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The authors state that they have no conflicts of interest.
We examined the combined effects of exercise and calcium on BMC accrual in pre- and early-pubertal boys. Exercise and calcium together resulted in a 2% greater increase in femur BMC than either factor alone and a 3% greater increase in BMC at the tibia–fibula compared with the placebo group. Increasing dietary calcium seems to be important for optimizing the osteogenic effects of exercise.
Introduction: Understanding the relationship between exercise and calcium during growth is important given that the greatest benefits derived from these factors are achieved during the first two decades of life. We conducted a blinded randomized-controlled exercise–calcium intervention in pre- and early-pubertal boys to test the following hypotheses. (1) At the loaded sites (femur and tibia–fibula), exercise and calcium will produce greater skeletal benefits than either exercise or calcium alone. (2) At nonloaded sites (humerus and radius–ulna), there will be an effect of calcium supplementation.
Materials and Methods: Eighty-eight pre- and early-pubertal boys were randomly assigned to one of four study groups: moderate impact exercise with or without calcium (Ca) (Ex + Ca and Ex + placebo, respectively) or low impact exercise with or without Ca (No-Ex + Ca and No-Ex + Placebo, respectively). The intervention involved 20 minutes of either moderate- or low-impact exercise performed three times a week and/or the addition of Ca-fortified foods using milk minerals (392 ± 29 mg/day) or nonfortified foods over 8.5 months. Analysis of covariance was used to determine the main and combined effects of exercise and calcium on BMC after adjusting for baseline BMC.
Results: At baseline, no differences were reported between the groups for height, weight, BMC, or bone length. The increase in femur BMC in the Ex + Ca group was ∼2% greater than the increase in the Ex + placebo, No-Ex + Ca, or No-Ex + Placebo groups (all p < 0.03). At the tibia–fibula, the increase in BMC in the Ex + Ca group was ∼3% greater than the No-Ex + placebo group (p < 0.02) and 2% greater than the Ex + Placebo and the No-Ex + Ca groups (not significant). No effect of any group was detected at the humerus, ulna–radius, or lumbar spine for BMC, height, bone area, or volume.
Conclusions: In this group of normally active boys with adequate calcium intakes, additional exercise and calcium supplementation resulted in a 2–3% greater increase in BMC than controls at the loaded sites. These findings strengthen the evidence base for public health campaigns to address both exercise and dietary changes in children for optimizing the attainment of peak BMC.
Exercise and calcium are modifiable environmental factors known to be important determinants of peak BMC.(1) Exercise during growth influences bone modeling locally at the regions being loaded, whereas calcium is thought to act systemically to influence bone remodeling. Despite acting through different mechanisms, a growing body of evidence suggests that exercise and calcium may not operate independently. Low dietary calcium intake or reduced bioavailability may minimize the adaptive response of bone to loading from exercise.(2–5) Conversely, high calcium intakes may enhance the effect of exercise at regions being loaded.(6–8)
There have been only four exercise and calcium interventions where both calcium and exercise have been randomized (i.e., four-group intervention); all were in women and two were in children.(6–9) The results have been equivocal possibly a reflection of differences in exercise levels and calcium intakes at baseline and/or the intervention. Whether additional benefits from exercise may be achieved from calcium supplementation in individuals who have adequate dietary calcium intakes remains unclear. Similarly, it is not known if a similar relationship exists in men, who are typically more active and have a higher total caloric and dietary calcium intakes compared with women.(10) Little is known about how the relationship between exercise and calcium is affected by maturity; sex, baseline levels of exercise and dietary calcium, and the intervention dose of exercise and calcium delivered.
Understanding the interaction between exercise and calcium is important to advance the understanding of the role of these modifiable environmental factors in optimizing the accrual of BMC during growth. Furthermore, both exercise and dietary-based calcium supplementation have the advantage of being inexpensive and widely accessible to large cross-sections of the community and now underpin primary osteoporosis prevention strategies.(11,12) In this exercise-calcium blinded randomized-controlled trial (RCT) in pre- and early-pubertal boys, we studied whether, at loaded sites, the combination of exercise and calcium is more osteogenic than either factor alone.
MATERIALS AND METHODS
We conducted a double-blind, 8.5-month prospective RCT in pre- and early-pubertal boys to test the following hypotheses. (1) At the loaded sites (femur and tibia–fibula), exercise and calcium will produce greater skeletal benefits than either exercise or calcium alone. (2) At nonloaded sites (humerus and radius–ulna), there will be an independent effect of calcium supplementation but not exercise.
