of osteoclasts. This leads to a greater resorption of calcium and phosphate ions. High levels of calcium in the blood, on the other hand, leads to decreased PTH release from the parathyroid gland, decreasing the number and activity of osteoclasts, resulting in less bone resorption. Vitamin D increases absorption of calcium and phosphate in the intestinal tract, leading to elevated levels of plasma calcium, and thus lower bone resorption. Calcitriol (1,25-dihydroxycholecalciferol) is the active form of vitamin D3. It has numerous functions involved in blood calcium levels. Recent research indicates that calcitriol leads to a reduction in osteoclast formation, and bone resorption. It follows that an increase in vitamin D3 intake should lead to a decrease in bone resorption — it has been shown that oral administration of vitamin D does not linearly correlate to increased serum levels of calcifediol, the precursor to calcitriol. Calcitonin is a hormone secreted by the thyroid in humans. Calcitonin decreases osteoclast activity, and decreases the formation of new osteoclasts, resulting in decreased resorption. Calcitonin has a greater effect in young children than in adults, and plays a smaller role in bone remodeling than PTH. In some cases where bone resorption outpaces ossification, the bone is broken down much faster than it can be renewed. The bone becomes more porous and fragile, exposing people to the risk of fractures. Depending on where in the body bone resorption occurs, additional problems like tooth loss can arise. This can be caused by conditions such as hyperparathyroidism and hypovitaminosis D or even decreased hormonal production in the elderly. Some diseases with symptoms of decreased bone density are osteoporosis, and rickets. Some people who experience increased bone resorption and decreased bone formation are astronauts. Due to the condition of being in a zero-gravity environment, astronauts do not need to work their musculoskeletal system as hard as when on earth. Ossification decreases due to a lack of stress, while resorption increases, leading to a net decrease in bone density. Alcoholism The effects of alcohol on bone mineral density (BMD) are well-known and well-studied in animal and human populations. Through direct and indirect pathways, prolonged ethanol exposure increases fracture risk by decreasing bone mineral density and promoting osteoporosis. Indirect effects of excessive alcohol use occur via growth hormone, sex steroids, and oxidative stress. Growth hormone is an important regulator of bone growth and remodeling in adults, and it acts via insulin-like