Chapter 9:
The Fat-Soluble
Vitamins
Vitamins
Defining a vitamin
Essential organic substances
Body cannot synthesize enough to maintain health
Absence from the diet will produce deficiency signs and symptoms
Yield no energy, but facilitate energy- yielding chemical reactions
May be fat- or water-soluble
Natural versus synthetic
Historical Perspective
Foods found to cure illnesses
Liver extract – vision problems
Citrus fruits – scurvy
All vitamins have probably been
discovered – TPN can support life
Storage of Vitamins in the Body
Fat-soluble are generally stored
except vitamin K
Water-soluble are generally excreted
except vitamin B12 and B6
Vitamin deficiency occurs when that
vitamin is lacking in the diet and body
stores are exhausted
Vitamin Toxicity
Toxicity of vitamin A is most likely
Unlikely to develop toxicity of any vitamin unless individual vitamin supplements are used
Balanced multivitamin and mineral
supplement supplies < 2 x RDA
Malabsorption of Vitamins
Fat malabsorption leads to deficiencies of fat-soluble vitamins
Alcohol abuse affects absorption of some B vitamins
Intestinal diseases affect absorption of
some B vitamins
Digestion and Absorption
of Fat-Soluble Vitamins
Absorption of Fat-Soluble Vitamins
Adequate absorption depends of absorption of fat
40 – 90% of ingested fat-soluble vitamins absorbed (less efficient when consumed in higher quantities)
Distribution of Fat-Soluble Vitamins
Transported by chylomicrons and
lipoproteins
Vitamin A
Deficiency of vitamin A is the most common cause of non-accidental blindness, worldwide
Preformed
Retinoids (retinal, retinol, retinoic acid)
Found in animal products
Provitamin A
Carotenoids
Must be converted to retinoid form
Intestinal cells can split carotene in two (molecules of retinoids)
Found in plant products
Terminal Ends of Retinoids
Conversion of Carotenoids to Retinoids
Enzymatic conversion of
carotenoids occurs in liver or intestinal cells, forming
retinal and retinoic acid
Provitamin A carotenoids
Beta-carotene
Alpha carotene
Beta-cryptoxanthin
Other carotenoids
Lutein
Lycopene
Zeaxanthin
Absorption of Vitamin A
Retinoids
Retinyl esters broken down to free retinol in small intestine - requires bile, digestive enzymes,
integration into micelles
Once absorbed, retinyl esters reformed in intestinal cells
90% of retinoids can be absorbed
Carotenoids
Absorbed intact, absorption rate much lower
Transport and Storage of Vitamin A
Liver stores 90% of vitamin A in the body
Reserve is adequate for several months
Transported via chylomicrons from intestinal cells to the liver
Transported from the liver to target tissue as
retinol via retinol-binding protein, which is
bound to transthyretin
Retinoid Binding Proteins
Target cells contain
cellular retinoid binding proteins
Direct retinoids to
functional sites within cells
Protect retinoids from degradation
RAR, RXR receptors on the nucleus
Retinoid-receptor
complex binds to DNA
Excretion of Vitamin A
Not readily excreted
Some lost in urine
Kidney disease and aging increase risk of
toxicity because excretion is impaired
Functions of Vitamin A:
Vision
Retinal turns visual light into nerve signals in retina of eye
Retinoic acid required for structural components of eye
Cones in the retina
Responsible for vision under bright lights
Translate objects to color vision
Rods in the retina
Responsible for vision in dim lights
The Visual Cycle
Functions of Vitamin A:
Growth and Differentiation of Cells
Retinoic acid is necessary for cellular differentiation
Important for embryo development, gene expression
Retinoic acid influences production,
structure, and function of epithelial cells that line the outside (skin) and external
passages (mucus forming cells) within the
body
Functions of Vitamin A:
Immunity
Deficiency leads to decreased resistance to infections
Supplementation may decrease severity of
infections in deficient person
Vitamin A Analogs for Acne
Topical treatment (Retin-A)
Causes irritation, followed by peeling of skin
Antibacterial effects
Oral treatment
Regulates development of skin cells
Caution regarding birth defects
Possible Carotenoid Functions
Prevention of cardiovascular disease
Antioxidant capabilities
≥5 servings/day of fruits and vegetables
Cancer prevention
Antioxidant capabilities
Lung, oral, and prostate cancers
Studies indicate that vitamin A-containing foods are more protective than supplements
Age-related macular degeneration
Cataracts
In general, foods rich in vitamin A and other phytochemicals are advised rather than
supplements
Vitamin A in Foods
Preformed
Liver, fish oils, fortified milk, eggs, other fortified foods
Contributes ~70% of vitamin A intake for Americans
Provitamin A carotenoids
