Most modern day athletes spend the day burning bagels. It wasn’t always so for runners.

In the 60s, legendary New Zealand coach Arthur Lydiard’s runners did their long runs fasted, as this was the norm for the day. They took only water during the hilly 22-mile Waiatarua Route and worked up an appetite that was quenched at local breakfast buffets. The end result was fat adaptation and recruitment of fast-twitch oxidative fibers in a glycogen-depleted state.

Lydiard runners on Waiaturua Route
“Arthur’s Boys” training on the 22-mile Waiaturua Route / photo courtesy Garth Gilmour Collection

In the 1970s, Dr. Phil Maffetone focused on aerobic training in relation to fat burning. Maffetone had a more proactive nutritional approach. Never having bought into the high-simple-carb and low-fat mantra of the day, he encouraged little to no grain, healthy fats, and removing processed “junk” food. This was not absolutely “low carb” for the elite but a healthy 40/30/30 (40% carb/30% fat/30% protein).

For Lydiard and Maffetone athletes, high mileage weeks with long, slow, distance training was the norm. Only after the base was built was intensity added. The base built mitochondria, capillaries, enzyme processes to oxidize fat, tissue adaptations, and countless other physiological changes aligning with health and fitness. Modern methods which attempt to hack this slow but necessary process—emphasizing intensity over quantity—have become the trend in training.

Higher speed, intense intervals and constant “hard cardio” necessitate an immediate need for carbohydrates. An entire sports-nutrition industry of high glycemic bars and goos has been born to supply this need.

What is Metabolic Efficiency

Why care about the ability to use more fat for energy while using less carbohydrate? A best a trained runner can carry only 1,500–2,000 calories of carbohydrates (less for the many insulin-resistant or pre-diabetic/diabetic athletes) but at least 50,000 calories of fat—even the leanest runners. It does not make sense that carbs are our preferred fuel.

If you’ve driven a hybrid car, you’ve watched the subtle shifts between gas and electric on the dashboard. Your body is a similar, two-energy-source hybrid. Your engines (muscles) run on a mixture of gas (sugars) and electric (fats). Utilizing gas or electric power depends on the effort.

Imagine you are starting the race with ten gallons in the gas tank—assuming you have eaten a nice meal the night before with a light breakfast top off. If you race in all gas mode, your engines will run about 1.5 hours at a strong pace….then you are out of gas. If your effort is mostly electric you can run for hours, but not as swiftly.

Running utilizes about 1kcal/Kg/Km. So for a lean marathoner of 80 kg (175 lbs) you needabout 3360 kcals (80kg x 42 km) to make it to a marathon finish line. Even fully carbo-loaded, your stored liver glycogen (300–500kcal), muscle glycogen (1000–1500kcal), and blood glucose (less than 20 kcal) don’t add up to enough. Glucose is easy to access for ready energy, but your stores add up to less than 2000 kcal.

The fat-utilizing pathway is the electric. In marathons you must be in hybrid mode to make it. Hybrid mode is where your energy is coming from both fuel sources. Conserving the gas and using electric early in the race is critical.

Many runners are in great “10k shape” (an all gas event). They train in all gas mode, start their marathon in the all gas mode….and crash. Glycogen-sparing strategy need not apply in races of less than an hour as long as you had a good pre-event meal to fill the tank. In marathons and ultras, however, top-end anaerobic fitness matters little and can only be applied very near the finish.

If you’re going long, you must tap into the fat-burning tank. Enter an old concept called “The Crossover Point”—the point where the effort rises to the level where we’re burning more carbs than fat.

crossover point
naturalrunningcenter.com

Below this point, easy efforts can be almost all electric (fat-burning) for a healthy athlete. Below the AeT (Aerobic Threshold) the body is primarily using fat as fuel. As the intensity of exercise increases, your body prefers to use more carbohydrate for fuel and you will exceed your AeT. Like a switch the body shifts from fat to sugar—but your brain may not be aware. The best signal for the AeT is an increase in ventilation: You need to start breathing harder and faster.

