They're touted as an insurance policy against osteoporosis, but which offers the best payoff?
We almost learn it at our mother's knee: calcium is good for our bones and teeth, and we must drink lots of milk to give us enough calcium to grow up to be strong, healthy adults. For years, this health message has been an article of faith. However, recent evidence is beginning to challenge the value of milk - certainly in later life (see box, p 2).
Calcium is the most common mineral in the human body - 99 per cent of it is in the bones and teeth, where it plays a structural role. The remaining 1 per cent is in body tissues and fluids, where it is essential for cell metabolism, blood-clotting, muscle contraction and nerve-impulse transmission.
However, the amount of calcium in the bones is not fixed. There is a continuous exchange of calcium between the bones and the bloodstream to maintain adequate levels in each. This process is controlled by hormones. Vitamin D and magnesium are also involved as they help calcium in the blood become reabsorbed into the bones.
By and large, the body regulates its own calcium levels, maintaining homoeostasis (a normal, balanced state) by compensating for any excess or shortage by changing calcium absorption rates.
Calcium is lost from the body in the usual processes of waste elimination; much more is lost during breastfeeding. Natural sources of calcium are dairy products, eggs, nuts, seeds, dried fruit and leafy vegetables.
Official recommendations for an ideal daily calcium intake have been somewhat volatile in recent years as opinions change about calcium's importance. In fact, a recent scientific review all but described the current Recommended Daily Allowance guidelines as a mess, complaining of 'the lack of quantitative data on which to base estimates of average calcium requirements' (Calcif Tissue Int, 2002; 70: 83-8).
The standard adult RDA was 700 mg a day until 1997, when the US National Academy of Sciences Institute of Medicine substantially increased it to 1200 mg. They also recommended 200-500 mg a day for young children, rising to about 1300 mg for adolescents and those over 50. The dose is even higher for breastfeeding and adolescent mothers.
Average daily intakes of calcium from foods are in the 500-800 mg range so, if the latest Dietary Reference Intakes are to be believed, a normal adult diet will not provide enough calcium.
Calcium supplements, therefore, appear to be necessary. However, here again, there is confusion as to how much calcium to advise people to take. The reason for this is apparently because the basic experimental science simply hasn't been adequately done (J Int Med Res, 1999; 27: 1-14).
The picture becomes even more complicated when assessing the evidence for the benefits of calcium. Least controversial are the claims that low calcium is associated with colon cancer, high blood pressure and premenstrual syndrome (Am J Obstet Gynecol, 1999; 181: 1560-9).
Colon cancer, in particular, has been shown to be less common in people with high calcium intakes. This has been recently confirmed experimentally. People at high risk of colon cancer were given a daily dose of 900 mg of calcium carbonate, which was found to significantly reduce the incidence of cancerous polyps in the intestinal tract (Nutr Cancer, 2001; 41: 150-5).
Also non-controversial is the relationship between calcium intake and dental health - in both childhood and old age. A recent placebo-controlled study showed that old people lost fewer teeth while taking a daily 1000-mg calcium supplement for three years, a benefit that persisted even after the supplements were stopped (Am J Med, 2001; 111: 452-6).
Calcium's major role, however, is not in teeth, but in the bones, and this is where the evidence becomes complicated.
The major medical problem involving the skeleton is the condition called osteoporosis, popularly known as 'brittle bone disease'. It causes the vast majority of bone fractures in the elderly. It's not easy to diagnose; typically, the first sign of it will be a fracture after a fall which a younger adult would take in his stride.
Such fractures often occur just above the wrist and at the top of the thigh bone. In more advanced cases, the spinal vertebrae may crumble, causing both pain due to compression of the spinal nerves and a progressive loss of height.
Healthy bone consists of a hard outer shell with softer, spongy bone cells underneath. Its principal components are collagen, a protein that gives bones their elasticity, and calcium, which provides their hardness and strength.
