Effects of Cycling Duration on Markers of Bone Metabolism in Active Young Men

Introduction:

Exercise has been shown to positively benefit bone health. It's well documented that an acute bout of weight bearing, high load and dynamic strain on the skeleton mobilizes markers of bone formation in normal weight males and females. Acute bouts of low-impact high intensity exercise such as cycling, have also been investigated for their effects on bone formation and resorption, showing an overall anabolic effect (Mezil et al. 2015). One study exists investigating an acute two-hour bout of moderate intensity cycling, which showed increases in parathyroid hormone, associated with bone metabolism, although the effects of these transient hormonal changes on bone remain unknown (Barry & Kohrt, 2007). Minimal research has investigated the effects of acute bouts of moderate intensity continuous cycling (70% of VO2 max) on bone metabolism. In contrast to these acute controlled studies, at least two studies have reported that athletes who regularly participate in non-weight bearing sports such as cycling present with higher rates of osteopenia in the lumbar spine and hip region (Rector et al. 2008; Sherk et al. 2013). Another study investigated the bone status of adolescent male cyclists, over 17 and under 17 years of age, compared to healthy age matched controls. Cyclists had lower BMD at the legs, pelvis, and total hip. In cyclists over 17, reported BMD was 8.9% to 24.5% lower for the whole body, pelvis, femoral hip and legs, suggesting cycling performed during adolescent years may negatively affects bone health and one's ability to reach peak bone mass during this critical time (Olmedillas et al. 2011). However, a small study investigating 5 male elite racers during a 6-day road cycling stage race, while meeting energy needs, showed an increase markers of bone formation and a decrease in markers of bone resorption (Hinton et al. 2010). Thus, the evidence provided above appears contradictory. On the one hand, an acute bout of high intensity cycling has an osteogenic effect on bone, while longer-term studies or observational data, which allow for the time required to see changes in BMD, appear to show that road cycling can also have an osteocatabolic effect on bone (Olmedillas et al. 2012). This contradiction raises interesting questions about the impacts of larger loads of cycling, energy intakes, as well as the impacts of longer durations of cycling on bone health.

There appear to be conditions related to the sport of road cycling, not specific to the mechanics of cycling itself, that can predispose an athlete to low BMD over time. These conditions remain unclear. Possible factors contributing to the lower BMD found in cyclists include low energy availability and its associated hormonal and nutritional implications, carbohydrate availability, the omission of weight bearing loads due to long hours spent performing non-weight cycling, weight loss, as well as excess calcium losses through sweat and urine. Recent studies have also provided evidence of the importance of consuming carbohydrates in attenuating markers bone resorption and supporting bone health during exercise (Heikura et al. 2019). Of interest, is the fact that gymnasts, athletes who also often train under conditions of low energy availability, appear to gain a protective effect from their high impact sport, that overrides the bone resorption typically associated with calorie deficits. The high mechanical forces in this sport have large osteogenic effects, maintaining BMD, unlike what has been observed in cyclists (Robinson et al. 1995). This makes one question if it is the non-weight bearing nature of cycling, certain conditions surrounding cycling or something inherent to the mechanical aspects of cycling itself, that have a seemingly negative impact on bone?

To date, researchers have investigated the effects of an acute bout of high intensity cycling on markers of bone formation and resorption, as well as moderate intensity cycling and its effects on bone. Field studies have been executed where energy needs have been met and where energy deficiency was present. There is a gap in the research as far as the impact of duration of moderate intensity cycling on bone, in an energy replete state, in a controlled setting. Filling this gap through a systematic approach would help to better understand if and how duration impacts the metabolic bone response.

There are a variety of methods to measure the bone's response to acute mechanical loading. Since changes in BMD are not immediate, and only occur over longer time periods, this measurement is not appropriate to use to investigate changes in bone metabolism after one acute bout of exercise. After an acute bout of exercise, it is common to measure circulating bone turnover markers. These markers are products of bone formation or resorption. There are a variety of bone turnover markers, however some of the more commonly used are procollagen I intact N-terminal (PINP) and C-terminal crosslinking telopeptides of type I collagen (CTX), which are recognized by the International Osteoporosis Foundation (IOF) and are products of osteoblastic or osteoclastic cell activation, respectively. More recently, bone metabolism has been investigated through measures of the glycoprotein sclerostin, an inhibitor of the Wnt pathway, which leads to decreased bone formation. The Wnt/β-catenin signalling process has influence on the mobilization of OPG (Osteoprotegerin). OPG binds to receptor activator of nuclear factor kappa-β ligand (RANKL), preventing RANKL from binding to RANK (an osteoclast cell surface receptor), acting as a decoy receptor. RANKL binding to RANK would otherwise increase bone resorption. Therefore, the two pathways, Wnt-B-catenin and OPG/RANKL have a relationship that can help us to better understand the processes of bone resorption and formation.