Vitamin D: a meal containing fat improves absorption

Taking vitamin D supplements during a meal containing fat, mono- or polyunsaturated, can increase its absorption by 32 percent compared to when the same dose of the vitamin is taken without lipids.

This is the finding of the authors of a study conducted at Tufts University in Boston, USA, and published in the Journal of the Academy of Nutrition and Dietetics.

What is vitamin D

More than an actual vitamin, vitamin D is a pro-hormone. It owes its nickname ("sunshine vitamin") to the fact that it can be synthesized by the action of sunlight on the skin. Its precursor (7-dehydrocholesterol) is first converted into pre-vitamin D and, later, into vitamin D3 (or cholecalciferol)-which is, indeed, the form of vitamin D produced by the human body.

As synthesized in the skin, vitamin D3 is not active. To perform its functions, it must be modified in the liver and kidneys. The former produces 25-hydroxyvitamin D3 (25(OH)D), which is the form dosed in blood tests to assess vitamin D3 values. The latter (along with a few other tissues), on the other hand, produce the active form of this pro-hormone, 1,25-dihydroxyvitamin D3 (1,25(OH)D), which carries out its actions by interacting with its receptors.

Vitamin D3: what is it used for? The properties

Vitamin D3 has more than one property:

• in the pancreas, increases the secretion of insulin, the hormone that controls blood sugar levels (i.e., blood sugar);
• in bones, participates in the metabolism and mineralization of bone;
• in the intestines, increases the absorption of calcium and phosphorus;
• Regulates cell proliferation and differentiation;
• Participates in the regulation of the immune system.

According to the European Food Safety Authority (EFSA), a vitamin D3 supplement can claim these benefits: 

• Vitamin D is necessary for normal bone growth and development in children;
• Vitamin D helps reduce the risk of falls associated with postural instability and muscle weakness. Falls son a risk factor for bone fractures in men and women 60 years of age and older;
• Vitamin D contributes to the normal absorption/utilization of calcium and phosphorus;
• Vitamin D helps maintain normal blood calcium levels;
• Vitamin D helps maintain normal bones;
• Vitamin D contributes to the maintenance of normal muscle function;
• Vitamin D helps maintain normal teeth;
• Vitamin D contributes to the normal functioning of the immune system;
• Vitamin D contributes to the normal functioning of the immune system in children;
• Vitamin D plays a role in the process of cell division.

The blood concentration of vitamin D3 has normal values if it is at least 30 ng/ml. When 25(OH)D is between 20 and 29 ng/ml, it is called insufficiency, while below 20 ng/ml it is in a state of deficiency (very serious if the measured value is less than 7 ng/ml). And when vitamin D3 is low, symptoms such as bone pain, muscle pain, and muscle weakness may appear.

Given the link between a nutrient known for its benefits to bone, calcium, and vitamin D3, it is not surprising that in children, vitamin D3 deficiency causes rickets and in adults leads to osteomalacia, a condition in which the collagen matrix within the bones, inadequately mineralized, is weak and unable to provide the structural support the bone would need, thus increasing the risk of fractures. In addition, vitamin D deficiency prevents young children from optimizing bone development and hinders their growth.

Sources of vitamin D3: where it is found

It is therefore clear how important it is to be able to meet the body's daily vitamin D requirements. Unfortunately, however, these days the feat seems far from simple.

Foods that contain vitamin D3 are few; these are mainly certain types of fish (such as herring, mackerel and salmon), cod liver oil, butter, some cheeses and eggs. Some plant-based foods, such as mushrooms and chocolate, are also sources of vitamin D, but within them is a different form: ergocalciferol, also called "vitamin D2."

Both forms of vitamin D are converted by the liver and kidneys into their active forms, but there is still some doubt whether active vitamin D2 has the same properties as active vitamin D3. And the differences do not end there, because vitamin D2 has a harder time binding to the main protein that allows vitamin D (which is a fatty substance) to be transported into plasma (which is, by contrast, an aqueous solution). In addition, vitamin D2 is degraded faster by the body.

Considering all these aspects, the vitamin D present in food is generally not sufficient to meet the body's needs. And while in Mediterranean countries the amount of ultraviolet radiation from the sun's rays is usually sufficient to cover this vitamin D requirement, in some northern countries exposure to the sun is very weak. Not only that, people in Mediterranean countries also often spend too much time indoors, significantly reducing sun exposure.

