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14 Calcium
14.1 Introduction
Calcium is the most abundant mineral in the body and makes up 1.9% of the body by weight. Nearly all (99%) of this is in the skeleton. The remainder is in the teeth (0.6%), the soft tissues (0.6%), the plasma (0.03%) and the extracellular fluid (0.06%). Calcium provides a “structural role” in providing rigidity (structure and strength) to the skeleton. This function is provided by a form of calcium phosphate that is generally known as hydroxyapatite [Ca (^) 10(OH)2(PO4) (^) 6] crystals which are embedded in collagen fibrils (Nordin, 1997).
Calcium ions on the surface of bone can interact with ions in body fluids and act like a large ion exchanger. These properties are important in relation to the role of bone as a reserve of calcium to help maintain a constant concentration of blood calcium (Gurr, 1999). Blood calcium plays an important role to regulate vital body processes such as blood coagulation, muscle contraction, nerve transmission and mediation of some hormonal actions across cell membranes.
14.2 Food sources
Besides milk and dairy products, other calcium-rich foods in the Malaysian diet are fish with edible bones such as canned sardines and anchovies, beans and bean products including yellow dhal, tofu and tempeh (fermented soybeans), locally processed foods such as shrimp paste, cincaluk and budu , as well as vegetables like spinach, watercress, mustard leaves, cekur manis , tapioca leaves, kai-lan and broccoli (Tee et al ., 1997). Currently, food manufacturers in Malaysia have also made available in the market calcium fortified products such as high-calcium milk, yogurt, breakfast cereals, biscuits and even rice.
According to IOM (1997), when evaluating the food sources of calcium, the calcium content is generally of greater importance than bioavailability. Calcium absorption efficiency is fairly similar from most foods, including milk and milk products and grains. Bioavailability of calcium from plant foods however can be affected by calcium chelators such as oxalate and phytate. Oxalic acids are found in high amounts in plant foods such as spinach, chocolate or cocoa products and in lesser quantities in dried beans, sweet potato, tea infusion, wheat germ, kale, okra and soybean products. However, a clinical study in humans has shown that calcium absorption from low-oxalate high calcium dark green vegetables from the kale is comparable to milk (Heaney & Weaver 1990). The authors concluded that the bioavailability from other Brassica family vegetables such as broccoli, mustard green, Chinese kale ( kai lan ) and cabbage can be considered as good as milk.
Phytate, the storage form of phosphorous in seeds, such as soybeans and pulses, is a modest inhibitor of calcium absorption. The phytic acid content of seeds depends on the phosphorous content of the soil of which the seeds are grown. Calcium absorption from low phytate soybeans have been shown to be similar with milk (Heaney, Weaver &
Fitzsimmons, 1991). The difference in absorption fraction from low and high phytate soybeans was reported to be 25%. This magnitude of difference may be important to individuals who consume soy products and no dairy foods as principle source of calcium. Other concentrated sources of phytate such as wheat bran or dried beans also substantially reduce calcium absorption.
Table 14.1 shows food sources of bioavailable calcium and their fractional absorption based on experiments carried out in Caucasians. Even though plant foods from the Brassica family such as broccoli, cabbage and kale have high fractional calcium absorption, milk still has the highest amount of absorbable calcium per serving compared to these foods. Thus, adding milk and milk products such as yogurt and cheese in an individual’s diet makes meeting calcium requirements easier.
