Researchers in Spain have shown a definitive link between low serum vitamin D and the prevalence of obesity. Rather than being a consequence of obesity, the vitamin deficiency may play a causative role.
This prospective trial begun in 1996 involved 1,226 individuals who were evaluated 6 and 9 years after their initial baseline assessments. The cohort was deemed representative of the Spanish population at large.
Obesity, defined as a body mass index of greater than 30 kg/m2, clearly increased as the cohort aged. At baseline, the prevalence was 28%, and increased to 36% after 6 years. By the 9th year, prevalence had increased to 39.5%.
The researchers, led by Inmaculada Gonzalez-Molero, of the Servicio de Endocrinología y Nutrición, Hospital Regional Universitario Carlos Haya de Málaga, reported that the prevalence of vitamin D deficiency at baseline, defined as a 25-(OH)-vitamin D level of 20ng/ml or less, was 34.7%, and did not vary significantly over the course of the study.
Presence of obesity at baseline did not seem to increase the odds of vitamin D deficiency, though there was a slightly higher prevalence of deficiency among the obese versus non-obese individuals on the second testing (39% vs 33%).
The vitamin deficiency very definitely increased the odds of obesity. In subjects who were non-obese at the second evaluation, a vitamin D value of 17 ng/ml (42.5 nmol/l) was associated with a marked increased risk of developing obesity in the next 4 years (OR=2.35, 95%CI: 1.03–5.4, P=0.040). The data were published last summer in the European Journal of Clinical Nutrition.
Deficiency Precedes Gain?
A number of epidemiological studies have shown associations between obesity and vitamin D deficiency, but researchers have struggled to determine which factor is a cause and which is a consequence.
Obesity is a recognized risk factor for vitamin D deficiency, which makes common sense because the larger an individual, the more of the vitamin he or she would need.
A 2010 study from Norway’s University of Tromsø involved over 10,000 people, with 2,656 individuals followed for fourteen years. The findings showed that increased BMI was directly associated with lower vitamin D levels: higher BMI was highly predictive of vitamin D deficiency (Jorde R, et al. Eur J Nutr. 2010;49(7):401-7).
While some investigators have suggested that vitamin D plays a role in fat metabolism, and that deficiency may promote weight gain, nobody had been able to prove a causal relationship.
Earlier this year, Fernanda Reis de Azevedo and Bruno Caramelli of the University of Sao Paulo Medical School published an excellent review of the current science on the obesity-vitamin D association, including thoughtful commentary on the challenges of proving causality.
The advantage of Dr. Gonzalez-Molero’s study is that its prospective design and long duration enabled researchers to evaluate whether people with various vitamin D levels gained or lost weight over roughly a decade.
The authors conclude that rather than being a secondary sequela of obesity, vitamin D deficiency may precede weight gain.
If vitamin D deficiency does, in fact, promote weight gain the big question is whether supplementation could play a role in preventing it. Though the findings are suggestive, the Spanish study cannot answer this question, nor can the Norwegian trial.
However, 93 subjects in the Tromsø study were given daily doses of 40,000 IU of the vitamin and followed for one year. While the investigators did not report any change in BMI as a direct result of supplementation, they did find that the rise in serum vitamin D in response to supplementation was inversely correlated with baseline BMI.
In other words, those with the highest BMI had the smallest serum increases.
In clinical terms, this means that obese people need to take more vitamin D to reach a given serum level than people with lower BMIs.