The challenges of calcium

Fortifying foods with functional levels of calcium requires due diligence on therapeutics, bioavailability and especially on the interactions of the food or beverage matrix on the calcium salt. Researcher Robert P Heaney, MD, explores the issues to consider when working with this beneficial ingredient

It is now official US nutritional policy that calcium intakes higher than those provided by typical American diets are necessary to ensure the health of the American population. Two NIH Consensus Development Conference reports,1,2 the DRIs for Calcium and Related Nutrients from the Institute of Medicine,3 the Surgeon General?s Report on Osteoporosis,4 and the Dietary Guidelines for Americans 20055 all come to the conclusion that calcium intakes should be in the range of 1,000-1,500mg/day for all adults.

By contrast, median calcium intakes for both sexes are below 800mg/day for most adults, and below 600mg/day for older women.5 Moreover, the recent Surgeon General?s Report stated explicitly: ?Calcium has been singled out as a major public health concern today because it is critically important to bone health, and the average American consumes levels of calcium that are far below the amount recommended for optimal bone health.?

And bones are not the only health issue at stake here. Optimal calcium intake is beneficial for a multitude of body systems. Diseases related to inadequate calcium intake include not only osteoporosis, but colon cancer, kidney stones, obesity, hypertension and preeclampsia, among others.6 Our recent published analysis of cost savings associated with augmented calcium intake estimates five-year direct health cost reductions for nine diseases in excess of $200 billion.6

For all these reasons, therefore, it is not surprising that food fortification with calcium has become an attractive option. Partly from a genuine interest in providing good nutrition and partly from competitive pressures, food manufacturers have produced a large variety of calcium-fortified products. Indeed, their doing so was formally invited in an earlier Surgeon General?s Report on Nutrition.7

Foods generally — and that includes calcium added to foods — are regulated by various federal agencies (FDA, Agriculture, Interior), but these controls are primarily with regard to purity and safety. There are few or no requirements that fortification be ?effective? — that an added nutrient deliver what it purports to offer. Nor is there any formal recognition that added calcium might vary in effectiveness. Calcium is implicitly considered to be simply calcium. Hence manufacturers, choosing to fortify, have had little reason or incentive to ensure that added nutrients actually contribute to nutrition — that is, that the additional calcium added to food actually was in amounts and forms that were significant and easily absorbed and utilised by the body.

Instead, many have simply added one or another form of calcium, and, acting out of what may perhaps be called ?milk envy,? have chosen a level per serving comparable to the calcium content of milk, often claiming explicitly that the product concerned provided as much calcium as a glass of milk. That approach can lead to problems, not least of which is that it has sometimes resulted in marketplace failures.

Such was the case in the 1990s for the first calcium-fortified bread, Wonder Calcium, with 300mg calcium (as the sulfate) per slice. A good idea — however, the form and amount chosen added measurably to the weight of the bread! Continental Baking chose, because of the high level of added calcium, to launch it as a separate product, competing with older, established Wonder varieties. This was a failure.

By contrast, General Mills, choosing for several of its ready-to-eat cereals a more modest level of fortification (about 100mg calcium/serving as the carbonate), has produced ?new and improved? varieties, simply replacing the unenriched versions with enriched.

Because both manufacturers had been careful to ensure good bioavailability of the calcium they added, nutrition was not a concern. The difference between these examples is one of marketing strategies — one successful (General Mills), the other not (Continental Baking). But the contrast at least raises the question of the optimal level of fortification. Is 300mg calcium per serving feasible for all products? Why emulate milk? Marketing issues aside, it would seem most reasonable to fortify to a level that 1) realistically improves nutrition; 2) does not impair the organoleptic properties of the food; 3) is shelf stable; 4) minimises retooling or reconfiguring of the manufacturing process; and 5) takes into account the typical number of servings that will be consumed daily.

A calcium dose of 100mg per serving satisfies the first criterion since, for otherwise wholesome foods, it triggers the possibility of a ?good source of calcium? claim on the label. The other three criteria will vary with the salt used to fortify and with the food matrix into which it is incorporated. All of the first four criteria, however, presume that the salt selected and the means of its incorporation into the foods do not substantially alter the absorbability of the added calcium. That presumption is often incorrect.

What are the facts about calcium absorbability? At first blush, it might seem that calcium salts, per se, would exhibit a range of differing intrinsic absorbabilities. However, there is actually relatively little difference in absorbability between the more commonly used salts when tested by themselves. Even calcium from insoluble salts is absorbed normally in the absence of stomach acid so long as the calcium is ingested with (or incorporated into) food. More to the point, solubility has virtually no effect.8 In fact, some of the more soluble calcium salts are substantially less well absorbed than quite insoluble preparations (eg, calcium glycero-phosphate vs calcium carbonate).

