Nutritionists agree that most diets aren't worth the paper they're printed on. They realize that the key to losing weight is adopting a sound, sustainable eating plan and then sticking with it, rather than opting for some wacko quick fix.
Problem #1: Diets don't last.
The problem with virtually all diets is the short-term mindset into which they feed. Most guys approach diets as an all-or-nothing proposition. Rather than making small, even incremental changes in lifestyle that can last a lifetime, diets encourage you to turn your life inside out for two weeks or so. Yet once those two weeks are over and you return to your old habits, guess what? Your body returns to its former state as well. If there's a rule of thumb to be had in this regard, it's that small changes last and big ones don't. Saying that you'll change everything you're doing wrong starting on Monday morning and straight-line it from there might sound impressive — and earn you some pats on the back — but it doesn't change your underlying behavior patterns. It's the slow, steady route that ultimately leads to success.
Problem #2: Diets make you hungry.
Diets typically treat fat loss as a function of nutrition only, when training is equally important. The diet world is about tearing down, and sports nutrition is about building up. You'll lose weight by creating a calorie deficit — burning more calories than you eat. You'll create that deficit, however, mostly through training and not through drastic dieting. The calories you burn in the weight room added to the metabolism boost you get from muscle growth will kick your body into fat-burning mode — without making you hungry.
Problem #3: Diets make you tired.
A chronic problem with diets is that so many of them are simply too low in calories. Because they don't provide enough energy for you to do your workouts and accomplish everything else you need to do in a day, they're a short-term solution at best. Even when weight-loss programs incorporate exercise — and, astonishingly, many don't — they typically ask you to eat like a gerbil and then train like a hamster by running or cycling endlessly in place. You may shed a few pounds in the short run, but you'll also forsake muscle, and the resulting metabolic downshift will soon take you back to square one. Whether it's being done on a treadmill, a stationary bike, or a squeaky metal cylinder, endless cardio performed on restricted calories is a road to nowhere, literally and figuratively.
This is especially true if you're following one of the ultra-low-carb diets that are so popular now. Carbohydrates are the primary energy source for physical activity, and decades of research has shown that low-carb diets don't adequately support strenuous physical activity or athletic performance for extended periods of time. In contrast, a diet moderate in carbs will supply enough energy for the average Joe to stay active and still burn fat. Endurance exercise requires more carbohydrates than strength training does, but in neither case will training be optimized without sufficient carbs.
Problem #4: Diets cannibalize your muscle.
Diets also tend to pay too little attention to supporting muscle mass during periods of caloric restriction. This is important for more than just aesthetic reasons. And when you lose muscle, your basal metabolic rate drops, and you don't burn as many calories.
Most guys try to burn off the fat first and then build the muscle. To do that, you have to lower your calories so far that you don't have the energy to train hard in the gym. You burn more muscle than fat, lowering your metabolic rate and setting the stage for weight gain.