According to a recent study in Italy by Luisella Vigna et al, supplementing of vitamin D could help obese people lose weight, if they are deficient in the vitamin. It may be useful to note that deficiency in vitamin D in the northern hemisphere is at epidemic levels. With some studies showing the figures to be as much as 80%.
This is for the main, in my opinion, due to the lack of regular sunshine combined with the fear of skin cancer, as we tend to avoid the sun or cover our self in sunscreen (which blocks the formation of vitamin D), when the sun does show.
Make D While The Sun Shines
For the average pale white guy, it only takes around 10-15 minutes for the body to make enough of its own vitamin D though exposed (unprotected) skin. This increases as the skin colour darkens.
For most of us 10-15 minutes isn't long enough to burn, so applying a high factor after 10 minutes could result in a safer tan as vitamin D is also cancer protective.
What To Do For Rainy D's
On the many occasions the sun doesn't shine it may be worth supplementing a good quality Vitamin D3 (important! D3 is over 4 times more effective than D2 used to fortify many breakfast cereals) in high dosages, up 10,000 iu every 4 - 5 days, rather than the arbitrary drip feed of 500 iu often recommended, has shown to be more beneficial for overall health benefits.
Over Vitamins For Weight Loss
Vitamin B12 has also been linked to weight loss, but once again the studies to date only show B12 to be effective for those deficient in the vitamin. The most common symptom of vitamin B12 deficiency is fatigue, and can be resolved with high dosage injections of the vitamin.
Those most at risk of B12 deficiency are vegetarians, vegans, people with cut related problems such as Chron's or celiac disease, but also those taking the diabetes drug metformin.
If you know you have good levels of these vitamins additional supplementation is unlikely to assist you in your weight loss goal, though optimal levels of both these vitamins is far higher than the RDA's. If you are deficient in either of these vitamins or think you may be get your GP to carry out a simple blood test so you know were you stand.
Luisella Vigna et al