INTRODUCTION

The potential health hazards of sedentary life and the health benefits of increased daily physical exercise have long been recognized. The very high and growing prevalence of obesity has prompted international bodies (WHO, FAO, etc.) to launch multiple initiatives to encourage a healthy diet and daily physical activity[1,2]. Millions of individuals worldwide now practice regular exercise to prevent or combat disease and enhance their quality of life or to improve their physical condition for participation in sports activities[3].

The performance of sportspeople is influenced by numerous physical, psychological, nutritional, and environmental factors. Nutritional status plays a key role, and minor dietary errors at critical times can ruin months or even years of hard training[4]. In fact, as the margin between victory and defeat becomes ever narrower, sportspeople increasingly resort to nutritional supplements[5,6]. There has been an exponential growth over recent years in the consumption of supplements, largely composed of proteins, amino acids, vitamins and minerals, and this has been especially marked among sportspeople[7].

Many supplements are promoted, with no sound scientific foundation, for enhancing performance and muscle mass as part of normal training or competition routines and for facilitating recovery after training or sports injury[8]. Around 85% of elite athletes are reported to take supplements, including vitamins, minerals, proteins, and/or creatinine, among others. These should be used with care in order to minimize risks, but it appears that sportspeople rarely wait for convincing proof of their effectiveness or safety[9]. In general, vitamin and mineral supplements are not considered necessary if the usual diet contains an appropriate variety of foods that provide sufficient energy to maintain the body weight[10,11]. However, supplements may be required by sportspeople who restrict their energy consumption or remove one or more food groups in order to lose weight or by those whose diets are unbalanced and deliver a low density of micronutrients[12].

Sportspeople frequently consume supplements that are not always appropriate for their bodies or for their sports activity and that may have no effect or may even exert adverse effects[9]. The American College of Sports Medicine, American Dietetic Association, and Dieticians of Canada have issued warnings about the frequently incorrect and unsafe utilization of nutritional ergogenic aids, which should only be taken after careful evaluation of their safety, effectiveness, potency, and legality; it was emphasized that individual nutritional guidance and advice should be based on a complete nutritional assessment by experts[12]. Nutritional indexes used for this purpose should evidently be as comprehensive as possible, and there is a need for an instrument that takes account of the consumption of supplements and includes all micronutrients (e.g., ramified or sulphured amino acids).

The objectives of this study were to evaluate the main nutrients used as supplements in sports and to propose a nutritional quality index (NQI) that enables sportspeople and other groups to optimize their use of supplements and detect and remedy possible nutritional deficits. Two NQI scores were obtained for each individual: one based on FFQ data alone and the other also including nutrient intake from supplements, as recorded in the life habits questionnaire, with the aim of comparing and evaluating the advantages or drawbacks of the supplement intake of the participants.

MATERIALS AND METHODS

RESULTS

Study inclusion criteria were met by a total of 515 volunteers. After exclusion of those who failed to complete the questionnaires in a correct manner, the final study sample comprised 485 healthy sportspeople (72.2% males and 27.4% females) aged between 18 and 69 yrs. The mean age was 27.95 (SD 11.69) yrs in the males and 19.89 (SD 7.83) yrs in the females.

Table 1 lists the estimated value reported for each nutrient considered in the index (without and with supplementation) alongside the percentage of the dietary reference intake (DRI) to which it corresponds (DRI source is specified in the footnotes). The mean intake of all nutrients was >100% of the DRI, with the exception of valine, leucine, methionine/cysteine and vitamin D. The mean intake of all nutrients significantly differed from the mean DRI, with the exception of iodine, selenium and methionine/cysteine (p>0.05). The mean value of the index was 16.28 when supplements were not included and 15.49 when they were. For both indexes, the lowest value was 2 points and the highest was 21. In the males, the mean score was 16.47 (SD 3.14) without versus 15.69 (SD 2.85) with supplement intake, and in the females, it was 16.09 (SD 3.31) without versus 15.19 (SD 3.15) with supplement intake, with no significant gender differences.

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Table 1. Intake of nutrients with respect to Dietary Reference Intake (DRI) and NQI mean value for both groups.^*

https://doi.org/10.1371/journal.pone.0125630.t001

Table 2 shows the consumption of each nutrient in tertiles: first, above the UL; second, between 2/3 of the recommendation and the UL, and third below 2/3 of the recommendation. The consumption of each intake was in the third tertile for >60% of participants.

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Table 2. Distribution of the population (%) according to the relationship of nutrient intake with UL and DRI values.

https://doi.org/10.1371/journal.pone.0125630.t002

Stepwise regression analysis revealed that only nine of the items significantly contributed to the indexes; these were the same items for both indexes but made distinct percentage contributions to the scores. For instance, protein accounted for 35% of the index score when supplements were not included compared with only 6% when they were (Table 3).