14.3 Deficiencies
Inadequate intake, poor calcium absorption and excessive calcium losses contribute to reduced mineralization of bone. For example, in rickets and osteomalacia, vitamin D deficiency causes poor absorption of calcium and reduced mineralization of bone resulting in soft, pliable bones that deform easily. A reduction in absorbed calcium causes serum ionized calcium concentration to decline. This stimulates the parathyroid hormone (PTH) that will act in one of three ways to increase and maintain the level of serum calcium. The parathyroid hormone can increase the production of calcitriol (1,25- dihydroxycholecalciferol), which in turn increases calcium absorption through active transport in the gut and tubular reabsorption in the kidneys. Bone resorption may also
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Table 14.1 Food sources of bioavailable calcium Food Serving Calcium Fractional Estimated Servings size content absorption absorbable needed to (g) (mg) (%) calcium/serving equal 1 (mg) glass of milk Milk 240 300 32.1 96.3 1. (or 1 glass yogurt or 1.5 oz cheddar cheese) Beans, dried 177 50 15.6 7.8 12. Broccoli 71 35 61.3 21.5 4. Cabbage 85 79 52.7 41.6 2. Kale 65 47 58.8 27.6 3. Spinach 90 122 5.1 6.2 15. Tofu , calcium set 126 258 31.0 80 1. Source: Weaver & Heaney (1999)
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variability of Ca absorptive capacity. Nonetheless, a large part of the variability remains unexplained. Dietary components that play a role in inhibiting calcium absorption include phytic acid from cereals and pulses, and oxalic acid from some varieties of vegetables. In contrast, dietary components that enhance calcium absorption include lactose and non-digestible polysaccharides (van den Heuval et al., 1999).
Dietary phosphorus
Phosphorus is a major structural component of bone in the form of calcium phosphate or hydroxyapatite. The regulation of blood calcium and phosphorus levels is interrelated through the actions of parathyroid hormone (PTH) and vitamin D. A decrease in the blood calcium levels, such as in the case of inadequate calcium intake, leads to a response by calcium-sensing proteins in the parathyroid glands, which secrete PTH. PTH in turn stimulates increased conversion of vitamin D to its biologically active form (calcitriol) in the kidneys. Calcitriol activates the vitamin D-dependent transport system in the small intestine, increasing the absorption of both dietary calcium and phosphorus. Although PTH stimulation results in decreased urinary excretion of calcium, it results in increased urinary excretion of phosphorus. Increased urinary excretion of phosphorus is beneficial because it helps to bring blood calcium levels up to normal, and also because high blood levels of phosphorus suppress the conversion of vitamin D to its active form in the kidneys.
Concerns for phosphorus intake have arisen due to presence of phosphoric acid in soft drinks and phosphate additives in a number of commercially prepared foods (Calvo & Park, 1996). Unlike calcium, phosphorus absorption is not as tightly regulated by the body. Hence, high dietary intake phosphorus gives rise to high blood phosphate levels which in turn, reduce the formation of calcitriol in the kidneys, reduce blood calcium and lead to elevated levels of PTH that may be detrimental on bone mineral content. Abnormally low Ca:P ratio (1:6) has been shown to cause bone loss in animal studies, and there have been reports of excessive fracture rates and low bone density in children with a history of high intake of phosphate-containing coal beverages (Weaver and Heaney, 1999). The concern is also for drinking of soft drinks displacing calcium- containing beverages including milk.
Sodium, protein and caffeine interactions
Sodium intake is an important determinant of calcium excretion because sodium competes with calcium reabsorption in the renal tubules as they share the same transport system in the proximal tubules. (Nordin, 1997). High sodium chloride intake results in increased absorbed sodium, increased urinary soldium and an increased obligatory loss of urinary calcium. Although indirect evidence indicates that dietary sodium chloride has a negative effect on the skeleton, the effect of a change in sodium intake on bone loss and fracture rates has not been reported. Thus, IOM (1997) felt that available evidence does not warrant different calcium intake requirements for individuals according to their salt consumption.
Animal protein has the same effect as sodium as its acid component is able to reduce tubular reabsorption of calcium. It is also likely that animal protein yields sulphate and phosphate residues which tie up calcium as complexes in the renal tubules (Nordin, 1997). It is estimated that for every gram of protein metabolized, urinary excretion of calcium increases by 50% or 0.025 mmol calcium taken out. Nevertheless, while protein intake appear to increase urinary calcium excretion, the effect of protein on calcium retention is controversial. IOM (1997) also pointed out that inadequate protein intakes (34 g/day) have been associated with poor general health and poor recovery from osteoporotic hip fractures. It was thus felt that available evidence does not warrant adjusting calcium intake recommendations based on dietary protein intake.
