Vitamin D: sources, absorption and influence on the body

Vitamin D - is a fat-soluble vitamin (like A, E and K.) However, unlike many beneficial substances that are obtained only from food, it can also be produced by the human body. However, it's not as simple as it sounds - to start synthesizing it, a person needs to be exposed to the sun, i.e. get a tan.

In 1922, this substance was first isolated from fish oil by the American biochemist Elmer McCollum. Scientists called it vitamin D because vitamins A, B and C had already been discovered. And in 1923, Harry Steenbock and Alfred Hess clearly demonstrated that the human skin can synthesize vitamin D under the influence of ultraviolet rays.

Vitamin D is a complex of biologically active substances - from D1 to D6. The most important are two - D2 (ergocalciferol) and D3 (cholecalciferol), although these are not active in the body. To "work", ergocalciferol and cholecalciferol are converted in the liver and kidneys into calcitriol, or active vitamin D.

In short, the benefits of vitamin D for adults and children cannot be described in a few words. Its main effects are:

• strengthening the immune system. Vitamin D is essential for the immune system. By activating it, it helps fight infection and inflammation. The lower the level of this substance, the more often a person gets sick and catches acute viral respiratory infections literally "on the fly";
• supporting heart and blood vessel health. Vitamin D maintains the elasticity of artery walls, reduces the risk of developing dangerous atherosclerosis, regulates blood pressure. All these effects significantly reduce the risk of cardiovascular disease;
• helping to maintain normal insulin levels. Vitamin D plays a significant role in its regulation and can be used for both prevention and control of diabetes (in combination with diet therapy, an active lifestyle and sugar-lowering drugs);
• regulating mood and combating depression. It is thought that vitamin D is involved in the synthesis of serotonin (the 'happy and peaceful' hormone) - perhaps that's why it has a positive effect on mood and mental health. Vitamin D deficiency, on the other hand, is associated with an increased risk of anxiety disorder and depression - in some cases it's enough to make up the deficiency to bring back the usual joys in a person's life.

However, this list is not complete in terms of the need for vitamin D - for example, without it, the skeletal system cannot function properly. In the form of calcitriol, vitamin D promotes the absorption of phosphorus and calcium from the gut, and these minerals are essential components of bones - they ensure the mineralization process, i.e. the strength and density of bone tissue. Without calcitriol, bone growth and regeneration would be impossible.

What foods contain vitamin D?

Vitamin D can be found in natural food sources - animal products and plants. However, the content of this beneficial substance varies widely.

Much vitamin D is found in fish, especially fatty species - salmon, tuna, sardines, mackerel. For example, 100 g of salmon contain almost the daily dose - 570 IU (international units), while 100 g of hake contain much less - just 54 IU.

Another accessible source of "vitamins" is eggs, specifically their yolks. They contain less of the beneficial substance than fish (about 37 IU) but more than other foods. And let's not forget liver, which many people ignore: it is rich not only in vitamin D but also in iron.

Unlike foods of animal origin, plants are not high in vitamin D. But it can be found in significant amounts in mushrooms, grown under the influence of ultraviolet light - for example, shiitake.

Where else is vitamin D found? To prevent hypovitaminosis, many manufacturers market foods fortified with this beneficial substance - oatmeal and cornflakes, bread, cereals, ready-made breakfasts, margarine and even orange juice.

An excellent source of vitamin D is fortified dairy products. On average they contain 50-100 IU of the substance per 100 ml. It can also be added to yoghurts, cheeses and plant milks (almond, oat, etc.).

How much vitamin D should we consume per day?

The daily intake of vitamin D varies depending on a person's age and individual characteristics. In 2011, the National Academy of Medicine published recommended daily intakes of vitamin D (from diet):

• newborns up to one year - 400 IU;
• children 1 to 18 years - 600 IU;
• adults aged 19 to 70 years - 600 IU;
• people over 70 years of age - 800 IU;
• pregnant and breastfeeding women - 600 IU.