In this RCT, there were two factors (exercise and calcium), each with two levels (moderate-impact versus low-impact exercise and calcium supplementation [Ca] versus placebo). The low-impact group served as the exercise control group. Calcium supplementation was double-blinded (the project coordinator was the only member in the research team who had access to the calcium participant list). We have previously reported the same study design and protocol in girls.(6) Participants were stratified by age, Tanner status, and ethnicity (white and Asian). The stratified groups were randomly assigned (using computer-generated random numbers) to one of four study groups: moderate-impact exercise with or without calcium supplementation or low-impact exercise with or without calcium supplementation. Power calculations were based on data from similar exercise and calcium interventions.(13,14) A 4% difference between the exercise + calcium and placebo group was considered to be a clinically relevant difference; thus, the sample size required for an adequately powered study would be 88 (four groups of 22 participants), with 80% power at the p < 0.05 level.(15) The Ethics Committees of Deakin University and the Directorate of School Education approved the study. Informed consent was obtained from all boys and their parents.
Inclusion and exclusion criteria
Participants were excluded from the study if they had (1) prolonged immobilization; (2) another illness known to affect bone metabolism; (3) exposure to medications known to affect bone health (e.g., thyroid hormones, anticonvulsants, glucocorticoids—except laxatives); or (4) been classified as being obese (because of the associated difficulty of undertaking the exercise program).
One hundred pre- and early-pubertal boys 7–11 years of age were recruited from a school in Melbourne, Australia. Twelve boys were not included in the final analysis: seven did not have the follow-up scan because of changes in the radiation risk statement associated with the DXA scans, one did not start the intervention, two voluntarily withdrew (no reason given), and two overweight and obese (body mass index [BMI] > 18.44 and 21.6 kg/m2, respectively) participants were excluded. Of the remaining 88 boys at baseline, 54 (61%) and 35 (39%) were in Tanner stages 1 and 2, respectively (for genital development; Table 1). Twenty-two (22%) boys were of Asian decent, and the rest were white.
Table Table 1.. Age, Maturity Status, Hours of Weight-Bearing Exercise, and Dietary Intakes in 88 Pre- and Early-Pubertal Boys Involved in the Exercise–Calcium Intervention (Mean ± SE)
The boys participated in either the moderate-high– or low-impact exercise program for 20 minutes three times a week as part of their typical physical education classes. The moderate-high–impact sessions included hopping, jumping, and skipping-based activities that produced ground reaction forces (GRFs) between two and eight times body weight (BW).(16) The loading of the moderate-high–impact program was progressively increased over the study period by increasing the magnitude and changing the direction of the impacts. Participants assigned to the low-impact sessions followed the same schedule, but all activities were low impact (∼1 BW) and included stretching, low-impact games, and circuits. The program was implemented by a trained physical education teacher used to deliver the specified curriculum. In a subgroup of 12 randomly selected boys, the number, type, and direction of impacts in eight lessons over the year were determined by video analysis. The GRFs of each activity identified from the video analysis were determined using the Kistler force plate. Detailed biomechanical analysis related to this program is reported elsewhere.(16)
Calcium supplementation and placebo
Calcium phosphate derived from milk (milk minerals) was added to foods in a powdered form. Food products were fortified with 4 g of milk minerals (Murray Goulburn Co-operative Ltd.), which provided participants with an additional 800 mg of calcium per day. Sample analysis was performed on each batch of milk mineral using standard methods(17); each batch consisted of, on average, 20% calcium, 10% protein, 2% fat, 10% lactose, 7% free moisture, and 39% phosphate, with the remaining 12% containing citrate, lactate, and trace minerals such as sodium and magnesium. Participants were required to consume one food product per day (seven food products each week). To assist with compliance, participants could choose from 10 varieties of muffins and cookies. The placebo group selected from the same variety of food products, but without added calcium. Therefore, the placebo food products had the equivalent of 4 g more of the recipe ingredients than the calcium-enriched foods. This represents a negligible difference in nutrient and energy intakes between the groups. The participants were asked to consume these food products in place of cakes and cookies regularly consumed before the intervention. To assist compliance, the milk minerals or a placebo equivalent (Poly Joule; Sharp Laboratories) was offered in powdered form to include in regular food products and drinks. Each satchel of powder contained a dose equivalent to the food product. Compliance was assessed through the weekly return of uneaten products or supplement powder.