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Table 3. Nutrients that together contribute to > 95% of the NQI scores (stepwise regression).

https://doi.org/10.1371/journal.pone.0125630.t003

Logistic regression analysis (Table 4) showed that the use of nutritional supplementation was influenced by educational level, type of sport (group v. individual), current or past smoking habit and the history of a dietary regime. There was a greater likelihood of supplementation in participants with a medium educational level (OR = 0.36, 95%CI: 0.13–1.04), those involved in group sports (OR = 2.59, CI 95%: 0.95–7.03), non-smokers (OR = 4.20, 95%CI: 0.91–19.48) and those who had never been on a diet (OR = 2.52, 95%CI: 0.94–6.78).

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Table 4. Factors influencing the intake of supplements in the study population (logistic regression).

https://doi.org/10.1371/journal.pone.0125630.t004

DISCUSSION

There has been a continuing increase in the use of supplements, especially by sportspeople[5]. In the present study of 485 healthy active sportspeople, around two-thirds consumed some type of supplement, mostly preparations based on proteins, ramified amino-acids, vitamins and/or minerals, similar to reports by other researchers[29].

Macronutrient and micronutrient requirements may be higher in sportspeople, due to their physical exertions, but these were amply met by the daily diet of almost all of the participants. Thus, the requirement for protein was met by the dietary intake of almost all (96.5%) of the participants; however, 59.5% of the participants exceeded the UL for protein by taking supplements, potentially putting their health at risk. A study in the UK associated a protein intake above the UL with various adverse effects, and concluded that it should not exceed two-fold the recommended level[20,22]. Other authors claimed that a protein intake >35% of the energy intake has no adverse effects but that a percentage above 45% can be life-threatening if maintained for several weeks[30]. It should be taken into account that the protein UL values considered in the present study were two-fold higher than the recommendations for adult sportspeople, which are much higher than those for the general population[31].

Because the replacement level of minerals is low, deficits can usually be avoided by consuming a healthy and balanced diet. Nevertheless, athletes frequently have problems in this respect due to increased excretion (from exercise or other factors) and problems of intestinal absorption[32]. This is mainly a concern with iron and calcium, whose deficit can be responsible for anaemia and osteoporosis, among other diseases, although long-term supplementation may do more harm than good[33]. The dietary intake of minerals was adequate for the majority of participants in the present study, and the use of supplements offered no advantage. In fact, supplementation increased the percentage of participants with an excessive intake of magnesium (>UL).

Vitamins cannot be synthesized and must therefore be obtained daily in the diet. Sportspeople require more vitamins than sedentary people due to their higher energetic metabolism. However, they generally consume more food to compensate for the greater caloric output generated by exercise, thereby increasing their vitamin intake. Consequently, vitamin deficits are rare unless the diet quality is inadequate, and supplements are unnecessary. Furthermore, there is increasing evidence from animal and human studies that supplementation with antioxidants can have a negative effect on sports performance[34–36]. On the other hand, the dietary vitamin D intake was below 2/3 of the DRI in 80% of our study participants, and an intake between this value and the UL was achieved by some of these through supplementation. Table 1 depicts the percentage of the mean value of all nutrients, showing that all DRIs are exceeded except for vitamin D. The mean intakes of selenium, vitamin C, vitamin B₂, niacin (B₃), and B₆ were between 2.5- and 3.8-fold higher than recommended levels. Recent studies have reported that nutritional supplementation does not offer a clear benefit for patients with chronic diseases and may even have negative effects; nevertheless. It was recently concluded in widely-read editorial in the Annals of Internal Medicine[37] that although available evidence does not rule out minor or major benefits or harm in small population subgroups, it is well established that the addition of (most) mineral or vitamin supplements to the diet of well-nourished adults offers no benefits and might even be harmful.

Study limitations include the preponderance of male participants, although this reflects the fact that the regular performance of intense physical activity is more than two-fold more frequent among males than among females, at least in Spain[38]. There is also a need for further research on specific sports disciplines and types of participant (e.g., professional or amateur).

A major strength of our index of nutrient intake is that it can be applied to any population group, not solely sportspeople. Supplements are marketed to the general population and are used in work and study settings and for “self-medicating” purposes. The benefits of these supplements are poorly understood and may be outweighed by their negative effects. Consumers may often be unaware of their components or their action mechanism. The method proposed here allows evaluation of the need for supplementation and the potential adverse effects associated with specific nutrients in any individual.

In conclusion, supplementation appears to offer no performance benefits to healthy active sportspeople and may pose a health risk. We propose a method to estimate the quality of diets and the need for supplementation by using the same index to rate the intake of individuals with and without supplementation. Our findings confirm that sportspeople do not need nutritional supplementation if they follow a varied and balanced diet.

Acknowledgments

The authors are grateful to the High Performance Centre of Sierra Nevada and the Sports Medicine Centres of Andalusia for their collaboration in this study and thank Richard Davies for his assistance with the English version. Furthermore, the authors are grateful to the Junta de Andalucía, Spain (Research Group AGR-255“Nutrition. Diet and Risk Assessment”), a collaboration agreement with the Andalusian Centres of Sports Medicine (Junta de Andalucía) and the FPU program of the Spanish Ministry of Education and Science. Study participants were recruited through the project “Nutritional and diet assessment methodologies applied to the Andalusian sportsperson in Andalusian Sports centres”, Research project FMD2010SC0071 of the Junta de Andalucía.