Fat-Soluble Vitamins

Part of Nutrition Science

Vitamins A, D, E, and K — their functions, sources, deficiency signs, and the special considerations they require compared to water-soluble vitamins.

Why This Matters

Fat-soluble vitamins have a distinct behavior that sets them apart from water-soluble B vitamins and vitamin C: they are stored in the body’s fat tissues and liver rather than being excreted in urine. This has two implications that matter practically. First, deficiency takes longer to develop — the body has reserves. Second, toxicity is possible — excessive intake accumulates rather than being flushed out.

In traditional whole-food diets, fat-soluble vitamin deficiency was primarily a disease of poverty and geographic limitation. Vitamin A deficiency (causing blindness) remains the world’s leading preventable cause of childhood blindness. Vitamin D deficiency is now recognized as near-universal in populations with limited sun exposure. Vitamin K deficiency in newborns can cause fatal brain hemorrhage. These are not obscure theoretical concerns — they are ongoing causes of death and disability that a community health system needs to address.

Fat-soluble vitamins also require dietary fat for absorption. A low-fat diet reduces absorption of all four. Eating beta-carotene-rich vegetables without any fat in the meal provides little usable vitamin A. This is a critical practical point: fat is not just a calorie source, it is a nutrient absorption vehicle.

Vitamin A

Functions: Vitamin A is required for:

Vision: Specifically the visual cycle in the retina — rhodopsin synthesis for night vision and cone function for color vision
Immune function: Critical for integrity of mucosal barriers (the inner lining of the respiratory tract, gut, and genitourinary tract) that are the first line of defense against infection
Cell differentiation: Required for normal development of skin, cornea, and other epithelial tissues
Reproduction and fetal development: Essential for normal organogenesis in the first trimester

Forms:

Preformed vitamin A (retinol): Found in animal products; directly usable by the body
Provitamin A carotenoids (beta-carotene): Found in orange and yellow vegetables and dark leafy greens; converted to vitamin A by the body, but conversion efficiency is only about 6:1 to 12:1 — you need 6-12 mcg of beta-carotene to get 1 mcg of vitamin A

Best sources:

Source	Vitamin A content	Notes
Beef liver	26,000 mcg RAE per 100g	Extraordinary source; limit to once/week in pregnancy
Sweet potato (baked)	961 mcg RAE per 100g	Excellent plant source
Carrot	835 mcg RAE per 100g	Requires fat in the same meal for absorption
Spinach	469 mcg RAE per 100g	Good source; eat with fat
Pumpkin/squash	400-500 mcg RAE per 100g
Eggs	149 mcg RAE each
Full-fat dairy	40-150 mcg RAE per 100g

Deficiency signs:

Night blindness: The earliest sign — difficulty seeing in dim light or adapting from bright to dark environments. In communities, ask: do people stumble or fall more at dusk? Do children have trouble seeing at night?
Xerophthalmia: Dry, dull cornea progressing to keratomalacia (corneal ulceration and liquefaction) — permanent blindness in severe deficiency
Bitot’s spots: Foamy, white triangular deposits on the white of the eye (beside the iris) — a reliable clinical sign of vitamin A deficiency
Increased infection severity: Vitamin A deficiency weakens mucosal defenses; children with deficiency have dramatically higher mortality from measles, diarrheal diseases, and respiratory infections

Treatment: High-dose vitamin A (100,000-200,000 IU, available as oil capsules) is given as mass supplementation in deficiency-endemic regions. This is the most cost-effective child survival intervention known. For clinical deficiency, 3 doses over 6 days. For measles with deficiency: two high doses.

Toxicity: Retinol (preformed vitamin A) is toxic in excess — symptoms include headache, nausea, hair loss, and in chronic excess, liver damage. Liver should be eaten only once per week in pregnancy. Beta-carotene is not toxic — excess simply causes orange discoloration of the skin (carotenemia), which is harmless.

Vitamin D

Functions: Vitamin D functions more as a hormone than a vitamin. It:

Regulates calcium absorption from the gut (the primary determinant of calcium status)
Promotes calcium deposition in bone
Modulates immune function (both innate and adaptive immunity)
Regulates hundreds of genes
Supports muscle function

Production: The skin produces vitamin D3 when UVB radiation from sunlight converts 7-dehydrocholesterol in the skin to previtamin D3. This is the primary source for most humans throughout evolutionary history.