Sexual maturity was determined using a parental-assisted self-report questionnaire containing illustrations and a description of each Tanner staging for pubic hair and penis development.(18) Body composition (lean mass and fat mass) and total body (TB) and lumbar spine (L2–L4) BMC were assessed using DXA (DPX-L; Lunar Corp., Madison, WI, USA). The in vivo CV established in our laboratory was 1.3 ± 0.4%. The femur, tibia–fibula, humerus, and radius–ulna were assessed from the TB scans (using the regional analysis tool) by the same investigator (SA) who was blinded to intervention groups. The precision, based on three measurements of eight healthy individuals, was 0.8% for TB and regional BMC, 1.1% for TB and regional BMD, 0.7% for TB soft tissue, 2.9% for TB fat mass, and 1.6% for TB fat-free mass. The repeatability of the operator for measuring regional BMC was 0.8 ± 0.7% (SE). Pediatric or adult software (Version 4.6d) was selected based on the participant's baseline BMI (kg/m2). Standing and sitting heights were measured using a Holtain wall stadiometer. Limb lengths were measured using a Harpenden anthropometer (CV, 0.1 ± 0.1%). Weight was measured using a SECA electronic scale.
Hours of organized exercise were assessed pre-, mid-, and postintervention by a modified parental-assisted physical activity questionnaire.(19) Dietary intakes were assessed by using a 3-day weighted food diary collected during the course of the intervention and were analyzed by the same qualified nutritionist (SI-B) using FoodWorks Version 2 (Xyris Software Pty. Ltd.).
To assist with clarity, the moderate-impact and low-impact exercise groups are referred to as the exercise (Ex) and No-exercise (No-Ex) groups, respectively. All data were analyzed using SPSS for Windows, Version 12.0 (SPSS, Chicago, IL, USA). After checks for normality, comparisons between groups at baseline were made using ANOVA. Total calcium intake equaled the mean dietary calcium intake plus the supplementation dose. Within-group anthropometric and body composition changes and gains in BMC were determined using repeated-measures ANOVA. The following comparisons were conducted: (1) to determine the combined effect of exercise and calcium (four-group comparison), analysis of covariance and posthoc linear contrast were used to assess the gains in BMC after adjusting for baseline regional BMC over the 8.5-month intervention period; (2) the main effect of exercise (two-group comparison) was determined by comparing the exercise with nonexercise groups: (Ex + Ca and Ex + placebo) versus (No-Ex + Ca and No-Ex + Placebo); (3) the main effect of calcium supplementation (two-group comparison) was determined by comparing the calcium-supplemented groups with placebo groups: (Ex + Ca and No-Ex + Ca) versus (Ex + placebo and No-Ex + placebo).
An α level of p < 0.05 was accepted as significant, and p < 0.1 was reported as a trend. Data are presented as means ± SE in absolute terms and adjusted for baseline BMC or as percentage change.
Exercise and food product compliance
The boys attended an average of 2.1 ± 0.3 exercise sessions per week. This represented an average total compliance of 89% (range, 61–100%), with no differences between the groups. The moderate impact exercise sessions consisted of ∼535 ± 10 impacts per session; 23% low impact (1.2–1.8 BW), 69% moderate impact (2.1–3.9 BW), and 8% high impact (4.3–8.0 BW). The low-impact sessions consisted of ∼511 ± 64 impacts per session (mean magnitude, 1.5 ± 0.1; range, 1.4–1.6 BW). Participants in the calcium and placebo groups consumed on average 3.4 and 4.7 food products per week, respectively (range, 0.3–7.0). Compliance was greater in the placebo group (placebo 67% versus calcium 49%, p < 0.001). Accounting for individual compliance over the 8.5 months, the average calcium supplementation was 392 ± 29 mg/day. Thus, in the supplemented groups, mean dietary calcium intakes increased from 913 ± 30 to 1302 ± 62 mg/day (Table 1).
Effect of exercise and calcium at the loaded sites
The four groups were homogeneous in age, pubertal status, weight-bearing exercise, and dietary intakes of calcium, protein, and total energy (Table 1). Height, weight, BMC, and bone length did not differ at any site at baseline (Table 2). The site-specific absolute increases in BMC in the four groups and mean percent changes in BMC after 8.5 months are presented in Table 2. Changes in BMC adjusted for baseline BMC are reported in Fig. 1.
