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. Author manuscript; available in PMC: 2017 Aug 11.
Published in final edited form as: Int J Sport Nutr Exerc Metab. 2004 Feb;14(1):18–29. doi: 10.1123/ijsnem.14.1.18

A Pilot Intervention to Increase Calcium Intake in Female Collegiate Athletes

Robyn S Mehlenbeck 1, Kenneth D Ward 1, Robert C Klesges 1, Christopher M Vukadinovich 1
PMCID: PMC5553541  NIHMSID: NIHMS893388  PMID: 15129927

Abstract

Calcium intake in adolescent and young adult female athletes often is inadequate to optimize peak bone mass, an important determinant of osteoporosis risk. The purpose of this study was to determine if calcium supplementation in eumenorrheic female collegiate athletes increases intake to recommended levels and promotes increases in bone mineral density (BMD). Forty-eight eumenorrheic female athletes from several college teams (15 soccer, 7 cross-country, 8 indoor track, and 18 basketball) were randomized at the beginning of a competitive season to receive either an oral calcium supplement (1000 mg calcium citrate/400 I.U. Vitamin D) or placebo daily throughout the training season (16 weeks). Self-reported daily pill intake was obtained every 2 weeks to assess adherence. Calcium intake was evaluated using the Rapid Assessment Method, and total body and leg BMD was measured at pre-, mid-, and post-season using dual energy x-ray absorptiometry (DEXA; Hologic QDR-2000). Pre-season calcium intake was lower than national recommendations for this age group (12), averaging 842 mg/d (SD = 719) and was lower in the placebo group compared to the supplemented group (649 ± 268 vs. 1071 ± 986 mg/d, respectively; p = .064). Adherence to supplementation was good, averaging 70% across the training season. Supplementation boosted total calcium intake to a mean of 1397 ± 411 mg/d, which is consistent with recommended levels for this group (37). Supplementation did not influence BMD change during this 16-week intervention. Across teams, a small increase of 0.8% was observed in leg BMD. Change in total body BMD was modified by team, with a significant increase of 1.5% observed in basketball players. These results indicate that providing calcium supplements of 1000 mg/d is adequate to boost total intake to recommended levels during athletic training. Longer intervention trials are required to determine whether calcium supplementation has a positive effect on BMD.

Keywords: athletes, bone mineral density, calcium, DEXA, exercise, training

Introduction

Adolescence and young adulthood is a critical time for increasing bone mass. Peak bone mass (PBM) is a primary determinant of osteoporosis risk in later life (23). Bone growth and maintenance have been likened to a “three-leg chair” (10, 21) maintained by physical activity, calcium intake, and reproductive hormone functioning. Each of these three “legs” can independently or interactively have an impact on skeletal health. Among females, serum estrogen levels predict the rate of bone loss (32), and estrogen deficiency permits greater efficiency of bone resorption by osteoclasts (21). Secondary amenorrhea among athletes is associated with decreases in bone mass (8, 34) and bone injuries (1).

Calcium intake is positively related to bone mass in observational studies (6, 39), and several randomized controlled trials have demonstrated that calcium supplementation enhances bone accrual in children and young adults (9, 14, 20, 22, 27). Several meta-analyses of intervention trials in adults indicate that weight-bearing activity enhances bone mass in both men and women (16, 17, 40). Fewer trials have evaluated the effects of weight-bearing exercise on bone accrual in children and young adults, but results indicate a positive effect (9, 40).

Adaptations of bone to mechanical loading, via weight-bearing exercise, may be compromised by low levels of calcium intake. A quantitative review of 17 physical activity and bone density trials (33) found that physical activity had beneficial effects on lumbar BMD at high calcium intakes but no effect at lower calcium intakes (<1000 mg/d). At low calcium intakes, small decreases in BMD were observed for both exercise and no-exercise groups. In contrast, at high calcium intakes, exercise groups had increases in BMD, while no-exercise groups did not change. Similar interactive effects of calcium intake and physical activity have been observed in cross-sectional (15) and longitudinal (28) studies of young adult women. Recker and colleagues (28) prospectively assessed the contributions of calcium intake and physical activity among a 156-member cohort of 18–26-year-old women. The ratio of calcium to protein intake, and physical activity level, exerted independent positive effects on spinal BMD. Holding age, protein intake, and physical activity level constant, BMD was substantially increased at calcium intake levels above 1000 mg/d. These results indicate the importance of adequate calcium intake for optimizing the bone accrual benefits of physical activity.

