Nutrition 331: Nutrition for Health
Study Guide: Unit 9
The word vitamin is derived from the Latin noun vita (life) and amine, referring to the nitrogen-containing group detected in the first vitamins discovered. The word was originally spelled vitamine; however, when it was realized that not all vitamins contained an amine group, the e was dropped.
In this unit, we discuss the characteristics, sources, functions in the body, food sources, and stability of five major vitamins.
After completing this unit, you should be able to
- explain the concept of dietary patterns.
- define phytochemicals.
- describe the general characteristics of vitamins.
- discuss the differences between water-and fat-soluble vitamins regarding the body’s absorption, storage, and excretion; potential toxicity to the body; and retention in food through processing and storage.
- describe functions of vitamins A, D, C, B12, and folate.
- identify the food group(s) that typically contain each vitamin in objective 5, and list three to five food sources of each vitamin.
- list Canadian population groups vulnerable to deficiencies of the vitamins listed in objective 5.
- discuss vegetable storage and preparation methods that maximize retention of water-soluble vitamins and minerals.
- discuss the consequences of refining and enriching grains on vitamin and mineral content.
Note: You will only be tested on the vitamins listed above.
Section 1 A New Way to View Vitamins
The way that nutrition scientists see vitamins has undergone a major change in recent years. This changed viewpoint is reflected in this unit. Section 1 will review how this new thinking has evolved.
Over the last two centuries, medical scientists have battled the challenges posed by many serious and widespread diseases. Slowly, they have figured out what causes many of these diseases and how they can be prevented. While the majority of such diseases proved to be infectious, several were found to be related to diet. During the twentieth century, nutrition scientists discovered that several diseases are caused by diets lacking particular foods. This led to the discovery of organic (carbon-containing) substances in food that prevent disease. These substances were given the name vitamins. Further research revealed the mechanism of action of different vitamins and explained why deficiency of a particular vitamin causes its associated symptoms. We can summarize this viewpoint of vitamins as follows:
- Lack of a particular vitamin in the diet causes a particular disease.
- The disease can be cured (in most cases) by giving supplements of the vitamin (or consuming foods containing the vitamin).
- The symptoms of each vitamin-deficiency disease can be explained by the biochemical role of the vitamin in body function.
Based on these discoveries, vitamins achieved an exalted status in nutrition. This high valuation extends to other food substances that are essential to health as well, namely minerals, amino acids, and essential fats. This perspective has been enormously important in explaining the relationship between diet and health. It has also been of enormous benefit for public health as we now know how to prevent serious vitamin-deficiency diseases such as scurvy and beriberi.
The legacy of the vitamin connection is that nutrition scientists have attempted to understand different diseases based on the influence of individual substances. However, the most common diseases today, such as cardiovascular disease and cancer, can rarely be explained by single substances but are generally related to multiple aspects of lifestyle. When diet plays a role, as it often does, studies of single substances seldom provide a credible explanation for the diet-disease relationship. Much more often the dietary pattern as a whole best explains the evidence. Based on this new viewpoint, dietary patterns appear to hold the secret of the cause of many diseases and of how to prevent them.
Dietary patterns refer to the complex mixture of many different foods that people eat. A dietary pattern is the sum of thousands of individual substances. As we shall see in later units, when we look at the chronic diseases of lifestyle, including cardiovascular disease and cancer, many different substances are involved in the causation of these diseases. This typically reflects an excess of some substances in the diet combined with a relatively low intake of others.
Research on phytochemicals adds support to this viewpoint. Phytochemicals are an extremely diverse group of substances that share three features: they are organic, they are found in plant foods, and they are not vitamins (i.e., they are not essential). Carotenoids are one group of phytochemicals; they include beta-carotene (the yellow substance in carrots which the body can convert to vitamin A) and lycopene (the red substance in tomatoes). There has been an enormous amount of research and speculation regarding the possible role of phytochemicals in preventing various diseases, such as heart disease and cancer. However, at present, nutrition scientists can make few firm statements regarding the role of individual phytochemicals. The one “firm” conclusion that can be made is that a diet rich in phytochemicals is associated with a low risk of such diseases as heart disease and cancer. In practice, this means that people should eat plenty of fruit, vegetables, and whole grain cereals.
