1. Jamie Pope, Steven Nizielski, and Alison McCook
NUTRITION
for a Changing World
FIRST EDITION
Chapter 10
Fat-Soluble Vitamins
Soak Up the Sun?
As my classes include two class periods in one day, I generally teach chapters 10 and 11 in one day. I have been using polling questions as competition, asking questions just prior to covering concepts instead of following concepts to assess understanding. I have included several polling slides. Feel free to delete them or move them.
© 2016 by W. H. Freeman and Company & Scientific American

2. Chapter 10
Objectives
Identify the fat-soluble vitamins and their primary functions
Describe the properties of the fat-soluble vitamins
Describe the role of vitamin D in blood calcium regulation
Identify excellent dietary sources of the fat-soluble vitamins
Describe the role of vitamin A in healthy vision
List sources of free radicals and describe how antioxidants can block oxidative damage
Explain how vitamin E works as an antioxidant in the cell membrane

3. Characteristics of vitamins include all of the following EXCEPT they:
Are considered micronutrients
Do not provide energy
Contain carbon molecules
Are incorporated into the structure of cells
Are necessary for development and growth
Correct answer is D. Vitamins do not have structural roles in cells.

4. Vitamins are… Organic compounds Micronutrients Essential nutrients
Contain both carbon-carbon and carbon-hydrogen bonds
Micronutrients
Needed in small quantities
Essential nutrients
Must be consumed in the diet to meet the body’s needs
Individual units
Are not a source of energy (calories)
But need to extract energy from macronutrients
Vitamins are chemical constituents (organic compounds) in food, required in small amounts for very specific functions. Consequently, vitamins (and minerals) are considered micronutrients because they are required in much smaller amounts than macronutrients (carbohydrates, fat, protein, and water). In nearly all cases, human cells cannot synthesize vitamins, so it is essential to obtain them through diet. Vitamins neither provide energy nor are they incorporated into structures of the body’s cells. They are needed to extract energy from macronutrients, as well as provide many regulatory and metabolic functions. The few vitamins manufactured by the body (or by intestinal bacteria) are not produced in amounts sufficient to meet our needs. Vitamins, unlike macronutrients, are individual units that do not have to be broken down into smaller components for utilization by the body.

5. There are 14 known vitamins.
FAT-SOLUBLE VITAMINS
WATER-SOLUBLE VITAMINS
B-complex vitamins
Thiamine (vitamin B1)
Riboflavin (vitamin B2)
Niacin (vitamin B3)
Vitamin B6 (pyridoxine)
Folate
Vitamin B12
Biotin
Pantothenic acid
Vitamin C
Choline
Vitamin A
Vitamin D
Vitamin E
Vitamin K

6. Vitamins are classified according to their solubility.
Ability to disperse or dissolve in water or lipids
Fat-soluble
Vitamins A, D, E, and K
Water-soluble
B vitamins, choline, and vitamin C
Determinant of how absorbed and transported in body
Bioavailability influenced by many factors
Affects where they are stored and risk for toxicity
Bioavailability refers to the proportion of a vitamin that can be absorbed, transported, and utilized by the body once it has been consumed. It is influenced by a number of physiological and dietary factors.

7. Fat-soluble vitamins have diverse functions in the body but share common characteristics.
Insoluble in water
Dissolve in lipids
Small amounts needed
Absorption in small intestine
Leave small intestine in chylomicrons
Stored in tissue
Potential for toxicity
Often present in fatty portion of foods
Vitamins A, D, E, and K are insoluble in water, dissolve in lipid, and require bile required for digestion. They are absorbed like fat from the small intestine into the lymph and require chylomicrons for transport. Fat-soluble vitamins can be stored in fatty tissues and liver. Not excreted as easily as water-soluble vitamins. Dietary deficiency takes longer to develop when dietary intake is low, but the potential exists for toxicity. Each vitamin has its own distinct functions, metabolism, and mechanism of action, but they share general properties.

8. The Fat-Soluble Vitamins and Their Primary Functions
The fat-soluble vitamins have diverse functions in the body. For example, vitamin A is involved in eyesight, while others are involved in such roles as bone growth, blood clotting, and antioxidant activity.

