Dietary Reference Intakes for Calcium and Vitamin D.
INTRODUCTION
Calcium as a nutrient is mostly related to the formation and metabolism of bone. Over 99 % of complete physique calcium is discovered as calcium hydroxyapatite (Ca10[PO4]6[OH]2) in bones and enamel, the place it gives arduous tissue with its power. Calcium within the circulatory system, extracellular fluid, muscle, and different tissues is crucial for mediating vascular contraction and vasodilatation, muscle operate, nerve transmission, intracellular signaling, and hormonal secretion. Bone tissue serves as a reservoir for and supply of calcium for these crucial metabolic wants by the method of bone reworking.
Calcium metabolism is regulated largely by the parathyroid hormone (PTH)–vitamin D endocrine system, which is characterised by a sequence of homeostatic suggestions loops. The fast launch of mineral from the bone is important to keep up satisfactory ranges of ionized calcium in serum. Throughout vitamin D deficiency states, bone metabolism is considerably affected on account of decreased energetic calcium absorption. This results in elevated PTH secretion because the calcium sensing receptor within the parathyroid gland senses modifications in circulating ionic calcium. Elevated PTH ranges induce enzyme exercise (1α-hydroxylase) within the kidney, which converts vitamin D to its energetic hormonal type, calcitriol. In flip, calcitriol stimulates enhanced calcium absorption from the intestine. Not surprisingly, the interaction between the dynamics of calcium and vitamin D usually complicates the interpretation of information relative to calcium necessities, deficiency states, and extra consumption.
SOURCES OF CALCIUM
Ingested calcium comes from meals sources and dietary dietary supplements. On this report dietary calcium refers to each meals sources and dietary supplements mixed (though some researchers reserve the time period dietary calcium to imply solely meals sources) and is most sometimes called complete calcium consumption for readability. With greater than one-half of the U.S. inhabitants (Bailey et al., 2010)—and between 24 and 60 % of Canadians (2004 Canadian Neighborhood Well being Survey, private communication, D. Brulé, Well being Canada, April 29, 2010)—reporting use of dietary dietary supplements of some kind, dietary dietary supplements have to be taken under consideration when contemplating the sources of calcium within the eating regimen and, in flip, estimating complete calcium consumption. Present estimates from 2003 to 2006 point out that the median complete consumption of calcium from all sources for individuals > 1 yr of age ranges from 918 to 1,296 mg/day, relying upon life stage (Bailey et al., 2010). Solely small quantities of calcium are contributed by water, relying upon geographic location. Chapter 7 of this report incorporates an evaluation of quantitative calcium consumption within the U.S. and Canadian populations.
METABOLISM OF CALCIUM – “calcium 11”
FUNCTIONS AND PHYSIOLOGICAL ACTIONS OF CALCIUM
Calcium is an integral part of the skeleton, and the skeleton gives a reservoir of calcium for different important calcium-dependent capabilities all through the physique. The skeleton serves not less than three predominant capabilities. First, calcium, as a part of the mineral hydroxyapatite, deposited into the natural matrix of the skeleton, is crucial for its construction and is critical for tissue rigidity, power, and elasticity. This operate permits for regular motion and train. Second, the skeleton capabilities as a supply of minerals and alkali and due to this fact is crucial for general mineral homeostasis. The skeleton is the principal depot for calcium, containing 98 % of complete physique calcium. It may be known as on repeatedly, by the processes of bone formation and resorption (known as reworking, as mentioned under), to keep up circulating ranges of calcium at a relentless stage. Whereas the identical qualitative processes apply to skeletal calcium metabolism throughout the life cycle, there are quantitative variations by age and hormonal standing. These life cycle variations for skeletal development and reworking are mentioned in a piece under. Extreme calcium resorption can compromise the integrity and power of the skeletal tissues. Third, the marrow cavity of bone serves as a significant website for the event of hematopoietic cells and as a significant compartment of the immune system. A number of of the cell sorts concerned in bone reworking originate within the bone marrow compartment. Stromal or connective tissue cells are discovered within the bone marrow; at one time, these have been regarded as inert, however they’re now thought-about multi-potent stem cells that may develop into both fats or bone cells underneath the affect of particular differentiation elements (Muruganandan et al., 2009).
A principal physiological operate of calcium aside from its function in sustaining the skeleton, is as a necessary intracellular messenger in cells and tissues all through the physique. Though this pool of calcium is quantitatively small, the ionized calcium current within the circulatory system, extracellular fluid, muscle, and different tissues, is crucial for mediating vascular contraction and vasodilatation, muscle operate, nerve transmission, and hormonal secretion. Ionized calcium is the most typical sign transduction ingredient in biology, owing to its potential to reversibly bind to proteins and to advanced with anions equivalent to citrate and bicarbonate (Weaver and Heaney, 2006b).
CALCIUM ACROSS THE LIFE CYCLE
The physique’s want for calcium relative to skeletal development and reworking varies by life stage. The main physiological actions embody bone accretion throughout skeletal development and upkeep of bone mass after development is accomplished. Later in grownup life, web calcium is misplaced from the physique when bone formation now not retains up with bone resorption. For all life phases highlighted under, particular research and conclusions are detailed in Chapter 4.