Let us now return to vitamins and also minerals. Across western countries, including Canada, vitamin deficiencies are rare: few people suffer from such diseases as scurvy or beriberi. A healthy diet (by definition) supplies the body with adequate amounts of all vitamins. This point applies equally to several minerals, such as potassium and magnesium. It seems highly probable that the dietary patterns that prevent such diseases as heart disease and cancer feature an optimal content of various vitamins, minerals, phytochemicals, dietary fibre, etc. It also seems highly likely that these many substances act synergistically in their large number of actions at many points in body function. In other words, it is the complex mix of substances in a healthy diet that makes the diet healthy.
As should now be clear, we have little real understanding of how vitamins (and other substances) act to prevent the chronic diseases of lifestyle. For that reason, this course presents a simplified approach to vitamins. In this unit, we focus on just five out of the 13 known vitamins. The ones selected will serve to illustrate where we obtain vitamins in the diet and what they do in the body. We stress that the ones left out are every bit as important as the ones included. Feel free to read all of Chapter 7 of the textbook to learn more about vitamins.
As emphasized above, it is generally true that dietary patterns have a much clearer relationship with risk of most diseases than do individual nutrients. However, there are some exceptions to this. In a small number of instances, evidence indicates that a particular vitamin or mineral has a relationship with health that is largely independent from the rest of the diet. In Canada, the clearest example of this is vitamin D, which will be discussed later in this unit and in other units. In the next unit, we will see how this principle also applies to iron and sodium (iron intake is often too low, and sodium intake is usually too high). Students should be aware that the nutritional norms in many developing countries are very different from those in Canada; deficiencies of some nutrients, such as vitamin A and iodine, are still a serious problem for millions of people in developing countries.
Section 2 General Characteristics of Vitamins
Read pages 249–250 “Definition and Classification of Vitamins” of Chapter 7: “The Vitamins.”
Vitamins are a group of organic compounds essential for the normal functioning of the body. They vary widely in their chemical structures and functions. Many act as coenzymes, helping enzymes in regulating metabolism (e.g., B vitamins). Others act like hormones (e.g., vitamin D), or are antioxidants (e.g., vitamins C and E). Some assist in the formation of bones and tissues (e.g., vitamins A and C). Although much is known about the vitamins, research into their roles and mechanisms of action is ongoing.
The beneficial effects of vitamins are often overrated by the general public; vitamins are sometimes regarded as having near magical properties. Consumers are frequently exploited by drug and health food companies. Routine vitamin supplementation is usually a waste of money and may sometimes be harmful. Healthy individuals who eat well-balanced diets seldom need vitamin supplements.
Foods within the four food groups of Eating Well with Canada’s Food Guide have different vitamin profiles, so eating a variety of foods and the recommended number of servings is important for achieving adequate vitamin intakes.
Most vitamins cannot be manufactured in the body; they must be obtained solely from food sources. Vitamin A and niacin can be synthesized by the body from dietary precursors; provitamin D can be synthesized under the skin with the help of sunlight.
Vitamins fall into two classes: water-soluble and fat-soluble. The differences between the two are described on page 250. Besides the differences described in the textbook, one additional characteristic should be noted. During cooking, water-soluble vitamins are generally less stable than fat-soluble vitamins. This will be discussed further in Section 8.
Read pages 289–291 titled “Food Feature: Choosing Foods Rich in Vitamins,” of Chapter 7: “The Vitamins.”
Review the sources of vitamins listed in Figure 7.17 (pp. 292 and 293). You will refer to this information throughout this unit. The section “Nutrient Enrichment of Foods Sold in Canada” (pp. 289–291) may be helpful for some parts of the assignment.
You do not need to memorize the RDAs for any of the vitamins or minerals.
Section 3 Introduction to the Fat-Soluble Vitamins
Read pages 250–251 (the introductory paragraphs) of “The Fat-Soluble Vitamins.”