A more important consideration is the food or supplement matrix and its interaction with the fortificant. We have only begun to scratch the surface here, but there are a few specifics that can be mentioned. For example, foods high in phytate or oxalate are likely to degrade the absorbability of any added calcium. But aside from a few such well-studied interactions, there is no general body of theory that allows a manufacturer to predict absorbability for any given combination, and surprises are common.

Early on in its development of its patented citrate-malate (CCM) technology, Procter and Gamble reported that CCM was well absorbed from orange and grapefruit juices, but poorly absorbed from lemon juice. This could not have been anticipated. Hence, for the moment, absorbability of the calcium added to a product can be securely established only by actual testing, though a method for screening does exist that may be helpful for fluid products (See "Screening of calcium-fortified beverages," below).

A variety of relatively inexpensive calcium salts is available. Principal among them are the calcium salts of carbonate, phosphate, citrate, lactate, sulfate, glycero-phosphate and CCM. Each has distinctive properties and will affect (or be affected by) different food matrices in ways that can only be assessed by testing (both of absorbability and of consumer taste preference). Common to all these calcium sources is a feature calcium shares with most divalent cations: It is relatively poorly absorbed.

Gross absorption efficiency at food calcium loads ingested in the neighbourhood of 300mg is in the range of 30-40 per cent in healthy young but mature adults (and net absorption is only 10-20 per cent).9 Many attempts have been made by food and supplement technologists to improve that absorption efficiency by various additives, but the figure stubbornly resists bumping up. Indeed, there is serious reason to question the desirability of doing so, since the benefits of calcium in prevention of colon cancer and kidney stones, for example, are produced precisely by the unabsorbed calcium that remains in the lumen as the food residue travels through the intestine.

Many attempts have been made to improve absorption efficiency by various additives
Nevertheless, calcium is generally added to most or all foods precisely so that some of it can be absorbed. The benefits of calcium for sensible body weight (See "The link between calcium and obesity," below) and bone health — perhaps the two principal health targets of current fortification efforts — manifestly require that calcium be absorbed. Thus, absorbability remains an important consideration, and especially so during growth. Also, for products fortifying to a level comparable to that of a glass of cows? milk, the implicit assumption is that absorbability of the added calcium will itself be comparable to that of cows? milk. Indeed, there is a trust issue involved, since the consumer will reasonably expect comparable health benefits from comparable intakes.

Figure 1 illustrates several points related to the foregoing matters. First, it displays per cent absorption for pure calcium carbonate as a function of ingested load, ranging from 100 to 1,000mg (regression line at top of graph). It also shows both the observed decline in absorption efficiency with increasing load size (a feature of most poorly absorbed substances) and the predicted gross absorbability for pure calcium carbonate at a 300mg load (ie, nearly 40 per cent).

Also included in Figure 1 are mean absorption percentage values for several tests of calcium carbonate supplement preparations performed in the author?s laboratory, using the identical calcium carbonate, but combining it with various excipients, flavourings and coatings to produce a variety of calcium supplement tablet formulations in several common dosage sizes.

As the figure shows graphically, some of these supplements exhibited the absorbability predicted for their calcium content, but others suffered substantial impairment of absorbability. Somehow, the pharmaceutical process and the added ingredients had degraded absorbability — in two of the preparations by 50 per cent or more. Ironically, in several instances, the formulations were experimental and had been designed by their manufacturers to augment absorbability — and the opposite turned out to be the case.

Limited experience with food suggests that matrix interactions with the added salt are similar in effect to those just described for supplement tablets. For example, tricalcium phosphate added to yoghurt exhibits close to expected absorbability, while the same salt, added to a soy beverage, is substantially less well absorbed.10Possibly soy phytate is an explanation, but that is not certain.

As these examples illustrate, there is currently no good substitute for trial-and-error testing of fortified products. Consumers are increasingly being educated to ask if the added calcium is available, and food manufacturers owe it to them to provide evidence that their products do what the consumer ought reasonably to expect.

Robert P Heaney, MD, is an endocrinologist and professor at Creighton University in Nebraska who has worked for 50 years in the field of calcium and bone biology. He has 153 scientific papers in peer-reviewed literature, and his lab has performed most of the bioavailability testing for food and supplements manufacturers in North America.
Respond: [email protected]
All correspondence will be forwarded to the author.

Screening of calcium-fortified beverages
Most of the calcium added to various beverages is in particulate form, since fluid volume of a typical serving is rarely large enough to dissolve nutritionally significant quantities of most calcium salts.

Such beverages can be screened by adding at the bench a suitable calcium tracer (eg, one of the calcium isotopes or possibly even strontium), incubating for several hours, and then separating the dissolved and solid-phase calcium by low-speed centrifugation.1,2

The proportions of the total calcium in the solid and fluid phases provide a good measure of the effect of the other beverage constituents on solubilisation and suspension of the fortificant, and the partition of the calcium tracer between the phases is a measure of the physical ?reactivity? of the suspended fortificant particles. Ideally, the tracer should exchange actively between the fluid and solid phases, and hence partition between the two phases in the same proportion as the carrier calcium.

