Antioxidants are free radical scavengers. They help the body deal with them. Free radicals occur in normal, healthy cells, but can cause damage to cellular mechanisms over time. Antioxidants help the body heal itself.
If you have a sick body, you won't get well from taking bunches of antioxidants. And, unfortunately, antioxidants won't affect your metabolism directly, though they will help your body to heal after exercise. The benefit in weight loss is indirect, with improved muscle motion burning more calories. If you have a reasonably healthy body, and especially if you're engaged in extreme workouts like P90X, antioxidants will help your cells heal faster and more effectively.
Interestingly, the type of antioxidants used seems to make a bit less difference in most situations than the fact that the antioxidants are present. You can get them in vitamins and nutritional drinks and supplements (most of them have at least a few) and you can get them in your diet. You can purchase them separately. Be advised that it is possible to take too many of them and that can cause health problems.
Here's a link to the US Preventive Services Task Force website where you can read more about antioxidants and other nutritional recommendations:
www.ahrq.gov/clinic/uspstfix.htm
Hope this helps...
Kevin