Table Table 2.. Baseline Values and the Percentage Changes in Body Composition, Anthropometry, and Bone Mass in 88 Pre- and Early-Pubertal Boys Undertaking an 8.5-Month Exercise–Calcium Intervention (Unadjusted Mean ± SE)
At the femur, there was a trend for an effect of exercise: BMC increased 1.1% more in the Ex than the No-Ex groups (15.2% versus 14.1%, respectively; p = 0.056). There was also a trend for an effect of calcium. BMC increased 1.1% more in the calcium-supplemented than placebo group (15.2% versus 14.1%, respectively; p = 0.06). The increase in femur BMC in the Ex + Ca group was, on average, 2% greater than the increase in either the Ex + placebo, No-Ex + Ca, or No-Ex + Placebo groups (all p < 0.03; Table 2; Fig. 1)
At the tibia–fibula, there was a trend for an effect of exercise: BMC increased 1.6% more in the Ex than the No-Ex groups (15.1% versus 13.5%, respectively; p = 0.07). There was also a trend for an effect of calcium. BMC increased 1.4% more in the calcium-supplemented than placebo group (15.0% versus 13.6%, respectively; p = 0.11). At the tibia–fibula, the increase in BMC in the Ex + Ca group was on average 3% greater than the No-Ex + placebo group (p < 0.02) and 2% greater than the Ex + Placebo and No-Ex + Ca groups (p = 0.20 and p = 0.16, respectively). These findings were replicated in the unadjusted analysis.
Effect of exercise and calcium at the nonloaded sites
No main or combined effects of exercise and calcium were detected at the humerus, ulna–radius (Table 2; Fig. 1), or lumbar spine BMC, height, area, or volume.
This blinded RCT was designed to determine the combined effect of exercise and supplemented calcium on BMC in pre- and early-pubertal boys. After 8.5 months, we report that exercise and calcium together resulted in a 2% greater increase in BMC at the femur than either exercise or calcium alone. At the tibia–fibula (lower leg), exercise and calcium together resulted in a 3% greater increase than the placebo group (no exercise or calcium). There was no combined or main effect of exercise or calcium alone or in combination at the arms or spine.
There have been only four exercise and calcium interventions where both calcium and exercise have been randomized (i.e., four-group intervention).(6–9) Of these trials, two have been conducted in children(20); similar to the results of this study, increased dietary calcium was reported to enhance the effect of exercise on BMC during growth.(6,8) In our previous study of 9-year-old pre- and early-pubertal girls with calcium intakes below recommended levels, 8.5 months of moderate impact exercise combined with calcium resulted in an exercise–calcium interaction at the femur and a main effect of exercise at the legs and calcium at the arms.(6) Similarly, in a study of 4-year-old children with baseline calcium intakes above the recommended level for age, 12 months of exercise (gross motor skills) combined with calcium supplementation (1000 mg/day) resulted in a greater increase in BMC than exercise or calcium alone.(8)
The clinical relevance of this and other studies lies in the findings that combining exercise and calcium seems to provide greater benefits to BMC accrual during growth than either factor alone.(6–9) However, it is more difficult to place into clinical context the benefits of a 2–3% increase in BMC in prepubertal children. A 10% increase in peak BMC has been reported to be associated with a halving of fracture risk(21) or being equivalent to delaying the onset of osteoporosis by 13 years.(22) Many exercise interventions in children have resulted in either no effect or small benefits (∼1%), which are often limited to a single site such as the trochanter.(23–25) In this study, however, there was a 2–3% greater increase in BMC at both the femur and the tibia after just 8.5 months of exercise and calcium supplementation. Whether these benefits will be sustained long term to enhance peak BMC remains uncertain, and further study is required to place into the context the importance of exercise during growth for long-term improvements in bone health. In particular, the following needs to be addressed: how can the benefit achieved during growth be increased further with longer-term exercise; what is the structural and architectural basis of the increased BMC; will the benefit be maintained if exercise is reduced or ceased; and how long do children need to exercise for a permanent benefit to be achieved?