Caffeine intake in large amounts acutely increases urinary calcium losses in adults and postmenopausal women. While moderate consumption of coffee (1-2 cups a day) had little effect on calcium balance, caffeine may cause a small decrease in calcium absorption. The addition of milk into coffee could ameliorate the adverse effect of caffeine. Available evidence does not warrant different calcium intake recommendations for people with different caffeine intakes.
Ethnicity
In terms of calcium metabolism, fractional calcium absorption was reported to be much higher in Chinese women compared to the Whites. This has led to a suggestion that calcium requirements may be lower in Asians than for the Whites for the equivalent of calcium to be absorbed. Whether this represents an ethnic difference or just adaptation to chronically low dietary calcium intakes since childhood in the Chinese is not known (Kung et al., 1998).
14.5 Setting requirements and recommended intakes of calcium
Calcium requirements are best derived from balance studies, which is a careful measurement of calcium absorbed and calcium losses across a range of calcium intakes. The intake which provides just enough absorbed calcium to meet losses (zero balance) is then derived and set as the mean calcium requirement of an adult. In children, adolescence and pregnancy, the factorial approach is used to estimate calcium requirement because these groups need to be in positive calcium balance.
The factorial method is approached by estimating losses as increased by growth (if applicable) and then correcting for an expected rate of absorption in the diet. The calculation of “obligatory losses” often still depends on calcium balance studies. Nevertheless, the factorial approach is commonly used especially in population groups in which calcium balance studies are not conducted (FAO/WHO, 2002).
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146 Recommended Nutrient Intakes for Malaysia 2005
Children
The amount of calcium that is accumulated in young children, mainly for bone growth, increases from 120 mg at age 2 to 400 mg at age 9. Hence, the daily accretion rate is estimated to be 120 mg. The urinary calcium loss is estimated at 60 mg/day and dermal loss 40 mg/day. These losses together with the accretion rate yield 220 mg/day that must be absorbed. Thus, the FAO/WHO (2002) RNI for children aged 1-9 years is 500 to 700 mg/day – the lower value for younger children and the higher value for the older children.
The study of Lee, Leung & Lui (1993) amongst Hong Kong children showed that those with habitually higher calcium intakes during the first 5 years of life had significantly higher bone mineral content than in children with lower calcium intakes of less than 400 mg/day. Lee & Leung (1995) also showed that amongst 7-year old Hong Kong Chinese children who habitually consume a low calcium diet (280 mg/day), gains in radial bone density (3.1 % more than controls) were seen when supplemented with 300 mg/day calcium carbonate for 18 months.
RNI for children 1 - 3 years 500 mg/day 4 - 6 years 600 mg/day 7 - 9 years 700 mg/day
Adolescents
There is a vast increase in the rate of skeletal calcium accretion at puberty – from age 10 to 17 years. This is the period of growth spurts and the attainment of ‘peak bone mass’. Achieving a higher peak bone mass is considered a better approach for prevention of osteoporosis. Several studies have shown that calcium supplementation of adolescents of 500-1000 mg/day led to increased bone mineral accretion, which can be sustained for at least 3.5 years (Cadagon et al., 1997, Bonjour et al., 2001).
The peak rate of calcium accumulation is 300-400 mg daily, which occurs earlier in girls but continues longer in boys. Thus, it is difficult to justify any difference between recommended allowances for boys and girls. Assuming a target value of 300 mg per day for the skeleton, urinary loss of 100 mg/day and losses in the dermal and feces of 40 mg/day, a total amount of 440 mg calcium needs to be absorbed per day. Assuming a high calcium absorption of 35% in a mixed diet (higher than adults), FAO/WHO (2002) has recommended 1300 mg/day and 1000 mg/day for populations whose animal protein intake is 60-80 g/day and 20-40 g/day respectively. Assuming Malaysian animal protein intake is closer to the latter level, the proposed RNI for calcium for Malaysian adolescents aged 10-18 years is set at 1000 mg/day.
