Your body contains more calcium than any other mineral. Of your calcium stores, 99% sits in your bones and teeth. The other 1% busies itself throughout your body - helping your blood to clot, your muscles to contract and relax, and your brain and body to communicate via nerve signals.
When you do not consume enough calcium to maintain these basic functions, your body steals the mineral from your bones. And when your bones lose calcium, you increase your risk for osteoporosis and fractures.
You need to add calcium back to your body every day, either through calcium supplements or through the foods you eat. As you get older, your body becomes less able to absorb calcium, so your recommended intake increases with age. Children require anywhere from only 200 milligrams of calcium per day as infants to upwards of 1,300 milligrams daily during the growth spurt of puberty. Adults should get about 1,000 milligrams each day, with needs increasing to 1,200 daily milligrams from the age of 51 for women, and from around the age of 71 for men.
Particular foods and nutrients can help or hinder how much of that calcium you absorb:
Vitamin D: Also called "the sunshine vitamin," vitamin D is calcium's best friend. When your body senses calcium, a hormone sends a message to your kidneys to activate vitamin D. Then vitamin D takes care of calcium business! It helps to move calcium through the intestines so it can be absorbed, and it kick-starts the circulation of the calcium that has been locked up in the bones. You should be getting 600 IU of vitamin D per day (800 IU if you are older than 71) from the sun, supplements, or foods like eggs, fatty fish, cheese, or fortified milks and cereals.
Skim milk: You don't need to fatten up your milk choice to get more calcium. That is because the calcium in milk is separate from the fat. This means that skim milk actually has a higher concentration of calcium than full-fat milk, because the part that doesn't contain calcium - the fat - has been removed. So, go for skim or non-fat milk to get more calcium for your buck.
Fruits and vegetables: Eating a diet full of fruits and vegetables is a good idea in the first place, but it also boosts your calcium absorption. When your body digests and metabolizes fruits and vegetables, your body produces bicarbonate, which reduces calcium loss.
Caffeine: For some, the cream stirred into their morning coffee may be the only calcium they get in a day. But the caffeine in coffee could cost you 2 or 3 milligrams of calcium. True, that's not many milligrams, but it can begin to add up - a few cups of coffee here, a caffeinated pop there, a tea in the evening. Especially if you are at risk of bone loss, consider cutting back on caffeine.
Alcohol: To absorb calcium, you know you need vitamin D. To get vitamin D to its job, your liver has to convert D into its active form. Unfortunately, drinking alcohol can interfere with this process. It is unclear how much alcohol it takes to affect your calcium levels, but moderation is likely the best approach to beer, wine, and spirits.
Oxylate: Though spinach is high in calcium, it is also high in oxylate, a type of acid that occurs naturally in many fruits and vegetables. When eaten together with calcium-rich foods, oxylate may affect your body's ability to absorb the calcium. That's why milk and dairy products tend to be better sources of absorbable calcium than green veggies like spinach.
Phytate: Another kind of plant-based acid that can reduce calcium absorption is phytate. Phytate shows up in whole-grain products, as well as in beans, seeds, and nuts. This acid can bind to calcium and make it harder for the body to digest and absorb.
Sodium and protein: Foods containing high levels of sodium or protein may prevent your body from absorbing as much calcium as it should. Instead, the calcium exits through the urine, and the body makes up for it by sapping it from the bones. Post-menopausal women may especially benefit from adding potassium to their diets to counter this effect.
Fibre: Like sodium and protein, fibre is a must for your body. But because it can both speed digestion and bind to calcium in the digestive tract, soluble fibre may affect calcium absorption. Those on a high-fibre diet, like some people with diabetes, should talk to their doctor about eating more calcium-rich foods or supplementing with calcium.