This indicates the supplement has been tested for purity. USP testing is voluntary, however, so many other quality supplements may exist without this symbol on the label. Moreover, it may not be available world-wide. Calcium carbonate has long been considered the best source of usable calcium because the body can absorb more of it than some other forms of calcium. For maximum absorption, it is best taken in small doses throughout the day with food, instead of in one big dose all at once.
Calcium carbonate is the active ingredient found in some antacids.
Calcium citrate is a newer form of calcium supplementation which is well-absorbed, particularly in older adults who produce less stomach acid in later years. Recently a study of women at risk for osteoporosis reported that calcium citrate was slightly better absorbed than calcium carbonate in women past menopause. Calcium citrate is usually more expensive than calcium carbonate, however. How much to take Recommendations for calcium intake range between 1,000 to 1,500 mg between health agencies and countries. Generally, it is agreed by experts that you need at least 1,000 mg per day.