Fat-Soluble Vitamins: Functions, Sources, and Health Implications

Objectives

• Identify fat-soluble and water-soluble vitamins.

• Understand the characteristics, solubility, and functions of vitamins.

• Define antioxidant and recognize which vitamins act as antioxidants.

• Describe absorption, transportation, and bioavailability of vitamins.

• List best sources of vitamins and factors affecting vitamin content in foods; understand vitamin fortification.

• Explain functions, recommended intakes, food sources, deficiency, and toxicity effects of Vitamin A, Vitamin D, Vitamin E, and Vitamin K.

• Define provitamins and provide examples.

• Discuss the role of dietary supplements and when supplementation is necessary.

Fat-Soluble Vitamins Overview

Definition and Characteristics

Fat-soluble vitamins are a group of vitamins that dissolve in fats and oils and are stored in the body's fatty tissues and liver. They include Vitamin A, Vitamin D, Vitamin E, and Vitamin K. These vitamins are absorbed along with dietary fat and can accumulate to toxic levels if consumed excessively, primarily through supplements.

• Solubility: Dissolve in lipids, not water.

• Storage: Stored in liver and adipose tissue.

• Transport: Packaged into chylomicrons for lymphatic transport.

Vitamin A

What is Vitamin A?

Vitamin A refers to a family of fat-soluble retinoids, including retinol, retinal, and retinoic acid. It is found in animal foods and synthesized in the body, while plant sources provide carotenoids (e.g., beta-carotene), which are provitamins converted to vitamin A.

• Retinol: Found in animal foods; supports reproduction.

• Retinal: Formed in the body; essential for vision.

• Retinoic acid: Regulates gene expression and cell differentiation.

• Carotenoids: Pigments in plants (e.g., carrots, squash) that can be converted to vitamin A.

Provitamin: Beta-carotene is a provitamin that is split into two molecules of retinal in the body.

Vitamin A Absorption and Transport

Vitamin A absorption depends on its form and dietary factors.

• Preformed vitamin A (retinol, retinal, retinoic acid) is absorbed by active transport with bile salts and micelles.

• Beta-carotene is absorbed by passive diffusion.

• Absorption is reduced with high fiber intake and improved in cooked foods.

• Most forms are packaged into chylomicrons and enter lymphatic circulation.

• Retinoic acid is attached to albumin and transported to the liver via the portal vein.

• Vitamin A is stored in the liver.

Metabolic Functions of Vitamin A

Vitamin A is essential for several physiological processes:

• Vision: Retinal is a component of rhodopsin, necessary for low-light and color vision.

• Reproduction: Retinol supports reproductive health.

• Bone Health: Involved in bone growth and remodeling.

• Immune System: Supports immune function and cell differentiation.

• Protein Synthesis: Regulates gene expression for protein synthesis.

• Antioxidant Activity: Carotenoids act as antioxidants, protecting cells from free radical damage.

Daily Needs—Vitamin A

• Males: 900 micrograms (μg) retinol activity equivalents (RAE) per day

• Females: 700 μg RAE per day

• Upper Limit: 3,000 μg RAE per day

• Beta-carotene: 3 to 6 milligrams daily

• Strict vegans: Should consume carotenoid- and beta-carotene-rich foods to meet vitamin A needs.

Vitamin A Toxicity

• Excessive preformed vitamin A can accumulate to toxic levels, primarily from supplements.

• Toxicity symptoms occur at >15,000 μg.

• Hypervitaminosis A: Can cause liver damage and death.

• During pregnancy, excess intake can cause birth defects.

• May result in osteoporosis and bone fractures in adults.

• Carotenoids in foods are not toxic, but excess can cause carotenodermia (yellowing of skin).

• High beta-carotene supplement intake may increase lung cancer risk in smokers.

Vitamin A Deficiency

• Chronic deficiency causes night blindness.

• Prolonged deficiency leads to xerophthalmia (dryness of the eye), a leading cause of preventable blindness in children in developing countries.

• Deficiency can cause keratinization of epithelial tissues, reducing mucus secretion and increasing infection risk.

Vitamin D

What is Vitamin D?

Vitamin D, known as the "sunshine vitamin," is produced in the skin from cholesterol upon exposure to ultraviolet rays. It is considered a conditionally essential nutrient and a prohormone.

• Cholecalciferol (Vitamin D3): Produced in the skin and found in animal foods.

• Ergocalciferol (Vitamin D2): Found in plants and dietary supplements.

Exposure to sunlight can synthesize most of the body's vitamin D needs.

Metabolic Functions of Vitamin D

• Stimulates intestinal absorption of calcium and phosphorus.

• Maintains healthy ratio of calcium and phosphorus in the blood.

• Regulates bone growth by controlling osteoblast and osteoclast activity.

• May help prevent certain cancers (e.g., breast, colon, prostate).

• May reduce risk of autoimmune disorders and type 1 diabetes.

• Associated with regulation of blood pressure.

Daily Needs—Vitamin D

• Recommended intake: 15–20 μg (600–800 IU) per day for adults.

• Children: 15 μg per day.

• Upper Limit: 50 μg (2,000 IU) per day.

