Losing muscle strength while on a cutting diet is common, but it is not unavoidable. Because weight loss inevitably involves the loss of both fat and muscle, you cannot expect to make any gains during your cutting phase, and you can expect your lean mass to decrease slightly as well. But the right planning can help you keep enough muscle so that you aren't likely to notice the loss in the weight room.
Don't Starve Yourself
As a bodybuilder, you've probably heard the stories about extreme low-calorie diets that strip the fat within weeks. While many people turn to these diets out of impatience, they are doing their bodies more harm than good. Severe calorie restriction does make the number on the scale go down, but it's not fat you're losing. It's mostly muscle and water -- the water will come right back when you return to normal eating, but the muscle mass you worked so hard for will be much more difficult to get back -- and your fat will still be there. Lose no more than two pounds per week to ensure that the majority of it is fat. Simply create a daily deficit of 1,000 calories between working out and eating less. If you drop too low, you'll notice your strength decrease. Divide your calories between five to eight meals, eating every two or three hours to stay fueled.
Eat Frequent Protein
Protein does more than repair and build muscle, so your body needs it, even during a cutting phase. If you don't give it the protein it needs, it will take it from your muscles, and you'll lose muscle mass and strength. During your cutting phase, eat about 0.54 to 0.77 g of protein per pound of body weight per day. Eat a little at every meal to keep a constant, ready supply in your bloodstream. Choose mostly complete proteins, like poultry, fish and lean meats to give your body all of the essential amino acids.
Time Your Carbs
Eat most of your carbs with the meals directly before and after your workout -- it will help with muscle recovery and will ensure you have enough ready fuel available to work as hard as you normally do despite eating fewer overall calories. Aim for 2.3 g per pound of body weight per day, and get most of it from fruits and vegetables. Save the whole grains for your pre- and post-workout meals. This practice can help keep your overall calorie intake within reason while keeping you fueled enough to lift as heavy and hard as you're used to.
Whatever you do, don't skip your workouts during your cutting phase. Cutting diets deprive your body, however slightly, and failing to use your muscles as you are accustomed to almost guarantees that you'll lose a higher percentage of muscle than fat. This will definitely be noticeable when you finally return to the weight room. Stick with your regular lifting schedule, just don't expect to make any major gains during this period. Cardio can help torch calories, but skip the marathons in favor of short interval sessions. Extended cardio can actually degrade muscle tissue over time.