Final answer:
Low body mass and body fat significantly decrease bone density, leading to risks such as osteoporosis. While increased physical activity improves health and decreases disease risk, optimal athletic performance depends on a balanced body composition. Low body mass can also reduce the risk of heart disease.
Step-by-step explanation:
Low body mass and body fat significantly decrease bone density. When the body has a low amount of fat tissue, it can lead to conditions such as osteoporosis, particularly if the low body fat is due to insufficient nutrition or certain eating disorders. Further, while increased physical activity can improve overall health by increasing cardiovascular and muscular fitness and bone density, having insufficient body mass or fat can negatively impact these areas. Moreover, it's important to balance body composition since too little body fat may not support optimal athletic performance in some sports. Nevertheless, a lower body mass can lead to a reduced risk of heart disease due to decreased strain on the cardiovascular system, yet this is only a part of a more complex picture involving overall health and lifestyle habits.