Calcium: an emerging issue for developing countries?

Calcium Requirements and Calcium Deficiency

There is no consensus on the dietary intake needed to ensure calcium requirements are met. Table 19 compares current recommendations by FAO/WHO (FAO/WHO, 1962), the European Union (Commission of the European Communities, 1993) and USA/Canada (Food and Nutrition Board, 1997). Similar contrasts are seen between other advisory bodies. The disparities largely reflect the fact that no nutritional deficiency syndrome for calcium has been defined amongst otherwise healthy individuals, despite a wide variation in calcium intake. In addition, there are resulting differences in emphasis placed on outcomes of calcium balance studies, on estimates of calcium accretion and losses, and, more recently, on the putative relationship between calcium nutrition and osteoporosis.

Animal milks and their products are particularly rich sources of calcium in the human diet. Estimates based on recent FAO Food Balance Sheets show that there is considerable variation (400-fold range) in milk supply between different regions of the world.

Table 19: Selected Examples of Calcium Recommendations

mCa/d	USA/CanadaA	EUB	FAO/WHOC
Adults <50y	1000	700	400-500
Adults >50y	1200	700	400-500
Children 1-3 y	500	400	400-500
Boys 11-18 y	1300	1000	500-700
Girls 11-18 y	1300	800	500-700
Pregnancy women	1000	700	1000-1200
Breastfeeding women	100	1200	1000-1200

In each case, the figures are equivalent to the intake which is deemed sufficient to meet the needs of practically all members of the population, although the nomenclature differs:

A = adequate intake; B = population reference intake; C = practical allowance

There are foodstuffs in the human diet with a calcium content similar to or greater than that of milk. Examples include certain leaves, nuts, shellfish and small, bony fish. In addition, local culinary practices can lead to calcium-rich materials, such as plant ash, lime and herbs, being incorporated into food (Prentice and Bates, 1993). In industrialized countries, there is an increasing trend towards use of calcium supplements and for commercial calcium fortification of foods. However, it is rare for these food components to increase calcium intake to the extent attained with milk and, in general, populations with a low intake of milk and dairy produce have a low total calcium intake.

In a recent WHO/FAO/IAEA (WHO, 1996c) study, the range of intakes for adults was from 210 to 1650 mg/d, with a median value of 760 mg/d. Despite the limited data for many areas of the world, it is apparent that low levels of intake are mostly found in developing countries. Highest intakes are found in countries with diets rich in dairy products. Calcium intake of populations with a low milk supply is generally close to or less than the FAO/WHO practical allowance for adults of 400-500 mg/d and considerably below the 1000 mg/d US/Canadian reference for young adults.

Detailed quantitative studies in a rural Gambian village, typical of many in the sub-Sahelian region, have shown that, although calcium is supplied by a variety of foods and condiments, the overall calcium intake is low, at 300-400 mg/d, and only 7% comes from milk (Prentice et al., 1993; Jarjou et al., 1993).

Epidemiology of Osteoporosis

Osteoporosis is a crippling disease that affects many millions of people worldwide. It is characterized by loss of bony tissue from the skeleton and deterioration of bone structure, and is associated with enhanced skeletal fragility and an increased propensity to fracture under minimal trauma (Consensus Development Conference, 1991). Worldwide variation in the incidence and prevalence of osteoporosis is difficult to determine because of problems of diagnosis. The WHO definition of osteoporosis is a bone mineral content (BMC) or bone mineral density (BMD) that is > -2.5 SD below the young adult mean for the population (WHO, 1994). This is a useful working definition within populations, but it is unhelpful in comparing populations as both BMC and BMD are strongly influenced by body size. Populations of short stature have lower bone mineral status than Western populations but do not have higher rates of clinical osteoporosis (Aspray et al., 1996; Prentice et al., 1994; Russel-Aulet et al., 1993).

Because of this, the most useful comparison between populations is the fracture rate amongst older people. However, this is not without problems. Many fractures, including those of the spine and wrist, are not life-threatening, can be asymptomatic, and may not come to medical attention. Only population-based screening can accurately determine prevalence rates for these fractures, and few such studies have been conducted. Hip fractures are the exception and many countries have hip fracture registers that can be used to estimate incidence. Even so, quantitative data from many developing countries are scarce, and may be unreliable, given the lack of access to medical facilities by older people in these regions and the uncertainties of determining exact age and cause of fracture in those populations.

Source: Abelow, Holford and Insogna (1992)

Despite these caveats, the current consensus is that approximately 1.66 million hip fractures occur worldwide each year. The incidence is set to increase fourfold by the year 2050 because of the increasing numbers of older people. Age-adjusted incidence rates are many times higher in Western countries than in Asia and Sub-Saharan Africa (Abelow et al., 1992). Countries in developmental transition, such as Hong Kong, have seen significant increases in age-adjusted fracture rates in recent decades while Western countries appear largely to have reached a plateau (Lau and Cooper, 1996). In countries with high hip fracture incidence, rates are higher amongst women (up to fourfold) but in the countries where rates are low, men and women are more equally affected (Maggi et al., 1991).

Role of Calcium in Osteoporosis

The reason for this large variation in fracture incidence is unknown. Many theories abound, including effects at the genetic, anatomical, biochemical, nutritional and lifestyle level (Prentice, 1997). There is increasing evidence that the variation is not due specifically to differences in the deterioration of bone mineral mass, since bone loss at the menopause and low bone mineral status in old age appear to be universal phenomena (Aspray et al., 1996). Other aspects of bone health, such as turnover, microstructure and resilience, or propensity to fall, are likely to be more important factors.

Because calcium is a major bone-forming mineral, it has long been assumed that primary or secondary calcium deficiency must, in some way, underlie osteoporosis and fracture risk. The evidence that a low calcium intake is implicated in osteoporosis is, however, equivocal. On a worldwide basis, calcium intake cannot explain variation in osteoporosis since, paradoxically, those countries with a low calcium intake have low hip-fracture incidence, while the highest rates of fracture occur in those populations with a high calcium intake (Figure 7).

A recent meta-analysis relating fracture incidence to habitual calcium intake showed a 4% reduction in fracture risk associated with an increment of 300 mg/d in groups of postmenopausal women with mean calcium intakes varying from 168-700 mg/d (Cumming and Nevitt, 1997). Whether these results represent the correction of an underlying nutritional deficiency or are due to a pharmacological action of calcium by suppressing bone resorption is not known. It is also unclear whether the same effects can be achieved by dietary modification without the use of mineral (or vitamin D) supplements, or indeed whether calcium alone is always the limiting nutrient.

Calcium supplementation has been shown to increase bone mineral in children and adolescents. However, there is evidence that this is reversed once the supplement is withdrawn and that calcium supplementation is associated, as with the elderly, with a decreased bone turnover. Whether this represents a benefit for the growing skeleton is uncertain (reviewed by Prentice, 1995).

Also unknown is whether the results of studies conducted in Western populations, where osteoporosis is common, are relevant to developing countries, where fracture rates are low. Recent studies in China, the Gambia and Hong Kong have shown positive effects of calcium supplements on bone mineral of older women and of children and adolescents (Lee et al., 1994; Dibba et al., 1997; Lau et al., 1992) but not on bone mineral status or breastmilk calcium secretion of breastfeeding women (Prentice et al., 1995). The results were similar to those obtained in studies conducted in Australia, the UK and the USA both in the skeletal regions that responded and in the magnitude of the effects. This suggests there are no fundamental differences in calcium biology between people in developed and developing countries.