What about all those "lose 10 pounds in four days" high-protein diets? At the very least, can't you rely on them to take off a few fast pounds at the beginning of a more "sensible" diet?
The trouble with high-protein diets aimed at quick weight loss is that the weight you lose consists primarily of water and glycogen (carbohydrates stored in the liver and muscles). Once you go off these diets and start eating more carbohydrates again, your body retains water and glycogen more stubbornly to compensate for the previous shortage, leading to a weight-gain "rebound." Even worse, because these high-protein diets are often too low in both carbohydrates and calories, the protein you eat must be used mostly for energy. This means that the protein needed for building and repairing body tissues may be drawn from your own lean muscle. So after you lose weight, you may be left with a higher proportion of fat to muscle than before you started.
If your goal is to slim down, this cycle could be self-defeating. Burdened with a higher proportion of fat to muscle, your body won't burn calories as quickly, since fat uses up energy more slowly than lean tissue. That means you'll appear flabbier and less "defined" than before, even if you end up weighing less (since muscle is slightly heavier than fat). In the more likely event that you gain back your lost weight ("quick" protein diets deliver only short-term results), you're less likely to lose it the next time around. What's worse, by raising levels of serum cholesterol and uric acid, these diets do exactly what excess protein does: They increase your risk for serious illness. So not only are high-protein weight-loss diets generally ineffective over the long haul, they're actually hazardous to your health.