9. Vitamin D Can be synthesized in the skin
Synthesis can be blocked by sunscreens
Good sources are found in fatty fish and fortified foods
Activated in the kidneys and liver
All of the above
Correct answer is E

10. Kimani the gorilla suffered from vitamin D deficiency.
Insufficient sun exposure and dietary inadequacy
Rickets
Bone malformation and muscle weakness
Inadequate intake or improper absorption and utilization of vitamin D
Received high doses of supplemental vitamin D
Responded and improved
Chapter 10 story overview: In 2005 Michael Holick, PhD, MD, (an endocrinologist) was called to see a 7-month-old gorilla born in captivity at Franklin Park Zoo. The gorilla exhibited signs of rickets, but a normal dose of vitamin D didn’t make much difference. Dr. Holick recommended 10-fold the normal dose, and Kimani responded. She will likely never be as tall as other gorillas, but she improved.

11. Vitamin D can be produced in the skin from cholesterol and exposure to UV light.
“Sunshine vitamin”
Insufficient sun exposure makes it essential to consume in diet
Also consumed through the diet
Must be activated in the kidneys and liver to function in the body
Most of us meet at least some of our vitamin D needs through sun exposure, but with limited exposure or impaired synthesis it becomes important to consume sufficient vitamin D through our diet. Whether through synthesis in the skin or through foods, vitamin D must be activated in the kidneys and liver to fulfill its biological functions in the body.

12. In 2010, the Institute of Medicine increased the RDA for vitamin D.
The RDAs for vitamin D intake through food sources are established on the basis of minimal sun exposure.
In 2010, the Institute of Medicine increased the RDA for vitamin D.
Increased threefold for adults 19 to 50 years of age, from 200 to 600 IU.
Although sunlight can be a significant source of vitamin D, the RDAs—established to maintain bone health and normal calcium metabolism in healthy people—are set on the basis of minimal sun exposure. In 2010, the Institute of Medicine increased the RDA for vitamin D for all age groups. It was increased threefold to 600 IU (15 mcg) for adults in the 19 to 50 years age range. However, the recent appreciation of the importance of vitamin D for functions beyond that of bone health has caused some scientists to question whether the new RDAs are actually high enough to provide for optimal health.

13. Fish and fish liver oils Whole grain foods
Most Americans get much of their vitamin D from fortified milk. What is the richest source?
Fruits and vegetables
Eggs and cheese
Fish and fish liver oils
Whole grain foods
Correct answer is C.

14. Vitamin D does not occur naturally in many foods besides fish; most common sources in the U.S. diet are fortified foods.
It can be difficult to meet the RDAs from food alone, for example because vitamin D is present in very few foods naturally. Some of the best sources include fatty fish (such as salmon, tuna, and mackerel) and fish liver oils. Beef liver, cheese, eggs, and some mushrooms (particularly those exposed to UV light) contain small amounts of vitamin D. Alternatively, most Americans get dietary vitamin D from fortified foods. Most of the U.S. milk supply is fortified with vitamin D (100 IU, or 2.5 mcg, per cup). This is the result of a milk fortification program that was instituted in the 1930s in the United States to reduce the incidence of rickets, a significant public health issue at the time.

15. Vitamin D must be activated in the kidneys and liver to fulfill its biological functions in the body.
Whether through synthesis in the skin or through foods, vitamin D must be activated in the kidneys and liver to fulfill its biological functions in the body.

16. Vitamin D has many important functions in the body.
Acts as a hormone
Helps bone growth and maintenance
Regulates calcium metabolism
Blood levels
Absorption
Excretion
Regulates the synthesis of certain proteins
Possibly regulates cardiovascular function
Ongoing research of other roles in the body
Vitamin D has many important known functions in the body. Mainly, as illustrated by the case of Kimani, vitamin D plays a key role in the growth and maintenance of bone by maintaining blood concentrations of minerals involved in bone development, such as calcium and phosphorus. In addition, vitamin D works in combination with other nutrients and hormones in bone growth and maintenance. But beyond bone health, research continues to explore and expand vitamin D’s role in human health.

