Nutrition 330 Introductory Nutrition

Study Guide: Unit 11

Nutrients for Bone Health

We now examine nutrients that are vital for bone health. This is a subject of great importance because of the high incidence of osteoporosis, especially among older women. The major focus of this unit is on calcium and vitamin D. However, several other nutrients also play an important role in bone health, including phosphorus, magnesium, and vitamin K.

This unit consists of two sections:

11.1—Calcium
11.2—Vitamin D

Objectives

After completing this unit, you should be able to

1. discuss the following about calcium:
   1. Identify its functions in the body.
   2. Describe the hormonal regulation of its blood level.
   3. Discuss the physiological and dietary factors that influence its balance.
   4. Discuss deficiency symptoms.
   5. List some of its major food sources.
   6. Discuss some of the factors to consider when using supplements.
2. discuss the following about vitamin D:
   1. describe the biosynthesis of its active form.
   2. identify its functions in the body.
   3. describe the prevalence of deficiency and the associated symptoms.
   4. describe the Canadian Cancer Society’s current recommendation for vitamin D intake.
   5. identify its dietary and non‑dietary sources.
3.  list the risk factors for osteoporosis and discuss preventive measures.

11.1 Calcium

Introduction

Living bone is composed of two different types of bone tissue: cortical bone and trabecular bone. Cortical bone, located on the outside of the long shafts, is characteristically dense, giving strength and rigidity to bones. Trabecular bone, located at the ends and insides of the long bones, is characteristically spongy and light, containing a network of blood and lymph vessels.

Two‑thirds of bone weight is contributed by minerals, the rest by water and collagen (a protein that makes up the organic matrix). This matrix provides a framework within which mineral salts can be embedded, giving strength and rigidity to bone. Throughout the matrix is a network of nerves, blood, and lymph vessels. The major minerals in the hydroxyapatite (the crystal structure characteristic of bone) are calcium and phosphorus, in the form of calcium phosphate and calcium carbonate salts. Other mineral ions present include fluoride, magnesium, zinc, and sodium. A similar crystal structure is also found in the teeth.

Objectives

After completing this section, you should be able to

• discuss the following about calcium.
• identify its functions in the body.
• describe the hormonal regulation of its blood level.
• discuss the physiological and dietary factors that influence its balance.
• discuss deficiency symptoms.
• list some of its major food sources.
• discuss some of the factors to consider when using supplements.
• list the risk factors for osteoporosis and discuss preventive measures.

Key Terms

After completing section 11.1, you should be able to define and use the following terms in context:

Reading Assignment

• Chapter 13: “About Bones,” pages 419–421
• Chapter 13: “Calcium,” pages 421–426
• Chapter 13: “Highlight 13: Osteoporosis and Calcium,” pages 441–447

Calcium

Calcium is the most abundant mineral in the body. Ninety‑nine percent of the body’s calcium is found in bones and teeth. The remaining one percent is distributed as calcium ions in the blood, extracellular fluids, and soft tissue, where it regulates many metabolic functions. The body gives higher priority to maintaining the level of this functional calcium in the blood and tissues than to retaining the calcium in bone. Hence, calcium can be withdrawn almost without limit from bone, as demonstrated by the severe bone loss in advanced osteoporosis.

The functions of calcium are listed below.

• Development and maintenance of bone and teeth. Calcium is one of the major minerals that make up the hydroxyapatite crystal of the hard tissues. During growth and throughout life, bone is remodelled and reshaped to adjust to the degree of bone stress. Therefore, weight on bone stimulates mineral deposition, resulting in thicker and stronger bone. Calcium also functions to maintain the integrity of collagen, the intracellular cementing substance.
• Nerve transmission. Calcium functions in several ways to facilitate normal nerve transmission. A current of calcium ions triggers the flow of an electric impulse from one nerve cell to another and then on to waiting target muscles.
• Muscle contraction. Calcium functions in cardiac muscle to help maintain normal heart rhythm and in smooth and skeletal muscles to maintain normal contraction and relaxation.
• Biological reactions. Many enzymes require the presence of calcium ions as a catalyst to function; for example, the absorption of vitamin B₁₂ by the intestinal mucosal cells requires calcium. Pancreatic lipase is activated by calcium, and some of the protein‑splitting enzymes also require calcium for activation.
• Cell membrane permeability. Ionized calcium located in the cell membrane plays a role in the cellular uptake of certain nutrients.
• Blood clotting. Calcium ions are required for the conversion of prothrombin to thrombin in the sequence of clot formation.

