Taking vitamin D does not mean the body is absorbing enough of it. Behind a level that will not rise there may be the way it is taken, fat absorption, excess weight, medicines or a lack of magnesium; and the answer is not to raise the dose on your own.
Your test showed low vitamin D and your doctor recommended a supplement. You took it for three months and had a follow-up test; the level has either risen very little or not moved at all. The first solution that comes to mind is to raise the dose. Yet behind vitamin D that is not rising there is usually a reason to do not with the amount but with the way it is taken or with absorption.
In this article we explain the path vitamin D follows in the body, ten reasons that may prevent the level from rising, the right way to take it and why raising the dose on your own is dangerous.
How is vitamin D processed in the body?
Vitamin D is obtained in two ways: it is produced in the skin under the effect of sunlight, or it is taken in with food and supplements. It is a fat-soluble vitamin; it is absorbed from the bowel together with fats. After absorption it undergoes a conversion in the liver. The value measured in a blood test is this storage form. It is then converted into its active form in the kidneys.
Every step of this path affects the result: absorption from the bowel, conversion in the liver and kidneys, and the distribution of the vitamin in the tissues. Active vitamin D enables calcium to be absorbed from the bowel and plays a part in bone and muscle health and in the working of the immune system.
Why does the vitamin D level not rise? 10 possible reasons
1. Irregular use, or not enough time having passed
This is the most common reason. Missed days, boxes left half finished and taking it “when I remember” do not raise the level. It takes weeks for vitamin D to reach its new balance; a test done after a few weeks does not show the true result.
2. Taking it on an empty stomach or with a fat-free meal
Because vitamin D is fat-soluble, its absorption depends on the fat in the meal. A supplement taken with water on an empty stomach in the morning, or with a breakfast of fruit alone, is absorbed less well than one taken with a meal containing eggs, cheese, olives or olive oil.
3. Excess weight
Vitamin D is held in fat tissue. In people with more body fat, the vitamin taken is spread over a wider area and the level in the blood rises more slowly. For this reason the requirement may be higher in people who are overweight; the doctor decides the amount.
4. Conditions that impair fat absorption
Coeliac disease, inflammatory bowel diseases, pancreatic insufficiency, problems with bile flow and weight-loss surgery on the stomach reduce the absorption of fat and, with it, of vitamin D. If there are greasy, foul-smelling stools, chronic diarrhoea or unexplained weight loss, this possibility must be assessed.
5. Medicines being taken
Some epilepsy medicines, long-term corticosteroid treatment, weight-loss medicines that reduce fat absorption and some cholesterol-lowering resins can reduce the absorption of vitamin D or speed up its breakdown. Tell your doctor about all the medicines you take.
6. Liver and kidney disease
For vitamin D to become useful in the body it has to be converted in the liver and kidneys. In diseases of these organs both the level and the effect may be impaired; the doctor decides on the form of treatment.
7. Lack of magnesium
The enzymes that convert vitamin D need magnesium. In people who are short of magnesium, raising the vitamin D level may be harder. We cover this in the article on forms of magnesium.
8. Not benefiting from the sun
Working indoors, covered clothing, dark skin, older age and the winter months reduce production in the skin. Sunlight through glass does not bring about vitamin D production.
9. The product itself, or a mistake in using it
With products in drop form, holding the bottle wrongly, or miscounting when dropping straight into the mouth instead of on to a spoon, changes the amount taken. Products kept in heat and light, or past their expiry date, may also be less effective.
10. Mixing up the units of the test
Laboratories may report vitamin D in two different units, and the figures in these two units are very different from each other. When comparing two tests done in different laboratories, make sure the unit is the same; otherwise you may take a level that has risen for one that has fallen.
How should vitamin D be taken?
- In the amount and for the time your doctor recommends: The requirement varies with the starting level, weight, age and health.
- With the meal of the day that contains the most fat: For most people this is breakfast or the evening meal.
- At the same time every day: Habit prevents forgetting.
- With attention to interactions with other medicines: Ask your doctor or pharmacist which medicines need a gap.
- Stored under suitable conditions: In a cool, dry place away from light.
There is no proven clear difference between taking it in the morning or in the evening; what counts is that it is taken regularly and with a meal containing fat.
How do you get vitamin D from the sun?
The natural source of vitamin D is the sun. A short period of sun, taken with arms and legs uncovered at the hours when the sun is high and without the skin reddening, brings about production. The time needed varies with skin colour, season and latitude. Even in a sunny city like İzmir, production may fall short in the winter months, in people who work indoors and in those who protect themselves from the sun all summer.
Sunburn increases the risk of skin cancer; so the aim is not to sunbathe for long but short, regular exposure. If you have a skin condition or take medicines that cause sensitivity to the sun, a doctor should be consulted.
Which foods contain vitamin D?
Vitamin D in food is limited; its main sources are oily fish such as salmon, sardines and mackerel, egg yolk, liver and some dairy products fortified with vitamin D. These foods are valuable, but it is usually not possible to correct a deficiency through diet alone. This is why doctors generally recommend a supplement when a deficiency is found.
Why is raising the dose on your own dangerous?
Unlike water-soluble vitamins, vitamin D is not easily passed out in the urine; it builds up in the body. When more is taken than is needed, calcium in the blood rises. The signs are nausea, vomiting, loss of appetite, constipation, excessive thirst, passing urine often, weakness and confusion; kidney stones and kidney damage may develop, and heart rhythm may be disturbed.
The idea that “the higher my level, the better” is not correct. Taking an extra dose on your own on top of what the doctor recommended, using more than one product containing vitamin D at the same time, or drinking high-dose ampoules haphazardly can have serious consequences. If the level is not rising, the thing to do is to look for the cause.
Should vitamin K2 and magnesium be taken with it?
Taking K2 together with vitamin D is very popular. K2 is known to play a role in directing calcium to the bones; but there is not enough evidence to show that everyone who takes vitamin D needs to take K2. In people on blood-thinning medication, vitamin K can alter the effect of the medicine, so it must never be used without consulting a doctor.
Magnesium, for its part, becomes important if not enough is being obtained from the diet or if a deficiency is suspected. For both supplements the decision should be made together with your doctor, who knows your health and your medicines.
In brief
- Vitamin D is fat-soluble; it should be taken regularly and with a meal containing fat.
- The most common reasons for a level that does not rise are irregular use, taking it without fat, excess weight and absorption problems.
- Some medicines and a lack of magnesium also affect the result.
- When comparing tests, check that the unit is the same.
- Do not raise the dose on your own; too much is harmful.
- If the level is not rising, see your doctor so that the cause can be investigated.
Frequently asked questions
Does vitamin D deficiency cause weight gain?
Vitamin D levels are more often found to be low in people who are overweight; but this does not show that the deficiency causes the weight. There is no sound evidence that a vitamin D supplement on its own leads to weight loss.
Should I take vitamin D in summer as well?
That depends on your level and on how much sun you get. Your doctor decides, on the basis of your follow-up test, whether to take a break.
When should the follow-up test be done?
It takes a few months for the level to reach its new value. Your doctor sets the time for the follow-up; a test done too early can be misleading.
