Feb 28, 2026
3 min read
Learn why vitamin D levels can remain low even with supplements.Factors such as absorption, magnesium, and weight can affect effectiveness.
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If the integration doesn't seem to work
Many patients are surprised that after a period of taking supplements, blood tests show that the level of 25-hydroxyvitamin D (the storage we measure) is below the requirement.This situation is common in the hospital, but it does not cause a problem.It is important to understand that vitamin D has slow pharmacokinetics: reaching a stable level in the blood (the so-called "plateau") requires time and persistence.The path from ingestion to the classification of blood pressure is a special process that depends on each change and does not happen randomly.
It's not always just a matter of underdosing, although underextraction is common.The problem often lies in adherence to treatment or in factors that affect pharmacokinetics, that is, how the body absorbs, distributes and metabolizes the molecule.Analysis of these aspects is necessary for the medicine to be effective and without correcting the forms of consumption to avoid increasing unnecessary doses.
Absorption: The importance of vehicle and meal
A related aspect concerns the chemical nature of the molecule.Vitamin D is fat-soluble, meaning it needs lipids (fats) to be absorbed in the intestine.Although newer oil or softgel formulations have improved bioavailability compared to older dry tablets, scientific evidence supports the need to take the supplement during the main meal of the day, which is usually high in fat, on an empty stomach.May increase absorption compared to
However, sometimes the overemphasis on cofactors in non-professional fields needs to be reduced.Although acute magnesium deficiency can theoretically interfere with vitamin D metabolizing enzymes, for a varied diet in the general population, unless there is a specific deficiency of this mineral, systemic magnesium support is not strictly necessary to increase vitamin D levels.Similarly, although vitamin K2 plays a role in bone metabolism, there is no strong evidence that its intake affects serum vitamin D levels or intestinal absorption.Therefore, if the goal is to increase vitamin D, the main clinical priority is always the correct intake of cholecalciferol itself.
Physiological barriers: body weight and malabsorption
There is a specific clinical condition that can affect the success of the treatment goal.Body mass index (BMI) is an indicator supported by strong evidence.Vitamin D, which is fat-soluble, is distributed in adipose tissue.In subjects who are overweight or obese, the phenomenon of "volume dilution" occurs: the vitamin is stored in fat stores and is not available in the bloodstream.Clinically, this means that an obese patient often needs two to three times higher doses than a normal weight person to achieve blood levels.
In addition, the health of the digestive system should be considered.Pathologies such as celiac disease, chronic inflammatory bowel diseases (such as Crohn's disease) or previous bariatric surgery cause malabsorption syndromes.In such cases, the intestine cannot absorb the vitamin properly, and sometimes it is necessary to use very high doses or different pharmaceutical forms (such as water-soluble or, in rare cases, parenteral) under the supervision of a specialist.Finally, medications Concomitant use of certain medications (such as cortisone, antiepileptics, or cholesterol medications) may accelerate the degradation of vitamin D or reduce its absorption.
Practical advice for truly effective integration
To optimize treatment and correct hypovitaminosis, the method should be systematic.The first step is compliance: the most common cause of treatment failure is forgetting the dose.Irregular intake of the supplement prevents the necessary accumulation in the tissues.
From a practical point of view, the strongest recommendation is to take vitamin D (preferably cholecalciferol) during the largest meal of the day.It is essential to avoid "do it yourself": the doctor must calculate the dose based on the starting levels and, above all, the patient's body weight.If the values do not increase after 3-6 months of regular and correct intake, the doctor will have to investigate secondary causes, such as undiagnosed celiac disease or pharmacological intervention, instead of adding additional supplements that are not supported by the guidelines.Vitamin D is an essential pleiotropic hormone: its management requires patience, consistency and a rational clinical approach.