Regulation of Blood Calcium

As explained later in this unit, one of the roles of vitamin D is to maintain blood calcium levels. 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) 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. Therefore, PTH, calcitonin, and vitamin D₋₃ (a vitamin with the characteristic of a hormone) work together to regulate blood‑calcium levels (see Figure 13‑1 on page 422).

This mechanism is an example of homeostatic control. As emphasized in the textbook, even when dietary intake of calcium is very low, blood‑calcium levels are maintained at the expense of bone calcium. Calcium rigour resulting from a high blood calcium level and calcium tetany from a low blood‑calcium level are caused by malfunctioning of one of the above three regulatory systems.

Balance

The following physiological and dietary factors affect calcium balance:

• Acidity of the digestive mass. Calcium is more soluble in an acidic medium than in a basic medium. Anything that reduces gastric acidity, such as antacids, can reduce calcium absorption.
• Vitamin D status. If the vitamin D status is poor, the amount of calcium being absorbed from the intestine will be low, regardless of how much calcium is in the diet. Because of Canada’s northern location, much of the population may have a poor blood level of vitamin D for much of the year. As a result, calcium absorption may be rather low. For this reason, improving calcium stores requires improving the body’s supply of vitamin D, either from sunshine or diet.
• Lactose. The absorption of calcium can be improved 15–50% by the presence of lactose. Hence, calcium from milk and milk products has a higher bioavailability (the extent to which a nutrient is absorbed) 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). 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. Fibre‑containing foods, such as bran, whole wheat breads, and cereals, can also inhibit the absorption of these minerals, but only to a small extent.
• 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.
• Physical activity. Another factor that affects calcium balance is the degree of 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.

Deficiency

The failure to deposit sufficient calcium in the bone can cause growth retardation in children and bone loss in adults. Suboptimal and marginal intakes of calcium throughout life can result in 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.

Dietary Sources of Calcium and Supplements

The DRIs for calcium are stated at the front of the textbook. Notice that the highest DRIs are for adolescents and for women over age 50. Milk and milk products are the best sources of calcium, both qualitatively and quantitatively. In the Canadian diet, dairy products supply a large majority of the total calcium intake. The calcium in milk is of high bioavailability because of the presence of lactose and because of the fortification with vitamin D, both of which enhance the absorption of calcium. Figure 13‑4 on page 424 shows the dominance of milk and milk products as sources of calcium. 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, but some vegetables, such as spinach and swiss chard, provide little absorbable calcium. The bioavailability of calcium in selected foods is illustrated in Figure 13‑5 on page 425.

Canada’s Food Guide recommends two servings per day of milk and alternatives for adults up to age 50, but this should be increased to three servings for adults over age 50. Consuming calcium from food sources is by far the preferred way to meet calcium needs. 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.

Osteoporosis

Osteoporosis, is a highly prevalent yet preventable disease affecting many older women in Canada. In Table H13‑1 (p. 443), the textbook lists the many factors that contribute to increased risk of osteoporosis. Several of these factors—age, gender, and family history—cannot be changed. Other risk factors, such as diet, physical activity, body weight, smoking, and alcohol use can be altered. The earlier that preventive strategies are adopted, the better. 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.

Study Questions

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11.2 Vitamin D

Introduction

The cure for rickets (sunlight and cod liver oil) was discovered in the early 1900s. However, there was some confusion between vitamins A and D, delaying the isolation and identification of vitamin D until 1930. Since then, the metabolites of vitamin D have been identified. The mode of action of vitamin D in mineral metabolism was understood by the late 1960s. In recent years, more has been discovered about vitamin D, and it has been shown to possess a multivariant role in the body tissues. Its action includes the production and development of red blood cells, cellular differentiation, and insulin secretion. Vitamin D has been the subject of much interest in recent years as evidence has emerged indicating its importance not only for protecting bone health but also for enhancing body defences against cancer and possibly some other diseases.

