Multiple sources of dietary calcium—some aspects of its essentiality
Introduction
The essentiality of calcium spans infancy, childhood, adolesence, adulthood, and the aging populations. A broad galaxy of metabolic processes and variable physiologic requirements for calcium are manifest during the stages of skeletal development and maintenance involving bone growth, bone mineral density (BMD), peak bone mass (PBM), bone mass index (BMI) as well as in the latter osteoporosis stage (American Academy of Pediatrics, 1999; Berdanier, 2002; Bonjour et al., 1997; Ensminger et al., 1994; Gueguen and Pointillart, 2000; Gurr, 1999; Heaney, 2000, Heaney, 2001a, Heaney, 2001b, Heaney, 2001c; Institute of Medicine, Food and Nutrition Board, 1997; National Institutes of Health Consensus Conference, 1994; National Research Council, 1989a, National Research Council, 1989b; Nordin and Marshall, 1988; Pocock et al., 1987; WHO, 1998).
In a continuous process of resorption and formation, bone is constantly turning over, while in children and adolescents the rate of formation of bone mineral predominates over the rate of resorption, while in later life, resorption predominates over formation resulting in gradual loss of bone during normal aging. The deposition and resorption of bone are regulated by several hormones: parathyroid hormone (PTH), calcitonin, 1,25-dihydroxycalcitrol, and estrogens (American Academy of Pediatrics, 1999; Berdanier, 2002; Gurr, 1999; Heaney, 2000, Heaney, 2001b, Heaney, 2001c; Institute of Medicine, Food and Nutrition Board, 1997; National Institutes of Health Consensus Conference, 1994; National Research Council, 1989a, National Research Council, 1989b; Nordin and Marshall, 1988). Approximately 1200 g of calcium are present in an adult human, (accounting for about 2% of body weight); more than 99% of that amount is found in bones and teeth which act as a vital physiological tissue providing a readily available source of calcium for maintenance of normal plasma Ca2+ levels (about 45% of adult skeletal mass is accounted for by skeletal growth during adolescence) (Berdanier, 2002). Serum calcium exists in three fractions: ionized and physiologically active calcium (approximately 50%), protein-bound calcium (approximately 40%), and a small amount of calcium that is complexed, primarily to phosphate, citrate, and bicarbonate ions (about 10%). The remaining 1% (about 10–12 g in an adult) of body calcium is found in extracellular fluids (ECF), intracellular structures and cell membranes. Bone mineral serves as the ultimate reservoir for the calcium circulating in the ECF. (Calcium enters the ECF from the gut by absorption and from bone by resorption) (Gueguen and Pointillart, 2000; WHO, 1998). The ionized form of calcium is physiologically important and its concentration is regulated through the integrated action of three hormones (parathyroid hormone, calcitriol, and calcitonin). Calcium circulates in the blood plasma at a concentration that normally lies between 90 and 110 mg/L; it is involved in the communications between cells and assists in regulating their behavior. In human cells, the total calcium concentration can vary from as little as 0.8 mg/L in red blood cells to more than 200 mg/L in muscle cells or platelets (more than 99.9% of this intracellular calcium is bound to internal cellular structures including nucleus, mitochondria, and endoplasmic reticula) (American Academy of Pediatrics, 1999; Berdanier, 2002; Gurr, 1999; Heaney, 2000, Heaney, 2001a, Heaney, 2001b, Heaney, 2001c; Institute of Medicine, Food and Nutrition Board, 1997; National Institutes of Health Consensus Conference, 1994; National Research Council, 1989a, National Research Council, 1989b; Nordin and Marshall, 1988; WHO, 1998).
In addition to calcium consumption, a variety of other factors is believed to affect bone-mass accumulation and retention of calcium during childhood and adolescence and later risk of osteoporosis (a complex, multifactorial disorder) (Heaney, 2000, Heaney, 2001a, Heaney, 2001b, Heaney, 2001c) including: physical activity, environmental factors, hormonal status, and other dietary components. Unique host characteristics may also modify the effects of dietary calcium on bone health including: age, ethnic and genetic background, other dietary constituents, presence of gastrointestinal disorders such as malabsorption, and the presence of liver and renal disease. Interactions among the above factors and these diseases, as well as co-factors may affect calcium imbalance thus altering the optimal levels of calcium intake (American Academy of Pediatrics, 1999; Berdanier, 2002; Gueguen and Pointillart, 2000; Gurr, 1999; Heaney, 2001a, Heaney, 2001b; Institute of Medicine, Food and Nutrition Board, 1997; National Research Council, 1989a, National Research Council, 1989b; Nordin and Marshall, 1988; WHO, 1998).
Calcium is one of the most important and widely occurring intracellular signal involved in regulating a broad array of cellular processes and physiological events including embryonic development, cell-division, proliferation, cell relaxation, cell wall permeability, apoptosis, cardiac contractility, muscle contraction, enzyme and hormone activation, activity of prothrombin for blood clotting, and learning and memory. Contributing to the versatility of calcium in the above processes is the spatial and temporal dynamic of calcium signalling which rely on the coordinated actions of various pumps, exchangers, and calcium-binding protein (American Academy of Pediatrics, 1999; Berdanier, 2002; Gurr, 1999; Heaney, 2000, Heaney, 2001a, Heaney, 2001b; National Institutes of Health Consensus Conference, 1994; National Research Council, 1989a, National Research Council, 1989b; WHO, 1998).
