Endurance exercise is frequently recommended as a means to reduce the risk of cardiometabolic diseases and to reduce the risk of osteoporotic fracture. However, bone does not always adapt in the way that would be expected, and there is evidence that endurance exercise may lead to bone loss under certain conditions. It is our contention that disruptions in calcium homeostasis during exercise, resulting in decreases in serum ionized calcium (iCa) and increases in parathyroid hormone (PTH) and c-telopeptide of type I collagen (CTX; a marker of bone resorption) at the onset of exercise, may be responsible for the lack of improvements in bone mineral density that are anticipated. Acute studies in this area have predominantly focused on young, healthy adults, primarily men, during stationary cycling exercise. Few studies have been conducted in older adults, but those studies have found that older adults experience similar disruptions in calcium homeostasis in response to an acute bout of endurance exercise. Preliminary comparisons of our studies, as well as research from other labs, also suggests that that mode of exercise conducted during these acute exercise bouts may be an important determining factor in the catabolic bone response to exercise. It appears that weight-bearing exercise (i.e., treadmill) results in smaller increases in PTH and CTX compared to weight-supported exercise (i.e., stationary cycling). This has never been tested using a within-subjects design, so it is unclear if these observed differences are due to the mode of exercise, lab-to-lab differences, or other underlying factors. To address this gap in knowledge, 30 Veterans (15 men, 15 women), aged 60+ years, will complete two 1-hour acute exercise bouts: 1) brisk treadmill walking at 70-80% of maximal heart rate; 2) vigorous stationary cycling at 70-80% maximal heart rate. Blood samples will be collected to measure iCa, PTH, CTX and procollagen type I n- terminal propeptide (P1NP) before, during, and after each exercise bout. The order of the exercise bouts will be randomized and counter-balanced. The primary aim is to determine if mode of exercise results in a differential bone biomarker response in older Veterans. This information is essential for understanding how future exercise interventions should be designed to benefit both cardiometabolic health and bone health. This is especially relevant to Veteran health due to the high burden of both cardiometabolic diseases (e.g., diabetes, heart disease) in the population, as well as evidence of increased osteoporotic fracture risk. The proposed research is significant because it is addressing a knowledge gap that has prevented the ability to design exercise and lifestyle interventions aimed at preserving multiple components of Veteran health, which could have a lasting impact on Veteran quality of life and functional independence. The proposed research is innovative because it is testing a novel hypothesis, the mode of exercise on ...