Calcium is a soft gray alkaline earth metal with symbol Ca. Calcium is a mineral that is essential for living organisms especially for bones and teeth. Calcium is the most abundant metal by mass in many animals. More than 99 percent of calcium is in the bones and teeth. The rest is throughout the body in blood, muscle and the fluid between cells.
Calcium and bone loss
Calcium supplementation may help maintain bone mass in athletes susceptible to premature osteoporosis (weak bones due to low bone density) . Its supplementation slows the rate of decline in bone density by 30 to 50%. Co-ingestion with vitamin D further enhances its absorption. Vitamin D converts into a hormone in the body, which induces the synthesis of intestinal proteins responsible for calcium absorption . However, the efficacy of calcium and vitamin D co-ingestion for preventing fractures in healthy postmenopausal women remains equivocal. Jackson et al.  recruited 36,282 postmenopausal women who were randomly assigned to receive 1000 mg of elemental calcium with 400 IU of vitamin D3 daily or placebo. Researchers found small but significant improvement in hip bone density, but that did not significantly reduce hip fracture. There is also recent evidence that dietary calcium may help manage body composition .
Calcium as a weight loss supplement
Low-calcium diets cause an increase in 1, 25-dihydroxyvitamin D (Vitamin D3) which as a consequence, stimulates lipogenesis (conversion of simple sugars into fatty acids), and suppresses lipolysis (breakdown of fats) whereas increasing dietary calcium inhibits these effects and may accelerate fat loss in animals and humans . Michael B. Zemel and colleagues  have demonstrated that increasing dietary calcium accelerated weight and fat loss. Davies and others  noted the calcium-treated subjects in the controlled trial exhibited a significant weight loss across nearly 4 year of observation.
These findings and many others suggest a strong relationship between calcium intake and fat loss. However, more research needs to be done before definitive conclusions can be drawn. Kreider and associated  marked calcium as possibly effective for weight loss in their “Summary of categorization of dietary supplements based on available literature”.
Calcium supplementation does not provide ergogenic effect on exercise performance.
Calcium side effects, hazards and toxicity
Most calcium compounds, unlike other metals, have low toxicity. At recommended doses it is likely safe. Some recent research suggests that doses over the recommended daily requirement of 1 – 1,3 grams daily (for adults) can increase the chance of heart attack.  So, until more is known you should not exceed recommended daily requirement. It was also noted that calcium and Vitamin D co-ingestion increases the risk of kidney stones . It can also cause some minor side effects such as belching or gas .
Reid, Ian R. “Therapy of osteoporosis: calcium, vitamin D, and exercise.” The American journal of the medical sciences 312.6 (1996): 278-286.
Combs, G (2008). The Vitamins. Academic Press. p. 161.
Jackson, Rebecca D., et al. “Calcium plus vitamin D supplementation and the risk of fractures.” New England Journal of Medicine 354.7 (2006): 669-683.
Kreider, Richard B., et al. “ISSN exercise & sport nutrition review: research & recommendations.” Sports Nutr Rev J 1.1 (2004): 1-44.
Zemel, Michael B., et al. “Calcium and dairy acceleration of weight and fat loss during energy restriction in obese adults.” Obesity research 12.4 (2004): 582-590.
Davies, K. Michael, et al. “Calcium intake and body weight.” Journal of Clinical Endocrinology & Metabolism 85.12 (2000): 4635-4638.
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