Although each of the fat-soluble vitamins—A, D, E, and K—has distinct functions in the body, all share some characteristics. They are present in fatty or oily portions of foods, and they form aggregates (micelles) with long-chain dietary fats for absorption.
The fat-soluble vitamins are absorbed from the gut and are then taken up by the liver where they are stored or changed into biologically active forms to be transported by special protein carriers to various body sites.
Since the fat-soluble vitamins are insoluble in water, they cannot be excreted in the urine. Only small losses occur via bile, which are excreted in the feces. The majority of fat-soluble vitamins remain in the liver and adipose tissue, allowing the body to draw on stores when dietary intakes are low. Hence, deficiency symptoms may take a long time to develop. When intake is extremely high, stores can accumulate to toxic levels.
Section 4 Vitamin A
Study Snapshot 7.1, “Vitamin A and Beta-Carotene” on page 255, “Food Sources of Vitamin A” on pages 254–256, and “Does Eating Carrots Really Promote Good Vision?” on page 256.
As indicated in Snapshot 7.1 (see “Chief Functions”), vitamin A has many roles in the body, and it is critical for the functioning of many cells.
Adequate vitamin A is easy to achieve when the recommended amounts of milk, milk alternatives, and dark green and orange vegetables are consumed. The addition of vitamin A to milk and margarine means that vitamin A deficiency is quite rare in Canada. However, a significant fraction of the population consume a diet providing well below the RDA (Kirkpatrick & Tarasuk, 2008).
Section 5 Vitamin D
Study Snapshot 7.2: “Vitamin D” on page 260.
In the early 1900s, it was discovered that rickets can be cured by sunlight and cod-liver oil. 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 is now known to play multiple roles in body tissues. Its action includes the production and development of red blood cells, cellular development, and insulin secretion.
Vitamin D is different from other nutrients, both because it can be synthesized by the body and because it functions like a hormone in the regulation of mineral metabolism. Vitamin D is synthesized from cholesterol. This process requires sunlight (ultraviolet rays).
Vitamin D maintains blood calcium and phosphorus at the concentrations essential for normal mineralization of bones and teeth as well as for neuromuscular activity (nerve-muscle impulses) and other cellular processes that depend on these elements. If dietary calcium is inadequate, vitamin D can help remove calcium and phosphorous from bones to maintain blood levels.
Vitamin D deficiency most notably affects the bones. The childhood version of vitamin D deficiency is known as rickets; the adult version, as osteomalacia.
Rickets is characterized by enlargement 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 bones become bowed with body weight (see Fig. 7.5, p. 258).
Osteomalacia is characterized by gradual demineralization of the bones, particularly those of the pelvis, spine, and extremities. Bones become thin, and spontaneous fractures occur frequently. Painful joints and muscles are symptoms that distinguish osteomalacia from osteoporosis.
An inadequate intake of vitamin D is an important factor in osteoporosis. Osteoporosis is most common in older females and is closely associated with the risk of bone fractures. This topic is discussed in more detail in the next unit.
Other functions of vitamin D have been studied based on the detection of its receptors in a wide array of tissues It has been postulated that vitamin D is also involved in gene transcription (i.e., in controlling protein synthesis).
Exciting and potentially important evidence has emerged in recent years, suggesting that vitamin D may play an important role in the prevention of cancer. The relationship between vitamin D and cancer is explored further in Unit 13. The growing body of evidence linking vitamin D with the prevention of various diseases, especially osteoporosis and cancer, has led to recognition that an inadequate intake may be an important health problem, especially in northern countries such as Canada. At the same time, there has been much debate concerning the optimal intake of the vitamin, with many experts arguing that most adults require 25 micrograms (1000IU) per day, an amount that is well above the current Adequate Intake.
Vitamin D deficiency occurs in environmental conditions where exposure to sunlight is limited. Vitamin D deficiency is making a resurgence in Canada in certain population segments (Canadian Paediatric Society, 2004). Vulnerable populations include children of low income, inner-city families, breast-fed infants who do not receive supplemental vitamin D, dark-skinned people who cover up their skin (e.g., some South Asian women), vegetarians who do not drink vitamin D fortified milk, and elderly people living in institutions.