17. Vitamin D functions as a hormone to regulate calcium metabolism and blood concentration.
Calcitriol increases absorption of calcium in the intestine.
Active vitamin D and parathyroid hormone decrease calcium excretion by the kidneys.
Vitamin D also plays an important role in the regulation of calcium. Most calcium in the body is stored in bones (see Chapter 13), with less than 1% circulating in the blood; however, this circulating calcium is critical for normal muscle and nerve functioning and must be maintained. The regulation of blood calcium requires the interaction of bones, kidneys, and intestines. For example, when the concentration of calcium in blood falls, vitamin D is converted into its fully active hormone form (calcitriol) in the kidneys. This active hormonal form of vitamin D then increases the absorption of calcium in the gastrointestinal tract—for this reason, you will often find calcium supplements that also contain vitamin D. Activated vitamin D also works together with parathyroid hormone to decrease calcium excretion by the kidneys and increase its release from bone to raise blood calcium concentrations.

18. Vitamin D is considered a nutrient of concern in the Dietary Guidelines for Americans.
~8% of Americans at risk for vitamin D deficiency
Groups at risk
Avoid vitamin-D fortified dairy foods
People with dark skin
Inadequate sun exposure
Sunscreens block UV rays and prevent vitamin D synthesis
Exclusively breast-fed infants
Breast milk not rich source of vitamin D
Elderly
Reduced ability to synthesize vitamin D
Reduced sun exposure
Potential diminished intake
Vitamin D is listed as a nutrient of concern in the 2015 Dietary Guidelines for Americans. Although 80% of Americans are reported to receive enough of the nutrient, according to data from the 2001–2006 National Health and Nutrition Examination Survey, about 8% of Americans were at risk for vitamin D deficiency. People most at risk are those who avoid vitamin D-fortified dairy products (because of allergies, intolerance, or a vegan diet); people with dark skin (since the melanin that darkens skin can interfere with the body’s ability to make vitamin D from sunlight); and those with little sun exposure, or who regularly use sunscreen. Although sunscreen can block UV rays and reduce the risk of skin cancer, it also prevents vitamin D synthesis. Other groups at risk include infants who are exclusively breast-fed because breast milk is not a rich source of vitamin D, and the elderly, who have a reduced ability to synthesize vitamin D when exposed to sunlight. Many elderly individuals are also more likely to stay indoors and therefore have less opportunity to synthesize vitamin D.

19. Evidence that deficiency may increase risk of:
Having extremely low levels of vitamin D can, over time, cause serious bone diseases.
Evidence that deficiency may increase risk of:
Rickets (in children)
Osteomalacia (in adults)
Other chronic diseases and conditions:
Cancer
Autoimmune disease
Kidney disease
Type 2 diabetes
Cardiovascular disease
Rickets is caused by low vitamin D levels, leading to impairments in the maturation and mineralization of cartilage in regions of the bone where growth is occurring, causing the characteristic “bowed” legs or “knocked” knees. In adults, the same deficiency can cause osteomalacia, in which the bone mineral is being depleted, causing the bones to become soft or weak and putting people at risk of fractures or falls, as well as creating pain in the pelvis, lower back, and legs. Research also now provides evidence that sufficient intake and stores of vitamin D may reduce the risk of a number of important diseases, including cancer, autoimmune diseases, kidney disease, type 2 diabetes, and even cardiovascular disease.

20. The best source of vitamin A is carrots.
True
False
Correct answer is B. Carrots contain beta-carotene which has limited bioavailability. Preformed sources are superior.

21. Vitamin A encompasses a group of fat-soluble compounds.
Retinoid compounds
Retinal
Retinol
Only retinoid is present in significant amounts in our diet.
It is called preformed Vitamin A because it is already in the active form in foods.
Retinoic acid
Carotenoids
Provitamin as only has vitamin activity after conversion to active form in the body
Beta-carotene is primary carotenoid in our diet.
Retinol is the only retinoid present in our diet. It is readily converted to retinal or retinoic acid in the body. These are the active forms of vitamin A. Carotenoids are provitamins, as they must be converted in the body into one of the active forms of the vitamin. The primary carotenoid is beta-carotene.

