Nutrition

Forget Counting Calories: Here's How to Fuel For Running

By Susan Kitchen Updated on

Weight loss is more complicated than calories in vs. calories out. Here's how substrate utilization comes into play when you're trying to shed pounds.

An array of healthy foods, including fish, lean meat, fruits and vegetables

Losing weight can be tricky.

Whether it’s to shed a few unwanted pounds or improve running efficiency, striving for leaner body composition is an ongoing process for many athletes, while a few extra pounds are part of staying healthy and injury free for other runners.

Achieving the ideal physique for your lifestyle is like a fork in the road, and the weight loss GPS may guide you on an off-road journey leading to a dead end. Here’s how to find your way.

The Calorie Counting Problem

Many fitness apps try to squeeze weight loss into a simple equation: Calories in minus calories out.

A pound of body weight is generally equal to 3,500 calories. So reducing your intake below 3,500 calories through a combination of eating less and burning more should result in losing a pound, right? Not quite.

The problem with the simplicity of calories in vs. calories out is that it doesn’t take into account substrate utilization.

An assortment of healthy foods for runners

What is Substrate Utilization?

Whether you’re sleeping or running an all-out sprint, your body constantly fuels itself with a combination of carbohydrates, fat and a small amount of protein. That fuel mix changes, though, depending on how hard you’re working: You burn a higher percentage of fat when your heart rate is low, and you burn a higher percentage of carbs when your heart rate spikes.

But there’s always a mix of fuel sources no matter what you’re doing.

For example:

You go for a one-hour run, and your fitness app reports you burned 700 calories. Knowing this, you might think there is some cushion in your daily caloric allotment. What the app doesn’t take into account, though, is the fuel source of those 700 calories, which depends on the intensity and duration of the workout.

During that one-hour run, you might burn a 50/50 ratio of fat to carbs—350 calories from fat and 350 from carbohydrates. The fat calories don’t need to be replaced if the goal is to shed fat, but the carbohydrate calories do, and 350 calories will naturally occur at your next timely meal.

Substrate utilization makes the calories-in, calories-out approach is a bit more complicated than merely entering your activities and meals into a clean equation.

(It should be noted the reported 100 calories burned per mile from some apps is a rough estimate and may not be accurate for everyone. Also, people who are more aerobically fit will burn a higher percentage of fat for fuel when a workout intensifies.)

Fuel Sources Based on Training Intensity Zones

Here's a more specific breakdown of how your body uses macronutrients for fuel based on perceived effort.

Effort zones 1-3* primarily burn a mix of blood glucose, muscle glycogen and fat.

  • Zone 1 (very easy effort): 40 percent carbs/60 percent fat
  • Zone 2 (easy effort): 65 percent carbs/35 percent fat
  • Zone 3 (moderate effort): 80 percent carbs/20 percent fat

Zones 4-5 use less fat and more blood glucose and muscle glycogen.

  • Zone 4 (half marathon to 10K pace): 92 percent carbs/8 percent fat
  • Zone 5 (5K to 3K pace): 98-99.5 percent carbs/<2 percent fat

*This should not suggest a runner spend more time in Zone 1-2 for the purpose of burning more fat but rather for aerobic endurance and resilience training. In higher effort zones, runners burn more calories, which increases the total percentage of fat burned.

Balance Macronutrients, Not Calories

This may come as a surprise, but the makeup of the calories you consume is more important than the number.

Calories are made up of three main macronutrients: carbohydrates, protein and fat. Athletic success, body composition, injury prevention and overall health rely on proper nutrient timing and the right balance of those three nutrients.

Most athletes* should tune their diets to include:

  • 40 to 60 percent of calories from carbs
  • 20 to 25 percent of calories from protein
  • 20 to 30 percent of calories from healthy sources of fat

*Calorie requirements can change depending on training cycle, activity, intensity, gender and age.

The proper nutrient timing, along with the correct ratio of macronutrients, stabilizes blood sugars and insulin response, decreases food cravings, and ultimately improves body composition. Working with a sports-certified dietitian can help you customize a macronutrient plan that fits your needs. Additional resources, like apps and websites, show macronutrient breakdowns for thousands of food items.

A man and woman running together while wearing ASICS apparel

Frequency and Balance

If you’ve ever felt lethargic or moody after missing a meal or snack, you’re probably well aware that the frequency and timing of meals is critical. But did you know timing is just as important as what you eat?

Always start your day off with a balanced breakfast that includes carbs, protein and fat. Aim to eat a snack or meal every 3-4 hours during the day.

Keep in mind that a meal can look healthy while still being unbalanced. But a few simple changes can make a big difference. Just looking at your plate should give you a good indication of whether it’s healthy and balanced.

Here are a few examples:

Breakfast (unbalanced):

  • 1 cup cooked oatmeal
  • 1 small banana
  • 1 cup skim milk

Totals: Carbs: 55g; Protein: 13g; Fat: 2g

Breakfast (balanced):

  • 1 cup cooked oatmeal
  • 1 small banana
  • 2 hard-boiled eggs
  • 1 cup skim milk

Totals: Carbs: 55g; Protein: 25g; Fat: 12g

Lunch (unbalanced):

  • 1 large salad with leafy greens, tomatoes, cucumbers, carrots, strawberries, pineapple slices, and few pecans with non-fat raspberry vinaigrette
  • 1 whole wheat roll

Totals: Carbs: 40g; Protein: <1g; Fat: 5g

Lunch (balanced):

  • 1 large salad with leafy greens, tomatoes, cucumbers, carrots, strawberries, 1/4 cup edamame, 1/4 cup corn, 1 oz. goat cheese, few pecans, 3 oz. grilled chicken breast, and non-fat raspberry vinaigrette
  • 1 whole wheat roll
  • 1 tsp. butter

Totals: Carbs: 55g; Protein: 36g; Fat: 22g

Takeaway

Adequately fueling your body in motion is important, but knowing where those calories come from is what facilitates the real magic. The body uses and processes carbs, protein and fat differently. So dial in the macronutrient balance that works for you to rev your fat-burning engine, ward off cravings and insulin spikes, and achieve optimal body composition.


By Susan Kitchen. Susan is a Sports Certified Registered Dietitian, USA Triathlon Level II Endurance Coach, IRONMAN Certified Coach, published author and founder of Race Smart, a sports nutrition and coaching company.