The margin of safety of vitamin D intake is particularly narrow compared to that of other vitamins. Vitamin D toxicity is not known from normal dietary sources but can occur with excessive intakes from supplements. For adults, intakes should not exceed 100 micrograms (4000IU) per day. Prolonged exposure to sunlight does not pose a risk of toxicity because the synthesis of vitamin D is carefully regulated.
Food sources of naturally occurring vitamin D are limited to liver, eggs, fish liver oils, and butter. In Canada, fluid milk, skim milk powder, margarine, and some brands of yogourt are fortified with synthetic vitamin D. Adequate milk intake (e.g., 2 cups, or 500mL), as recommended by Canada’s Food Guide, is the most important dietary source of vitamin D. By contrast, human breast milk does not contain enough vitamin D; hence, breast-fed babies should be given supplemental vitamin D.
Adult vitamin D requirements can be met through exposure to the sun, ranging from 10–15 minutes, two or three times a week (for fair-skinned people) to three hours a day (for darker-skinned people). It is important to limit exposure to intense sunshine, however, because of the increased risk of skin cancer associated with excessive sun exposure. In Edmonton, Alberta (52° North), UVB rays that synthesize cholesterol to vitamin D are not adequate from October to March—six months of the year (Webb, Kline, & Hollick, 1988). Overall, about one in 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).
Maintaining adequate body levels of vitamin D is clearly a challenge for people living in Canada, which is significant because a low body level of vitamin D poses a risk to health. Based on this information, Canada's Food Guide recommends that adults over the age of 50 consume a supplement supplying 10 micrograms (400IU) daily.
Section 6 Vitamin C
We now turn our attention to water-soluble vitamins.
Read page 246, “The Water-Soluble Vitamins.”
Vitamin C is also known as ascorbic acid. Ascorbic means free of scurvy, as this vitamin was discovered to be essential in preventing and curing scurvy. Today, vitamin C is probably the best known and most controversial of all the vitamins. Linus Pauling’s 1970 book, Vitamin C and the Common Cold, initiated an era in which megadoses of vitamins started to be used in attempts to cure or prevent diseases. Since then, vitamin C has been the most commonly taken single vitamin supplement, based on claims that it cures or prevents colds, flu, cold sores, cancer, atherosclerosis, stress, arthritis, and allergies.
Read Snapshot 7.5, “Vitamin C,” on page 269 and study the vitamin C section of Figure 7.17 (p. 293).
A diet rich with foods high in vitamin C is the ideal way to provide an adequate intake of this vitamin. Such foods also provide many other nutrients along with dietary fibre and phytochemicals. The dominant food sources of vitamin C are fruit and vegetables. Fruit, especially citrus fruits such as oranges and grapefruit, is an excellent source of the vitamin. Broccoli, tomatoes, green peppers, cauliflower, asparagus, and potatoes are also good sources.
If you compare the amount of vitamin C provided by a serving of fruit and vegetables with the RDA (75–90mg per day in non-smokers), you can see that it is easy to reach the RDA through dietary sources. Indeed, most people consume well above the RDA. However, a substantial minority of people fail to reach the RDA (Kirkpatrick & Tarasuk, 2008).
The textbook describes the possible hazards from consuming extremely high amounts (mega doses) of vitamin C (“Is too much Vitamin C Hazardous to Health?” p. 268). Low doses of supplemental vitamin C (100–200mg per day) are free of hazards.
In Canada, several juices are fortified with vitamin C, as are many artificial fruit drinks. However, fruit drinks contain few other vitamins, minerals, and phytochemicals that natural juices contain.
The prophylactic (disease preventing) and therapeutic uses of vitamin C in treating cardiovascular disease, the common cold, flu, and cancer are highly controversial. There are various hypotheses about the possible benefit of vitamin C supplementation in cardiovascular disease (CVD). Research studies have provided little supporting evidence. This will be examined further in the final section of the next unit.