Unfortunately, high reactivity does not by itself guarantee good absorption. A highly soluble salt, such as calcium glycero-phosphate, will score well in this test; however, its actual absorbability is less than that of the relatively quite insoluble calcium carbonate.

Nevertheless, poor reactivity, especially when combined with a high particulate fraction, almost surely means poor absorbability. Using such an approach, manufacturers can exclude unpromising formulations prior to incurring the expense of formal human absorbability testing.


1. Heaney RP, Dowell MS, et al. Bioavailability of the calcium in fortified soy imitation milk, with some observations on method. Am J Clin Nutr 2000; 71:1166-1169.
2. Heaney RP, Rafferty K, et al. Not all calcium-fortified beverages are equal. Nutr Today 2005; 40(1):39-44.

The link between calcium and obesity
A large body of epidemiological data1,2 indicates that high calcium intakes are associated with decreased risk of obesity. The effect is relatively small, but highly consistent across many types of studies. In population-based studies, observed obesity risk is from 30-85 per cent lower for individuals with calcium intakes in the highest quantiles (usually above 1,100mg/day) than those in the lowest quantiles.3 Additionally, several randomised controlled trials have shown that high calcium intakes — particularly high dairy intakes — substantially augment both total weight loss and trunk fat loss — and preserve lean body mass as well — when incorporated into energy-restricted, weight-loss regimens.

In the aggregate, the evidence indicates that from 3-10 per cent of the variability in adult weight is due to variations in calcium intake (probably closer to the lower end of that range),4 and average weight differences between adult populations with low and high calcium intakes are probably no more than 15-16 pounds. However, that is enough, at a population level, to affect the prevalence of obesity substantially.5

Given the burgeoning epidemic of obesity in this country, it would seem important to take such steps to control the problem as may be within our power. One of these is to ensure adequate calcium intake across the population. Calcium-fortified foods with good absorbability characteristics are feasible and inexpensive and may be a useful public health strategy.


1. Parikh SJ, Yanovski JA. Calcium intake and adiposity. Am J Clin Nutr 2003; 77:281-287.
2. Zemel MB, Miller SL. Dietary calcium and dairy modulation of adiposity and obesity risk. Nutr Rev 2004; 62:125-131.
3. Zemel MB, Shi H, et al. Regulation of adiposity by dietary calcium. FASEB J 2000; 14:1132-1138.
4. Heaney RP, Davies KM, et al. Calcium and weight: clinical studies. J Am Coll Nutr 2002; 21(2):152S-155S.
5. Heaney RP. Normalizing calcium intake: projected population effects on body weight. J Nutr 2003; 133:268S-270S.

Per cent absorption of calcium plotted as a function of the size of the ingested load. The regression line and its confidence intervals at the top of the graph represent the values found in more than 100 measurements of calcium carbonate ingested at various loads. Hence they reflect the predicted absorption for this salt. The horizontal axis is logarithmic. The symbols (? 1 SEM) are mean values for 10 tests of specific calcium carbonate supplements in their marketed formulations, each involving 16-30 subjects. Three of these products fell in or close to the predicted value for their loads; seven were substantially below predicted.

(Copyright Robert P Heaney, 2005. Used with permission.)

1. Consensus Conference on Osteoporosis. JAMA 1984; 252:799-802.
2. Consensus Conference on Optimal Calcium Intake. JAMA 1994; 272:1942-8.
3. Dietary Reference Intakes for Calcium, Magnesium, Phosphorus, Vitamin D, and Fluoride. Food and Nutrition Board, Institute of Medicine. National Academy Press, Washington, DC, 1997.
4. Bone Health and Osteoporosis: A Report of the Surgeon General. DHHS (PHS), 2004.
5. Nutrition and Your Health: Dietary Guidelines for Americans, 2005. USDA.
6. McCarron DA, Heaney RP. Estimated healthcare savings associated with adequate dairy food intake. Am J Hypertension 2004; 17:88-97.
7. The Surgeon General?s Report on Nutrition and Health. DHHS (PHS), 1988.
8. Heaney RP, Recker RR, et al. Absorbability of calcium sources: the limited role of solubility. Calcif Tissue Int 1990; 46:300-304.
9. Heaney RP. Vitamin D: Role in the calcium economy. In: Vitamin D, 2nd edition, Volume 1, pp. 773-787. Feldman D, Glorieux FH, Pike JW, eds. Academic Press, San Diego, CA, 2005.
10. Heaney RP, Dowell MS, et al. Bioavailability of the calcium in fortified soy imitation milk, with some observations on method. Am J Clin Nutr 2000; 71:1166-1169.

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