22. The bioavailability of vitamin A differs upon the food source.
Preformed vitamin A (retinol)
More readily absorbed
Animal foods and fortified foods
Provitamin A carotenoids
Yellow-, orange-, and red-pigmented fruits and vegetables
RDA given as mcg of retinol activity equivalents (RAE)
Conversion of beta-carotene to retinol is never complete.
Absorption of carotenoids from food is poor.
Slicing, chopping, cooking, juicing, and cooking can improve bioavailability.
The bioavailability of vitamin A differs upon the food source. Preformed vitamin A (retinol), found in foods from animal sources, is more easily absorbed than the carotenoids found in foods from plant sources. In the United States and Canada, reduced-fat milk and yogurt must be fortified with vitamin A to make up for the loss of this vitamin during the removal of the fat. Provitamin A (beta-carotene) must be converted into an active form in the body before it can fulfill biological functions. To account for these differences in bioavailability, the DRI values are expressed as micrograms (mcg; one millionth of a gram) of retinol activity equivalents, or RAE. The RDA for vitamin A for men aged 19 to 50 years is thus 900 mcg RAE; for women of the same age, it is 700 mcg RAE. Of the provitamin A carotenoids, beta-carotene is best converted into retinol, but this conversion is never complete. Because the release of carotenoids from food is difficult, processing the food by slicing, chopping, and even juicing can improve bioavailability. The smaller food particles are more completely broken down by mechanical digestion than are larger particles, allowing nutrients to disperse more readily into digestive fluids. Cooking can also often increase nutrient bioavailability because it ruptures plant cells, releasing the nutrients that otherwise might be trapped within those cells and subsequently excreted. Thus, the potential availability of vitamin A is actually higher from sliced and cooked carrots than from raw carrots.

23. Most of the vitamin A consumed in the U. S
Most of the vitamin A consumed in the U.S. diet comes from preformed sources.
In the United States, approximately two-thirds of the vitamin A is consumed as preformed vitamin A (retinol) from fortified foods, supplements, and foods that naturally contain retinol. Preformed vitamin A occurs in animal products; it is highest in liver, but fish, eggs, and dairy foods (containing fat) are also good sources. In the United States and Canada, reduced-fat milk and yogurt must be fortified with vitamin A to make up for the loss of this vitamin during the removal of the fat. The rest of our vitamin A comes from provitamin A carotenoids, which are the yellow, orange, and red pigments of fruits and vegetables—think sweet potatoes, carrots, cantaloupe, and apricots, as well as dark, leafy greens (which, despite their color, contain a lot of beta-carotene). Even when beta-carotene is consumed as a supplement in oil (to improve absorption), it takes two micrograms of beta-carotene to provide the equivalent of one microgram of retinol. Because the absorption of carotenoids from foods is even poorer than from supplements, you need to eat 12 micrograms of dietary beta-carotene to reach the equivalent RAE of only 1 microgram of retinol.

24. Vitamin A serves many critical biochemical and physiological functions in the body.
Vision
Functions as a hormone in:
Cell development
Immune function
Growth
Bone health
Reproduction
Vitamin A serves many critical biochemical and physiological functions in the body related to vision, cell development, immune function, and growth. Along with these important functions, vitamin A also plays key roles in bone health and reproduction, such as sperm and fetal development. In all functions except vision, vitamin A acts as a hormone by controlling the synthesis of numerous proteins encoded in our genes. The hormonal actions of vitamin A play a key role in normal, healthy cell development, such as in cell differentiation (the process by which cells become progressively more specialized). Vitamin A is particularly important to epithelial cells, which form the skin and mucous membranes inside the body, such as those present in our eyes, lungs, and intestines. It also plays a role in the development of immune cells, which affect how well our immune system functions and, therefore, influences our susceptibility to disease. In developing countries, a child’s vitamin A deficiency significantly increases his or her risk of death from an infectious disease.

25. Vitamin A is a key component of rhodopsin which is required to absorb light and trigger nerve impulses in the brain for vision.
Insert infographic 10.6
One of the forms of vitamin A (retinal) is a necessary component of the visual pigment rhodopsin that detects light in our eyes. The cells at the back of the eye (the rods in the retina) that are primarily responsible for allowing us to see in low light are particularly sensitive to a vitamin A deficiency.
Vitamin A is required by our eyes to convert light into nerve impulses that bring messages to the brain, telling us what we’re seeing. Specifically, vitamin A is a key component of rhodopsin, the visual pigment that is formed when retinal binds to the protein opsin. Rhodopsin is found in light-sensing cells within the retina at the back of the eye. When rhodopsin absorbs light, retinal changes its shape and is then released, triggering a chain of events that generate a nerve impulse that transmits the visual signal to the brain. When the vitamin A level is low, these light-sensing cells are unable to quickly regenerate rhodopsin, which can make it difficult to see in low light. We also need vitamin A to maintain the health of the cornea, the clear outer covering at the front of the eye.