Dark leafy green, yellow-orange
vegetables/fruits
Measuring Vitamin A
International unit (IU)-crude method of measurement
Retinol activity equivalent (RAE) -current, more precise method of measurement
RDA for Vitamin A for Adults
900 RAE for men
700 RAE for women
Average intake meets RDA
Much stored in the liver
Vitamin A supplements are unnecessary
No separate RDA for carotenoids
Deficiency of Vitamin A
Most susceptible populations:
Preschool children with low F&V intake
Urban poor
Older adults
Alcoholism
Liver disease (limits storage)
Fat malabsorption
Consequences:
Night blindness
Decreased mucus production
Decreased immunity
Bacterial invasion of the eye
Conjunctival xerosis
Bitot’s spots
Xerophthalmia
Irreversible blindness
Follicular hyperkeratosis
Poor growth
Upper Level for Vitamin A
3000 μg retinol
Hypervitaminosis A results from long- term supplement use (2 – 4 x RDA)
Toxicity
Fatal dose (12 g)
Toxicity of Vitamin A
Acute – short-term megadose (100 x
RDA); symptoms disappear when intake stops
GI effects
Headaches
Blurred vision
Poor muscle coordination
Toxicity of Vitamin A
Chronic – long-term megadose; possible permanent damage
Bone and muscle pain
Loss of appetite
Skin disorders
Headache
Dry skin
Hair loss
Increased liver size
Toxicity of Vitamin A
Teratogenic (may occur with as little as 3 x RDA of preformed vitamin A)
Tends to produce physical defect on developing fetus as a result of excess vitamin A intake
Spontaneous abortion
Birth defects
Health Effects of Vitamin A
Toxicity of Carotenoids
Not likely, as rate of conversion of carotenoids to retinoic acid by liver is slow and efficiency of absorption of carotenoids decreases as
intake increases
Hypercarotenemia
High amounts of carotenoids in the bloodstream
Excessive consumption of carrots/squash/beta- carotene supplements
Skin turns a yellow-orange color
What are the functions of vitamin A?
What are the two forms of vitamin A and in what foods can they be found?
How does vitamin A help with night vision?
What are the effects of a diet that is deficient in vitamin A?
What are the effects of a diet that is toxic in vitamin A?
Content Review
Vitamin D
Prohormone
Derived from cholesterol
Synthesis from sun exposure
Insufficient sun exposure makes this a vitamin
Activated by enzymes in liver and kidneys
Deficiency diseases
Rickets
Osteomalacia
Vitamin D 3 Formation in the Skin
Sunlight converts provitamin D 3 to previtamin D 3 in skin
Previtamin D 3 converted to vitamin
D 3 and released into bloodstream
Absorption of Vitamin D 2
~80% of vitamin D consumed is incorporated into micelles
Absorbed in the small intestine and transported via chylomicrons
Fat malabsorption impairs vitamin D
absorption
Metabolism, Transport, Storage and Excretion of Vitamin D
Transported from small intestine to liver in form of chylomicrons in lymphatic
system
Activation by the liver and the kidneys
Stored in fat tissue
Activate vitamin D when calcium is inadequate
Excretion of vitamin D mainly via bile
Functions of Vitamin D
Regulate blood calcium level
Increased intestinal absorption of calcium from food
With parathyroid hormone, releases calcium from bone
Cell differentiation
Linked to reduction of
breast, ovarian, colon,
and prostate cancer
Food Sources of Vitamin D
Fatty fish (salmon, herring)
Fortified milk
Some fortified cereal
Vitamin D Needs
Due to variation in sunlight exposure, no RDA set, but AI established as:
5 μg/d (200 IU/d) for adults under age 51
10-15 μg/d (400 - 600 IU/d) for older adults
Light skinned individuals can produce enough vitamin D to meet the AI from casual sun exposure
Infants are born with vitamin D, but AAP recommends supplementing breastfed
infants with 5 μg (200 IU)/d until weaned to
fortified infant formula
Vitamin D Deficiency
Rickets – poor bone
mineralization in children
Osteomalacia – soft bones in adults
Vitamin D resistance – problem
with synthesis of active form or
defective receptor binding
Pharmacologic Use of Vitamin D Analogs
Psoriasis
Skin disorder
Topical treatment
Upper Level for Vitamin D
UL = 50 μg/d (2000 IU/d)
Regular intake of 5-10x the AI can be toxic
Over-absorption of calcium (hypercalcemia), increase calcium excretion
Calcium deposits in kidneys, heart, and blood vessels, narrowing of pulmonary arteries and aorta, facial changes, mental retardation
Results from consuming megadoses, not
excess sun exposure
Why is vitamin D considered a pro- hormone?
How is vitamin D metabolized?
What are the functions of vitamin D?
What are good sources (food and non- food) of vitamin D?
Content Review
Miracle Vitamin D
Vitamin D Sources
Sun exposure
80 – 100% from sun exposure
SPF 8 reduces synthesis by 97.5%
Foods contain low levels
