Unit 11: Nutrients for Bone Health Flashcards
Discuss the following about calcium:
Identify its functions in the body.
- Development and maintenance of bone and teeth.
- Nerve transmission.
- Muscle contraction.
- Biological reactions.
- Cell membrane permeability.
- Blood clotting.
Discuss the following about calcium:
Describe the hormonal regulation of its blood level.
A low blood calcium level triggers the release of parathyroid hormone (parathormone, PTH). In response to PTH, active vitamin D (1,25‑(OH)2 D3) (Vitamin that acts like a hormone) is produced. The active vitamin D functions by increasing intestinal absorption of calcium, increasing mobilization of bone calcium, and increasing calcium reabsorption by the kidney. As calcium levels reach the upper end of the normal range, the thyroid gland releases the hormone calcitonin. Calcitonin favours the deposition of calcium in bone. The simultaneous decline in PTH results in reduced active vitamin D synthesis; consequently, the blood calcium level stops rising and starts falling.
Discuss the following about calcium:
Discuss the physiological and dietary factors that influence its balance.
- Acidity of the digestive mass: Calcium is more soluble in an acidic medium than in a basic medium
- Vitamin D status: If the vitamin D status is poor, the amount of calcium being absorbed from the intestine will be low despite calcium intake
- Lactose: The absorption of calcium can be improved 15–50% by the presence of lactose. Calcium from milk and milk products has a higher bioavailability than calcium from other sources.
- Need for calcium: Several periods during the life cycle are characterized by an increased calcium requirement: infancy, childhood, adolescence, pregnancy, and lactation. To meet additional demands, the body becomes more efficient at absorbing dietary calcium; absorption increases from the average of 10–30% to as high as 50%. When demand for calcium is lower, the rate of absorption decreases proportionally.
- Phytic acid (or phytates): Forms insoluble complexes thus reducing bioavailability. Fibre‑containing foods can also inhibit the absorption of these minerals, but only to a small extent.
- Oxalic acid (or oxalates): Can combine with calcium to produce calcium oxalate, an insoluble complex that precipitates in the gut and cannot be absorbed.
- Physical activity: Bed rest, or immobilization of bones, causes loss of bone calcium as does the weightlessness experienced by astronauts. It is thought that bone requires the stress of weight upon it to maintain a balance between bone deposition and bone resorption. Without the stress of body weight and exercise, bone resorption exceeds bone deposition, resulting in a net loss of bone.
Discuss the following about calcium:
Discuss deficiency symptoms.
Growth retardation in children and bone loss in adults. Suboptimal and marginal intakes of calcium throughout life can result in osteoporosis
Discuss the following about calcium:
List some of its major food sources.
- Milk and milk products are the best sources of calcium, both qualitatively and quantitatively. (High bioavailability because of fortification with Vitamin D)
- Sardines (with bones) are another excellent source.
- Clams, salmon with bones, soybeans, and tofu are also good sources.
- Green, leafy vegetables, such as turnip greens, broccoli, and mustard greens, are fairly good sources,
- Some vegetables, such as spinach and swiss chard, provide little absorbable calcium.
Discuss the following about calcium:
Discuss some of the factors to consider when using supplements.
Consuming calcium from food sources is preferred. However, with approximately 250 mg calcium per cup of milk, even following the Guide will not provide enough calcium to meet the RDA. This is even more likely in situations of allergy, lactose intolerance, food dislikes, poor appetite, or illness.
For these reasons, millions of people, especially women over the age of 50, are regular users of calcium supplements.
Discuss the following about vitamin D:
describe the biosynthesis of its active form.
The precursor of vitamin D, 7‑dehydrocholesterol, is synthesized from cholesterol by the body. In the presence of sunlight (ultraviolet rays), 7‑dehydrocholesterol in the skin is converted to cholecalciferol (previtamin D3). In the liver, previtamin D3 is hydroxylated at the 25 position to form 25‑hydroxycholecalciferol (25‑OH D3). The crucial activation of vitamin D occurs in the kidney where a second hydroxyl group is added on the number one position of 25‑OH D3, producing 1,25‑dihydroxycholecalciferol or 1,25‑(OH)2 D3. This final product is the active vitamin D.
Discuss the following about vitamin D:
identify its functions in the body.