Despite the importance of calcium intake for bone development, numerous studies have confirmed that physically active females have inadequate calcium intakes (2, 3, 5, 19, 24, 31, 38). A recent study of several NCAA Division I-A women’s athletic teams found that calcium intake averaged only 898 mg/d (31), which is substantially lower than recent recommendations from the Food and Nutrition Board of the Institute of Medicine, which sets adequate intake as 1300 mg/d for 14–18-year-old males and females, and 1000 mg/d for men and women 19–30 years of age (12). Calcium requirements are possibly even greater for competitive athletes (5, 18, 37).

Because of the synergistic relationship between calcium intake and physical activity for bone mineralization, it is imperative that physically active women consume adequate levels of calcium. To date, however, no randomized controlled studies have been conducted to determine whether supplementation increases calcium intake to recommended levels and increases (or prevent decreases in) BMD among female athletes; therefore, the purposes of this study were to evaluate if female athletes will reliably take daily calcium supplements during a training season, to evaluate if supplementation of 1000 mg/d is sufficient to boost total calcium intake to recommended levels, and to examine if supplementation positively affects BMD during training.

Methods

Participants and Procedures

Fifty-eight subjects were recruited for this study through the head coaches of each team. Five athletic teams at two colleges in the mid-South participated. Across the season, 10 women (4 from the supplement group, 6 from the placebo group) did not complete the study due to injury or moving from the city. Thus, the final sample included 48 athletes (83% of the original randomized sample). Athletes were from a Division I-A soccer team (final sample n = 15; 7 randomly assigned to treatment vs. 8 placebo), cross-country team (n = 7; 4 treatment vs. 3 placebo), indoor track team (n = 8; 5 treatment vs. 3 placebo), and basketball team (n = 10; 5 treatment vs. 5 placebo), as well as a Division III basketball team (n = 8; 5 treatment vs. 3 placebo). Of the final sample, 33.3% were African Americans; the remainder were Euro-American.

This study was a double-blinded, randomized, controlled investigation of the effects of calcium supplementation on total calcium intake and bone mineral density (BMD) in female collegiate athletes. Exclusion criteria included (a) pregnancy or the possibility of pregnancy; (b) irregular menstrual cycles, defined as missing more than one cycle in the previous 6 months; and (c) a personal or family history of kidney stones. Only 1 athlete was excluded based on these criteria (for menstrual irregularities). To ensure that normal menstrual functioning was maintained throughout the study and to guard against fetal exposure to radiation, dual energy x-ray absorptiometry (DEXA) scans (described below) were performed only during the first 10 days of participants’ menstrual cycles. As such, all participants maintained regular menstrual cycles throughout the athletic season. The protocol was approved by the University of Memphis Institutional Review Board, and written informed consent for all measures and procedures was obtained prior to beginning the study.

A laboratory assessment was performed on each participant within 2 weeks prior to the start of training (pre-season assessment), approximately halfway through the season (mid-season assessment), and within 2 weeks of the end of the season (post-season assessment). The assessments were approximately 8 weeks apart, with the average length of active treatment being 16 weeks. At each assessment, participants completed a demographic and health survey, which included use of hormonal contraceptives and other medications, had their height and weight measured, completed measures of calcium intake and physical activity, and had a body composition assessment performed using DEXA.

After completing the pre-season assessment, athletes were stratified by team, then randomly assigned, using a random numbers table, to the treatment group or a placebo control group. The treatment group received, in the form of oral supplements, 1000 mg of calcium citrate along with 400 international units (I.U.) of Vitamin D. Calcium citrate was used, as it appears to be the form of calcium with the fewest side effects and is easiest on the digestive system. Vitamin D was included as it helps maximize the intestinal absorption of calcium (26). This dosage combination was chosen based on previous studies completed in this lab (18), as well as one study assessing the incidence of stress fractures in athletes (25). Myburgh and colleagues (25) found no stress fractures in athletes with dietary calcium levels exceeding 1200 mg per day. Thus, given the likelihood that athletes ingest at least 200 mg calcium from dietary sources and that adherence rarely is perfect, 1000 mg of calcium was considered a minimum level for supplementation. Also, as the mean calcium intake for women has been found to be 500–700 mg, a 1000-mg supplement puts the average woman in a desirable range of calcium intake.

Following randomization, each participant was given a medicine bottle marked with her name. The bottle contained either the calcium/vitamin D supplement or a placebo. Both pills were similar in shape and color. Each participant was asked to take 2 pills a day, starting at the beginning of her sport season. The athletes were given a 2-week supply of pills at a time. No identifying data regarding treatment group was on the packaging.