Osteoporosis is mainly caused by the loss of collagen which also takes away calcium from both spongy and hard bone, causing it to become less dense and more brittle. Bone size, though, is not affected.
Gradual loss of bone density seems to be an inevitable part of the ageing process. Both men and women start losing bone density from age 30 but, over the years, women become much worse off than men. By age 70, women on average lose 25 per cent of their bone density whereas men only lose half that.
It is thought that women are more severely affected because of a progressive loss of oestrogen, particularly after the menopause. One indication that oestrogen is the culprit is that bone loss accelerates if the ovaries no longer function, for example, after a hysterectomy. However, the fact that not all postmenopausal women develop osteoporosis suggests that oestrogen loss is not the only factor (Am J Obstet Gynecol, 1987; 156: 1342-6).
Osteoporosis appears to have reached pandemic proportions, affecting one in three women and one in 12 men, making it one of the West's most common diseases. In Europe, it's estimated to cause nearly 400,000 hip fractures a year (Publ Health Nutr, 2001; 4: 547-59).
As a result, taking calcium supplements has skyrocketed - particularly among women. A recent US survey discovered that nearly 70 per cent of elderly women now supplement with calcium to avoid osteoporosis (Osteoporos Int, 2002; 13: 657-62). Calcium supplement sales are described as 'big business', with more than a sevenfold increase in the early 1980s, representing then a $130 million business (Am Diet Assoc, 1989; 89: 397-400).
Money well spent?
It seems axiomatic that the more calcium in the diet, the more calcium in the bones and, by the same token, the more calcium in the bones, the stronger the bones and the less risk of osteoporosis. The logic appears unassailable and, until recently, was almost a universally acknowledged conclusion.
However, hard evidence has been hard to come by. A recent US survey of middle-aged Mexican women found no association between dietary calcium and bone strength, measured by bone mineral density (BMD) (Osteoporos Int, 1997; 7: 533-8). Adolescent girls were also assessed by calcium intake, but again with no correlation to their BMD - not even with intakes as high as 1500 mg/day (Pediatrics, 2000; 106: 40-4).
A study of over a hundred women aged 23-84 could find no link between calcium in the diet and BMD. The researchers concluded: 'These data do not support the hypothesis that insufficient dietary calcium is a major cause of bone loss in women' (J Clin Invest, 1987; 80: 979-82).
So, in light of such evidence, are calcium supplements of any use? The answer is yes and no. Evidence that they're useless comes from various studies. In one, women aged about 40 taking calcium supplements were monitored for four years. Despite using two different measures of bone density, no evidence could be found that calcium arrested the normal decline in bone strength. 'Premenopausal women in the fifth decade lose about 1 per cent of [bone] mineral yearly, in spite of . . . ample calcium intake,' reported the researchers (Osteoporos Int, 1995; 5: 228-33).
So, supplemental calcium before the menopause appears to be ineffective, but the 'change of life' changes everything. About 40 trials have investigated the benefits of calcium postmenopause. One study pooled them together and concluded that, although the results were not clear-cut, the weight of evidence suggested that supplementation is valuable.
In these studies, calcium effectiveness was measured by crude numbers of bone fractures. By and large, increased calcium intake correlated with fewer fractures. In four randomised trials, more than 1000 mg of calcium a day reduced fractures by up to 70 per cent (J Bone Miner Res, 1997; 12: 1321-9). A later study of over a hundred 60-year-old Swedish women revealed that only the high calcium intakes - in a range of 1417-2417 mg/day - had any effect on BMD and, therefore, the severity of osteoporosis (Osteoporos Int, 1997; 7: 155-61).
So, calcium supplements appear to help keep osteoporosis at bay after the menopause as long as total calcium intake is at least 1400 mg a day. Given that women of that age already consume about 500-600 mg of calcium in their diet, this suggests that a daily supplement of not less than 800 mg is advisable (J Int Med Res, 1999; 27: 1-14).
