Participation in road cycling vs running is associated with lower bone mineral density in men

Abstract

The effects of regular non–weight-bearing (NWB) exercise on bone health are largely unknown. The objective of the study was to determine the effects of participation in NWB sports on bone health in adult male recreational athletes. Male cyclists (NWB; n = 27) and runners (weight-bearing [WB]; n = 16) aged 20 to 59 years were recruited from the community. Whole-body and regional bone mineral content and bone mineral density (BMD), and body composition were assessed using dual x-ray absorptiometry. Bone formation and resorption markers, and hormones were measured in serum. Bone-loading history was estimated from a sports participation history questionnaire. Nutrient intake and current physical activity were estimated from 7-day written logs. The NWB athletes had significantly lower BMD of the whole body and spine than the WB athletes, despite having similar age, weight, body mass index, body composition, hormonal status, current activity level, and nutrient intakes. Sixty-three percent of NWB athletes had osteopenia of the spine or hip, compared with 19% of WB athletes. Cyclists were 7 times more likely to have osteopenia of the spine than runners, controlling for age, body weight, and bone-loading history. There were no group differences in serum markers of bone turnover. Based on the results of this study, current bone loading is an important determinant of whole-body and lumbar spine BMD. Therefore, bone-loading activity should be sustained during adulthood to maintain bone mass.

Introduction

Osteoporosis affects more than 2 million men in the United States, and nearly 12 million more have osteopenia [1]. In addition, 85% to 90% of all hip and vertebral fractures in men occur in osteoporotic individuals. Furthermore, the number of fractures associated with osteopenia is nearly double that of osteoporotic fractures [2]. Nevertheless, osteopenia and osteoporosis in men often remain undiagnosed and inadequately treated. Unlike women who are often identified as osteopenic or osteoporotic via routine dual x-ray absorptiometry scans, men usually present with fragility fractures, back pain, or diminishing stature [3]. In addition, even after having a fracture, men are less likely to receive follow-up care and to be prescribed antiresorptive pharmacotherapy than women [4].

Risk factors for osteoporosis in men are similar to those identified in women: family history, age, low body weight, smoking, excessive alcohol consumption, inadequate calcium or vitamin D intake, low reproductive hormone levels, physical inactivity, and disease or medication affecting bone metabolism [5], [6]. One might expect that men who participate in endurance sports, such as running and road cycling, would be at reduced risk for osteopenia and osteoporosis because of their healthful lifestyle and high levels of physical activity [7], [8]. Surprisingly, the prevalence rates of osteopenia and osteoporosis are alarmingly high in adult male road cyclists [9], [10], [11], [12], but not in distance runners [11].

Low body weight and weight loss have been associated with reduced bone mineral density (BMD) [13], [14] due to reduced mechanical loading on the skeleton and hormonal changes associated with inadequate energy intake [15], [16]. However, because runners and cyclists both have relatively low body weight compared with sedentary men [11], the discrepant BMD has been attributed to lack of ground reaction forces (GRF) on the skeleton during cycling [9], [11], [12].

The importance of mechanical stress in maintaining the balance between bone formation and resorption is evident in the rapid loss of bone that occurs during weightlessness and prolonged bed rest [17], [18]. Road cycling has little osteogenic effect on bone because of the type of forces it exerts on the skeleton [18]. Based on longitudinal studies of bed rest [19], [20] and space flight [20], one might expect bone resorption to be elevated in cyclists, at least transiently, and bone formation to be unchanged. However, the effects of cycling on rates of bone formation and resorption and, therefore, on the remodeling imbalance that leads to bone loss in cyclists remain to be identified. Therefore, the objectives of this study were to (1) determine the effects of participation in non–weight-bearing (NWB) vs weight-bearing (WB) sports on bone mineral content (BMC) and BMD of the whole body, hip, spine, and appendicular skeleton and (2) compare rates of bone turnover between male athletes in NWB (cycling) and WB (running) sports.