Participants were educated about the potential benefits of the supplements, including the possible reduction of risk of stress fractures, other health implications, and the possibility of increased performance. Every 2 weeks, the athletes were also asked to fill out a short form indicating the number of pills that they had taken over the past 2 weeks. Each team was visited during at least one practice per week as well as several meets or games during the season to answer questions, encourage adherence, and address any concerns. Athletes were counseled to avoid excess calcium supplementation. NIH (26) sets 4 g (4000 mg) of calcium per day as the upper limit.

Measures

Calcium Intake

Dietary calcium intake was assessed using the rapid assessment method (RAM; 11). The RAM was developed for use with adolescents and has been shown to compare well with longer dietary recalls and records (11, 36). The measure originally covered 27 items. Several calcium-fortified foods that have recently come onto the market were added, including bread, orange juice, and fruit-flavored drinks. The RAM covers intake for 1 week. Participants were assisted in completing the RAM by a trained research assistant.

In addition to dietary calcium intake, total calcium intake was calculated by multiplying percent adherence to calcium supplementation (for the treatment group) and adding this to the dietary calcium intake derived from the RAM. Adherence to supplementation was determined by calculating the ratio of the self-reported number of pills consumed (reported every 2 weeks) to the total number of pills provided during the trial. For example, if an athlete was in the placebo group, her dietary calcium intake would equal her total calcium intake. If an athlete in the treatment group was adherent to the 1000-mg supplement 75% of the time, 750 mg/d would have been added to her daily dietary calcium intake to yield a total calcium intake value.

Physical Activity Level

Level of physical activity was assessed using the Stanford Physical Activity Recall (PAR; 30). This self-report instrument estimates energy expenditure across the previous week. The PAR compares well with other longer activity recalls and records (13) and is sensitive to total energy expenditure (kcals per day; 30).

Participants reported their daily sleep habits and the number of hours spent in moderate, hard, or very hard activity. Participants had an extensive example page of classifications, and a research assistant also gave assistance in making classifications. To obtain an accurate assessment of physical activity, subjects completed the PAR five times during the study, including for the week prior to the onset of training, and four additional times during training. Overall physical activity level was calculated as the mean of the five assessments, and expressed as metabolic equivalents (METS).

Bone Mineral Density (BMD)

BMD was assessed by DEXA using a Hologic QDR-2000 absorptiometer (Hologic, Inc., Waltham, MA, USA). Each participant underwent a total-body scan. Because the legs are a major site of stress fractures in athletes, leg BMD also was examined, using the average of the right and left leg scans.

As one of the concerns regarding DEXA technology is lack of standardization (29), careful quality control, standardization, and calibration procedures were followed. Quality control scans done on this unit have demonstrated an extremely low error rate, with the coefficient of variation (n > 100) being .47%. Test-retest reliability on actual subjects was measured by conducting serial DEXAs on 14 athletes (Division I-A baseball players) taken approximately 30 min apart. Both intra-operator (n = 4) and inter-operator (n = 10) reliability were assessed and found to be equivalent. Mean deviation for total BMD, leg BMD, total body lean tissue, and leg lean tissue averaged .31%, and the mean intra-class correlation (ICC) was .985.

Subjects were positioned on the scanning table according to standardized protocol. Initial scan analysis, including the placement of baselines (cuts) distinguishing bone and soft tissue, edge detection, and regional demarcations for all sites, was done by computer algorithms (v. 7.1, Hologic). Subsequent visual inspections and adjustments were made as needed by the same trained technician to obtain whole body and regional BMD.

Statistical Analysis

The treatment and placebo groups were compared on several baseline characteristics, using t tests and chi squares, to determine whether randomization resulted in equivalency between groups. Average percent adherence with supplements (calcium or placebo) was calculated for each group. Repeated measures analysis of variance (RMANOVA) models were used to ascertain whether total calcium intake or BMD changed during the season and whether any such changes were a function of treatment group or team. RMANOVA models included two between-subjects factors (supplementation group and team), a within-subjects factor (time: pre-season, mid-season, post-season), and interaction terms for Time by Group, Time by Team, and Time by Group by Team. F statistics were corrected for sphericity using the Greenhouse-Geisser epsilon.

Results

Baseline Comparisons of Treatment Groups

Baseline differences were explored to determine the effectiveness of the randomization (see Table 1). There were no significant differences between the treatment group and the placebo control group for most of the key variables, including age, race, weight, change in weight during the training season, height, oral contraceptive use, adherence to supplementation (reported as a percentage of total pills), or baseline BMD for the total body or legs.

Table 1.

