You can measure your vitamin D levels by using a blood test to determine if you have enough vitamin D in your body. In fact, not the active form of vitamin D is measured, but the concentration of 25(OH)D (25-hydroxyvitamin D) in the blood.
The 25 (OH)D value is a precursor of vitamin D, in which form is is transported in the blood. If necessary, this form is converted into the active form of vitamin D (calcitriol).
The 25 (OH)D value shows how much vitamin D you got through nutrition or produced during sun exposure. But this value does not tell you anything about your vitamin D levels throughout the year, which will vary depending on the sun exposure and vitamin D uptake from food.
The half-life of 25(OH) Vitamin D is 2 month. This means, that the vitamin D level will fall to half of its value, provided that you no longer get any vitamin D at all.
Nobody can say what the optimal 25(OH)D value is. Therefore, the suggestions vary by author. After viewing many different suggestions, we came to the following conclusion:
The ability of each person to produce vitamin D through sun exposure or to increase the vitamin D level by taking vitamin d supplements, can vary significantly and depends on age, body weight, skin colour, sun exposure, and life style.
It is not said, that each person can reach the same vitamin D level by taking the same amount of vitamin D or having the same sun exposure. Some people may need more or less vitamin D per day to achieve the same level.
Therefore, the recommended rules for calculating the optimal vitamin D dosage are only indicative. To know your personal vitamin D level, a 25(OH)D blood test has to be performed. It shows you exactly how your vitamin D level is changing depending on the vitamin D intake and sun exposure.
In Europe the 25(OH)D value is usually measured in ng/ml. But any laboratories also use nmol/l.
- To calculate a test result from nmol/l to ng/ml, divide the nmol/l value by 2,5. For example, 50 nmol/l are equal to 20 ng/ml (50 ÷ 2,5).
- To calculate a test result from ng/ml to nmol/l, multiply the ng/ml value with 2,5. For example, 20 ng/ml are equal to 50 nmol/l (20 x 2,5).
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