Role of vitamin D is to maintain blood calcium and phosphorus at the concentrations essential for normal mineralization of bones and teeth, for neuromuscular activity, and for other cellular processes dependent on these minerals. Vitamin D raises blood levels of calcium and phosphorus in three ways:
- it stimulates the intestinal mucosa to increase absorption;
- it stimulates the kidney to reabsorb calcium and phosphorus so that they are not lost in the urine; and
- it stimulates bone resorption, in conjunction with parathyroid hormone (PTH), by mobilizing calcium and phosphorus from the bones to the blood.
When calcium and phosphorus levels in the blood reach the upper end of the normal range, the hormone calcitonin initiates the synthesis of an inactive form of vitamin D: 24,25‑(OH)2 D3. This process results in reduced calcium and phosphorus absorption and increased bone mineralization.
Discuss the following about vitamin D:
describe the prevalence of deficiency and the associated symptoms.
Vitamin D is essential for calcium absorption, a deficiency most notably affects the bones. (Rickets and Osteomalacia)
Vitamin D deficiency occurs in environmental conditions where exposure to sunlight is limited. Reports of vitamin D deficiency in certain population segments:
- children of low‑income, inner‑city families;
- breastfed infants who do not receive supplemental vitamin D;
- dark‑skinned people who cover up their skin (e.g., some East Indian women);
- vegetarians who do not drink milk fortified with vitamin D;
- elderly people who stay indoors.
Overall, about one on four Canadians have a blood level of vitamin D that falls below the healthy range
Discuss the following about vitamin D:
describe the Canadian Cancer Society’s current recommendation for vitamin D intake.
Exposure to the sun is the most important risk factor for skin cancer prompting the Canadian Cancer Society to recommend reliance on diet and supplements for vitamin D throughout the year.
Discuss the following about vitamin D:
identify its dietary and non‑dietary sources.
Food sources of naturally occurring vitamin D are limited to liver, eggs, fish liver oils, and butter. In Canada, milk and margarine are fortified with synthetic vitamin D. Human breast milk does not contain enough vitamin D; hence, it is recommended that breastfed babies be given supplemental vitamin D (400 IU/day).
Adult requirements for vitamin D can be met by unprotected exposure of the face, arms, and hands to the sun for five to ten minutes, two to three times a week. Dark‑skinned people may require as much as three hours of daily exposure. Latitude, season, and time of day can each have a dramatic effect on vitamin D synthesis. Sunscreens of 8 SPF and higher also prevent vitamin D synthesis. In Canada—vitamin D synthesis essentially ceases for four months of the year. Dietary sources thus become essential to meet the body’s need.
Define Bioavailability
The extent to which a nutrient is absorbed
Define Phytic Acid
Phytic acid (or phytates). This organic acid, found in the outer husks of cereal grains, forms insoluble complexes with a number of minerals, including calcium, iron, and zinc, thus reducing bioavailability.
Define Oxalic Acid
Oxalic acid (or oxalates). This organic acid, found in foods such as spinach, rhubarb, and beet greens, can combine with calcium to produce calcium oxalate, an insoluble complex that precipitates in the gut and cannot be absorbed. Fortunately, these foods contain sufficient calcium to tie up the oxalic acid, leaving no excess to bind with calcium in other foods.
Explain Osteoporosis
Osteoporosis (porous, brittle bones), mainly in post‑menopausal women. Although this disease of bone loss is multifactorial in origin, a growing body of evidence suggests that a low calcium intake is a significant factor in osteoporosis. A person with osteoporosis has a long history of being in negative calcium balance: much calcium was withdrawn from bone, resulting in porous bones of low mass. As a result, bones become more susceptible to fracture, especially at the wrist, spine, and hip. The level of peak bone mass at approximately 30 years of age is a major factor determining the risk of developing osteoporosis. A diet that meets the recommended intakes of all bone nutrients, most notably calcium, phosphorus, magnesium, and vitamin D, is important throughout life. It must be stressed that calcium intake is only one factor among many that determines whether a person develops osteoporosis; other important factors include smoking, alcohol, exercise, and genetics.
High intakes of caffeine and sodium may contribute to increased calcium loss in the urine, so these substances should also be considered as risk factors for osteoporosis. Protein also appears to increase calcium loss. However, in older adults, protein intake is associated with higher bone calcium and less risk of fracture.