As mentioned earlier, Linus Pauling received a great deal of publicity for his claim that massive doses of vitamin C (between 1 and 10 grams per day) prevents and cures the common cold and flu. Subsequent clinical trials have shown that the effect of vitamin C on colds—if any—is small. There is evidence that an increase in vitamin C intake may help you get over your cold sooner, but contrary to Pauling’s claim, massive doses will not prevent or cure the common cold in well-nourished people. However, there is evidence that supplements may have a preventive effect when taken by people who are marginally deficient in vitamin C. The amount of vitamin C that might help shorten the duration of a cold is quite easily obtained from the diet. The Consumer Corner section titled “Vitamin C and the Common Cold,” on pages 270–271 may be of interest (optional).
Section 7 B Vitamins
The B vitamins are often referred to as a group or complex, because they were originally thought to be a single vitamin. Later work demonstrated that “vitamin B” actually consists of several distinct chemicals. B vitamins
- act as coenzymes that serve as helpers to the enzymes involved in the metabolism of food.
- help cells to multiply (e.g., red blood cells and cells of the digestive and nervous systems).
- are usually consumed in adequate amounts. Folate and vitamin B12 tend to be the B vitamins that are sometimes inadequate in Canadian diets.
- are widely distributed in all food groups, except vitamin B12, which is found only in food of animal origin.
- tend to be concentrated in the outer coating of grains; milling, therefore, causes a major loss.
- are water-soluble, are not stored in the body to any appreciable extent (except B12), are relatively non-toxic, and are excreted in the urine when consumed in excess.
Pages 270–282 of the text cover the B vitamins. Skim through these pages. Here we will focus on just two of the B vitamins: folate and vitamin B12.
Read Snapshot 7.9, “Folate” on page 279.
Folate derives its name from Latin folium, meaning foliage or leaf, because it was first isolated from spinach leaves and was known to be concentrated in green, leafy plants. It functions chiefly as a part of coenzymes needed for new cell synthesis. This role is critical for many cells throughout the body. For that reason, a lack of folate negatively affects cells that divide quickly or have a high turnover, especially red blood cells, white blood cells, and a developing fetus. This explains why a low intake of folate creates a risk of neural tube defects (NTDs).
The best time for a woman to take folic acid is before becoming pregnant, but as many as 40% of all pregnancies are unplanned, so use of folic acid supplements is appropriate for women during their childbearing years. Canada’s Food Guide recommends that women take a multivitamin supplement with folic acid daily, including women who could become pregnant or who are pregnant or breastfeeding. However, less than 20% of women follow this recommendation.
In 1998, a policy was introduced requiring that folic acid (supplemental folate) be added to grain products in Canada. As a result, there has been a sharp drop in the incidence of NTDs since then (see textbook p. 278). However, a relatively low intake of the vitamin is still fairly common, especially among people living in poverty (Kirkpatrick & Tarasuk, 2008).
Some outstanding sources of folate are shown in Snapshot 7.9 (p. 279). They include green leafy vegetables such as spinach, asparagus, and broccoli. Dried beans, lentils, and liver are also excellent sources. As noted above, folic acid is added to many refined breads and ready-to-eat cereals in Canada. The most significant food sources of folate in the diet are enriched grains and vegetables, followed by meat alternatives, especially dried beans, lentils, and eggs. Whole grains are also a good source.
Read Snapshot 7.10: “Vitamin B12” on page 281.
Vitamin B12 is needed to activate folate and works cooperatively with folate in the synthesis of DNA during cellular reproduction. Accordingly, tissues that have a high turnover, such as red blood cells and cells of the gastrointestinal tract, are affected by B12 deficiency.
In the stomach, vitamin B12 is released from protein complexes in food by hydrochloric acid. This gastric secretion also contains intrinsic factor, which combines with vitamin B12 and allows it to be absorbed.
Primary deficiency of vitamin B12 is mainly confined to strict vegans (persons living exclusively on foods of plant origin) since this vitamin is present only in foods derived from animals. The main cause of secondary deficiency is the lack of intrinsic factor (that may emerge in middle age) or the atrophy of stomach cells. Deficiency may take a long time to develop (three to six years) because the liver maintains a large store of the vitamin.