26. What does the plant form of vitamin A have in common with vitamin C and E?
You can’t get enough unless taken as a supplement.
They function as antioxidants.
They are all fat-soluble vitamins.
They all have a role in blood clotting.
Correct answer is B.

27. Beta-carotene has an important role as a phytochemical with potent antioxidant properties.
Plant forms of vitamin A
Carotenoids and beta-carotene
Function as antioxidants
Help protect cells from oxidative damage caused by reactive molecules called free radicals
Eating foods rich in carotenoids is associated with less risk for cancer, but research suggests that supplements may actually increase cancer risk. The plant form of vitamin A, beta-carotene, as well as the other dietary carotenoids have important roles as health-promoting phytochemicals. One of the health-promoting functions of carotenoids may be their ability to function as antioxidants in the body, which helps protect cells from damage caused by highly unstable molecules called free radicals.

28. High levels of free radicals can cause cellular damage (oxidation).
Sources of free radicals and their effects
Free radicals are molecules that have an unpaired electron and that are naturally formed in the body or present in the environment. At high levels they damage cells, including DNA, through a process called oxidation. Oxidative damage can lead to chronic diseases like cancer and heart disease, so eating a plant-rich diet that contains beta-carotene and other antioxidants may reduce the risk of those diseases. We actually need some of the free radicals generated in the body for several important functions, such as cellular communication and killing bacteria.

29. Vitamin A Deficiency Can lead to osteoporosis
Is prevalent in the United States
Is primary cause of blindness in children worldwide
Can turn skin yellow or orange
Correct answer is C.

30. Vitamin A deficiency is a leading cause of blindness in children worldwide.
Hypovitaminosis A
Night blindness
Permanent blindness
Impaired immunity
Rough, dry, or scaly skin
Keratinization
Given its crucial roles in the body, vitamin A deficiencies can have major effects on health. Although uncommon in the United States, people in developing countries develop hypovitaminosis A if they have little access to preformed vitamin A in animal foods and beta-carotene. The hallmark symptoms of vitamin A deficiency affect the eye. Night blindness occurs first, the result of problems in the synthesis of rhodopsin. Then, production of mucous in tears is decreased, and the cornea of the eye begins to dry. If the deficiency continues, permanent blindness is the result. Vitamin A deficiency is the leading cause of preventable blindness in children worldwide.
Other symptoms include impaired immunity (increasing the risk of infections), and rough, dry, or scaly skin, resulting from a loss of moisture in the mucous membranes. This last symptom, known as keratinization, occurs as the epithelial cells that cover the surface of our bodies (like the cornea of the eye) and line body cavities cannot develop properly. As a consequence, they become filled with a fibrous structural protein called keratin. These are the same type of proteins that make up our nails, hair, and even the horns of animals.

31. Consuming too much vitamin A can result in hypervitaminosis A.
Excess beta-carotene from plant sources does not cause toxicity.
Can temporarily turn skin yellow or orange
Usually as a result of excess supplementation
Result in defects and weaknesses in bones
Can lead to osteoporosis
UL for adults is 3,000 mcg RAE
Pregnant women need to be cautious of excess vitamin A.
Can cause birth defects

32. A good source of vitamin E in the diet is:
Almonds
Eggplant
Milk
Strawberries
Correct answer is A.

33. Vitamin E is found in a variety of foods, but the best sources are many nuts and some oils.
Dietary sources
Plant-based oils
Nuts and seeds
Wheat germ
Fortified foods

34. Incorporated into cell membranes Healthy immune function
Vitamin E encompasses a group of fat-soluble compounds called tocopherols.
Functions
Incorporated into cell membranes
Antioxidant protecting against oxidative damage
Healthy immune function
Little evidence that vitamin E supplementation reduces risk of heart disease or cancer
Diet rich in vitamin E from foods sources may be protective
Despite low intake among many Americans, documented deficiency rare
Because of its unique ability to effectively be incorporated into cell membranes, vitamin E acts within membranes as an antioxidant, protecting cells throughout the body from the oxidative damage that results from exposure to free radicals produced in the body or present in the environment. Vitamin E appears to be of particular importance in maintaining healthy immune function by protecting white blood cells from oxidative damage, particularly in aging adults. Adequate intake of vitamin E is also necessary to prevent damage to nervous tissue.

