Look at just about any race-training plan out there, and you’ll see a mix of longer, slower runs and faster, shorter runs. That’s because both have physiological benefits that help develop the speed and endurance you need to run your best race.

But what if you’re less concerned about the finish line than you are your waistline? Is it necessary to include both low-intensity and high-intensity running in your weight-loss strategy, or can you slim down quicker by narrowing your focus?

Let’s find out.

Types of Running Defined

If you’re new to running, you’ve probably heard the terms distance, tempo, and interval, or seen them in a training plan. But do you know what separates one from another? Because this distinction is critical to understanding the role each type of running plays in weight loss, I am going to start off by defining each for you. (If you’re an experienced runner and know what sets these three types of running apart, feel free to skip to the next section.)

• Long Slow Distance (LSD)
  Ah… the easy run. These are usually run at 60 – 70% of your max heart rate (Here’s how to find yours.), or a perceived effort of 6-7 on a scale of 1-10. As part of a race-training program, these long runs are intended to help build endurance and teach your body to more effectively use fat as an energy source. (More about this later.)
• Tempo Runs
  These are run at a higher intensity than LSD runs, typically 80 – 90% of your max heart rate, with the goal of increasing your lactate threshold. This is the point at which your working muscles produce the metabolic by-product lactic acid faster than it can be cleared from the muscle. According to renowned running coach and author Jack Daniels, tempo means running at a comfortably hard pace, roughly the fastest you can maintain for an hour of running.
• Interval Training
  Intervals are most commonly used to build speed in competitive runners by increasing VO2 max, the maximum amount of oxygen your body can use. This is accomplished by alternating short efforts at close to your maximum heart rate (95% – 98%) with easier recovery efforts of lesser or equal duration. The goal is to spend as much of the run as possible at VO2 max, roughly the fastest pace you can maintain for 10 to 15 minutes of racing. To put it another way, intervals are damn hard!

Where Your Body Gets Its Energy

Now that we know how these types of runs differ in intensity, it’s time to explore how they also differ in the way they use energy.

While running, your muscles have two primary fuel sources—fat (that stuff you want to get rid of) and glycogen (a supply of fast-burning carbohydrate stored in your muscles and liver). At low levels of intensity, like our long slow distance runs, your body will rely on a mix of fat and glycogen, with the balance tipped in favor of fat.

As the intensity increases, this ratio changes, and your muscles get more of their fuel from glycogen than fat. This shift continues until you reach your lactate threshold, and your muscles run almost exclusively on glycogen.

It is important to understand this distinction because it is the basis for the long-held belief among runners that slower runs, by virtue of the fact that they draw a greater percentage of calories from fat, are inherently better for weight loss than harder efforts that burn more glycogen. But is this really the case?

The Argument for Faster is Better

Several articles I came across when researching this subject cite the following data from The 24/5 Complete Personal Training Manual (24 Hour Fitness, 2000) to support a faster-is-better viewpoint. All values are based on a 130-pound (59kg) female runner.

As we might expect, the high-intensity running derives a lower percentage of fuel from fat (38.85 vs. 50) but burns a higher total number of calories per minute (6.86 vs. 4.86). Over the course of 30 minutes, the high-intensity running burns 41% more total calories, including 9 more fat calories than the low-intensity running.

More total calories AND more fat calories? It certainly seems like an open-and-shut case for higher intensity. That is, until we take these numbers off the chart and apply them in the real world.

The Argument for Slower is Better

Higher-intensity running burns more calories per minute, but it also limits the number of minutes you can spend running. If we assume that our 130-pound female runner can hold her low-intensity pace for much longer than her high-intensity pace—let’s say 60 minutes instead of 30—the numbers start to look different. Over that 60-minute run, she burns 292 total calories with 146 being fat calories.

If she can perform either run 3 times/week, she will burn 876 total calories and 438 fat calories at low intensity (180 minutes) as opposed to 618 total calories and 246 fat calories at high intensity (90 minutes).

Why It’s Not That Simple

Before we crown longer low-intensity running the king of weight loss, we should consider one more piece of the equation. To this point, we have been looking at low- and high-intensity running in terms of the total amount of calories (regardless of source) that are being burned per session. In the world of fitness and nutrition, this is known as gross calories. To get a clearer picture of what’s really happening in our example runner, and one that applies most directly to weight loss, we need to look at net calories burned.

To find net calories, we have to take the total number of calories our example runner burned and subtract out the number of calories her body would have burned over that same period of time were she at rest. For this calculation, we need to know her basal metabolic rate or BMR, the number of calories she will burn at rest over a 24-hour period. We will use a calculator, like this one, and assume the following data: our 130-pound female runner is 5-feet 4-inches tall (162cm) and 30 years old. This gives us a BMR of 1,295 calories per day or 54 calories per hour.

Now that we know she burns 54 calories/hour at rest, we can take a second look at our two example runs. 60 minutes at low intensity burns 292 gross calories – 54 BMR calories = 238 net calories. 30 minutes at high intensity burns 206 gross calories – 27 BMR calories = 179 net calories.

It may still look like the shorter faster run burns significantly fewer calories, but keep in mind that it only takes 30 minutes to complete. Based on our example runner’s BMR, she will burn another 27 calories over the next 30 minutes, even if her body immediately went into a state of rest at the end of her run. So, 179 net calories for 30 minutes of running + 27 BMR calories for the next 30 minutes = 206 calories burned in 60 minutes.

Mix Things Up for the Best Results

Even though the low-intensity running burns more net calories per hour in our example, the difference in the real world may not be as noticeable. High- and low-intensity running both have their own distinct advantages and should be part of your running plan. You will have days when you have more time or tired legs. These days are perfect for longer easier training. You will have days when you are short on time, feel great, or just want the satisfaction that comes from completing a hard workout. These are perfect for tempo and interval runs. The mix will make you a happier runner and help keep your body’s natural tendency toward adaption from getting in the way of your fitness and running goals.

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