Slimming down muscular thighs presents an interesting challenge because you must lose muscle from the legs to achieve your goal. To do this, you need to use the right exercise and diet program. For example, some resistance training exercises cause an anabolic, or muscle-building effect, which you want to avoid. Even some forms of cardiovascular training can trigger muscle size increase in the thighs. Your diet also plays an important role because what you eat determines whether you will gain, maintain or lose muscle mass.
Decrease your total daily caloric intake by 500 to 750 calories per day. A caloric deficit is needed to lose weight. You can find your maintenance calorie intake by using a standard calorie calculator. Maintenance calories are those needed to maintain your body as it is. To lose weight from the thighs, you must subtract your calorie reduction from the daily maintenance total.
Do long-duration medium- to high-intensity cardiovascular exercise on an empty stomach upon waking up in the morning. When you awaken, your body's cortisol levels reach their highest point, according to "The Cortisol Connection" by Shawn Talbott. A natural stress hormone, cortisol breaks down muscle tissue when your body does not have enough calories for energy. Doing cardio in a fasted state forces the body to turn to muscle protein for a source of energy, helping to slim down muscular thighs.
Do high repetition, low-weight isolation exercises for the thighs immediately after your cardio session. For example, one to five sets each of 20 to 50 reps on leg extensions and leg curls. This strategy allows your body to burn up any glycogen or muscle-stored carbohydrates during your cardio. When you get to the resistance training, your body will be in a very catabolic state, readily breaking down muscle. Isolation exercises have more of a catabolic effect than compound exercises, for example, according to "Xtreme Lean" authors Jonathan Lawson and Steve Holman.
Eat less frequently to allow cortisol to break down muscle. Frequent meals -- eating every two to three hours -- blunts cortisol and creates a muscle-building effect, according to "The Abs Diet" by David Zinczenko. Waiting four to six hours between meals encourages muscle loss. Keep saturated fats and sugars low to prevent storing body fat in unwanted places while dieting to slim the thighs.
Decrease your total daily protein intake. Taking in less than one gram of protein per pound of your body weight each day will keep you in a negative nitrogen balance, meaning protein synthesis cannot overtake catabolism. Stick to healthy low-glycemic carbohydrates such as fresh fruits and vegetables, as well as healthy fats such as olive oil, avocados, and nuts and seeds.