Fat-soluble vitamins: A, D, E and K
The four that accumulate: what each one does, how much you actually need, the dose at which they stop being harmless, and why only one of them is supplemented routinely.
Fat-soluble means they dissolve in fat, and everything else follows from that: they are absorbed better with a meal that contains fat, the body stores them —in the liver and in fat tissue— and, because they are stored, they have a ceiling above which they stop being harmless. It is the exact opposite of the water-soluble ones, which you urinate out.
This is the detail; the criterion is in the general chapter
If what you want is to decide whether to buy something, start with vitamins, minerals and health: the underlying rule —supplementing without a deficiency improves nothing— does not change by reading this page. The numbers are here.
Of the four, only one is routinely supplemented by people who train, and the other three show up mostly in multivitamins and in "joint health" and "antioxidant" products. It is worth knowing what each does before paying for it.
Vitamin D: the one almost everybody should measure
It is not exactly a vitamin: the body makes it in the skin from ultraviolet B radiation and it acts like a hormone. The problem is that usable UVB disappears from northern Europe between October and March, so for half the year the only possible source is food or a tub.
Around half of the athletes studied have inadequate levels
Farrokhyar’s meta-analysis of 23 studies found inadequate levels in 56% of athletes, with higher prevalence in indoor sports, high latitudes and winter. Owens and Close’s review describes the mechanism —vitamin D receptors in skeletal muscle, bone and immune cells— and the same conclusion as always: correcting a real deficiency improves strength and bone health, topping up someone who is already fine does nothing measurable.
Serum 25-OH vitamin D
ng/ml
- Deficiency< 20
Bone and muscle risk. Correcting this is what produces real improvements.
- Insufficiency20-30
The commonest zone at the end of winter in northern Europe.
- Sufficient30-50
The reasonable target. Above it there is no demonstrated benefit.
- No added benefit50-100
Neither better performance nor better bone. Just more spending.
- Toxicity risk> 100
Hypercalcaemia. Almost always from badly prescribed megadoses.
- Cod liver oil (5 ml)~1,300 IU
It also carries a lot of vitamin A: not a source to take daily without checking.
- Typical supplement capsule1,000 IU
The best-selling catalogue dose.
- Wild salmon (100 g)~700 IU
Farmed salmon carries considerably less: between 100 and 250 IU.
- UV-exposed mushrooms (100 g)~400 IU
The only meaningful plant source, and only if they have been irradiated.
- Canned sardines (100 g)~270 IU
- Fortified milk or plant drink (250 ml)~100 IU
- Two egg yolks~80 IU
- How to know
- A 25-OH vitamin D blood test
- Usual maintenance dose
- 1,000-2,000 IU/day (25-50 µg)
- Form
- D3 (cholecalciferol), better than D2
- What to take it with
- The fattiest meal of your day
- Tolerable upper level
- 100 µg/day (4,000 IU) in adults
- When to retest
- Three months after starting
It is cheap and it is the only way. Symptoms diagnose nothing. What to ask for.
Correcting an established deficiency needs a medical protocol, not the tub’s dose.
D3 raises and holds serum levels better than D2 at the same dose. Vegan D3 from lichen exists.
Fasted, absorption is considerably worse.
EFSA’s. Above that, only under medical supervision with calcium monitored.
And after that, once a year at the end of winter, which is the worst point.
Monthly or annual megadoses are not "the same, spread out"
A 100,000 IU ampoule every few months is convenient and widely prescribed, but it produces peaks and troughs that daily dosing does not, and in older populations very large boluses have not gone well in trials. If the protocol comes from your doctor, fine; if you are doing it yourself because "that way I do not forget", the small daily dose is the better-supported option.
Vitamin A: the easiest one to overshoot
It is involved in vision, epithelial integrity, immune function and gene expression. Deficiency is rare in countries with access to dairy, eggs and coloured vegetables, and excess is far easier to reach than people think, because there are two forms and they do not behave the same.
| Form | Where it comes from | How it behaves |
|---|---|---|
| Retinol (preformed vitamin A) | Liver, fish liver oil, supplements, full-fat dairy | Absorbed almost entirely and accumulates in the liver. This is the one that causes toxicity |
| Beta-carotene (provitamin A) | Carrot, sweet potato, squash, leafy greens | The body converts it as needed, and conversion self-limits. Eating it does not cause toxicity: at worst it turns your skin orange |
High-dose beta-carotene supplements increased lung cancer in smokers
The CARET trial, with more than 18,000 high-risk participants, was stopped early: the group taking 30 mg of beta-carotene and 25,000 IU of retinol a day had 28% more lung cancer and 17% more deaths than placebo. It is the canonical example of why "it is an antioxidant from fruit" does not license taking it at pharmacological doses.
- Reference intake
- 650-750 µg RAE/day in adults
- Upper level
- 3,000 µg RAE/day of retinol
- Pregnancy
- Avoid retinol supplements and do not eat liver
- Who should check it
- Almost nobody
EFSA reference values. RAE = retinol activity equivalents.
Only preformed counts. Dietary beta-carotene has no established limit.
Excess preformed vitamin A is teratogenic. One serving of liver can exceed the daily limit several times over.
Deficiency is exceptional on a varied diet. The useful question runs the other way: how much are you already getting from tub plus food.
Vitamin E: the antioxidant that failed in trials
It is the fat-soluble antioxidant of cell membranes, and the promise was enormous: if oxidative damage ages us, an antioxidant should protect us. The large trials said otherwise.
