Introduction: The skeletal system serves a variety of purposes such as providing structural support, locomotion, protection and more. In order to sustain these functions, bone strength must be upheld. The process of replacing older, microdamaged bone with newer, healthier bone is known as bone remodelling. Bone remodelling involves the balance/coupling of bone resorption and bone formation. Mechanical loading is the magnitude of stress placed on the bone that stimulates bone remodelling, which determines bone mineral density (BMD) and promotes bone growth (Olmedillas et al. 2011). Thus, human research on the determinants of bone growth and strength, is focused on two common factors, exercise and nutrition. Although cycling, a predominantly aerobic exercise, is exceptional for cardiovascular fitness, it is commonly questioned whether it provide adequate mechanical strain on bone. To date the literature surrounding cycling and its impact on bone is contradicting and limited. Inadequate energy intake may also influence BMD via reductions in bone mineral content. Energy restriction is known to reduce insulin-like growth factor 1 (IGF-1), which plays an important role in bone formation 5. Acute dietary energy restriction has been found to be accompanied by an imbalance of bone remodelling with reduced bone formation, which is especially dangerous in aerobic sports that expend a great amount of energy, such as cycling. Many professional and master cyclists are classified as osteopenic (Medelli et al. 2000). Specifically, female cyclists have shown significant lower values of whole-body BMD, and a lower bone strength, than male cyclists (Olmedillas et al. 2011). In addition, female cyclists are known to be energy deficient (low calorie intake) with the average energy intake being 85% of the Recommended Dietary Allowance (RDA). Therefore, it is important to understand whether cycling is a poor form of exercise for bone health or whether the energy restriction, alone or in combination with cycling, is the true underlining issue.

Objective: The objective of this study is to examine whether short-term energy restriction leads to changes in markers of inflammation, oxidative stress, bone turnover at rest and in response to cycling in young adults. Specifically, the study will examine changes in circulating bone markers at rest and following one bout of low impact 45-minute spinning (cycling) both before and after one week of restricted energy intake.

Methods: Fifteen healthy males and females, aged 20-25 years, will be invited to participate in this study, which involves one control trial and two exercise trial visits scheduled one week apart. Females will be on monophasic oral contraceptives, During the control trial, participants will be briefed on the purpose and procedures, will fill out an exercise screening questionnaire, and will provide 4 resting blood samples. One week later, participants will perform the first exercise trial of one 45 minute spin session. Following the first trial, the participants will go on a predetermined energy deficient diet (25% of their habitual diet) for one week, at the end of which they will perform the second trial following the same cycling protocol. Between the first and last visit the participants will be receiving a Fitbit to monitor their activity (steps, heart rate, physical activity participation) and nutritional habits in a food log. To keep the diet consistent and standardized, participants will also be asked to record their habitual food intake during the control week using the "Eat This Much" app, which will provide an overall portion plan and a 25% caloric restriction measure for the intervention week. Thus, the intervention diet will mirror the control diet in terms of types of food consumed, with the difference being in the amount consumed. There will be no restriction regarding water, but drinks will be restricted according to the overall calorie intake plan.In both trials, blood will be drawn at rest (i.e., pre-trial, fasted), and 3 times post-trial (5 min, 1h and 24h). The blood will be centrifuged and the serum separated, aliquoted and then stored at -80 °C until analysis 8. The serum will be analysed for biochemical markers of bone turnover (e.g. osteocalcin, bone-specific alkaline phosphatase \[BAP\]; osteoprotegerin \[OPG\]) and bone resorption (C-telopeptides of type I collagen \[CTX\]; receptor activator of nuclear factor κB ligand \[RANKL\], sclerostin) to assess the impact of energy restriction on bone at rest and in response to exercise. Metabolic and oxidative stress markers, inflammatory cytokines, adipokines, growth factors and hormones will also be assessed to examine potential mediating effects.

Impact: This innovative work has the potential to make significant advances in understanding the impact of energy restriction and cycling on bone health. The findings from this work will be translatable to understanding tissue growth and development in response exercise and malnutrition.