CALCIUM, MAGNESIUM AND BONE MASS DENSITY
Unless your calcium intake is very low indeed, calcium supplementation alone makes little difference. A review of all the evidence for calcium in the British Medical Journal concludes that increasing calcium intake from dietary sources or by taking calcium supplements produces small non-progressive increases in bone mass density, which are unlikely to lead to a clinically significant reduction in risk of fracture. 
Both magnesium intake and measure of serum magnesium predict fracture risk.  When post-menopausal women supplemented magnesium over a two-year period their fracture risk reduced and bone mass density increased.  According to data collected in the UK’s Biobank which tracks the health of over 150,000 people, higher dietary magnesium intakes correlate with slower muscle deterioration that supports the skeleton.  Also, measures of magnesium levels within cells predicts knee extension strength.  Stronger muscles means less osteoporosis and arthritis risk.
While calcium intake appears to effectively delay the tendency towards bone mass density decrease in postmenopausal women with a total intake of 1200 mg a day (diet plus supplements) your intake of magnesium, and vitamin D is just as important. With the average dietary intake of calcium being around 700mg (eg vegans) to 900mg (high dairy consumers) this suggests that supplementing 300mg or maybe 500mg in those, such as vegans or those not eating dairy products, whose intake might be lower might have a small advantage. However, estimates of calcium intake of our Paleolithic ancestors, and intakes of African communities, both with no evidence of substantial bone mass loss with age, are much lower, between 400mg and 600mg a day. Their diets, and living outdoors, would have meant more magnesium and vitamin D. Perhaps, with sufficient magnesium and vitamin D, even this extra calcium intake is not so necessary.
VITAMIN D DRIVES CALCIUM INTO BONE
More effective than calcium or vitamin D on its own, however, is the combination of calcium with vitamin D. The latest ‘meta analysis’ of the results of six good quality randomised controlled studies that used calcium with vitamin D supplementation versus placebo showed 6 RCTs that the combined supplementation with vitamin D (daily doses of 400-800 IU) and calcium (daily doses of 1000-1200 mg) was associated with a 6 per cent reduced risk of any fracture and a 16 per cent reduced risk of hip fracture.  Based on the included trials, it appears that the minimum effective dose of calcium is likely to be 1,000mg 1,200mg while vitamin D should not be below 800iu, which is twice the current winter ‘top up’ supplemental guideline for vitamin D alone. However, it is possible that the vitamin D dose given in most earlier trials was just too low.
The optimal daily dose of vitamin D is whatever gets your blood level above 75nmol/l (30ng/ml). You may need between 2,000iu and 5,000iu a day to achieve this. If your level is low, below 50nmol/l, having 100,000iu a week for four weeks, then 3,000iu a day (or the same dose times seven once a week, is more likely to get you there faster. Choose vitamin D3, which is more effective than D2. HOLFORDirect have a high strength vitamin D in the Essentials range.
It is best to take vitamin K2 especially if you are using these higher doses of vitamin D. This is because K2 helps keep calcium in bone and low calcium in the arteries.  My one concern with high dose vitamin D is it’s potential to increase calcium levels in the blood. High coronary artery calcium is a risk factor for heart disease.
A recent double-blind placebo-controlled trial showed that vitamin K, at a dose of 90mcg a day, but not 50mcg significantly reduced loss of bone density in post-menopausal women but not men.  I recommend a daily dose of at least this, or 100 to 200mcg, of vitamin K2, which is more effective than K1. Some supplements combine these. You may find vitamin K in your multivitamin. Vitamin K1 is found in brassica family of vegetables but then has to be converted to the more active K2 version, which is found in natto, a fermented soya product and in supplements of nattokinase.
Calcium balance in the body depends on many factors, not only vitamin D and K intake. A person with a relatively low intake of calcium, but none of the factors shown, may have better calcium status than someone apparently consuming enough calcium, but scoring high on the above factors.
You can read the Full Report Osteoporosis - the Skeleton in the Cupboard here.
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