Running or Weights

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A number of studies have been done, and it comes down to simple math: your caloric deficit is what matters for weight loss. In controlled studies, those folks who perform both strength training and cardio lost more weigh than the people who did only one or the other.

Strength training has the benefit of helping you maintain muscle mass while cutting calories (otherwise you're going to loss muscle along with fat). So if you can ONLY do one, then strength train.

But for best results, do both.

BoxerBrawler

Both. Although I am sort of a cardio junkie because I dance. I found that cardio was best for losing weight in the beginning but as you get smaller and your body adapts to the exercise you don't burn as many calories. Lifting weights gave me a much needed boost! I also found that quick hit cardio, like a quick 20 minutes is way better and more effective than hours and hours.