Fat-soluble vitamins
Vitamins A, D, E and K, which dissolve in fat, are stored by the body for a long time, need fat to be absorbed and can build up to toxic levels.
The fat-soluble vitamins, A, D, E and K, dissolve in fat rather than water. This property governs how they are absorbed, stored and how risky they are, in the opposite way to water-soluble vitamins.
First consequence: they need fat to be absorbed. Eating grated carrots without a trace of fat means missing out on some of the beta-carotene. A drizzle of oil on a salad or in a vegetable soup is not a pointless extra calorie, it is what makes the vitamins available.
Second consequence: they are stored, so they can build up. The liver and fatty tissue hold reserves that last several months. This is an advantage, because an irregular intake remains tolerable, but it is also the main risk of overdosing with supplements, particularly vitamins A and D, where an excess is toxic.
Vitamin D has a special status: food provides only a small share of it, and most is made by the skin under the effect of sunlight. At our latitudes, this synthesis becomes negligible in winter, which is why deficiency is common and why seasonal supplementation is widely recommended.