RNI for adolescents Boys 10 – 18 years 1,000 mg/day Girls 10 – 18 years 1,000 mg/day
Adults (ages 19 to 50 years in women & 19 to 65 years in men)
After peak bone mass attainment, bone formation and resorption is balanced during adulthood. Among Asians, calcium balance is achieved at intakes of 340- mg/day (Chinese Nutrition Society, 2000). Bone mass density is relatively stable between ages 20-50, and hence there are relatively few intervention studies on the role of calcium during young and middle adulthood. Two intervention studies showed that 1000 mg of calcium supplement was able to slow down bone loss by about 1% in premenopausal women. The results of two meta-analysis found significant positive effect of calcium intake and bone maintenance (Welten et al ., 1995; Andersen & Rondano, 1996). A study amongst young Chinese women concluded that an intake above 600 mg/day would have beneficial effect on bone mass (Ho et al ., 1994). FAO/WHO (2002) has recommended 750 mg/day for populations with animal protein intake of 20- 40 g/day. Studies conducted in Thailand and China has led to their recommendation of 800 mg/day (Chinese Nutrition Society, 2000). Based on these considerations, the proposed RNI for calcium for adults aged 19 to 50 years in women and men is set at 800 mg/day.
RNI for adults Men 19 - 65 years 800 mg/day Women 19 - 50 years 800 mg/day
Older adults (women aged above 51 years; men aged above 65 years)
Women generally attain menopause at approximately 50 years of age. The combined effects of aging and menopause lead to a 20-25% decrease in absorption efficiency for calcium. Menopause is also associated with a rise in excretion of obligatory calcium or fasting urine of about 20 mg–40-mg daily. All these factors are taken into consideration in recommending a higher amount of calcium intake for menopausal adults. There is no evidence of differences between recommendations for men and women (IOM, 1997). In fact, after the age of 65, men are also at risk of age- related osteoporosis. There is also not enough evidence to support different recommendations based on menopausal status or use of HRT.
In a study by Dawson-Hughes et al. (1990), it was shown that in subjects taking less than 400 mg calcium per day, increasing their calcium intake to 800 mg/day reduced bone loss significantly. Supplementation of Malaysian postmenopausal women with 1200 mg calcium per day using milk has been shown to reduce rate of bone loss (Chee et al ., 2003). Similarly, results from several reviews and meta-analysis of randomized- controlled trials indicate that calcium supplementations of up to 500-1000 mg/day
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Discussions on revised RNI for Malaysia
The previous recommended dietary intakes for calcium (Teoh, 1975) were lower than the current recommended calcium intake (2005) for Malaysian children, adolescents and adults but higher for infants, pregnant and lactating mothers. The calcium intake recommendations by FAO/WHO (2002) are similar to the recommended values by IOM (1997) for almost all age groups. For infant and children age groups, the Malaysian RNI adopted similar values as FAO/WHO (2002). However, for adolescents and adults, assuming a lower animal protein intake (20-40 g/day) in Malaysia, the recommended calcium intakes are lower than the values indicated by FAO/WHO (2002) for similar age groups (Appendix 14.1). FAO/WHO (2002) recommended higher calcium intakes for adolescents and adults based on populations whose animal protein intake is 60-80 g/day.
14.6 Toxicity and tolerable upper intake levels
Calcium levels in the body are very closely controlled so that excessive accumulation in blood or tissues arising from over consumption is unknown. Abnormally high calcium concentrations may occur but usually secondary to diseases such as bone cancer, hyperthyroidism and hyperparathyroidism. The efficiency of calcium absorption decreases with intake, thereby providing the body with a protective mechanism to lessen the chances of calcium intoxication. Currently, the available data on the adverse effects of excessive calcium intake in humans primarily concerns overuse of calcium supplements (National Institute of Health, 1994).
