The Role of Fat-Soluble Vitamins: A, D, E and K

This is why the most effective approach to fat-soluble vitamin nutrition isn't supplementing them in isolation. It's eating the foods richest in all of them: organ meats, particularly liver and heart.

What makes vitamins fat-soluble

Fat-soluble vitamins (A, D, E, K) are absorbed in dietary fat and require lipase enzymes and healthy bile production to be properly absorbed. This means that people with poor fat digestion, bile acid problems, or insufficient stomach acid may absorb these vitamins poorly even if they eat enough of them.

These vitamins are stored in fat tissue and can accumulate in the body. This is both an advantage (you can build up reserves over time) and a potential risk (you can theoretically overdose, though this is rare from food and more common with high-dose synthetic supplements).

They require adequate fat intake to be absorbed properly. Eating fat-soluble vitamins with a fat-free meal means minimal absorption. Eaten with fat, particularly with saturated fat and cholesterol (which enhance absorption), absorption becomes efficient. This is one reason organ meats are so effective sources. They contain both the vitamins and the fat needed for their absorption.

Vitamin A: The vision and immunity vitamin

Retinol (preformed vitamin A) is found in animal foods.¹ Beta-carotene (provitamin A) is found in plants and must be converted to retinol by your body. The conversion is inefficient and varies based on genetics, gut health, and current nutrient status. Some people convert as little as 3-6% of consumed beta-carotene to usable retinol, while others convert at higher rates.

Vitamin A is essential for vision (particularly night vision, which requires rhodopsin, a vitamin A-dependent protein), immune function, skin health, and gene expression. Deficiency causes night blindness, poor immunity, dry skin, and reproductive problems. Excess from food is virtually impossible; toxicity occurs primarily with high-dose supplementation.

The richest sources are organ meats, particularly beef liver, which contains over 500,000 IU per 100g. Organ meats generally deliver bioavailable retinol. Fatty fish and eggs provide moderate amounts. No plant food comes close to organ meat for retinol density or bioavailability.

Vitamin D: The hormone vitamin

Vitamin D is technically a hormone your body synthesises from cholesterol when your skin is exposed to UVB radiation from sunlight.² It's found in food primarily in fatty fish (particularly wild salmon and mackerel), organ meats, and egg yolks, but in smaller amounts than would be needed to maintain optimal levels if you had no sun exposure.

Vitamin D regulates calcium absorption, immune function, gene expression, bone health, and mood. Deficiency is pandemic in modern populations, particularly in northern latitudes and in people who avoid sun exposure. Optimal blood levels are probably 50-70 ng/mL, while recommended minimums are often too low (20 ng/mL).

The critical point: vitamin D doesn't work well in isolation. It requires vitamin K2 to activate osteocalcin, the protein that directs calcium to bone. Without adequate K2, supplemental D can actually cause calcification in soft tissues (arteries, kidneys) instead of bone.

Vitamin E: The protection vitamin

Vitamin E is a potent antioxidant. Its primary function is to protect other fat-soluble vitamins and polyunsaturated fats in the body from oxidation.³ It's found in nuts, seeds, and vegetable oils, but also in meat and organ meats, particularly in grass-fed sources.

The best form is mixed tocopherols (alpha, beta, gamma, delta-tocopherol), not isolated alpha-tocopherol, which is what most supplements contain. Grass-fed beef contains more vitamin E than grain-fed beef. Organ meats from well-raised animals contain significant E.

Deficiency causes oxidative stress, neurological problems, and impaired function of other fat-soluble vitamins. But massive excess (from supplements) can interfere with other nutrients and actually increase oxidative stress, so balance is important.

Vitamin K: The direction vitamin

Vitamin K exists in two main forms: K1 (phylloquinone) and K2 (menaquinone). They have different functions.

Vitamin K1 is found in plant foods and is essential for blood clotting (this is what warfarin interferes with). It's abundant in leafy greens but doesn't have significant roles beyond clotting.

