In a word: it made me feel very sloooow.

But it worked. The idea was to lose about a pound each week over four weeks to jump-start my training for a high-altitude trail marathon later in the year. At the end of a month in which I doubled my running mileage, more than doubled my protein intake and burned at least 300 calories a day more than I ingested, I lost about six pounds, 50 percent more than the goal I set for myself.

But during the process I could also see my physical fitness start to decrease in measurable ways, exercise became more of a slog when it should have been getting easier and everything physically just seemed to slow down, from digestion to my state of mind.

In other words, the diet was a success, but I am so glad it's over.

The Motivation

Most people probably wouldn't consider me overweight, but I definitely carry around more body fat than your average ultramarathoner. When you're dragging yourself up and down mountains for more than twenty miles you can waste a lot of energy carrying around extra weight. Just try jogging a few miles carrying a backpack full of books and you'll quickly understand my motivation to get a little lighter before starting to train in earnest.

So I followed a 4-week "quick start" weight loss plan outlined by Matt Fitzgerald, author of the book "Racing Weight: How to Get Lean for Peak Performance" and the associated Quick Start guide. The guide details plans to lose 5-20 pounds over 4 - 8 weeks as part of the initial phase of an extended training cycle.

The basic idea is that dieting can drop a few pounds, but it also compromises athletic performance. So Fitzgerald suggests an initial "Quick Start" period that focuses on beginning training while restricting calories and eating more protein to replace fat with muscle.

It's really nothing revolutionary: Eat better and exercise more; drop some pounds before really ramping up your training to get more out of your workouts once you do start to hit it hard.

The Price Paid

But jumping from protein making up about 10-15 percent of my daily calories to 30 percent while also doubling exercise truly is not a recipe for peak performance during those first four to eight weeks. 

Here's what to expect should you take the same approach to losing some pounds relatively quick:

Performance is sacrificed: Boosting protein intake so significantly inevitably means eating fewer carbohydrates, which provide the simplest source of energy for exercise. About midway through my four-week plan I started to notice that my energy level during workouts was going down rather than up as it would during a normal training period. 

This wasn't only in my head: my Garmin watch with heart rate monitor also reported a corresponding drop in VO2 max - a measure of how efficiently the body processes oxygen. Garmin's algorithms also reported that my workouts were unproductive in terms of building fitness, likely due to improper rest or nutrition.

Get ready to bonk: The whole point of eating fewer carbs while boosting protein is to burn fat. But frying that fat in your body for fuel during exercise results in a fatigue-filled bonk-fest. You've probably heard the phase "hitting the wall" used by runners. This refers to the awful feeling when your body has burned through all its available reserves of carbs and switches to consuming fat for energy. The result is a feeling of being totally wiped out that's also referred to as "bonking." This is why energy chews and gels are so popular: they provide a carb boost to avoid the bonk. 

The best breakfast of your life: Fitzgerald's plans call for a weekly long "fasting run" to hasten fat burning. Just get up and start running for 90 to 120 minutes without consuming anything except water. This might sound absolutely awful, and while it can be a bit uncomfortable, it's not so terrible. Plus, if you do it right and go out for a big (but not too big) breakfast afterwards, you'll enjoy some of the best meals in recent memory. I always met my family for a breakfast focused on protein but also on getting some carbs back in my system. These meals were by far the best part of the program.

Worthwhile results: It's now a week since I wrapped up my quick start diet, and like I mentioned earlier, I lost more weight than I was promised and dropped my overall body fat percentage by a few points. I couldn't really ask for more from the process, but once I went back to a more normal balance of protein and carbs, the immediate energy boost to my workouts was impressive and felt great. We'll see how long that lasts.

Another unexpected bonus is that all that extra protein has left me feeling stronger than before. Tiny little muscle tweaks and pulls healed notably faster during the quick start phase and I seemed to build up at least a minimal amount of new muscle mass. Combine this little bonus with the extra energy I have this week after returning to a normal diet and I'm feeling fantastic. This is also reflected in my VO2 max immediately increasing and my Garmin again registering my workouts as productive. 

Lessons learned: Perhaps most of all, the not entirely pleasant experience of manipulating my diet and habits has motivated me to maintain healthy diet and training habits going forward and hopefully even during breaks. Anything to keep from having to do a month of bonk runs again!