supplement..?

That depends on what your end goal or desired outcome would be?

Two supplements popular with bodybuilders, which help increase endurance and stamina and aid in recovery from strength training or heavy cardio sessions, are glutamine and creatine. You can check bodybuilding websites for dosages, like I did. This would be more important if you are trying to build muscle/strength. I have been taking glutamine, and just got creatine in the mail, to take after workouts. I am hoping they will help me recover faster so that I can work the same muscle groups again sooner and try to increase the weights I use in my strength training circuit routines.

Taking a Vitamin D supplement may help you to lose weight if you are deficient in the vitamin, as many people are. I would suggest getting tested by your doctor to check your levels, however. I don't know if it will help you if you have normal levels of the hormone already. I've just started taking it and am hoping for the best; I don't get much sun at all and believe I'm deficient, and I've noticed that I'm sort of "lagging." I'm hoping to replenish my stores of Vitamin D for more energy.

Green tea (either in the form of a supplement or taken as a drink) has been shown in clinical studies to boost metabolism, and many are now touting a new fiber-rich supplement called "African Mango" for that same purpose. The latter has become popular in the news lately because it was endorsed by Dr. Oz. I think he has also endorsed something called "Raspberry Ketones" for a similar purpose, but I admit I don't know much about it.

Lastly, Resveratrol, an antioxidant found in red wine but sold in doses thousands of times higher than that found in a glass of red wine, has been found in studies of mice to, in effect, create the positive side-effects of losing weight (I believe an example was lower cholesterol) without actual weight loss. Proponents, however, say that it increases metabolism and decreases appetite. I myself gave it a chance but didn't really notice a big difference.

There are also a few other ways to cut a few extra calories/increase your metabolism besides supplementation:

-Some foods also aid in boosting the metabolism, especially spicy foods. Adding chopped hot peppers to a dish or including a dash of cayenne can give a temporary boost for a short period after your meal.

-I'm sure you've heard this tons of times but spreading your meals out over the day, eating breakfast, drinking A LOT of pure water (not including tea/coffee, unfortunately) and not eating or drinking calories for ~3-4 hours before you go to sleep keep your metabolism revved.

-Vigorous cardio is the best way to increase your metabolism for the SHORT term. A bout of intense aerobic exercise increases your resting metabolism for about 8 hours. This is called "after-burn." Studies have shown that the type of aerobic exercise that most increases your metabolism during "after-burn" is called "High Intensity Interval Training (HIIT)" or "Sprint Interval Training (SIT)". This is where you interchange very short periods (think 30 seconds to 1 or 2 minutes) of strong sprinting (on a scale of effort from 1-10, about an 8-10) with rest periods of more endurable exercise (about a 3-5 on a 1-10 scale) for about 1-3 minutes. The best part about HIIT is that you don't have to do it as long as a normal bout of aerobic exercise to get the same (and some additional) benefits! An intense HIIT session may take you 15-20 minutes as opposed to 45-60 minutes of a normal cardio session.

-Strength training is the best way to increase your metabolism for the LONG term. The more muscle you have, the higher your resting metabolism will be, the more fat you will burn on a daily basis outside of exercise.

Hope this helps!