Vitamin B12 deficiency causes the well-defined syndrome of pernicious anemia. More accurately, pernicious anemia is caused by a lack of intrinsic factor, rather than lack of B12. Pernicious anemia is characterized by the following symptoms:
- macrocytic or megaloblastic anemia
- nerve damage. Extremities are affected first, causing numbness and tingling in the hands and feet. Damage progresses to the spinal column, resulting in paralysis, and finally to the brain. It begins with mental confusion and agitation, progresses to depression, and finally leads to psychotic changes, such as delusions, hallucinations, and paranoia.
The only source of B12 in the human diet is from foods of animal origin. Since liver is the storage site for B12, it is an excellent food source of vitamin B12. Meat, milk, and eggs are other good sources. Snapshot 7.10 on vitamin B12 (p. 281) highlights the different animal sources.
Omnivores and lacto-ovo vegetarians usually achieve adequate intakes of B12. People over the age of 50 years may consume enough B12 but not absorb adequate amounts due to the lack of intrinsic factor that may occur in this population.
Section 8 Vitamin and Mineral Stability in Foods
Read pages 548–550, “Food Feature: Preserving Nutrients in Foods,” of Chapter 12, “Food Safety and Food Technology.”
Re-read pages 126–128, “Consumer Corner: Refined, Enriched, and Whole-Grain Bread” of Chapter 4, “The Carbohydrates: Sugar, Starch, Glycogen and Fibre.”
Minerals and water-soluble vitamins are lost in food processing. Water-soluble vitamins that are particularly sensitive to poor food storage, cooking, and extensive processing are folate and vitamin C. They are sensitive to light, heat, and air so they tend to be lost with almost any processing or cooking method. Minerals are easily leached out of vegetables into cooking water. As the textbook discusses on page 548 (with respect to orange juice), some losses are acceptable where a high nutrient level remains in the food.
Most discussions on maximizing vitamin content of foods tend to focus on vegetable preparation because cooks can control the cooking times and methods.
Suggestions to minimize loss of nutrients through cooking include purchasing vegetables and fruits often and in small quantities. This is not a popular suggestion in North America, where weekly grocery shopping is considered the norm. The reason for purchasing fruits and vegetables more often is that vitamin C synthesis in plants stops and degradation of vitamin C begins upon harvest. Refrigeration slows down vitamin degradation.
Cut up fruits and vegetables just before cooking or serving, as exposure to air speeds up the breakdown of vitamin C and folate.
Cook foods in as small a quantity of water as possible and for as short a time as feasible. As the textbook discusses, steaming, baking, and microwaving are excellent ways of conserving vitamins, and are preferable to boiling. Avoid high temperatures and long cooking times (pp. 549–550) such as that which occurs with roasting skinless vegetables (e.g., potatoes) or with cooking food in steam tables in cafeterias or buffets. Consume food as soon as possible after cooking.
Avoid using baking soda. The addition of baking soda in cooking water softens tough vegetables and enhances their colour (e.g., cooked broccoli or peas), but destroys vitamin C and some B vitamins. Baking soda may be used where food is served over a long period of time, such as a buffet or cafeteria line.
In addition to vegetables, another food group that experiences major vitamin and mineral losses are grains as a result of the process of refining. For that reason, whole grains provide much higher levels of several B vitamins and minerals than refined grains. To help compensate for this several nutrients are added to refined grains. Refined flour and bread with added nutrients are called “enriched.” Enriched flour (mandated in Canada) helps to ensure adequate thiamin, riboflavin, niacin, folate, and iron intakes. Fibre, vitamin B6, vitamin E, magnesium, and zinc are lower in breads and bakery products made with enriched flour than with whole grains (see Fig. 4.8, p. 127).
Vitamin B12 and fat-soluble vitamins are relatively stable. For example, vitamin A levels in vegetables remain stable despite a range of cooking methods such as steaming, boiling, or roasting.
See the textbook, pages 303–304, “Self-Check” questions 1, 2, 3, 5, 6, & 7.