Low calcium intake has also been implicated as a determinant of preeclampsia and several chronic conditions including hypertension (Appel et al., 1997; Barr et al., 2000; Berdanier, 2002; Dwyer et al., 1998; McCarron and Reusser, 1999, McCarron and Reusser, 2001), and colon cancer (Holt, 1999a, Holt, 1999b; Holt et al., 2001; Lipkin, 1999); although the role of calcium in the latter cases has not been unambiguously established (Institute of Medicine, Food and Nutrition Board, 1997; National Institutes of Health Consensus Conference, 1994).
Changes in calcium intakes have been observed in many communities throughout the world, especially in the last few decades (Gurr, 1999; WHO, 1998). The increasing recognition of the dietary changes and preferences of various population groups in a number of societies in terms of their decreasing consumption of milk and milk products as well as populations of individuals who are lactose intolerant (American Academy of Pediatrics, 1999; Berdanier, 2002; Fleming and Heimbach, 1994; Heaney, 2000, Heaney, 2001b, Heaney, 2001c; Institute of Medicine, Food and Nutrition Board, 1997; Nordin and Marshall, 1988; Vesa et al., 2000) and/or for a variety of regional, cultural, economic, ethnic or religious reasons do not consume (or consume less) milk or dairy products (American Academy of Pediatrics, 1999; Berdanier, 2002) has illustrated the need to highlight other dietary and supplemental sources of calcium to insure a sufficiency of its daily intake. For example, lactose intolerance affects about 25% of adults in the US; primarily lactase deficiency begins in childhood and may become clinically apparent in adolescence. In adults, the presence of lactose intolerance is highest in Asians (about 85%), intermediate in African Americans (about 50%), and lowest in Caucasians (about 10%). Although lactose intolerance may influence intake, there is no evidence that it interferes with calcium requirement (Institute of Medicine, Food and Nutrition Board, 1997). The principal focus of this assessment of the essentiality of calcium is to elaborate the variety of non-dairy food, calcium fortified foods, and supplemental sources of calcium. Initially it is useful to briefly consider: (a) the salient aspects of absorption and bioavailability of calcium, (b) changes in individual and societal dietary habits and preferences, and (c) various national and international Recommended Dietary Allowances (RDAs) and Dietary Reference Intakes (DRIs).
Section snippets
Absorption and bioavailability
The efficiency of calcium absorption is subject to a wide intersubject variation and is regulated by a number of dietary and host-related factors including the overall level of calcium in a food and the chemical specific composition of food and volume of food consumed, e.g., the presence of components that enhance (e.g., dietary protein, lactose, casein phosphopeptides, and non-digestable oligosaccharides) or inhibit (as oxalic acid and phytic acid) absorption and food components such as large
Dietary changes and preferences
It is increasingly recognized that there have been and are occurring dynamic changes generally manifest as decreases in calcium intake in regard to dietary habits, food intolerances, allergies, preferences, and concerns over the past half-century involving many societal sectors involving both sexes of all ages, ethnic, racial, religion, culture, moral beliefs, geographical, and economic backgrounds (in the US and principally other industrialized societies) as witnessed in the US by the
Recommended dietary allowances and dietary reference intakes for calcium
Recommendations on mineral and nutrient intakes differ (in some cases significantly) between countries and are variously referred to as RDAs (recommended daily or dietary, allowance or amounts), RDIs (recommended daily or dietary intakes), DRIs (dietary reference intakes) or RNIs (referred nutrient intakes) (Gurr, 1999) (employed now in Canada and the UK). The meaning of these terms has been widely misunderstood by health officials and the general public in terms of how they were formulated and
Non-dairy sources of calcium and calcium fortified foods
As noted earlier, the RDAs, RNIs, and AIs for calcium are the amounts needed to provide for adequate nutrition in mostly healthy individuals depending on age, sex, and physical condition (e.g., pregnancy and/or lactation) (American Academy of Pediatrics, 1999; Berdanier, 2002; Murphy et al., 2002; National Research Council, 1989a, National Research Council, 1989b; WHO, 1998). Milk and milk products such as cheeses are the principal and most available sources of calcium currently providing
Supplemental sources of calcium
As noted earlier, that while the required or optimum calcium intake can be generally obtained from conventional food sources (milk and other dairy and non-dairy sources), for some people, however, it can be difficult to obtain the amount of calcium they need for propylactic or therapeutic use on a daily basis through diet alone. Hence, calcium supplements which have been recommended for at least 40 years (Heaney, 2002b) are becoming an increasingly important ancillary source of calcium for a
Conclusions
The recognition of dynamic dietary changes in various populations transcending age, gender, ethnicity, geographic, and economic status has led to an increasing recognition and use of non-dairy foods coupled with an increasingly available broad panolpy of calcium fortified foods and supplemental sources of bioavailable calcium, in addition to the traditional dairy sources of calcium to ensure its sufficiency of intake for nutritional as well as for a host of essential physiological and medical
Acknowledgements
This work was funded by an unrestricted grant from the National Soft Drink Association, Washington, DC.
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