Carbs are everywhere in running, from the ritual of pre-race carbo loading to the gels and energy bars we eat in stride to the recovery drinks that jump out at us from the pages of every running magazine on the rack.

Carbs! Carbs! Carbs! And considering that carbohydrate, most notably in the form of glycogen, is your body’s preferred source of fuel for working muscles, it all seems to make perfect sense—carbs are the best thing for runners since sliced bread. (Hey, there’s a lot of carbs in bread. Let’s eat some!)

Not so fast.

Carbohydrates are an essential part of the human diet, making them a very good thing. However, an excess of carbs, like we see far too often in western diets, can be a bad thing, especially if you are trying to lose weight. This is what gave rise to the low-carb/no-carb diets championed by Dr. Atkins and other prominent nutritionists.

So, how can we, as runners, lose weight when the very thing we do is so dependent on the same carbs we are being lead to believe are making us fat? The answer lies in finding the right balance. And to find that balance, we need to start with the basics.

What It Takes to Lose Weight

As runners, we want to lose weight by losing body fat. To achieve this, we need to maintain some level of calorie deficit in our daily lives. In short, we have to take in fewer calories than we burn. Our bodies respond by dipping into our fat stores to make up for the difference. As our fat is converted into energy, we lose weight.

If you are new to the sport, the additional workload running places on your body may be all you need to create the calorie deficit you’re after. If you’re an experienced runner, additional miles or intensity may do the trick, but you’ll likely also have to make some modifications to your diet in order to see any significant fat loss.

The big question in either case: How large of a calorie deficit should you aim for?

Unfortunately, there is no one-size-fits-all answer. It depends on 3 important factors: the amount of weight you want to lose, the time period over which you want to lose it, and your body’s individual response to the deficit.

All too often, runners consider only the first 2 factors. It’s easy to latch onto the commonly accepted equation that 1 pound (.45kg) of fat = 3,500 calories (Something Denise Webb and other dietitians have challenged) and simply multiply the number of pounds you want to lose by 3,500. Divide that by the number of days, weeks, or months you want to lose it over, and you’ve got a roadmap that leads straight to your goal. Or so it would seem.

While the math (sound or not) may check out, your body may not respond the way you’d like.

How to Tell if Your Calorie Deficit Is Too Large

If you get overzealous with your calorie cutting, you could end up with a deficit that is greater than your body and your running can safely afford to maintain. Some of the warning signs that indicate your body needs more or better (more on this later) calories are:

Persistent muscle soreness

Persistent fatigue

Inconsistency between heart rate and perceived exertion

Increased susceptibility to illness

Irritability and/or depression

It’s easy to dismiss the first two as just part of running and something you should push through. Instead, try taking a closer look. If the soreness and fatigue from one workout are spilling over and negatively impacting your next workout, you are either running too hard (intensity, duration, or frequency) or not feeding your muscles properly. If you find yourself too sore or fatigued to follow your training plan, especially if you also notice an onset of any of the last 3 warning signs, it’s best to take corrective action.

If you created your calorie deficit by cutting your daily intake, try adding back some of the calories you removed. If you created your deficit by increasing your running load or intensity, you can either dial back your running a bit or try adding some healthy carb-based calories to your diet. Provided you don’t completely erase your calorie deficit, you should still achieve the body fat loss you’re looking for. It will just happen at a slower rate.

Note: If you are a frequent or competitive runner who is exhibiting the warning signs listed above, be aware that these are also signs of overtraining. Learn more about overtraining here.

Where to Find the Right Calories

The overly simplistic view of calories I have taken to this point is used for illustrative purposes but can lead to the dangerous misconception that all calories are created equal. Mathematically, this may be true enough, but, nutritionally speaking—especially when it comes to carbs—it’s not.

With carbs, you’ve got the choice between simple and complex. Simple carbs include things like refined sugar, corn syrup, pastas & bread made with white flour, candy, honey, milk, and most types of pastries. These carbs are digested quickly and can rush fuel to working muscles. Complex carbs, like brown rice, whole grain breads & pastas, beans, potatoes, seeds, and fruits & vegetables, must be broken down by our bodies before they can be used as fuel. This may seem like a small difference, but it is extremely important to runners.

As we run, we deplete the glycogen (a form of the simple sugar glucose) stored in our muscles. If we run long and hard enough without replenishing some of the glycogen by ingesting carbs, we bonk (been there) or hit the dreaded “wall” (done that). The best carbs to take in during exercise are simple carbs in the form of gels, sports drinks, or energy bars. These carbs get right to the muscles and help keep us running longer.

When we are not running, our bodies prefer a more gradual supply of energy. This is where complex carbs come in, and the primary reason they should make up the bulk of your daily carbohydrate intake. A second reason is that complex carbs tend to come from lightly processed or whole food sources. These are less calorie dense than the highly processed convenience foods whose simple carbs tempt us from countless billboards and radio commercials. Fewer calories per gram means you can eat larger portions of the complex carb foods, which is great news if you want feel full while limiting your calorie intake.

Note: I encourage you to read more about the differences between simple & complex carbs and how glycogen works. Both are important to understanding your body’s energy system.

How to Maintain the Balance Once You Find It

Let’s say you’ve followed my advice to this point. You’ve dialed back your calories, made adjustments to your running load based on your body’s feedback, and swapped out some of the processed simple carbs in your diet for whole food complex carbs. You’re now losing fat at a consistent pace, feeling strong when you head out for your runs, and, very likely, getting faster from the combination of running and weight loss. Now what?

Well, the hard reality is that things are going to change. That’s because the human body is incredibly good at adapting to a repeated stress, like running. Your body is constantly looking for ways to work more efficiently and burn as few calories as possible. In fact, your weight loss is likely to slow gradually or plateau once your body adapts to the new lower-calorie diet you are feeding it. In order to see further weight loss, you are going to have to shake things up again.

Depending on how many daily calories you are consuming, you may be able to cut a bit more from your diet. Again, you will need to pay close attention to your body and make sure your calorie reduction doesn’t impede your running. If you are uneasy about reducing your calorie intake any further than you already have or simply don’t want to cut any more, focus on your running instead. Add mileage or intensity in small increments and pay attention to how it affects you. If you get it right, this added stress should speed up or restart your weight loss. If you begin dreading or skipping planned runs because you are too tired or sore, you’re adding mileage or intensity too quickly. Dial back until you find your sweet spot again.

Once you have reached a comfortable, healthy weight, it’s time to go into maintenance mode. Continue to eat a balanced diet with a heavy concentration on whole food complex carbs. Supplement this with simple carbs before and during your runs, and use the scale and your body’s feedback to keep yourself on track. Don’t sweat small fluctuations in your weight, and don’t be afraid to give in to the occasional junk food craving. Just be sure to keep those delicious indulgences from becoming a regular occurrence.

Serious about running nutrition? Check out our “Run Lean, Run Strong” program. Your definitive guide to running nutrition, strength training and injury prevention/treatment.