The common effects of excessive calcium intakes are kidney stones (nephrolithiasis), milk-alkali syndrome and interaction of calcium with absorption of other essential minerals such as iron, zinc, magnesium and phosphorous. IOM (1997) had established a tolerable upper level (UL) of 2,500 mg per day for all age groups.
With regards to phosphorus implications, the most serious adverse of abnormally elevated blood levels of phosphate (hyperphosphatemia) is the calcification of non- skeletal tissues most commonly the kidneys. Such calcium phosphate deposition can lead to organ damage. Hyperphosphatemia from dietary cause is a problem mainly in people with kidney failure, since the kidneys are normally efficient at eliminating excess phosphate from the circulation. In order to avoid hyperphosphatemia, IOM set a UL for oral phosphorus intake of 4g/day for young adults and 3g/day for older adults.
14.7 Research recommendations
The following priority areas of research are recommended:
- Nationally representative data on calcium intakes of various population groups such as children, adolescents, adults and elderly.
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- Content of calcium in local foods and absorption efficiency, especially from non- dairy foods.
- Studies on the effects of increased calcium intakes on skeletal mass and bone loss. In adolescents, it is important to determine to what extent increased calcium intake can influence peak bone mass formation in conjunction with other nutrients and physical activity level. Identify calcium thresholds above which further calcium intake would be ineffective.
- Calcium balance studies on various age and ethnic groups to determine optimal recommendations of calcium intake.
- Effects of consumption of soft drinks and other processed foods high in phosphorus on blood phosphorus and bone health among various groups, especially children and young adults
14.8 References
Anderson JJ & Rondano PA (1996). Peak bone mass development of females: can young adult women improve their peak bone mass? J Am Coll Nutr 15: 570–574.
Bonjour JP, Chevally T, Amman P, Slozman D & Rozzoli R (2001). Gain in bone mineral mass in prepubertal girls 3.5 years after discontinuation of calcium supplements: a follow-up study. Lancet 358: 1208-1212.
Cadogan J, Eastell R, Jones N & Barker ME (1997). Milk intake and bone mineral acquisition in adolescent girls: randomised, controlled intervention trial. Br Med J 315: 1255-
Calvo MS & Park YK (1996). Changing phosphorus content of the U.S. diet: potential for adverse effects on bone. J Nutr 126 (4 Suppl): 1168S-1180S.
Chee WSS, Suriah AR, Chan SP, Zaitun Y & Chan YM (2003). The effects of milk supplementation on bone mineral density of postmenopausal Chinese women in Malaysia. Osteoporos Int 14(10): 828-834.
Chinese Nutrition Society (2000). Chinese dietary reference intakes. Chinese Nutrition Society, China.
Cooper C, Campion G & Melton LJ III (1992). Hip fractures in the elderly: a worldwide projection. Osteoporosis Int 2: 285-289.
Dawson-Hughes B, Dallal GE, Krall EA, Sadowski L, Sahyoun N & Tannenbaum S (1990). A controlled trial of the effect of calcium supplementation on bone density in postmenopausal women. N Engl J Med 323: 878-
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National Institute of Health (1994). Optimal Calcium Intake. NIH Consensus Development Conference Statement. JAMA 12 (4): 1-31.
Nordin BEC (1997). Calcium in health and nutrition. Fd Nutr Agric. 20: 13-23.
Shea B, Wells G, Cranney A, Zytaruk N, Robinson V, Griffith L, Hamel C, Ortiz Z, Peterson J, Adachi J, Tugwell P & Guyatt G (2004). Calcium supplementation on bone loss in postmenopausal women. The Cochrane Database of Systematic Reviews 2004, Issue 1. Art. No.: CD004526.pub2. DOI: 10.1002/ 14651858. CD004526.pub2.
Tee ES, Ismail MN, Mohd Nasir A and Khatijah I (1997). Nutrient Composition of Malaysian Foods. 4 th^ Edition. Malaysian Food Composition Database Programme, Institute for Medical Research, Kuala Lumpur; 310 p.