• Sunlight exposure, skin pigmentation (melanin), and sunscreen use affect vitamin D synthesis.

• Winter sunlight intensity may be insufficient for vitamin D production in some regions.

Food Sources of Vitamin D

• Fatty fish (e.g., salmon, mackerel)

• Fortified dairy products

• Egg yolks

• Mushrooms (exposed to sunlight)

Vitamin D Toxicity

• High intake can cause loss of appetite, nausea, vomiting, and constipation.

• Supplement overuse may lead to hypervitaminosis D and hypercalcemia (high blood calcium).

• Can cause calcium deposits in kidneys, lungs, blood vessels, and heart.

• May affect the nervous system and cause severe depression.

• Toxicity is rarely caused by food intake or sun exposure.

Vitamin D Deficiency

• Rickets: Deficiency disease in children; bones are inadequately mineralized, leading to bowed legs.

• Osteomalacia: Adult equivalent of rickets; softening of bones.

• Osteoporosis: Bones do not mineralize properly, leading to weakness and pain.

• Muscle and bone weakness and pain.

Vitamin E

What is Vitamin E?

Vitamin E is a group of compounds, with alpha-tocopherol being the most active form in the body. The synthetic form in supplements is less active than the natural form.

Vitamin E Absorption & Transport

• Absorbed with bile salts and micelles.

• Packaged in chylomicrons, transported via lymphatic system to circulation.

• 90% of vitamin E is stored in adipose tissue.

Metabolic Functions of Vitamin E

• Acts as a powerful antioxidant, protecting cell membranes from oxidative damage.

• Prevents oxidation of LDL cholesterol.

• Acts as an anticoagulant, inhibiting formation of harmful blood clots.

Daily Needs—Vitamin E

• Adults need 15 mg of alpha-tocopherol equivalents per day.

• Upper limit: 1,000 mg/day.

Vitamin E Toxicity and Deficiency

• No known risk from natural food sources.

• Overconsumption of synthetic forms in supplements can increase risk of hemorrhage.

• Deficiency is rare but can cause nerve problems, muscle weakness, and free radical damage to cell membranes.

Vitamin K

What is Vitamin K?

Vitamin K exists in three forms:

• Phylloquinone: Found in green plants.

• Menaquinone: Synthesized by intestinal bacteria.

• Menadione: Synthetic form.

Vitamin K Absorption & Transport

• Most vitamin K is absorbed in the jejunum.

• 10% is produced by bacteria in the colon.

• Incorporated into chylomicrons and transported to the liver.

• Storage forms are transported by VLDL, LDL, and HDL; stored mostly in the liver.

Metabolic Functions of Vitamin K

• Promotes blood clotting by synthesizing four clotting factors.

• Promotes strong bones by assisting with carboxylation of proteins (e.g., osteocalcin, matrix Gla protein).

• Essential for formation of a strong bone matrix.

Daily Needs—Vitamin K

• Men: 120 μg/day

• Women: 90 μg/day

• Recommendations are based on current consumption; intestinal synthesis contribution is not well known.

Vitamin K Toxicity and Deficiency

• No known adverse effects from high intake.

• Individuals on anticoagulant medications should keep intake consistent to avoid altering drug effectiveness.

• Infants are born with low vitamin K; deficiency severe enough to affect blood clotting is rare but serious.

• May cause hip fracture in older adults.

Vitamin Supplements

Role and Regulation

• No evidence supports supplements for prevention of chronic diseases or cognitive decline.

• Supplements are not substitutes for healthy eating and can cause adverse side effects.

• Supplements are not regulated like drugs and do not require FDA approval before marketing.

• Supplements can only be removed from the market if proven unsafe.

How to Choose a Supplement

• Look for the United States Pharmacopeia Convention (USP) seal.

• Read labels carefully for dosage and ingredients.

Are Vitamin Supplements Necessary for Good Health?

• May be helpful for certain individuals:

  • Pregnant and lactating women

  • Older adults needing vitamin B12

  • Individuals lacking dietary vitamin D or sun exposure

  • Individuals on low-calorie diets

  • Strict vegetarians

  • Individuals with food allergies or lactose intolerance

  • Individuals who abuse alcohol

  • Food insecure individuals

  • Breast-fed infants

Table: Comparison of Fat-Soluble Vitamins

Vitamin	Main Functions	Sources	Deficiency Effects	Toxicity Effects
Vitamin A	Vision, immune function, reproduction, cell differentiation	Liver, dairy, carrots, sweet potatoes, leafy greens	Night blindness, xerophthalmia, keratinization	Liver damage, birth defects, osteoporosis
Vitamin D	Calcium/phosphorus absorption, bone health, immune regulation	Sunlight, fatty fish, fortified milk, egg yolks	Rickets, osteomalacia, osteoporosis	Hypercalcemia, kidney damage
Vitamin E	Antioxidant, protects cell membranes, anticoagulant	Vegetable oils, nuts, seeds, green leafy vegetables	Nerve problems, muscle weakness	Hemorrhage (from supplements)
Vitamin K	Blood clotting, bone health	Green leafy vegetables, intestinal bacteria	Bleeding, bone fractures	None known