What are fat-soluble vitamins?
As their name indicates, they are not soluble in water, but in fats (lipids). They are absorbed and stored in the liver and fatty tissues. This group of vitamins is made up of:
- vitamin A (retinol, retinoids and carotenoids)
- vitamin D (ergocalciferol, D2 and cholecalciferol D3)
- vitamin E (tocopherols and tocotrienols)
- vitamin K (K1 or phytoquinone, K2 or menaquinone, and K3 or menadione)
Fat-soluble vitamins are usually heat stable but unstable to light and oxygen.
- vitamin A: stable to moderate heat treatments, reducing agents and alkaline medium and unstable to light and oxygen.
- vitamin D: is highly stable, and is not destroyed either by heat or by technological processes.
- vitamin E: stable to heat and unstable to light and oxygen.
- vitamin K: stable to heat and unstable to light.
Digestion and absorption
Being fat-soluble substances, their digestion is the same as that of fats, depending on bile salts and pancreatic enzymes that hydrolyze the ester bonds of the vitamins that are esterified.
Their absorption also depends on the absorption of fats and will be more or less effective depending on the individual’s ability to absorb fats. Thus, individuals with fat malabsorption problems are more susceptible to vitamin deficiencies.
- vitamin A has a variable absorption depending on whether it is in the retinol form, which has an absorption of 80-90%, or whether it is in the carotenoid form with an absorption of 40-60%. In addition, this absorption is affected by the amount and type of fats, as well as the quantity and quality of proteins ingested. Antioxidants such as α-tocopherol and lecithin improve absorption and mineral oil type laxatives worsen absorption.
- The absorption of vitamin D is quite efficient and depends on the individual’s ability to absorb fats.
- Vitamin E is absorbed in the middle part of the intestine and approximately 50% of the daily intake is absorbed.
- The absorption of vitamin K from the diet (K1) takes place in the upper intestine by active transport. In the case of endogenous vitamin K (K3, from microbiota synthesis), absorption is in the ileum and colon and is by passive transport. Absorption is highly variable and is estimated at 40-70 %. Once absorbed by the enterocyte, they are incorporated into the chylomicrons to be transported from the lymphatic system to the bloodstream.
Storage and excretion
The vast majority of fat-soluble vitamins are stored as follows:
- vitamin A: 50-80 % in liver and 15-20 % in adipose tissue.
- vitamin D: Plasma bound to protein, in adipose tissue.
- vitamin E: has a low storage capacity. Widely distributed in all tissues (adipose, membranes of all tissues).
- vitamin K: has a low storage capacity (some in liver, mainly in the form of menaquinone).
And the excretion mechanism is mainly through biliary secretions:
- vitamin A: 70% in bile, 30% in urine.
- vitamin D: bile, small losses in urine.
- vitamin E: bile, 1% by urine.
- vitamin K: bile.
Food sources of fat-soluble vitamins
Fat-soluble vitamins are abundant in fatty fish fats and offal (especially liver), egg, milk.
- vitamin A : as retinol is found in foods of animal origin such as liver, whole milk and butter. As carotenoids in plant foods: in green leafy vegetables (spinach) and vegetables (carrots, tomatoes) and in some fruits.
- vitamin D (ergocalciferol, D2 and cholecalciferol D3): 80% of vitamin D3 can be obtained due to an endogenous origin, which originates with the help of ultraviolet (UV) light, the human body transforms 7-dehydrocholesterol into vitamin D3. The remaining 20% of vitamin D3 needed can be found in cod liver oil, oily fish (tuna, salmon, mackerel and sardines), calf liver and egg yolk. Vitamin D2 can be found in mushrooms.
- vitamin E (tocopherols and tocotrienols): vegetable oils and nuts.
- vitamin K (K1 or phytoquinone, K2 or menaquinone, and K3 or menadione): 50% of vitamin K2 requirements can be generated endogenously by the microbiota. Vitamin K1 is found in green leafy foods: cabbage, spinach, Swiss chard, kale, chard, lettuce, broccoli, cauliflower, etc.
Deficiency and hypervitaminosis
Vitamin A levels in individuals not only depend on intake, but also on other factors such as excretion. Thus, for example, chronic infectious diseases, metabolic stress, liver diseases, smokers, alcoholics, etc., accelerate its catabolism and excretion.
Vitamin A deficiency in adults is rare, and when it occurs, it is usually due to diseases rather than inadequate intakes. There are homeostatic mechanisms that in the initial stages of deficiency, mobilization of vitamin A stored in the liver occurs, excretion decreases and the conservation mechanisms increase, this causes normal levels of vitamin A to be maintained in plasma and retina. However, if the deficiency is maintained or increases, signs of deficiency occur, leading to various diseases, including night blindness, xerophthalmia, infections and skin disorders.
Hypervitaminosis A can be caused by ingesting more than 12 times the recommended dietary intake for several months.
Vitamin D deficiency causes rickets in children and osteopenia in adults. Groups at risk of deficiency are populations in which sun exposure is not sufficient (dark-skinned people, countries with few hours of sunshine, women in Islamic countries, the elderly, children).
Vitamin D is toxic at doses greater than or equal to 50,000 IU/day in adults and 1,000 IU/day in children. (IU is equal to the biological equivalent).
Vitamin E deficiency in healthy individuals is rare and vitamin E deficiency states are almost never observed. In individuals with intestinal malabsorption (celiac disease, cystic fibrosis, pancreatic disorders, etc.) supplementation may be recommended. Vitamin E is classified as a practically non-toxic substance.
Vitamin K deficiencies are very rare, due to its abundance in food and the contribution of the intestinal microbiota. Newborns are at higher risk of deficiency, since they have no reserves and lack intestinal microbiota. As prevention, an injection of vitamin K is administered at birth. No cases of overdose toxicity have been described in the form of phytoquinone, however menadione seems to be more dangerous.