The following article by T Nation gives some important information and progressions to help you maintain muscle while losing fat.

Running Man
Energy-System Work to Get Lean and Mean
by Christian Thibaudeau | 03/07/03

As most of you probably know, I’m not a big fan of slow-pace cardio work, especially when you’re trying to build muscle mass or improve your performance. Sadly, my reluctance to include aerobic work in my programs has been misinterpreted. Many people believe I’m against any type of road work. This isn’t so!
I do believe that energy-system work (note how I don’t use the term “aerobic”) is a must for everyone, from the athlete wanting to improve his performance, to the bodybuilder wanting to get shredded or to gain as much muscle mass as possible without getting too fat.
I believe some form of energy-system work should be included every week in your training schedule. Obviously, depending on your goals, the duration and frequency of these sessions will vary. So, let’s quickly review the various physique goals and the use of energy-system work to reach them.

Losing Fat

To get very lean and muscular you must have pretty much everything in order, from diet to rest to training. While a proper strength training program will help you get lean, it’s really hard to get a high degree of definition without some form of “road work,” unless you’re genetically gifted for leanness.
As I already mentioned, I’m not a fan of low-intensity cardio work. While it’s adequate for fat loss, I feel it can have a negative effect on strength and ultimately muscle mass. Interval training and/or long distance sprints are optimal to maximize fat loss while retaining muscle mass.

Gaining Muscle

No, I haven’t dropped a barbell on my head! I really do suggest including energy-system work even if your main goal is to gain muscle. The reason is simple: to gain a lot of muscle you must consume a lot of food; this will lead to great size gains but also some fat gains. With energy-system work, you’ll be able to minimize the amount of fat you gain while trying to pile on slabs of new muscle.
To accomplish this goal, we want to do just enough energy-system work to decrease fat storage, but not so much as to slow down muscle gains. The most important variable to play with in this case is the frequency of training. More on that later.

Athletic Performance

I’ve seen a lot of strong, fast and powerful athletes, but believe it or not, most of them aren’t well conditioned. As a result, they’re not able to utilize a large part of their potential for a long period of time. Ultimately, at the end of a football drive, a hockey shift, or anything similar, the athlete who can use the most of his capacities will win.
Most sports have a very important anaerobic/high-intensity component. Sadly, this is probably underdeveloped in most athletes! Having a super effective anaerobic energy system will give any athlete a huge advantage.
Now, I’m going to give you three possible methods to use for getting defined and conditioned: 400-meter runs, interval running, and my personal favorite, interval build-up running (IBUR).

400-Meter Running

I discovered the high fat-burning potency of 400 meter sprints without really looking for it. I use a lot of 400m running with my hockey players, mostly because it develops the energy system they require the most during a game. However, I quickly noticed how lean they were getting shortly after starting 400m runs. They were not only getting leaner but stronger!
I then experimented with the 400m for fat-loss purposes and found time after time how efficacious it truly was. To this day I still believe that few things can match up with 400m runs for fat loss.
Editor’s Note: For mathematically impaired Americans who never ran track in high school, 400 meters is one lap around a standard track.
I recommend using 400-meter sprints once per week at first as it’s very hard work! However, some of my athletes use up to three sessions per week, two being the norm. The following table will give you some basic guidelines:

400 meters

Interval Running

Interval running is another great way of burning body fat without jeopardizing your efforts to gain muscle and strength. It combines low-intensity and high-intensity work for a very large fat-burning effect. Basically you’ll alternate between slow-pace running (slow jog) and fast-pace running (sprint).
This form of training is a bit less intense and stressful than 400-meter sprints. It can be started at a frequency of twice per week, building-up to three or four times per week for maximum fat loss. Stay with two weekly sessions if you’re trying to build muscle.
A good program to use is illustrated in the following table:

Interval Running

Interval Build-Up Running (IBUR)

This is my personal favorite fat-burning strategy. IBUR is based on many of the same principles as regular interval training, but with each cycle (or each interval), the duration of the sprint and jog phases increase in length.
This is the workout I used myself three times per week and it led to a marked decrease in body fat. It may not be the most specific method available for athletes, but if all you’re interested in is fat loss, give IBUR a try. You won’t be sorry!
Here’s an example:

Interval Build-Up Running

Fitting It In
I recommend you only use one of those three methods in a training block. If you want to use all three methods I suggest using the following periodized approach (modify the volume according to your level):

periodized approach

Along with proper dieting, this program can help get you into top shape for summer, improve your athletic performance, and even help you minimize fat gain while on a mass program. Get ready to get ripped to shreds when the snow melts!