Vitamin K2 (found primarily in animal foods and fermented foods) directs calcium where it needs to go, to bone and teeth, not to soft tissues like arteries and kidney stones.⁴ K2 exists in several forms: MK-4 (found in animal foods) and MK-7 (found in fermented foods and some bacteria-fermented foods). Both are active, but MK-4 has a shorter half-life and requires more frequent intake, while MK-7 circulates longer in the body.

Beef liver is one of the richest sources of MK-4. Grass-fed dairy is another. Fermented foods like natto (fermented soybeans) provide extraordinary amounts of MK-7. Sauerkraut and kimchi provide small amounts.

K2 deficiency leads to a paradoxical pattern: soft-tissue calcification (arteries, kidneys, joints) alongside osteoporosis (weak bones). This paradox, weak bones and calcified soft tissues, is a hallmark of K2 deficiency.

Vitamin D without K2 is like giving someone directions without a destination. You're increasing calcium absorption, but D can't tell calcium where to go. K2 is the traffic director.

Synergies and antagonisms

Vitamin A and high-dose beta-carotene compete for absorption, so very high-dose beta-carotene supplementation can impair retinol absorption.

Vitamin E protects other fat-soluble vitamins from oxidation, so adequate E status supports A and K function. Without sufficient E, A gets oxidised and becomes unusable.

Vitamin D and K2 work synergistically. Together they regulate calcium metabolism far better than either alone. Without this synergy, calcium regulation becomes dysregulated, leading to the paradox of osteoporosis with vascular calcification.

Calcium, magnesium, and vitamin D balance affects all of these. Excess calcium without magnesium impairs D function. These minerals and vitamins need to be in appropriate ratios, not taken in isolation.

Deficiency patterns and testing

Vitamin A deficiency is rare in developed countries but common in developing countries. Signs include night blindness and dry skin.

Vitamin D deficiency is pandemic. Signs include fatigue, mood disturbance, bone pain, muscle weakness, and susceptibility to infection. Testing serum 25(OH)D is standard.

Vitamin E deficiency is rare from diet alone but can occur with fat malabsorption. Signs include neurological problems.

Vitamin K deficiency is uncommon for K1 (bleeding problems) but increasingly common for K2 (soft-tissue calcification, osteoporosis). There's no standard K2 test, but functional markers include bone density and arterial calcification.

Best food sources

The single most nutrient-dense source of all four fat-soluble vitamins is beef liver. A 100g serving provides:

• Vitamin A (retinol): 5000-10000 IU
• Vitamin D: 40-80 IU
• Vitamin E (mixed tocopherols): 1-2mg
• Vitamin K2 (MK-4): 1-3 mcg

Plus the cofactors these vitamins need: copper, zinc, B vitamins, selenium, and the fat needed for absorption. It's a genuinely complete package evolved over millions of years to support mammalian development.

Other excellent sources: other organ meats (kidney, heart, brain), grass-fed butter (high in K2 and A), pasture-raised egg yolks (high in all four), wild-caught fatty fish (high in D and E), grass-fed beef (moderate amounts of all four).

If you're supplementing any of these vitamins individually, you're probably creating imbalances. The more effective approach is eating the foods that naturally contain all four in synergistic ratios.

The bottom line

Fat-soluble vitamins are teammates. They work best together. Trying to supplement one in isolation often creates imbalances and reduces overall efficacy. Eating real foods rich in all four, particularly organ meats, gives you the full spectrum in ratios your body evolved to use.

If you must supplement, supplement them together as whole-food extracts, not individual synthetic vitamins. But better still: eat the foods that humans have eaten for millennia as the primary source of these vitamins. Organ meats remain unmatched for fat-soluble vitamin density and bioavailability.

References

1. 1. National Institutes of Health, Office of Dietary Supplements. Vitamin A and Carotenoids — Health Professional Fact Sheet.
2. 2. National Institutes of Health, Office of Dietary Supplements. Vitamin D — Health Professional Fact Sheet.
3. 3. National Institutes of Health, Office of Dietary Supplements. Vitamin E — Health Professional Fact Sheet.
4. 4. National Institutes of Health, Office of Dietary Supplements. Vitamin K — Health Professional Fact Sheet.