Participant Characteristics at Preseason

Variable Treatment group (n = 26)
Placebo group (n = 22)
p value
M SD M SD
Age 20.0 1.3 19.6 1.4 .350
% whites 69.2 63.6 .682
Weight (lbs.) 142.7 18.5 142.9 17.6 .978
Height (in.) 66.8 2.7 67.3 3.2 .582
Dietary calcium intake (mg/day) 649.0 267.6 1070.6 985.7 .064
Physical activity (METS/day) 40.4 5.9 44.2 7.4 .051
Use of hormonal contraceptives (%) 42.3 38.1 .770
Total body BMD (g/cm2) 1.025 0.064 1.040 0.069 .460
Leg BMD (g/cm2) 1.200 0.063 1.220 0.092 .392
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Across treatment groups, baseline calcium intake averaged 842 mg/d but was lower in the treatment group compared to the placebo group (649 ± 268 vs. 1071 ± 986 mg/d, respectively; p = .064). The magnitude of difference in baseline calcium intake between the groups was influenced by one statistical outlier in the placebo group who had an intake level of 4624 mg/d, which was 3.6 SD units above the mean for that group. When this participant was removed from analysis, the difference between the treatment and placebo groups declined somewhat (649 ± 268 vs. 901.3 ± 598.8 mg/d, respectively; p = .085). This individual’s data were included in all reported analyses, however, because (a) verification with the participant indicated that her reported intake was accurate (albeit high) and (b) no results were altered appreciably in any of our reported analyses by excluding this participant. Similarly, the treatment group had a lower physical activity level at baseline compared to the placebo group (40.4 ± 5.9 vs. 44.2 ± 7.4 METS, respectively; p = .051). As such, pre-season calcium intake and physical activity level were included as covariates in statistical models evaluating the effects of calcium supplementation on BMD.

Adherence

Adherence to supplementation across the season (expressed as the number of pills consumed to the total number of pills provided during the trial), averaged 70.0% with a range of 16–115%. Adherence was > 100% for 1 participant who used calcium supplements in addition to those provided in the study. Adherence did not differ significantly between the treatment and placebo groups (66% vs. 74%, respectively; p = .261). The majority of athletes (73%) took at least 50% of the supplements across the season.

Hormonal Contraceptive Use

Because change in hormonal contraceptive use could influence any observed changes in BMD (28), we evaluated the association of these medications with BMD. At the time of the first lab visit, 19 participants (39.6%) reported using a hormonal contraceptive (17 used oral contraceptives, 1 Norplant, and 1 Depo Provera). No changes in use were reported by any of the 48 participants at either the second or third lab visits. Users of hormonal contraceptives (n = 19) were compared to non-users (n = 29) via t test on total body and leg BMD at each of the three lab visits, and change in total body and leg BMD from lab 1 to lab 3. No significant differences between users and non-users were observed (all p values > .96).

Effects of Supplementation on Total Calcium Intake

Total calcium intake across the training season is shown for the intervention and placebo groups in Table 2. In a RMANOVA, a significant Time by Group effect was observed (F2,92 = 18.57, p < .0001). In post hoc analysis, total calcium intake was found to increase across the season in the treatment group (p < .0001) but not in the placebo group (p = .485). During intervention, total calcium intake was significantly higher in the treatment group (1364 ± 414 mg/d) than in the placebo group (917 ± 378 mg/d; p = .002).

Table 2.

Total Calcium Intake (mg/d) During Training

Group Pre-season
Mid-season
Post-season
p*
M SD M SD M SD
Treatment 649.0 267.6 1330.7 458.2 1397.2 410.9 <.0001
Placebo 1070.6 985.7 922.9 405.2 911.1 442.6 .485
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*

From a repeated-measure analysis of variance model comparing calcium intake at pre-, mid, and post-season.

Effects of Supplementation on Bone Mineral Density

BMD results are shown in Tables 3 and 4. Neither Time by Group, nor Time by Group by Team interactions, were significant in RMANOVA models for either total body or leg BMD (p values > .25), indicating that calcium supplementation did not influence BMD change. For total body BMD, a significant Time by Team effect was observed (F8,76 = 2.58; p = .022). When stratified by team, a significant Time effect was observed for the Division 1-A basketball team, with BMD increasing significantly by 1.5% over the season (F2,18 = 5.99, p = .0180). For leg BMD, the only significant effect observed was for Time, with an average increase of 0.8% occurring across the season (p = .013). Results did not change appreciably when pre-season calcium intake, physical activity level, or body weight were included as covariates. Likewise, including pre- to post-season, changes in physical activity level or body weight did not alter the pattern of results.

Table 3.

