Vitamins are vital organic substances for the human body, which have to be absorbed externally, as they cannot be synthesized sufficiently in the own body. According to current knowledge, humans can produce around 80-90% of the vitamin D they need through sufficient exposure to sunlight, which means that, strictly speaking, the vitamin is not classified as such. The remaining 10-20% is absorbed through food. In the body, vitamin D is converted into calcitriol, a steroid hormone, which is why it is also called a prohormone, i.e. a hormone precursor.
Essential functions of vitamin D are the formation and maturation of bone stem cells, as well as the regulation of calcium levels. The absorbed calcium can subsequently be incorporated into the bones together with phosphate, causing them to harden.
Vitamin D is said to have other functions, some of which are currently not clearly scientifically proven. These include:
- Lower risk of vascular disease and rickets.
- Stronger muscles and immune system or help with immune reactions
- Positive effect on cardiovascular disease, brain neurons, human psyche and cancer.
In addition to this, several studies showed that low vitamin D blood levels (i.e. 25-hydroxyvitamin D blood levels, i.e. the storage form of vitamin D3) could show a possible correlation with an increased risk of type 2 diabetes mellitus. Such a correlation seems at least plausible, since at low vitamin D blood levels the beta cells of the pancreas function in a restricted manner. These cells produce insulin - if they fail, an insulin deficiency can result, which is one cause of the development of certain types of diabetes mellitus. In this regard, a 2011 study showed a 40% improvement in beta cell function - with vitamin D supplementation. Since direct causality has not yet been proven, researchers now analyzed the possible direct association of vitamin D supplementation and the risk of type 2 diabetes in adult humans.
24 months of vitamin D3
Published in 2019, in the New England Journal of Medicine, the clinical trial analyzed the efficacy and safety of vitamin D3 as oral administration in the context of preventing type 2 diabetes mellitus, with calciol (i.e., vitamin D3) taken at 4000 IU per day by participants at increased risk for type 2 diabetes mellitus in the double-blind, randomized, placebo-controlled study. According to researchers, the manufacturer of the vitamin D3 supplementation was independent - and all aspects of the clinical trial were reportedly not sponsored by drug companies.
Adult humans who met at least two of three criteria for prediabetes but no criteria for diabetes were included in the study. Glycemic criteria were:
- Fasting plasma glucose level (100 - 125 mg /dl) - i.e., after 8 - 12 hours without food intake, or NPG.
- Plasma glucose level (140 - 199 mg/dl) - i.e., 2 hours after oral intake of a concentrated glucose solution, or OGTT for short.
- Glycosylated hemoglobin level (5.7 - 6.4%) - i.e., shows how high blood glucose has been in the last 8 - 12 weeks, HbA1C for short.
Other inclusion criteria were age of at least 30 years and a certain BMI value.
Study participants were randomized to receive 4000 IU of vitamin D3 or placebo daily. The primary endpoint was the occurrence of diabetes, with an upper limit of 508 cases of diabetes.
The 2423 participants were randomly assigned to two groups - vitamin D group: 1211 and placebo group: 1212. The intervention period was 2 years, and the mean vitamin D serum level after 24 months was 54.3 ng/ml in the vitamin D group and 28.8 ng/ml in the placebo group. In comparison, at baseline, this was 27.7 ng/ml in the vitamin D group and 28.8 ng/ml in the placebo group. After a follow-up period of 2.5 years, 293 study participants were found to have diabetes, with the placebo group recording 323 cases. As a result, the vitamin D group showed a potentially lower risk of diabetes by 12%, according to researchers. Side effects did not differ between the two groups.
Despite understandable biological plausibility of the hypothesis that vitamin D supplementation at high daily doses is associated with a lower risk of diabetes, the clinical trial failed to show a significant risk reduction (compared with placebo) with daily 4000 IU vitamin D administration. Nevertheless, scientists continue to conduct research to clearly find a possible link between vitamin D and risk prevention in type 2 diabetes mellitus - in theory, at least, this significant issue seems plausible.