Here are some studies about vitamin D and obesity:
Vitamin D Treatment Improves Levels of Sexual Hormones, Metabolic Parameters and Erectile Function in Middle-Aged Vitamin D Deficient Men (2017)
Conclusion: In a 12-month study of 102 vitamin D-deficient middle-aged men, monthly high-dose vitamin D therapy raised their testosterone levels, improved erectile function, and reduced body fat.
Conclusion: A study of 39 young adults found that those with higher vitamin D levels also had a higher level of cardiovascular fitness (Vo2 Max) and less body fat.
Serum 25-Hydroxyvitamin D Concentrations are Associated with Prevalence of Metabolic Syndrome and Various Cardiometabolic Risk Factors in US Children and Adolescents Based on Assay-Adjusted Serum 25-Hydroxyvitamin D Data from NHANES 2001-2006 (2011)
Conclusion: In a study analyzing 5,867 adolescents, there was a higher prevalence of metabolic syndrome among those with lower vitamin D levels.
A 16-Week Randomized Clinical Trial of 2000 International Units Daily Vitamin D3 Supplementation in Black Youth: 25-Hydroxyvitamin D, Adiposity, and Arterial Stiffness (2010)
Conclusion: A 4-month study of 44 black teenagers found that 95% had low vitamin D levels and that supplementing with 2,000 IU of vitamin D daily was effective for reducing arterial stiffness and increasing vitamin D levels. The study also found that the vitamin D levels of the obese subjects rose less than the leaner subjects. Although 2,000 IU daily was effective in raising levels, only 56% achieved sufficient levels after the four months.
Conclusion: A study of 340 black adults with type-2 diabetes found that those with the highest percentage of body fat also had the lowest vitamin D levels and the highest amount of carotid artery and aorta plaque. There is a correlation between high body fat and low vitamin D levels and a correlation between low vitamin D levels and increased risk for atherosclerosis.
Prevalence and Associations of 25-Hydroxyvitamin D Deficiency in US Children: NHANES 2001-2004 (2009)
Conclusion: A 3-year study of 9,757 U.S. children and young people found that: 61% had low vitamin D levels, those who spent more time watching/using TVs and computers had lower vitamin D levels, obese children had lower vitamin D levels, and those with lower vitamin D levels had higher blood pressure.
Conclusion: Obese children were found to have lower vitamin D levels than non-obese children. In the obese children, losing weight resulted in an increase of their vitamin D levels.
Low Serum 25-Hydroxyvitamin D Concentrations are Associated with Insulin Resistance and Obesity in Women with Polycystic Ovary Syndrome (2006)
Conclusion: A study of 120 adult women found that those with the lowest vitamin D levels also had the highest body fat percentages and highest rates of insulin resistance.
Conclusion: People with higher vitamin D levels are less likely to be diagnosed with having metabolic syndrome.
Conclusion: “The data show a positive correlation of 25(OH)D concentration with insulin sensitivity”
The Relationship between Obesity and Serum 1,25-Dihydroxy Vitamin D Concentrations in Healthy Adults (2004)
Conclusion: This study of 152 obese people and 148 non-obese people found that the obese people had lower vitamin D levels.
Conclusion: In a study involving 410 women, women with higher total body fat percentages had lower vitamin D levels.
Intakes of Calcium and Vitamin D Predict Body Mass Index in the Population of Northern Norway (2003)
Conclusion: A study of 12,866 men and women in Norway found that those with lower vitamin D levels had higher body fat percentages.
Conclusion: In vitamin D experiments carried out on a group of 19 obese people and a group of 19 normal weight people, the obese group experienced 57% lower vitamin D level increases following UVB irradiation compared to the non-obese group.
Conclusion: The study compared the serum pre-vitamin D levels of a group of obese individuals to the levels of a group of normal weight individuals and found the obese group to have significantly lower pre-vitamin D levels.