As a high-dose supplement it does not protect, and in some cases it harms
The SELECT trial, with more than 35,000 men, found 17% more prostate cancer in the group taking 400 IU of vitamin E daily versus placebo. Bjelakovic’s Cochrane review of antioxidant supplements finds no reduction in mortality and signals of an increase with beta-carotene and vitamin E. And in sport, Paulsen’s trial showed that 1,000 mg of vitamin C plus 235 mg of vitamin E a day blunt the cellular markers of adaptation to endurance training.
Do not confuse dietary vitamin E with the capsule
A handful of almonds carries about 7 mg and nobody has ever shown that does harm; the problem is the 268 mg (400 IU) in a capsule, which is nearly twenty times the reference intake. The practical conclusion is short: do not buy it on its own, and if your multivitamin carries a high dose, change it — especially during a building block.
Vitamin K: bone, and the anticoagulant problem
It is the cofactor that activates clotting factors and also osteocalcin, the protein that fixes calcium into bone. That is where its commercial appeal comes from —"K2 so calcium goes to bone and not to your arteries"— and it is also where its only real risk lives.
| Form | Where it is | Note |
|---|---|---|
| K1 (phylloquinone) | Leafy greens: spinach, kale, broccoli | One daily serving of green vegetables covers the requirement without effort |
| K2 (menaquinones: MK-4, MK-7) | Natto, aged cheese, eggs, fermented foods | MK-7 lasts much longer in blood than MK-4. It is the form sold in bone supplements |
The benefit on bone density and fractures does not hold across trials
Mott’s updated meta-analysis of randomised trials in adults finds no consistent effect of vitamin K on bone mineral density, and the fracture results depend heavily on a few Japanese studies using pharmacological-dose MK-4 that have not replicated elsewhere. Translated: as part of the diet, yes; as a capsule for your bones, the evidence does not justify the price.
If you take warfarin or acenocoumarol, this is a real interaction
Coumarin anticoagulants work precisely by blocking the vitamin K cycle. Suddenly changing how much you take —starting a supplement, or stopping eating greens— destabilises INR. This is not a "bear it in mind": it is telling whoever manages your dose before taking anything.
The four, at a glance
| Vitamin | What it does | Supplement? | Daily ceiling |
|---|---|---|---|
| D | Bone, muscle, immune function | Yes, if the blood test calls for it. It is the exception in the family | 100 µg (4,000 IU) |
| A | Vision, epithelia, immunity | Not routinely. Watch how much your multivitamin adds | 3,000 µg RAE of retinol |
| E | Membrane antioxidant | No. High-dose trials came out neutral or worse | 300 mg alpha-tocopherol |
| K | Clotting and fixing calcium into bone | Not routinely. Eat green vegetables | No established limit, but it interacts with anticoagulants |
Takeaways
- They are absorbed with fat and they accumulate: that is why they are the ones with a ceiling.
- Vitamin D is the only one supplemented routinely, and only with a blood test that justifies it.
- The sensible 25-OH-D target is 30-50 ng/ml: above that there is no extra benefit.
- Preformed vitamin A (retinol) counts against the ceiling; dietary beta-carotene does not.
- High-dose vitamin E does not protect, and during a building block it can blunt adaptations.
- Vitamin K is better from vegetables — and it is a real interaction if you take anticoagulants.
Carry on here
The other halves of the same topic, and the page that turns all of it into a decision.
Sources for this chapter
- [1]Farrokhyar F, Tabasinejad R, Dao D, et al. (2015). Prevalence of vitamin D inadequacy in athletes: a systematic review and meta-analysis. Sports Medicine. meta-analysis
- [2]Owens DJ, Allison R, Close GL (2018). Vitamin D and the Athlete: Current Perspectives and New Challenges. Sports Medicine. review
- [3]Holick MF, Binkley NC, Bischoff-Ferrari HA, et al. (2011). Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. Journal of Clinical Endocrinology & Metabolism. official guideline
- [4]Ross AC, Manson JE, Abrams SA, et al. (2011). The 2011 Dietary Reference Intakes for Calcium and Vitamin D: what dietetics practitioners need to know. Journal of the American Dietetic Association. official guideline
- [5]EFSA Panel on Dietetic Products, Nutrition and Allergies (2017). Dietary Reference Values for nutrients: summary report. EFSA Supporting Publications. official guideline
- [6]EFSA Scientific Committee on Food / NDA Panel (2006). Tolerable upper intake levels for vitamins and minerals. European Food Safety Authority. official guideline
- [7]Omenn GS, Goodman GE, Thornquist MD, et al. (1996). Effects of a combination of beta carotene and vitamin A on lung cancer and cardiovascular disease. New England Journal of Medicine. trial
- [8]Klein EA, Thompson IM, Tangen CM, et al. (2011). Vitamin E and the risk of prostate cancer: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA. trial
- [9]Bjelakovic G, Nikolova D, Gluud LL, Simonetti RG, Gluud C (2012). Antioxidant supplements for prevention of mortality in healthy participants and patients with various diseases. Cochrane Database of Systematic Reviews. meta-analysis
- [10]Paulsen G, Cumming KT, Holden G, et al. (2014). Vitamin C and E supplementation hampers cellular adaptation to endurance training in humans: a double-blind, randomised, controlled trial. Journal of Physiology. trial
- [11]Mott A, Bradley T, Wright K, et al. (2019). Effect of vitamin K on bone mineral density and fractures in adults: an updated systematic review and meta-analysis of randomised controlled trials. Osteoporosis International. meta-analysis
- [12]Sale C, Elliott-Sale KJ (2019). Nutrition and athlete bone health. Sports Medicine. review