35. Vitamin E functions as an antioxidant within cellular membranes.

36. Vitamin E is less likely to be toxic than vitamins A or D.
UL = 1,000 mg or 1,500 IU
60 times the RDA
Only observed with very high supplement intake
Increases tendency to bleed

37. A significant amount of our vitamin K requirement comes from:
Use of supplements
Intake of dairy foods
Production by bacteria in the intestine
Breakfast cereals
Correct answer is C.

38. Dietary sources Leafy greens Some fruits Some cheeses Some oils
A significant amount of our vitamin K requirement is produced by intestinal bacteria.
Dietary sources
Leafy greens
Some fruits
Some cheeses
Some oils
The most significant dietary sources of vitamin K are green leafy vegetables; some cheeses, fruits, and vegetable oils are also good sources. The requirements are given in Adequate Intake (AI) amounts since current knowledge does not provide sufficient evidence to establish a specific RDA.

39. Vitamin K has a key role in blood clotting.
Needed for synthesis of protein for blood clotting
Deficiency can cause uncontrolled bleeding
Bone metabolism
Modifies bone proteins
Vitamin K is required to complete the synthesis of several proteins in blood that cause the blood to clot when those proteins are activated. Deficiency can cause uncontrolled bleeding, or hemorrhaging. This can result from long-term use of antibiotics that destroy intestinal bacterial, or the use of medications that prevent clotting (anti-coagulants) and interfere with the metabolism of vitamin K. People on these types of medications must strive to keep vitamin K intake consistent, as the vitamin can interfere with the medication.
The vitamin also plays a key role in bone metabolism by modifying bone proteins, which allows them to bind calcium and regulate bone formation. Although studies suggest that a diet high in vitamin K is associated with lower risk of hip fractures in aging adults, current evidence is insufficient to recommend supplementation for the prevention of osteoporosis or fractures.

40. Vitamin K reduces the risk of hemorrhage in newborns.
Since the early 1960s, infants receive vitamin K at birth to reduce the risk of hemorrhage because they have limited stores and lack bacteria to produce vitamin K.
All babies are born with low levels of vitamin K. Babies generally have enough vitamin K stores to stop bleeding. However, one in 10,000 babies experience vitamin K deficiency bleeding. These babies don’t have enough vitamin K to make their blood clot. For this reason, doctors recommend that all babies receive vitamin K at birth.

41. Summary
Vitamins are organic compounds (containing both carbon–carbon and carbon–hydrogen bonds) that are needed in small quantities for very specific functions, such as the maintenance of regulatory and metabolic processes in the body.
Most vitamins are in their active form (preformed) in foods; however, in some cases, inactive precursor forms of vitamins, called provitamins, must be converted to fulfill biological functions in the body.
Inadequate or excessive intake of vitamins can lead to deficiency or toxicity, with adverse health effects.
The 14 known vitamins are classified by their solubility, as fat-soluble or water-soluble.
The fat-soluble vitamins (A, D, E, and K) have diverse functions in the body.

42. Summary (Cont’d)
Fat-soluble vitamins are transported in the body via the lymph, and excess intake is stored in fatty tissues and the liver, from which the vitamins are released when needed.
The body derives vitamin A from preformed A (retinol) found in animal foods and fortified foods, and to a lesser degree from provitamin A carotenoids in plant foods, as conversion to the active form is incomplete.
Vitamin A serves many critical biochemical and physiological functions in the body related to vision, cell development, reproduction, immune function, and growth, among others; the carotenoids have antioxidant properties in the body, which help protect cells from damage caused by free radicals.

43. Summary (Cont’d)
Vitamin D, which functions as a hormone in the body, is actually not considered an essential nutrient in the diet unless there is inadequate sun exposure because it can be synthesized from cholesterol through exposure of the skin to UV light; among other important functions, vitamin D plays a key role in the growth and maintenance of bone, in part through its role in maintaining calcium concentrations in the blood.
Serving primarily as an antioxidant in the body, vitamin E is a group of compounds called tocopherols, found primarily in nuts and vegetable oils.
Vitamin K is an essential nutrient found in foods such as leafy greens; a significant amount of our daily requirements may also be produced by bacteria in the intestine.
Vitamin K plays a key role in blood clotting and bone metabolism.