... SANS OUBLIER LE ROLE IMPORTANT DE LA VITAMINE D DONT NOUS MANQUONS SOUVENT....
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The mineral calcium is essential to a number of our bodies’ functions, including muscle contraction, blood clotting, the transmission of nerve impulses, and the release of enzymes and hormones. But it’s known first and foremost as the bone-building nutrient.
Healthy bones (which, along with teeth, are where 99 percent of our calcium is stored) are living tissue, so they are continuously breaking down and rebuilding in a process called remodeling. Children and teenagers make more bone than they lose, and even after they stop growing in height, their bones keep getting denser, until they reach what is called peak bone mass in their early 20s.
As you get older, your bones break down more quickly, and when rebuilding no longer keeps pace, they gradually become weaker. Both men and (particularly) women experience this decline in bone density; when it advances to a certain point, it’s called osteoporosis, which often results in frequent bone fractures and significant loss of height.
According to the National Osteoporosis Foundation, an estimated 48 million Americans have low bone density and about 9 million have osteoporosis. There are a number of risk factors, including heredity, for the disease, but according to medical experts at the Mayo Clinic and elsewhere, the higher your peak bone mass, the more bone you have in the bank, so to speak, and the less likely you are to develop osteoporosis as you age.
Calcium is key, then, for lifelong bone health, especially if you want to stay out of the emergency room or be able to see over the steering wheel when you’re 70. But how much calcium and in what form are the subject of considerable debate and ongoing research.
In 2010, after assessing more than 1,000 studies, the Institute of Medicine (IOM) issued new daily recommendations for calcium and vitamin D (which must be present for calcium to be properly absorbed). For teens ranging in age from 14 to 18, the recommended amount of calcium is 1,300 milligrams (mg) each day. For adult men and women (ages 19 to 50), the recommended daily amount is 1,000 mg. The same holds true for adult men ages 51 to 70, but adult women in that age group (i.e., postmenopausal, with decreasing estrogen levels) should bump up their daily intake to 1,200 mg. (Here’s a chart of recommendations for all ages.)
But critics such as Walter Willett, professor of epidemiology and chair of the department of nutrition at the Harvard School of Public Health, argue that calcium is overemphasized and dispute the methodology. “…the calcium DRI is based on measurements of calcium intake and losses in feces and urine over periods of less than 14 days, which probably don’t reflect bones’ long-term calcium content,” Willett wrote in The New England Journal of Medicine. “Although DRIs warrant consideration (and periodic reassessment), recommendations regarding foods and dietary patterns should be based primarily on evidence about health outcomes, not levels of specific nutrients.”
While Willett and other detractors don’t question the importance of the mineral in bone development and strength, they cite other studies that suggest that high calcium intake doesn’t seem to lower a person’s risk for osteoporosis or correlate with fewer fractures. And, for men, a diet high in calcium may be a risk factor for prostate cancer.
As Lesley Gore (okay, and Nikki Jean) would say, so what’s a girl supposed to do?
For young women, aiming for 1,000 mg of calcium (combined with 600 I.U. of vitamin D) a day doesn’t appear to have any drawbacks, especially when combined with frequent weight-bearing exercise, an undersold way to minimize bone density loss during adulthood and old age. Walking, running, rollerblading, hiking, dancing, weightlifting, stair-climbing, and racquet sports all count, and the resulting muscle strength, coordination, and balance all make it easier to avoid the falls that cause broken bones in the first place.
Foods rich in calcium
Nutrition and medical experts will tell you that foods, not supplements (more about them in a sec), are your best source of calcium. When you eat foods that contain calcium, you’re also getting all sorts of other nutrients bones need. As a general rule, don’t consume more than 500 mg of calcium at a single meal because that’s the most your body can absorb at one time.
Milk, yogurt, cheese, and other dairy foods are the main sources of calcium; they contain a lot of the mineral—an eight-ounce serving of plain yogurt provides about 400 mg, an eight-ounce serving of milk, 300 mg—and it is easy to absorb.
You may have heard or read that dairy products actually cause osteoporosis, based on the idea that their high protein and phosphate content make them acid-producing foods, thus increasing the renal acid load; in order to neutralize the acid, the thinking goes, calcium is urinated out with the acid.
Over the years, there has been a great deal of research done on protein and bone health, and this hypothesis has been refuted. For more information, check out this 2009 evidence-based review and meta-analysis in the Journal of Bone and Mineral Health, as well as any number of peer-reviewed journal articles on the subject. (For free, you can read the PubMed abstracts; here’s one, titled “Milk and acid-base balance: proposed hypothesis versus scientific evidence,” published in 2011, and another “Protein intake, calcium balance and health consequences,” published in 2012.)
Still, if you don’t want or can’t eat dairy, there are other calcium-containing foods to choose from—and, as always, variety is the way to go. A 3.7-ounce tin of sardines eaten with the bones (the canning process softens them) contains 351 mg calcium. One cup of almonds contains 378 mg. One tablespoon of sesame seeds provide 88 mg, so choose a sesame-seed bagel over another type, sprinkle them over a stir-fry or steamed greens, and work tahini into your salad dressing repertoire. Two teaspoons of blackstrap molasses meet almost 12 percent of your daily requirement. Varying amounts of calcium are found in dried beans and legumes, and a whole host of foods, from orange juice and breakfast cereals to tofu and soymilk, are fortified with the mineral.
Green vegetables such as spinach, collards, kale, bok choy, and broccoli, contain calcium, too, but the mineral’s absorption rate, or bioavailability, can be a bit tricky, depending on the level of oxalates in each plant. Spinach, for instance, has 245 mg of calcium per cup (in addition to loads of antioxidants), but its high oxalate content means that just five percent of it is absorbed. Studies show the calcium in collards and broccoli is absorbed moderately well, and that in kale and bok choy is absorbed well. You should also know that leafy greens are high in vitamin K, which is also good for bones.
Calcium supplements
Calcium is found in many multivitamins as well as in dietary supplements. In the form of calcium citrate (as opposed to calcium carbonate), it absorbs well and is well tolerated by most people, and the citrate may help prevent kidney stones.
But calcium supplements have been making news—big news. In 2012, Medical News Today reported that an analysis of data on nearly 24,000 people followed for more than a decade suggests taking calcium supplements may increase the risk of having a heart attack. But other researchers, including those in the Framingham Study, found no correlation between calcium intake and calcification of the arteries.
Heart health aside, in February of this year, after reviewing more than 135 studies, the United States Preventive Services Task Force (USPSTF) reported that there is insufficient evidence to support the view that calcium and vitamin D supplements prevent fractures in healthy premenopausal women or in men. Its recommendation for postmenopausal women was the same: “Do not supplement.”
The takeaway? Stick with food, real food—and take a walk instead of a pill.
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