To cope, it is helpful to take vitamin D3 supplements or fortified foods (i.e., fortified with vitamin D3). Some studies suggest that taking vitamin D3 is a more effective strategy than using vitamin D2 supplements. These research data are supplemented by data from experiments indicating that vitamin D2 intake may reduce blood levels of vitamin D3; however, the consequences of this effect are not yet known.

To date, vitamin D supplements are, after multivitamins, the most popular dietary supplements. Prescribing vitamin D3 to infants is routine, and taking supplements of this nutrient is recommended throughout the first year of life. And many adults also take it, precisely because many follow a lifestyle that greatly reduces direct skin exposure to the sun promotes vitamin D3 deficiency. 

Because vitamin D is a fat-soluble nutrient, it has been hypothesized that fat-containing foods may affect its absorption. Prior to the study published in the Journal of Academy of Nutrition and Dietetics there was, however, little clear information about this.

Proper fat intake improves vitamin D absorption

In the study Bess Dawson-Hughes colleagues recruited 50 subjects, men and women, elderly and healthy. The participants were divided into three groups: the first group consumed fat-free meals, while the second and third consumed meals in which 30 percent of the calories were provided by fat, but differed in one aspect: the ratio of monounsaturated fatty acids (MUFAs) to polyunsaturated fatty acids (PUFAs). In the meals of the second group, the MUFA:PUFA ratio was low (1:4), while in those of the third group it was high (4:1). 

After 12 hours of fasting, all participants had their blood drawn, ate breakfast and took a vitamin D3 supplement (50,000 IU, or about 125 micrograms). After 10 hours (during which time they ate nothing else), everyone had their blood drawn again and had dinner; after another 2 and 4 hours new blood samples were taken from all participants.

Blood tests showed that at 12 hours after taking the vitamin D3 supplement, plasma vitamin D levels were 32% higher in subjects who had consumed fat-containing foods. Absorption was also higher at 10 and 14 hours after breakfast (by 40% and 25%, respectively), but did not appear to be affected by the ratio of MUFAs to PUFAs.

A very useful discovery

These results helped to better understand how the composition of a meal may affect the absorption of a vitamin D3 supplement and suggests that it is best to take vitamin D during meals that contain fat.

How much vitamin d3 a day to take depends on individual needs: your doctor or nutritionist will determine this on a case-by-case basis. Not only that, the most suitable formulation may also depend on specific needs: sometimes a vitamin D3-only supplement is sufficient, sometimes a calcium carbonate plus vitamin D3 combination is more suitable, and sometimes it may be more useful to take vitamin D3 and Omega 3 from krill oil, an ideal pairing to simultaneously protect bones, the immune system, and the cardiovascular system.

In any case, it is best not to exceed the maximum tolerable intake level indicated by the Italian Society of Human Nutrition (40 micrograms for infants, 65 micrograms from 1 to 3 years, 75 micrograms from 4 to 10 years, and 100 micrograms from 11 years). In fact, vitamin D3 excesses have side effects such as nausea, vomiting, constipation, reduced appetite, weight loss, weakness, confusion, disorientation, kidney damage, kidney stones, and calcium deposits in soft tissues (e.g., in the heart and lungs).

Bibliographic references:

Chauhan K et al. Vitamin D. [Updated 2023 Apr 9]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK441912/

European Commission. EU Register of Health Claims. Last viewed: 25/01/24

Dawson-Hughes B et al. Dietary fat increases vitamin D-3 absorption. J Acad Nutr Diet. 2015 Feb;115(2):225-230. doi: 10.1016/j.jand.2014.09.014

Md Isa Z et al. The Impact of Vitamin D Deficiency and Insufficiency on the Outcome of Type 2 Diabetes Mellitus Patients: A Systematic Review. Nutrients. 2023 May; 15(10): 2310. doi: 10.3390/nu15102310

MedlinePlus. Vitamin D. Last viewed: 25/01/24

Ringe JD and Kipshoven C. Vitamin D-insufficiency. An estimate of the situation in Germany. Dermatoendocrinol. 2012 Jan 1; 4(1): 72-80. doi: 10.4161/derm.19829 

Sinu. LARN 2014 Tables. VITAMINS -. Maximum tolerable intake level (UL)