Teoh ST (1975). Recommended daily dietary intakes for Peninsular Malaysia. Med J Mal 30: 38-42.
van den Heuvel EG, Muys T, van Dokkum W & Schaafsma G (1999). Oligofructose stimulates calcium absorption in adolescents. Am J Clin Nutr 69: 544-
Weaver C & Heaney RP (1999). Calcium. In: Modern Nutrition in Health and Disease , 9th edition. Shils ME & YoungVR (Eds). Williams & Wilkins Publisher, Baltimore. pp. 141-155.
Welten DC, Kemper HC, Post GB & van Staveren WA (1995). A meta-analysis of the effect of calcium intake on bone mass in young and middle aged females and males. J Nutr 125: 2802–2813.
WHO (1994). Assessment of Fracture Risk and Its Application to Screening for Postmenopausal Osteoporosis. Technical Report Series 843. World Health Organization, Geneva.
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Appendix 14.1 Comparison of recommended intake for calcium: RDI Malaysia (1975), RNI Malaysia (2005), RNI of FAO/WHO (2002) and AI of IOM (1997)
Malaysia (1975) Malaysia (2005) FAO/WHO (2002) IOM (1997) Age groups RDI Age groups RNI Age groups RNI Age groups AI (mg/day) (mg/day) (mg/day) (mg/day)
Infants Infants Infants Infants < 1year 550 0 – 5 months 300 (bf) 0 – 6 months 300 (bf) 0 – 6 months 210 400 (ff) 400 (ff) 6 – 11 months 400 7 – 11 months 400 7 – 12 months 270
Children Children Children Children 1 – 3 years 450 1 – 3 years 500 1 – 3 years 500 1 – 3 years 500 4 – 6 years 450 4 – 6 years 600 4 – 6 years 600 4 – 8 years 800 7 – 9 years 450 7 – 9 years 700 7 – 9 years 700
Boys Boys Boys Boys 10 – 12 years 650 10 – 18 years 1,000 10 – 18 years 1,300 9 – 13 years 1, 13 – 15 years 650 14 – 18 years 1, 16 – 19 years 500
Girls Girls Girls Girls 10 – 12 years 650 10 – 18 years 1,000 10 – 18 years 1,300 9 – 13 years 1, 13 – 15 years 650 14 – 18 years 1, 16 – 19 years 500
Men Men Men Men 20 – 39 years 450 19 – 65 years 800 19 – 65 years 1,000 19 – 30 years 1, 40 – 49 years 450 > 65 years 1,000 > 65 years 1,300 31 – 50 years 1, 50 – 59 years 450 51 – 70 years 1, ≥60 years 450 >70 years 1,
Women Women Women Women 20 – 39 years 450 19 – 50 years 800 19 – 50 years 1,000 19 – 30 years 1, 40 – 49 years 450 51 – 65 years 1,000 51 – 65 years 1,300 31 – 50 years 1, 50 – 59 years 450 > 65 years 1,000 > 65 years 1,300 51 – 70 years 1, ≥60 years 450 >70 years 1,
Pregnancy Pregnancy Pregnancy Pregnancy 1 st^ trimester 450 1 st^ trimester 1,000 1 st^ trimester 1,000 14 – 18 years 1, 2 nd^ trimester 1,200 2 nd^ trimester 1,000 2 nd^ trimester 1,000 19 – 30 years 1, 3 rd^ trimester 1,200 3 rd^ trimester 1,000 3 rd^ trimester 1,200 31 – 50 years 1,
Lactation Lactation Lactation Lactation 1 st^ 6 months 1,200 0 – 3 months 1,000 0 – 3 months 1,000 14 – 18 years 1, 2 nd^ 6 months 450 4 – 6 months 1,000 4 – 6 months 1,000 19 – 30 years 1, 7 – 12 months 1,000 7 – 12 months 1,000 31 – 50 years 1,
bf=breast fed ff=formula fed
