(NaturalNews) Confirming what health professionals have long suspected, a new study published in the journal PLOS Medicine provides some of the strongest evidence yet that obesity is indeed a cause of vitamin D deficiency.
The large study was a collaborative effort between U.S. and European researchers, and was funded by the British Heart Foundation and the UK Medical Research Council.
Many prior studies have found a strong correlation between obesity and low levels of vitamin D. According to the researchers, however, the new study is the first that has actually been able to show a causal link, confirming that obesity causes vitamin D deficiency rather than the other way around (and rather than both conditions being caused by some third factor).
The researchers examined 21 separate studies on a total of 42,024 adults of European ancestry to collect data on not just vitamin D levels and body mass index (BMI, a measure of obesity), but also on 12 separate genetic variations related to BMI and four genetic variations related to vitamin D levels. Based on prior research into these genetic variations, all participants were assigned scores approximating their genetic predispositions to obesity and to lower vitamin D levels.
The researchers hypothesized that if obesity is a cause of vitamin D deficiency, then people with a high genetic predisposition to obesity should be more likely to have a lower vitamin D levels. In contrast, if it is vitamin D deficiency that causes obesity, then a genetic predisposition to vitamin deficiency should be associated with higher obesity rates.
Strong evidence of causal link
Confirming the findings of previous studies, the researchers found that every 1 kg/m2 increase in BMI was associated with a 1.15 percent lower vitamin D blood concentration. In a separate statistical analysis, the researchers also found that a 10 percent increase in BMI led to a 4.2 percent decrease in vitamin D levels. All statistical analysis was controlled for the influence of potential confounding factors.
Furthermore, the researchers found that people with a higher genetic predisposition to obesity had both a higher BMI and lower vitamin D levels. Yet while people who were genetically predisposed to vitamin D deficiency did indeed have lower vitamin D levels, they did not have higher BMIs than less predisposed individuals. The latter finding was further confirmed in an analysis of 123,864 people taking part in the Genetic Investigation of Anthropometric Traits (GIANT) study.
This strongly suggests that obesity is a cause of vitamin D deficiency, not the other way around.
"Population-level interventions to reduce BMI are expected to decrease the prevalence of vitamin D deficiency," the researchers wrote.
Get outside and take a walk
Although researchers remain unsure exactly why obesity would cause vitamin D deficiency, many believe that because the vitamin is fat-soluble, it can become "trapped" in excessive fat deposits and unable to enter the bloodstream.
This does not mean that obese individuals are doomed to vitamin D deficiency, however. Like all people, obese individuals can dramatically improve their vitamin D levels by increasing their dietary intake and their exposure to sunlight (without sunscreen).
David Haslam of the UK's National Obesity Forum noted that some of the same lifestyle changes may help fight both obesity and vitamin D deficiency.
"Food intake and genetics all play a part in obesity - but this research is a reminder that physical activity, like walking the dog or going for a run out in the sunshine, shouldn't be forgotten and can help correct both weight and lack of vitamin D," he said.