My last post covered the correct way to get your daily dose of calcium: via foods (and supplements if necessary), spaced throughout the day for maximum absorption. Calcium-loading is a no-no; when you take a 1000mg tablet at breakfast and swig down a smoothie for another 500mg 2 hours later, your body will actually only absorb 1/3 of what you’ve taken, about 500-600mg. Smaller amounts, taken more frequently, are the way to go.

Now that you’ve absorbed that news (pun intended), you need to know what else affects how your body absorbs calcium. There are three major factors: vitamin D; things that suppress calcium absorption; and things that increase calcium absorption.

Vitamin D
Vitamin D is to calcium what gas is to your car: without vitamin D, calcium just can’t get going. Vitamin D helps in the formation of a hormone, calcitriol, which in turn allows your body to absorb calcium. So insufficient amounts of vitamin D in your body prevent the most effective absorption of calcium.

While the current government recommendation for vitamin D is 400IU daily, it’s an accepted fact that new guidelines, to be released next year, will advise a much higher daily dose. A recent study disclosed that fully 70% of Americans are vitamin D deficient; we’ve got some catching up to do.

So, how do you get vitamin D? Through your skin, via about 10 minutes a day (or 15-20 minutes several times a week) of natural sunlight, no sunscreen. Through your diet: eggs, saltwater fish, and vitamin D-fortified milk are all good sources. And via vitamin D supplements, which are often paired with calcium supplements. Caltrate 600-D (and its generic knockoffs) offers 600mg calcium paired with 400IU vitamin D, both in tablet and chewable form.

Calcium inhibitors
Oxalates and phytates. Salt. Excess protein. Caffeine. Not enough stomach acid. All of these may prevent your body from absorbing calcium fully; or may encourage your body to excrete too much calcium via the kidneys.

Let’s look at oxalates and phytates first. Both are found naturally in certain foods, chiefly oxalates in spinach, chocolate, collard greens, sweet potatoes, rhubarb, walnuts, tannins (tea), and beans; and phytates in whole grains, beans, seeds, nuts, and soy isolates. Oxalates and phytates bind to calcium and prevent it from being absorbed in the intestine, so look at the calcium content of any of these foods with a jaundiced eye; you won’t be getting as much as you think.

And how about salt and protein? Both cause your kidneys to excrete calcium at a faster rate than desirable. But this is only a problem for those who don’t get the recommended daily amount of calcium; if you’re getting your 1500mg a day, don’t worry about salt and protein.

Ditto caffeine; while it both prevents absorption of calcium and increases excretion, its effect is so minimal that simply adding a tablespoon of milk to your coffee makes up for caffeine’s inhibiting effects.

For those of you lacking stomach acid—either naturally, or because you take an acid inhibitor, e.g., Prevacid®, Prilosec® or Nexium®—breaking down calcium in pill form is a problem. Lean towards a liquid or chewable supplement; and/or take calcium citrate (rather than calcium carbonate; read your supplement’s fine print). Calcium citrate doesn’t rely on stomach acid to break down.