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There aren't any guidelines to define what is a low-fat or low carb diet. However, a recent research study from DIETFITS came up with a good definition. They defined low fat as a diet with only 29% fat and 48% carbohdyrates of their daily meal plan; and low carb as 30% carbohydrates and 45% fat of their daily meal plan. The balance would be eating lean animal protein and vegetarian proteins. They didn't ask the participants to cut calories but the participants were eating 500 less calories a week. What was interesting about this study was that there was no difference between eith diet when it came to weight loss. Both groups loss about the same amount of weight!
Losing weight is a hot topic. This page gives some great tips on healthy weight loss.