What is Bone Mineral Density?
Bone mineral density (BMD) is a common measure of bone health, referring to the amount of mineral matter per square centimetre of bone (1). BMD can be used as a predictor of degenerative bone diseases and fracture risk. Lastly, BMD can be used as an estimate of bone strength. The majority of bone formation occurs through early developmental years and adolescence, BMD then peaks around age 30. After this, the interaction of bone remodelling maintains bone health until osteoclast activity (bone breakdown) outpaces osteoblast activity (bone rebuild) in later life. The major predictor of BMD appears to be exercise intensity as appose to duration, as such, efforts to increase BMD should prioritise high-impact loading protocols (2).
What are the Principles of Bone Turnover During Exercise?
Bone is an active tissue constantly changing through a process known as remodelling, where new bone cells replace old, damaged bone cells. Bone turnover, in relation to exercise, is the total volume of bone that is both resorbed and formed during the exercise bout, which can be measured using bone biomarkers. Exercise may increase osteoclastic, bone breakdown, activity with no short-term associated increase in osteoblastic, bone development, activity. Following an acute bout of exhaustive treadmill running (60-minutes at 65% VO2 max, 15-minute rest period, run to exhaustion at 70% VO2 max) an increase in bone resorption was seen, without any change in bone formation for the 4-day post-exercise period (3).
Should Endurance Athletes be Concerned About Bone Health?
Participation in sports involving lower-impact, repetitive loading or non-weight-bearing sports do not typically result in any exercise-induced skeletal benefits (4). BMD is often higher in distance runners compared to controls (5, 6). However, cycling populations tend to report lower than normal BMD (7, 8, 9) and cyclists undertaking higher training loads (>8 hours/week) were found to have lower BMD than those with lower training loads (<8 hours/week) (7). This is in part due to the low-impact nature of cycling and the trend is seen in other low-impact sports with swimmers reporting low BMD (6). Even though triathletes spend time training swim and bike disciplines, as an athletic group they report improved bone health (6). As low BMD is associated with bone stress injuries (10) and an increased risk of developing osteoporosis, athletes should look to prioritise nutritional and training strategies that optimise bone health.
How can Nutrition Support Bone Health?
With the concomitant bone health concerns in endurance athletes and short-term negative bone outcomes associated with exercise, nutritional strategies to support bone health should be prioritised. Initially, low energy availability (energy intake minus exercise energy expenditure) was thought of as the key driving factor, with reduced energy availability linked to both short- and long-term negative influences on bone health (4). However, the relationship between carbohydrate availability and bone health maybe more telling. Carbohydrate feeding (100-100 g) during a 120-minute treadmill run at 70% VO2 max significantly reduced markers of bone resorption, perhaps suggesting that low carbohydrate availability, as appose to low energy availability, could be more representative of the link between athlete and bone health (11).
Periods of low energy and carbohydrate availability are intrinsically linked with endurance sports, in particular, those times when changes in body composition are prioritised and a reduction in body weight is the main goal of training. At these dedicated times of restriction increasing protein intake has been suggested to be advantageous in preserving lean body mass, with intakes of 1.8 -2.0 g.kg.day-1 being recommended and these higher protein intakes should be accompanied by an adequate intake of calcium (11).
Take Home Messages:
- • Skeletal bone provides structure and protection for the body, the environment for white blood cell production and a storage site for minerals
- • Endurance athletes are at increased risk of poor bone health
- • During normal training blocks, carbohydrate availability should be prioritised to meet the demands of exercise and support bone health
- • Supplement with 1000mg of calcium pre-exercise and fuel with 60-90g of carbohydrate during exercise for long endurance efforts over 2.5 hours
- Santos, L., Elliott-Sale, K. J., & Sale, C. (2017). Exercise and bone health across the lifespan. Biogerontology, 18(6), 931-946.
- Karlsson, K. M., Karlsson, C., Ahlborg, H. G., Valdimarsson, Ö., & Ljunghall, S. (2003). The duration of exercise as a regulator of bone turnover. Calcified tissue international, 73(4), 350-355.
- Scott, J. P., Sale, C., Greeves, J. P., Casey, A., Dutton, J., & Fraser, W. D. (2010). The effect of training status on the metabolic response of bone to an acute bout of exhaustive treadmill running. The Journal of Clinical Endocrinology & Metabolism, 95(8), 3918-3925.
- Papageorgiou, M., Dolan, E., Elliott-Sale, K. J., & Sale, C. (2017). Reduced energy availability: implications for bone health in physically active populations. European journal of nutrition, 1-13.
- Tam, N., Santos-Concejero, J., Tucker, R., Lamberts, R. P., & Micklesfield, L. K. (2018). Bone health in elite Kenyan runners. Journal of sports sciences, 36(4), 456-461.
- Scofield, K. L., & Hecht, S. (2012). Bone health in endurance athletes: runners, cyclists, and swimmers. Current sports medicine reports, 11(6), 328-334.
- Mojock, C. D., Ormsbee, M. J., Kim, J. S., Arjmandi, B. H., Louw, G. A., Contreras, R. J., & Panton, L. B. (2016). Comparisons of bone mineral density between recreational and trained male road cyclists. Clinical journal of sport medicine, 26(2), 152-156.
- Mathis, S. L., & Caputo, J. L. (2018). Resistance Training Is Associated With Higher Lumbar Spine and Hip Bone Mineral Density in Competitive Male Cyclists. The Journal of Strength & Conditioning Research, 32(1), 274-279.
- Baker, B. S., & Reiser, R. F. (2017). Longitudinal Assessment of Bone Mineral Density and Body Composition in Competitive Cyclists. The Journal of Strength & Conditioning Research, 31(11), 2969-2976.
- Tenforde, A. S., Parziale, A. L., Popp, K. L., & Ackerman, K. E. (2018). Low Bone Mineral Density in Male Athletes Is Associated With Bone Stress Injuries at Anatomic Sites With Greater Trabecular Composition. The American journal of sports medicine, 0363546517730584.
- Sale, C., Varley, I., Jones, T. W., James, R. M., Tang, J. C., Fraser, W. D., & Greeves, J. P. (2015). Effect of carbohydrate feeding on the bone metabolic response to running. Journal of Applied Physiology, 119(7), 824-830.