Recently, some people have stopped supplementing with calcium due to fears that calcium supplementation will increase cardiovascular disease. We recently wrote about that issue.
Other people have stopped calcium supplements due to the belief that 5,000 IU/day of vitamin D will increase dietary intestinal calcium absorption to the point that calcium supplements are not needed. This belief is in part based on Professor Robert Heaney’s work showing that calcium absorption increases when vitamin D levels go from 20 ng/ml to 32 ng/ml, with no further increase in calcium absorption with levels higher than 32 ng/ml.
The question is, by taking 5,000 IU/day, do you absorb enough calcium from your diet without having to rely on supplements?
Vitamin D’s role in the body’s calcium economy is often referred to as vitamin D’s primary function. The reason for this belief is that adequate blood calcium is necessary for life. Blood calcium does not begin to fall until vitamin D levels are very low, such as in infants with rickets and hypocalcemia.
In order to maintain life, if you have a low input of vitamin D, the body reserves all that vitamin D to maintain blood calcium levels, and does not use much for vitamin D’s many other functions. That is, with low vitamin D input, all that vitamin D is triaged (selected over other priorities) to maintain serum calcium. I wrote about this triaging in 2011, using a mountain pool metaphor. Kate Saley has also blogged on the topic.
- Is reducing insulin resistance one of vitamin D’s top or low priorities? Posted on December 1, 2011 by John Cannell, MD
- The triage theory: Does vitamin D fit? Posted on November 22, 2012 by Kate Saley
Doctor Sue Shapses and colleagues of Rutgers University recently tried to find out how much vitamin D is needed to maximize calcium absorption. She used the gold standard test of calcium absorption, measurement of absorption of radioactive labeled calcium.
Shapses SA, Sukumar D, Schneider SH, Schlussel Y, Sherrell RM, Field MP, Ambia-Sobhan H. Vitamin D supplementation and calcium absorption during caloric restriction: a randomized double-blind trial. Am J Clin Nutr. 2013 Mar;97(3):637-45.
The authors designed a randomized controlled trial and enrolled 82 women into their trial. The trial lasted for 6 weeks, and all participants were on either a weight loss program or just a weight maintenance program. These women were randomized to either receive 2500 IU of vitamin D/day + 1200 mg calcium/day, or just 400 IU/day + 1200 mg of calcium/day.
Here is what they found:
- Five weeks of 2500 IU per day increased 25(OH)D by 8 ng/ml in the dieting group but only by about 4 ng/ml in the weight neutral group. There was no significant change in 25(OH)D whether dieting or not in the 400 IU group.
- For those who took 2500 IU/day of vitamin D, total fractional calcium absorption was not increased when 25(OH)D levels went from 25 ng/ml to 34 ng/ml in the weight loss group, nor from 25 ng/ml to 28 ng/ml in the weight neutral group.
Then the authors discussed the following points:
- For unknown reasons, caloric restriction (dieting) reduces calcium absorption by about 10%.
- Only about 25% of the calcium you ingest — be it from diet or supplements — is absorbed by the body. Seventy-five percent is not absorbed, and is eliminated in the stool. You also lose calcium in your urine.
- In another study that used radioactive calcium and vitamin D, administration of 50,000 IU/day for 15 days only increased fractional calcium absorption from 24% to 27%.
- The average calcium intake from diet in the women in Doctor Shapses’ study was only 650 mg/day. This is about average for Americans, which is why we may need calcium supplements.
- It has been estimated that at least 200mg absorbed calcium per day is required just to offset obligatory calcium loss, mainly in the kidneys. If you want to be in positive calcium balance, taking in more than you excrete, you need between 1,000 and 1,200 mg/day of total calcium intake from food and/or supplements combined.
This was an important paper.
I just want to add that the efficiency of absorption increases as calcium intake decreases. That is, the less calcium you ingest, the higher the percentage that is absorbed. Also, net calcium absorption is as high as 60% in infants and young children who need substantial amounts of calcium to build bone. Calcium absorption actually decreases to 15%–20% in adulthood (though it is increased during pregnancy) and continues to decrease as people age.
Recommended calcium intakes by the Food and Nutrition Board are:
- 1,300 mg/day for teenagers
- 1,000 mg/day for adults
- 1,200 mg/day for females older than 50 years
- 1,200 mg/day for both sexes older than 70 years
And remember, strong bones need more than just calcium and vitamin D.