEs and athletic performance is lacking, with only eight articles classi ed as strong. Nonetheless, there is limited but growing evidence for potential bene ts of some MTEs in relation to athletic performance (Fe and Mg), although the majority of research quality remains moderate-weak (Figure). One article was identi ed that presented evidence for a possible bene t of a particular combination of minerals on athletic performance-related phenotypes (see Section). Furthermore, the present review retrieved recent, although still limited, evidence for a `natural' mineral-rich compoundLithothamnionand its potential for athletic performance-related haematological phenotypes, although currently no evidence for functional performance. 4.1. Iron Twenty nine articles ful lled the inclusion criteria for iron (Fe), including 946 participants (females, n= 776). These consisted of 19 randomised control trials (RCT's), 10 of which referred to elite athletes. Iron is the most studied mineral in exercise and athletic performance, with the best quality research (Figure). Non-anaemic Fe de ciency (serum ferritin <20.0 g L 1 , Hb >115 g L
lled the inclusion criteria for iron (Fe), including 946 participants (females, n= 776). These consisted of 19 randomised control trials (RCT's), 10 of which referred to elite athletes. Iron is the most studied mineral in exercise and athletic performance, with the best quality research (Figure). Non-anaemic Fe de ciency (serum ferritin <20.0 g L 1 , Hb >115 g L 1 ) and anaemia (serum ferritin <12.0 g L 1 , Hb <115 g L 1 ) are common at all levels of athletic performance and are thought to affect physiological capacity. Fe de ciency with and without anaemia has been repeatedly shown to be attenuated following both oral and intravenous (IV) Fe supplementation in a variety of sports [5459]. However, the potential bene ts of Fe supplementation on physiological performance may be dependent of baseline ferritin level, Fe dose and route of administration. Baseline iron status or ferritin level is a major factor that could impact the ef cacy of Fe supplementation on Fe status and performance-related outcomes. For example, in a group of elite endurance athletes (n= 178, 80 females) divided by baseline ferritin levels prior to training at moderate altitude, those with high ferritin levels (>100 g L 1 ) were given no supplement, mid-ferritin levels (~76 g L 1 ) were supplemented with 105 mg Fe and low ferritin levels (~25 g L 1 ) were supplemented with 210 mg for two to four weeks [60]. Haemoglobin mass (HBmass) increased in those with low and mid-baseline ferritin levels supplemented with Fe, but there was no change in non-supplemented athletes. In addition, follow up ferritin levels increased by 37% in the group with the lowest baseline levels of ferritin, whereas in the other groups, ferritin decreased and total
Description
This systematic review evaluates the role of MTEs in exercise and athletic performance.