Remember the "a calorie is a calorie" argument? Well it's not... not nearly.
In order for the body to give off more heat, more of the calories consumed must be lost as heat. This means the body is working harder at using these calories and so 1000 calories of carbohydrate does NOT equal 1000 calories of protein in potential weight gain/loss. Many more calories are expended by the body in turning protein (and fat) into sugar, and the net calories actually used by the body for other functions is far less.
Postprandial Thermogenesis Is Increased 100% on a High-Protein, Low-Fat Diet versus a High-Carbohydrate, Low-Fat Diet in Healthy, Young Women"The recent literature suggests that high-protein, low-fat diets promote a greater degree of weight loss compared to high-carbohydrate, low-fat diets, but the mechanism of this enhanced weight loss is unclear. This study compared the acute, energy-cost of meal-induced thermogenesis on a high-protein, low-fat diet versus a high-carbohydrate, low-fat diet."
" The popularity of high protein diets for weight loss is unquestionable. Although this research did not assess weight loss or the long-term effects of a high protein diet, results indicated that the increased thermogenesis of a high protein diet may contribute to its efficacy."