This week, Australian researchers published a long-term weight loss study in healthy adult (49 + or - 9 years) overweight women (BMI initially ~32).
In this study, they followed 79 women for more than a year (64 weeks) to see how protein influences weight loss and compliance to a dietary program.
The women were placed on one of two diets: A energy-reduced high protein (HP) diet with 34% of energy from protein, 20% of energy from fat, and 46% of energy from carbohydrate, or a energy-reduced high carb (HC) diet with 17% of energy from protein, 20% of energy from fat, and 64% of energy from carbohydrate. For the first 12 weeks of the study, the women saw a dietitian very four weeks to ensure they were following the plan. Then for the remaining 52 weeks, they were asked to follow the same regimen as during the short-term study if they could, but those consuming the HP diet could substitute chicken, fish, or pork for red meat, whereas those consuming the HC diet could omit biscuits and substitute them with more bread, potatoes, or rice. During this time, they only saw a dietitian every 3 months.
As you probably guessed, not every woman was compliant with the program when they were left to do it on their own. During the free-living 52 weeks, when the results of all the women were analysed, it appeared that there was no difference in weight loss or fat loss(about 10 lbs on average and 6 lbs of fat). However, when the women were seperated into those that actually ate the HP diet and those that actually at the HC diet, there was a significant difference: Women who followed a HP diet lost MORE weight and more fat than those that ate the HC diet. Protein intake was inversly related to weight and fat loss such that for those that are more protein lost an average of 14 lbs and 10 lbs of fat compared to those that ate less protein (and more carbs) who lost 7 lbs and 5 lbs of fat over the course of a year.
Body composition was measured my DEXA, the gold standard for body comp assessment. After 64 weeks, body fat was higher in those that ate more carbohydrate and lower in those that ate more protein. The same was seen for belly fat - more belly fat in those that ate a higher carb diet compared to the higher protein diet.
Bone health was also assessed in response to the diets: Overall, decreases in the 24-h urinary bone turnover markers—ratio of dexoypyridinoline to creatinine and the ratio of pyridinoline to creatine were not different between diets and there was no relation with weight loss or any dietary components. Calcium excretion was also not different from week 0 to week 64 in the two dietary groups.
Overall, as you can plainly read here, eating more protein to help a woman lose weight and fat is more beneficial than eating more carbohydrates and less fat.
Now, there are differences among women such that some women will lose more weight and fat with less dietary fat, but on average, for the majority of women out there, bumping up your protein with fresh meats, fish, poultry, and dairy can help you be a bigger loser :)