Vitamin D levels in obese subjects have been studied extensively. Serum 25-hydroxyvitamin D (25(OH)D) has been shown be inversely related with BMI. It has been speculated that supplementing with Vitamin D may help reduce body fat in obese individuals.
Let me first explain what 25-hydroxyvitamin D is. It isn’t vitamin D, it is merely an accurate method of measuring the amounts of vitamin D present in the body. I’m not going to explain what exactly Vitamin D is because everyone and their dog knows about it. If you want to learn more about it check out this article in the CJASN.
Numerous studies have shown an association between low vitamin D levels and obesity. A cohort of over 25,000 individuals were studied in Norway. The researchers concluded that have serum (25(OH)D) levels <50 nmol/L was associated with new-onset obesity in adults. This study didn't discuss the reasons for this. Another study by Alemzadeh et al studied hypovitaminosis D (>75 nmol/L) in children and adolescents. The researchers discovered that obese 74% of obese adolescents were vitamin D deficient. So there’s another interesting tidbit.
Now let’s examine what happens when obese individuals are supplemented with vitamin D. Do they lose weight? Do their biomarkers of health improve? Jorde et al studied obese subjects over a one year period and found no positive effects of vitamin D on
glucose tolerance, blood pressure or serum lipids. No good…
Lastly, let’s see how vitamin D supplementation effects fat loss in obese subjects. Sneve et al studied 450 obese individuals over a one year time period. They set out to determine if vitamin D supplementation would results in fat loss. The participants were separated into 2 groups. One group received 20,000 IU’s of vitamin D per week, the other group received 20,000 IU’s of vitamin D twice per week. Both groups were supplemented with calcium. There was no reduction in weight in either groups. One huge hole in this study was the lack of tracking food intake. Since neither group lost weight, it’s still unlikely that their caloric intake changed much. It still would’ve been a good piece of data to record.
So we now see that vitamin D doesn’t really do anything in terms of improving biomarkers of health or reduce bodyfat in obese individuals. Why aren’t these individuals getting benefits from supplementing with a vitamin that they are deficient in?
The answer lies in a study published over a decade ago. Wortsman et al sought out to discover the bioavailability of vitamin D in obese subjects. Bioavailability is defined as: the proportion of a drug or other substance that enters the circulation when introduced into the body and so is able to have an active effect. The study “assessed whether obesity alters the cutaneous production of vitamin D(3) (cholecalciferol) or the intestinal absorption of vitamin D(2) (ergocalciferol).”
The researchers discovered that the vitamin D(3) precursor 7-dehydrocholesterol did not differ significantly between obese and non-obese subjects. Both groups received 50,000 IU’s of vitamin D2. The conversion of vitamin D2 to D3 was also equal between both groups in vitro. However, after 24 hours, obese subjects were shown to have 57% less incremental increase in vitamin D compared to non-obese subjects. Ergo the researchers concluded that “Obesity-associated vitamin D insufficiency is likely due to the decreased bioavailability of vitamin D(3) from cutaneous and dietary sources because of its deposition in body fat compartments.”
So it would appear that the presence of excess adipose tissue is the reason that the body cannot receive the proper amount of vitamin D. It’s simply being stored in the fat. Vitamin D needs to be metabolized by the liver and kidney’s in order to be used in the bloodstream. So to conclude, it would appear that obesity is causing vitamin D deficiency, it’s not the other way around.