Why are these figures so important? Because vitamin D deficiency is as dangerous as excess.

Vitamin D deficiency

Deficiency of this beneficial substance can occur for the following reasons:

• lack of sun exposure - without its rays, vitamin D will not synthesize in the skin;
• insufficient dietary intake;
• fat absorption disorders - fats are necessary for vitamin D absorption;
• damage to the liver and/or kidneys, where hydroxylation - the conversion of the inactive substance into the active form - takes place.

Vitamin D deficiency can cause the following symptoms:

• weakness, fatigue;
• poor mood or mood changes;
• depression
• frequent colds and acute respiratory infections;
• muscle and bone pain;
• slow wound healing;
• bone problems (in children, vitamin D deficiency can lead to rickets and in adults - osteoporosis).

Vitamin D excess

Hypervitaminosis is just as dangerous as vitamin D deficiency. However, it occurs much more rarely - usually as a result of improper administration of supplements containing this substance. Symptoms of vitamin D intoxication have been reported in people taking more than 60 000 IU of vitamin D per day.

The medical literature describes cases of hypervitaminosis due to excessive vitamin D fortification of milk.

How to get enough vitamin D?

Doctors are often asked which is better - a balanced diet or taking supplements containing vitamin D. The answer to this question depends on several factors.

In summer, healthy people can usually manage without dietary supplements. This is not because the body gets enough vitamin D from food, but because it is produced in sufficient quantities in the skin by the sun's rays. However, it's very important to remember that a nice tan (or rather excessive sun exposure) can cause skin cancer.

Winter is different - during the cold season, taking vitamin D is necessary for almost everyone, but doses can vary significantly. It is therefore first necessary to do a test called 'determining the level of 25(OH)D in the blood'.

If the 25(OH)D level is normal (greater than 30 ng/ml), - when the day begins to shorten, doctors recommend the following preventive doses of vitamin D for adults:

• 18-50 years - 600-800 IU per day;
• over 50 - 800-1000 IU per day.

As for pregnant and breastfeeding women, doctors' opinions are divided - recommended doses range from 600 to 1200 IU per day.

Pediatricians choose doses for children on an individual basis.

However, for certain conditions, extra vitamin D is needed. These include:

• thyroid disorders;
• cardiovascular pathologies;
• diseases of the liver and digestive tract;
• diabetes mellitus;
• malignant tumors;
• COPD (chronic obstructive pulmonary disease);
• overweight and obesity.

Therapeutic doses are recommended for people with a blood 25(OH)D level of less than 30 ng/mL, but these should only be adjusted by a doctor, depending on the initial vitamin D level and concomitant conditions. For example, if the 25(OH)D level is below 12 nm/ml, the endocrinologist may recommend taking 25 000 to 50 000 IU per week for 1.5-2 months.

However, diet also plays an important role - even the best supplements will not be effective if the diet is inadequate.

The importance of a comprehensive approach to maintaining normal vitamin D levels

Vitamin D is not just one of many vitamins needed for our health. It belongs to the progormones, and its active metabolites act as hormones: it stimulates insulin production and inhibits the synthesis of parathyroid hormone. Moreover, this vitamin has anti-tumor activity and may reduce the risk of autoimmune diseases.

However, it doesn't just appear out of nowhere. To keep vitamin D levels within normal limits, it is important to take the following steps:

• Spend more time outdoors, especially on sunny days (but only in the morning and afternoon);
• Avoid extreme diets, especially protein diets - fats are also needed by the body;
• Opt for healthy fats - better to eat 100g of salmon than a piece of pork;
• don't forget omega-3 fatty acids - they are found both in food and in food supplements;
• take preventative doses of vitamin D when the day starts to shorten;
• check your blood levels of 25(OH)D regularly and correct deficiency if necessary (only on doctor's advice).

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Following these steps will help you maintain your health as well as prolong your life and activity for many years.