Factors affecting cutaneous vitamin D production:

Sun angle (latitude and season): UVB is insufficient at latitudes above 35° in winter months
Skin color: More melanin requires more sun exposure for equivalent production
Age: Older skin produces less vitamin D per unit of sun exposure
Clothing: Covered skin produces no vitamin D

Dietary sources: Vitamin D is present in very few foods:

Fatty fish (salmon, mackerel, sardines): 200-500 IU per 100g
Fish liver oils (cod liver oil): 1,000-10,000 IU per teaspoon
Egg yolks: 40 IU each (varies with hen sun exposure)
Beef liver: 50 IU per 100g
Mushrooms exposed to sunlight: variable, can be significant

Deficiency: (See Rickets and Osteomalacia in the Deficiency Diseases article)

The global burden of vitamin D deficiency is enormous — estimates range from 40-80% of populations in many countries. Risk groups: northern latitudes in winter, indoor occupations, religious covering of skin, darker skin in low-sun environments.

Recommended daily sun exposure: For vitamin D production: 10-30 minutes of midday sun (10am-2pm) on arms and legs, 3+ times per week, is sufficient in most climates for people with light to medium skin. This is not possible in northern winters or for people who must cover their skin.

Testing: Serum 25-hydroxyvitamin D is the test. Without laboratory access, assess clinically through history, sun exposure, and signs of deficiency.

Supplementation: Where supplements are available (D3 / cholecalciferol is more effective than D2 / ergocalciferol), 1,000-2,000 IU per day is adequate for most adults; 400 IU per day for infants.

Vitamin E

Functions: Vitamin E (the most active form: alpha-tocopherol) is the primary fat-soluble antioxidant in cell membranes. It:

Protects polyunsaturated fatty acids in cell membranes from oxidation
Supports immune function
Is involved in gene regulation

Sources: Vitamin E is found in fat-containing plant foods:

Wheat germ oil (highest known food source)
Sunflower seeds and oil
Almonds
Hazelnuts
Olive oil (moderate amount)
Avocado
Dark leafy greens (small amounts)

Deficiency: Isolated vitamin E deficiency is rare in people eating any fat-containing diet. It occurs in people with fat malabsorption (certain liver, pancreatic, and intestinal diseases) and in premature infants.

Signs: peripheral neuropathy (numbness, tingling), muscle weakness, immune dysfunction, hemolytic anemia in newborns (red blood cells break down).

Toxicity: Vitamin E from food is essentially non-toxic. High-dose supplements (above 1,000 IU per day) may increase bleeding risk by interfering with vitamin K.

Practical note: Ensuring adequate dietary fat, nuts, seeds, and vegetable oils provides sufficient vitamin E for most people. Specific supplementation is rarely needed outside of medical conditions.

Vitamin K

Forms:

K1 (phylloquinone): From green plants; primarily involved in blood clotting
K2 (menaquinones, various forms): From animal products and fermented foods; primarily involved in calcium regulation in bone and arteries

Functions: Vitamin K activates clotting factors in the blood coagulation cascade — without it, the blood cannot clot normally and even minor injuries cause uncontrolled bleeding. Vitamin K2 also activates osteocalcin (bone matrix protein) and matrix GLA protein (which prevents calcium deposition in arteries).

Sources:

K1 Sources	K2 Sources
Kale, spinach, collard greens	Natto (fermented soybeans) — extremely high
Broccoli	Hard and soft cheeses
Brussels sprouts	Egg yolk
Fermented plant foods	Organ meats (especially liver)
Cabbage	Grass-fed butter

Vitamin K and newborns: Newborns are born with minimal vitamin K stores. Breast milk is very low in vitamin K. This creates a risk window: hemorrhagic disease of the newborn — typically presenting as bleeding from the umbilical cord, nose, or gut in the first days of life, or potentially fatal brain hemorrhage in the first weeks.

All national health authorities recommend vitamin K supplementation at birth (0.5-1 mg IM injection is most effective; oral drops are an alternative requiring multiple doses).

Without access to injectable vitamin K:

Ensure the mother eats plenty of vitamin K1-rich foods in late pregnancy and while breastfeeding
Feed infant early and frequently — breast milk vitamin K improves with colostrum and frequent feeding
Watch for bleeding (unusual bruising, blood in vomit or stool, bleeding from cord) — urgent response needed

Deficiency in adults: Unusual except in people on anticoagulant drugs (warfarin blocks vitamin K) or with severe fat malabsorption. Diet-related vitamin K deficiency is rare because green vegetables provide abundant K1.

Vitamin K and anticoagulants: People on warfarin (a rat poison/anticoagulant used medically) must maintain consistent vitamin K intake — fluctuations change the drug’s effectiveness. This is rarely a practical concern in resource-limited settings.