In a study of 217 obese children ages 7-18 years, vitamin D deficiency was associated with increased weight and blood pressure, and a lower "Good Cholesterol (HDL)." Over half (55.2%) of all children were found to be vitamin D deficient (defined as 25-OH-D < 20 ng/ml) which, given the importance of vitamin D, is frightening. Vitamin D deficient children were much more likely to be both obese and insulin resistant. This is important because insulin resistance causes weight gain (as well as causing diabetes).
Though this does not prove that the low vitamin D caused the obesity and insulin resistance (could just be that overweight children are more likely to sit inside in front of the TV and snack), many studies suggest that low vitamin D is associated with insulin resistance, as well as many other severe illnesses. The upshot of this information? Send your kids out to play in the sun and (hopefully) fresh air. Remember—Avoid Sunburn, Not Sunshine!
By enhancing insulin sensitivity, vitamin D and sunshine will not only keep you much healthier, it may help keep you skinny as well! A good multivitamin powder has 2,000 units per serving of vitamin D.
For more information on the importance of vitamin D, see "Vitamin D Could Save Your Life!"
"Prevalence of vitamin D insufficiency in obese children and adolescents," Smotkin-Tangorra M, Purushothaman R, et al, Journal of Pediatric Endocrinology and Metabolism, 2007; 20(7): 817-823.