Vitamin D is different from other nutrients because it can be synthesized by the body and because it functions like a hormone in the regulation of mineral metabolism.

Objectives

After completing this section, you should be able to

• describe the biosynthesis of the active form of vitamin D.
• identify its functions in the body.
• describe the prevalence of deficiency and the associated symptoms.
• describe current recommendations for vitamin D intake.
• identify its dietary and non‑dietary sources.

Key Terms

After completing section 9.2, you should be able to define and use the following terms in context:

Reading Assignment

• Chapter 13: “Vitamin D,” pages 427–433

Vitamin D

Vitamin D and its metabolites are sterols, which are a form of lipid. Like cholesterol, the chemical structure of vitamin D resembles chicken wire. Sterols are absorbed and transported like any of the fat‑soluble nutrients.

Synthesis

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 (see Figure 13‑6, p. 428). This final product is the active vitamin D.

Functions

The well‑known 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.

Studies of other functions of vitamin D have been based on the detection of its receptors in an astonishing array of tissues. In addition to being found in intestinal, renal, parathyroid, and skeletal tissues, vitamin D is also found in the skin, breast, pancreas, and connective tissues. We will discuss this in more detail later in this unit. It has been postulated that vitamin D is involved in gene transcription, a subject beyond the scope of this course.

Deficiency

Because vitamin D is essential for calcium absorption, a deficiency most notably affects the bones. The childhood version of vitamin D deficiency is known as rickets and the adult form as osteomalacia.

Rickets is characterized by enlargement of and deformities at the ends of the long bones. When calcium and phosphorus levels are inadequate, the bones fail to mineralize, while the cartilage at the growing ends continues to grow, producing enlarged and deformed areas at the joints. Unable to elongate and harden, the body weight causes the bones to become bowed.

Osteomalacia is characterized by gradual rarefaction of the bones, particularly those of the pelvis, thorax, and extremities. They become thin, leading to frequent, spontaneous fractures.

Vitamin D deficiency occurs in environmental conditions where exposure to sunlight is limited. This was certainly the case during the Industrial Revolution, when severe air pollution filtered out most ultraviolet light, making rickets common in young children. Supplementation of foods with vitamin D has almost totally eliminated rickets in industrialized nations. There are, however, 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; and
• elderly people who stay indoors.

Overall, about one on four Canadians have a blood level of vitamin D that falls below the healthy range, rising to about one in two among non‑white Canadians (Whiting et al., 2011).

Toxicity Versus Effective Intake

Until 2007, the margin of safety for vitamin D was thought to be particularly narrow compared to other vitamins, so public health officials routinely advised caution when using vitamin D supplements. The DRI give an UL (upper limit) of 100 μg (4000 IU). This is double the previous UL. As described in the textbook, excess vitamin D can certainly have serious consequences.

In recent years, evidence has emerged suggesting that vitamin D is protective against falls and fractures in older adults (Bischoff‑Ferrari et al., 2009a, 2009b). Other evidence has linked vitamin D with protection against cancer, especially colon cancer (Scragg, 2011). There is also evidence indicating that a low level of vitamin D may increase the risk of diabetes, heart disease, and faster cognitive decline in older adults. The intake necessary to achieve the blood levels of the vitamin associated with reduced risk of these conditions is around 25 μg (1000 IU) or more, which is higher than the current RDA of 15 μg (600 IU). This issue is the subject of much ongoing research.

Population groups for which the case for supplementation is strongest are older adults, those who have dark skin, people who cover their skin, and those who spend little time outdoors.

This recommendation for vitamin D attempts to balance risk reduction with the least potential for harm. Canada’s Food Guide recognizes the need for vitamin D by adults, but has recommended a daily supplement of only 10 μg (400 IU) for adults over 50 years (in addition to drinking two cups of milk or alternatives). Vitamin D intake should not exceed the current UL of 100 μg (4000 IU) per day.

For now, it is fair to say that the recommendations for vitamin D are controversial, and vitamin D will be a hot topic in nutrition in the years ahead.

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. As Figure 13‑8 on page 431 shows, above 40° north latitude—which includes all of Canada—vitamin D synthesis essentially ceases for four months of the year. Dietary sources thus become essential to meet the body’s need.

Study Questions

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References