The reason for the combination of calcium and exercise leading to a greater osteogenic effect than either factor alone remains inconclusive. Bone is a dynamic organ that adjusts its mass and architecture to the strain imparted as a result of external loads. Longitudinal growth increases lever arms and bending moments, which create greater loads on bone during exercise(26); as a result, bones are continually challenged to adapt to increases in bone length and muscle forces related to increased muscle mass and neural maturation. Thus, growing bone has to continually adjust its strength to keep strains (bone deformation) within the threshold range for modeling and remodeling. The addition of increased loading to growing bone can stimulate bone modeling, leading to increases in bone strength caused by changes in cortical bone shape and increased trabecular BMC (trabecular thickness). Calcium, on the other hand, is the major constituent and building substance of bone; it does not drive changes in bone geometric properties. Calcium supplementation decreases the rate of bone remodeling and influences bone resorption at the endocortical surface; it does not seem to act on bone modeling.(27)
Given the different mechanisms of facilitating the osteogenic response to exercise and calcium, it is not immediately clear how increased exercise and dietary calcium act together to result in either an additive or synergistic effect? Human and animal studies show that exercise is associated with increased calcium absorption from the gut(28–30) and that children have enhanced capacity to increase calcium absorption(31); neither of these actions provide an explanation of why this would lead to an enhanced osteogenic effect of exercise. Nor does it make sense that the skeleton would build a stronger bone in response to a similar amount of loading because dietary calcium was increased. A more likely explanation is that exercise-induced osteogenesis requires adequate calcium, and thus, the osteogenic adaptation may be compromised in the presence of inadequate calcium.(3,32) Further study is required to determine how the osteogenic adaptation to exercise may be impaired when calcium intakes are inadequate. It may be the case where bone strength may not necessarily be compromised if there is a change in bone geometry that results in an increase in bone strength without a corresponding net increase in BMC, as is the case with starvation and exercise in adults.(33)
In this study, we chose to fortify foods with milk-based calcium rather than calcium carbonate or calcium citrate malate. The rationale underpinning food fortification was to assist with compliance and to provide a positive health message about improving dietary intakes in preference to supplementing with tablets. Milk minerals were chosen in preference to calcium carbonate or calcium citrate malate because of the beneficial effects reported in previous studies on bone health in children.(34) Milk-based mineral supplementation has been reported to be associated with an increase in BMC accrual and bone size; effects that seem to be maintained once supplementation has ceased.(34,35) Milk-based calcium may increase bioavailability of calcium because of the presence of other nutrients such as protein fragments and nonprotein nitrogen(36,37) and increase bone modeling.(14,38) In this study, however, we did not detect an increase in bone size in the calcium-supplemented groups, perhaps because of the brevity of the study and the lower dose of supplementation compared with other studies.
This study is one of few studies where exercise and calcium were both randomized. This four-group study design is appropriate to test both the interaction and combined effect of exercise and calcium. We report the results of a combined effect; however, this study was not powered to detect interaction; >300 children would be required to provide adequate power to confidently detect an interaction for the change in BMC reported in this study. The 2–3% greater increase in BMC at the tibia–fibula and the femur in the exercise + calcium group (compared with exercise and calcium alone and control groups) over the 8.5-month study was marginally less than expected; this was likely to be caused by high baseline levels of exercise and dietary calcium combined with the average compliance of two rather than three exercise sessions per week. The inclusion of a control group participating in low-impact exercise rather than no exercise may also have resulted in a more conservative outcome. Strengths of the study included (1) the development of an exercise intervention for both exercise and controls (GN)(39) that was educationally sound and appropriate for the boys' level of motor skills, (2) the consistency of delivery of the exercise program by two teachers specifically used for this research project, (3) the use of weighed food records to determine dietary intakes, and (4) the use of dietary rather than tablet based supplementation.
The results of this study show that skeletal benefits can be achieved by small changes in behavior early in life; an additional 400 mg of calcium daily (one additional serving of dairy) and 20 minutes of weight-bearing exercise on average twice per week can enhance BMC at loaded skeletal sites. Whereas exercise may be more important in terms of changing bone size and shape,(40) increasing calcium intake seems to play an important role in optimizing the effect of exercise at the loaded sites and the systemic effect on the nonloaded sites in those who have low calcium intakes.(6)
In conclusion, in this group of normally active pre- and early-pubertal boys with adequate calcium intakes, additional exercise combined with calcium supplementation resulted in a 2–3% greater increase in BMC than controls at the loaded sites. Furthermore, the combination of exercise and calcium was more osteogenic at the femur than either exercise or calcium alone. These findings strengthen the evidence base for public health campaigns to address both exercise and dietary calcium intakes in children for optimizing the attainment of peak BMC.
The Dairy Research and Development Corporation and Murray Goulbourn Co-op Pty. Ltd. supported this study. The authors thank the following people for contributions to this study: Milgate Primary School, Principal Gabrielle Martin, Vice Principal John Warden, teachers Daniel Lynch, and Christine Papageorgiou, office staff, Ava McKinnon and Wendy Farr, and the parents and the boys who participated in the study. We thank Roman Shaw and Whole Foods for the production of the food products and research assistant Simon Austen.