If you give a patient calcium…

You have to tell them when to take it.

My prior blog warned about the unknowns of natural supplements in general. This follow up piece is focused on calcium-based products, which are commonly utilized for a variety of medical indications. (Also, am feeling particularly inspired about this topic as I am putting together a National Kidney Foundation webinar for dieticians that discusses calcium in chronic kidney disease – includes a lot of fun pathophysiology and pictures of high-calcium foods!)

The timing of when to take the calcium is quite critical. General rules: calcium taken with meals serves as a binder to prevent absorption of a certain food component. Calcium taken between meals is absorbed and will boost body calcium stores. Let’s go into more specific examples…

Prevention of oxalate kidney stones (dietary calcium with meals)
Calcium oxalate is the most common type of kidney stones. While drinking plenty of fluids to maintain dilute urine is the cornerstone of stone prevention (used “stone” twice there and it’s legit), calcium is a proven adjunct intervention to lower urinary oxalate. The concept here is that dietary calcium binds oxalate, and the calcium-oxalate complex is pooped out so less oxalate is absorbed by the gut, and less oxalate reaches the kidneys. In a randomized clinical trial, normal calcium intake of 1200 mg/day with restricted oxalate, protein and salt intake decreased stone recurrence compared to a low-calcium diet of 400 mg/day.

Prevention of hyperphosphatemia in dialysis patients (calcium-based binders with meals)
Chronic kidney disease (CKD) patients have a high prevalence of vascular calcification and heart failure. As kidney function declines, blood phosphorus levels increase and hyperphosphatemia has been linked with adverse cardiovascular events and death. Calcium-based phosphate binders, such as calcium carbonate and calcium acetate, are prescribed with meals to decrease phosphorus absorption (along the same lines as for calcium use with oxalate, see above). This approach becomes a slippery slope, because of the concurrent risk of positive calcium balance in CKD patients whereby excess calcium is also a risk factor for vascular calcification. Neither blood calcium nor urinary calcium levels accurately reflect total body calcium, so there is no easy way to gauge a patient’s calcium status. Clinical trials have noted less progression of coronary artery calcification in CKD patients on non-calcium phosphate binders (as compared to the calcium-based binders), thus nephrologists have become more cautious with prescribing calcium-based phosphate binders in CKD patients.

Treatment of osteoporosis (calcium supplements with meals or between meals… it depends)
Calcium and vitamin D for osteoporosis treatment is controversial but experts agree that high-risk patients would benefit from these supplements for fracture prevention. They also agree that it is important to avoid excessive dosing, e.g., adding elemental calcium 1000 mg per day in an individual who is already taking in a normal amount of dietary calcium (1200 mg/day) would increase the risk of adverse cardiovascular events or kidney stones. When taking calcium for osteoporosis therapy, the intention is to optimize calcium absorption from the gut (very different from the two scenarios presented above). The calcium formulation dictates timing with meals; for calcium citrate, there is good absorption regardless of whether the supplement is taken with or between meals. Calcium carbonate is taken with meals as gastric acidification is needed to release the calcium for gut absorption. Magnesium, zinc and iron should not be taken at the same time with calcium supplements as each can interfere with the other’s absorption.

Supplementation in hungry bone syndrome (may require both IV and oral calcium!)
Hungry bone syndrome is defined by a significant drop in blood calcium following surgical removal of hyperfunctioning parathyroid glands. The sudden decrease in parathyroid hormone levels leads to unopposed uptake of calcium by the bones. Hypocalcemia can be sustained for a few weeks, and patients often require IV calcium repletion within the first week followed by high doses of oral calcium 1-3 grams per day with vitamin D therapy. Similar to use of calcium for osteoporosis therapy, the goal here is to optimize calcium absorption from the gut. The rule of thumb is that elemental calcium should be spread out through the day up in doses of 500 mg as this is the gut’s threshold for effective calcium absorption.

Finally, it is a good idea to inform patients that it is the “elemental calcium” portion of supplements that we are referring to when discussing calcium doses. Calcium carbonate contains 40% elemental calcium, and calcium citrate contains 21% elemental calcium. An “ultra” strength calcium carbonate 1,000 mg tablet contains 400 mg of elemental calcium which is within the absorption threshold. Calcium supplements are a prime example where how much and when can impact response to therapy.

Wei Ling Lau Headshot

Wei Ling Lau, MD is Assistant Professor in Nephrology at University of California-Irvine. She is currently funded by an AHA Innovative Research Grant, and has been a speaker for CardioRenal University and the American Society of Nephrology. Follow her on Twitter @Kidneys1st