How should calcium supplements be taken? Experiments show that maximum absorption occurs when calcium is taken four times a day with meals, rather than all at once, with up to 2500 mg a day considered safe (Osteoporos Int, 1991; 1: 65-71).
What are the best kinds of calcium to take? Like other minerals, calcium cannot be absorbed in its pure mineral form; it has to be chelated with an acid. The most common form of chelated calcium is calcium carbonate (common chalk), which is cheap. But a minor disadvantage is that it can only be broken down by a healthy concentration of stomach acids, which tend to decrease with age. So there's a small risk that the very people most likely to benefit from extra calcium may be least able to absorb it.
Other forms of calcium claim to be much more bioavailable. The most common of these is calcium citrate, which one study showed is up to 2.5 times better absorbed than calcium carbonate (J Clin Pharmacol, 1999; 39: 1151-4).
However, the special value of citrate has been challenged. Trials at the Osteoporosis Research Center in the US concluded that 'when taken with food, calcium from the carbonate salt is fully as absorbable as from the citrate' (Osteoporos Int, 1999; 9: 19-23). The researchers claimed that the study favouring citrate had measured the wrong parameters (J Nutr, 2001; 131: 1344S-8S).
Another way to increase calcium uptake is to combine it with nutrients such as vitamins A, D and K, magnesium and boron. The most important is vitamin D as calcium cannot be metabolised without it. The official advice is that vitamin D should be taken at the same time as calcium supplements (J Am Diet Assoc, 2002; 102: 818-25). Adding amounts of vitamin D as low as 3 mcg (120 IU) to calcium supplements can double the uptake of calcium (J Clin Endocrinol Metab, 1997; 82: 4111-6).
Certainly, a vitamin D-calcium combination has proved effective in clinical trials. A recent study of 80-year-old women showed that 1200 mg calcium a day, taken with 800 IU of vitamin D3, arrested normal bone mineral loss and significantly reduced the risk of fracture (Osteoporos Int, 2002; 13: 257-64).
Magnesium is also an important adjunct to calcium as it is vitally involved in calcium deposition in bones. Not surprisingly, therefore, magnesium deficiency itself is a cause of low bone mineral density.
In fact, it may be an even more powerful factor in osteoporosis than calcium, as it can reverse the effects of ageing. Of more than 30 women taking 250-750 mg of magnesium a day for two years, nearly three-quarters showed bone density increases of up to 8 per cent while those in a control group lost 1-3 per cent, the standard age-related rate of bone mineral loss (Magnes Res, 1993; 6: 155-63).
For our road test of calcium supplement products, we asked the analytical laboratory to simply assess calcium content. But, in working out the best value for money, we have also taken into account the bioavailability of the calcium in these products.
Price: lb6.19 for 60 600-mg tablets
A generous-sized and -priced product, this delivers much more than it claims - 650 mg of calcium per tablet, or a total bottle content of 39,000 mg. Solgar has also included vitamin D and magnesium, increasing calcium absorbability, making it even better value for money. The cost is low - a modest 16 p per 1000 mg. This product is far and away the leader of the pack in terms of both quality and price.
Calcium 250 mg
Manufacturer: Nature's Plus
Price: lb8.65 for 90 250-mg tablets
This also delivers more than it claims, with each tablet containing 329 mg calcium, plus magnesium to help its absorption. The cost per 1000 mg is a reasonable 29 p.
Price: lb7.25 for 90 100-mg capsules
These capsules offer 110 mg of calcium, so the whole bottle delivers just under 10,000 mg in total. The fact that the calcium is in citrate form significantly adds to the cost of manufacture but, according to the latest research, does not increase bioavailability.
The cost per 1000 mg is 73 p.
Price: lb12.70 for 90 150-mg capsules
This is another product providing the expensive citrate form, which research now shows may only become more bioavailable than carbonate on a totally empty stomach. So, in practice, it has no advantages. This product will cost you 95 p per 1000 mg.