Total Body Bone Mineral Density (g/cm2) During Training

Team N Pre-Season
Mid-Season
Post-Season
% change
M SD M SD M SD
Soccer 15 1.028 0.054 1.024 0.048 1.026 0.054 −0.2
Cross country 7 0.972 0.063 0.971 0.062 0.966 0.066 −0.6
Track 8 1.018 0.048 1.019 0.047 1.023 0.049 0.4
Division 1A basketball 10 1.104 0.064 1.112 0.065 1.120 0.069 1.5*
Division II basketball 8 1.014 0.031 1.018 0.034 1.021 0.033 0.6
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Note. Percent change in BMD is from pre- to post-season.

*

Indicates that the change is statistically significant at p < .05.

Table 4.

Leg Bone Mineral Density (g/cm2) During Training (N = 48)

Training period M SD
Pre-season 1.209 0.077
Mid-season 1.212 0.079
Post-season 1.219 0.083
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Note. Percent change in BMD from pre- to post-season is 0.08, which is statistically significant at p < .05.

Discussion

This study explored whether providing calcium supplementation to eumenorrheic female collegiate athletes would increase total calcium intake and BMD. This issue is important, since several studies have documented low levels of calcium intake in collegiate athletes, particularly females (e.g., 2, 5, 31). Calcium intake is an important modifiable determinant of bone health. Inadequate calcium intake increases the risk of stress fractures, a common injury in athletes (25). Calcium intake also influences the achievement of peak bone mass during adolescence and young adulthood, which is an important determinant of osteoporosis risk (4, 20, 28, 35).

In addition to calcium intake, bone growth is affected by physical activity and reproductive hormone functioning, and all three factors can interactively influence bone accrual. Weight-bearing physical activity generally promotes bone accrual, but in the presence of deficiencies in estrogen and/or calcium intake—both strong stimuli for bone turnover and resorption—physical activity may be insufficient to prevent negative calcium balance, leading to reduced BMD (7). Work by Specker (33) indicates the importance of adequate calcium intake as a modifier of the effects of physical activity on BMD. Data combined from 17 exercise intervention trials in peri- or postmenopausal women indicated that at calcium intakes of < 1 g/d, exercise interventions failed to improve lumbar spine BMD. Similar interactive effects of calcium intake and physical activity were observed in a prospective observational study of 156 health college-aged women (28). Physical activity level was measured objectively by accelerometer. The effect of physical activity on the rate of change in spinal BMD was modified by calcium intake. Very low levels of calcium intake were associated with BMD decreases, whereas intakes at 1000 mg/d or more were associated with substantial increases. These results indicate the importance of adequate calcium intake for optimizing the bone accrual benefits of physical activity. Further work is needed to determine appropriate calcium intake levels to optimize BMD in competitive female athletes.

Our results indicate that increasing total calcium intake through oral supplementation is possible in college female athletes. Prior to intervention, participants were found to have calcium intakes that were substantially below recommended levels for this age group, averaging 842 mg/d. Calcium intakes in the range of 1200–1500 mg/d have been recommended for adolescents and young adults to optimize bone health (12, 26). Although calcium balance data are scarce for competitive athletes, it has been suggested that calcium requirements may be even higher than these national recommendations during training among collegiate athletes (5, 18, 37).

Adherence to supplementation was good, averaging 70% across an entire training season. No serious side effects were reported and no participant dropped out of the study due to inability or unwillingness to take the supplements. While total calcium intake did not change and remained inadequate throughout the season for athletes receiving placebo, significant increases in total intake were observed for athletes in the treatment group. Importantly, supplementation, at a target dose of 1000 mg/d, was found to increase calcium intake levels to recommended levels. Average total intake of calcium for the treatment group was 1397 mg/d, which appears to be a reasonable intake for this population (26, 37). As noted above, however, balance studies are needed on competitive female athletes during training to determine calcium requirements.

To our knowledge, the present study is the first randomized, controlled calcium supplementation trial in female athletes. Supplementation did not increase BMD over the course of a single training season, lasting approximately 16 weeks. Several previous trials in non-athletic samples of female adolescents and young adults have reported that calcium supplementation enhances bone accrual (9, 14, 20, 22, 27) over longer intervention periods of 1–3 years. However, a previous open label trial found that calcium supplementation during athletic training had a positive impact on BMD during a time interval similar to that of the present study (18). With 48 subjects, we were powered (at 80% with a two-tailed alpha of .05) to detect a 5% difference in BMD levels between the two groups. Although 5% is a reasonable treatment effect based on previous work (e.g., 18), future trials will want to increase the duration of treatment and possibly the intervention dosage to produce an effect on BMD.

Acknowledgments

We gratefully acknowledge the expert assistance of Karen Harmon in the conduct of this study. This research was supported by PHS grants R29 AR 448909 and R55AR043729.

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