High Strength 500mg
Price: lb5.99 for 60 100-mg capsules
The label boldly declares that these capsules contain 500 mg of calcium but, if you scrutinise the small print on the label, you will find that the true (elemental) calcium content is 100 mg - a figure confirmed by our lab. This product works out at 91 p per 1000 mg, but it loses a point because of the misleading label.
Price: lb7.35 for 60 30-mg tablets
The terms 'elemental' and 'chelated' used here sound impressive, but the first simply means the basic calcium content, and the second means that the calcium is bound to an acid - as, indeed, all mineral supplements have to be. Although these 30-mg tablets weigh in at 52 mg, they're still poor value for money at lb2.36 per 1000 mg.
Calcium 50mg (as Orotate)
Price: lb12.95 for 90 50-mg tablets
Lamberts is a top-notch manufacturer with a reputation for high-quality, well-researched products. Calcium orotate is a relatively new formulation derived from chelation of calcium with orotic acid. Orotate is much more absorbable than the carbonate form, says Lamberts, based on its own research. But the evidence hasn't been published yet, so the claim is difficult to assess.
In any case, however bioavailable these may be, the amount of basic calcium delivered is very low. Lamberts says it's meant for children, but this is nowhere stated on the label. For 1000 mg of this, you pay lb2.82.
Food State Calcium 30mg Elemental
Manufacturer: Nature's Own
Price: lb4.75 for 50 30-mg tablets
In this product, our lab found not 30 mg per tablet, but 46 mg. But, even so, this offering is not good value - given its cost of lb2.07 per 1000 mg.
NDS Calcium 65mg
Manufacturer: NDS Healthcare
Price: lb6.95 for 42 65-mg tablets
Despite the added magnesium and Lactobacillus bulgaricus probiotics, both of which may improve bioavailability, this product scores poorly. Each tablet only contains 52 mg of calcium - not what's claimed on the label - at a whopping lb3.18 per 1000 mg.
Milk, because it is rich in calcium, is often claimed to prevent osteoporosis, yet clinical research surprisingly shows otherwise.
The Harvard Nurses' Health Study, which followed more than 77,000 American women for 12 years, showed no protective effect with increased milk consumption on the risk of bone fracture - in fact, quite the reverse. The women who drank three glasses of milk a day actually had more fractures than those who rarely drank it (Am J Publ Health, 1997; 87: 992-7). This finding was replicated in a later study of women over 50, where again 'dairy-product intake was significantly associated with hip fracture' (Psychol Rep, 1999; 85: 423-30).
The Harvard researchers found the same results, in general, with over 43,000 male physicians although, in this group, the risk of osteoporosis did not increase with higher milk consumption (J Nutr, 1997; 127: 1782-7). The research team suggests that one explanation for these puzzling, apparently paradoxical, findings may be that the proteins in milk, when in the bloodstream, cause the body to release calcium from bone to neutralise the extra acidity, thereby causing a loss of bone density.
Some confirmation of this theory has come from vegetarians, who tend to have relatively low blood-protein levels. One study showed that the average bone density of 70-79-year-old vegetarians was greater than non-vegetarians who were 20 years younger (Am J Clin Nutr, 1972; 25: 555-8). Thus, low protein levels appear to be correlated with a lower risk of osteoporosis - and milk comprises 3 per cent protein.
On the other hand, calcium uptake from milk is higher than from other food sources because milk contains lactose, which improves the bioavailability of calcium (J Nutr, 1999; 129: 9-12).
There is other, intriguing evidence from international surveys of milk consumption. Osteoporosis is more common in Europe and North America, where people consume large amounts of milk products, than in African countries, where people consume almost none. The explanation is unlikely to be genetic as people of African descent living in the US have more osteoporosis than their African counterparts.
Clearly, however, other dietary factors than milk may also be involved.