This threshold is lower than the AT (Aerobic Threshold). Most of us know this as the “red line” above which acidosis occurs and fatigue sets in quickly. This is also called “lactate threshold” or “tempo” training pace. By the time you reach the intensity of the AT most runners have passed the crossover point and are burning more carbohydrates than fat.

How do you teach your body to be a better butter burner?

What are the nutrition and training principles needed to move the crossover point to the right, where you burn more fat and less carb at a given effort and speed? The key is tailoring your macronutrients (carbohydrates, fats and proteins) along with adjusting aerobic exercise pace to increased fat utilization.

The nutrition component has a largest effect on your fat-burning capacity and you must make changes to your diet to see results. Bob Seebohar, exercise physiologist and sports dietician, says that your nutrition and diet account for 75% of your metabolic efficiency, with exercise only 25%.

Junk food, sweetened beverages, and simple sugars/starches are off the table. The problem with these is that they spike your blood sugar levels higher than your muscles can burn, which, when you are at rest, causes the body to release excess insulin, whose role is to store those extra carbs. Insulin also shuts off fat-burning which is not needed in the presence of excess carbs.

Some athlete’s will even experience hypoglycemia with an insulin spike especially those with highly-active muscles and fit athletes with insulin sensitivity. The blood sugar goes up fast, the pancreas releases insulin, but the exercising body over-compensates and the sugar now drops quickly, leaving the athlete feeling depleted.  Without insulin the sugar does not drop.

low carb foods
photo: Shutterstock

Instead of sugars, eat “slow carbs” of natural sources, tailored to the training volumes and the individual’s carb tolerance. You don’t need a calorie-deficient diet. Healthy fat is the foundationalong with healthy amounts of real-food protein. Athletes require good amounts of protein—about 1 gram/pound of body weight is not excessive.

You can eat healthy and safe carbohydrates on this plan. Lots of above ground and colorful veggies, even some legumes—just get rid of the grains. Be cautious with sweet fruit and cut starchy veggies.

By keeping insulin low, fat burning rules the day and night, whether you are exercising or sleeping. Fiber from quality fruits and vegetables can stabilize blood sugar and add what some refer to as “fertilizer” for the healthy bacteria in your intestines (the microbiome). Focus on nutrient density and learn to cook for joy and variety. You can find an abundance of low carb cooking sites and books.

Build an Aerobic Base

To change your metabolism, you must, however, adjust diet and exercise together. Eat more healthy fat, eat few to no refined carbs—and slow the heck down!

When you have a large aerobic base your body can use fats AND carbohydrates more efficiently. Depending on the fuel available and the effort, an aerobically developed athlete can have metabolic flexibility.

How do you build this flexible-energy aerobic base? The work of renowned sports scientist Dr. Stephen Seiler has shown that the best athletes across endurance sport have mostly applied the “80/20” rule, meaning at least 80% is easy aerobic with less than 20% intense work.

In a 2009 SportsScience article, Seiler summarizes: “Research on the impact of interval and continuous training with untrained to moderately trained subjects does not support the current interval craze, but the evidence does suggest that short intense training bouts and longer continuous exercise sessions should both be a part of effective endurance training.”  He adds: “The available evidence suggests that combining large volumes of low-intensity training with careful use of high-intensity interval training throughout the annual training cycle is the best-practice model for development of endurance performance.”

Easy morning miles
The author and colleagues putting in easy miles / photo: 101 Degrees West

A good simple rule of thumb is to keep 80 percent of your running below the AeT or ventilatory threshold. Practically speaking, this is the pace where you can carry a comfortable conversation. If you’re able to breathe through your nose then you’re hybrid. If you’re breathing faster, and are reduced to short, gasping sentences, you’re using mostly glucose as fuel.

Higher-intensity work demands carbohydrates as easy access fuel; this produces more carbon dioxide which needs to be expired—that is the reason you breathe faster at higher paces.  Slow efforts have a lower respiratory exchange ratio, which means your tapping fat and expiring less carbohydrates. Therefore you can talk at these slower paces.

Many runners minimize the importance of aerobic foundation because we don’t see the immediate results as we often do in speed training. Hard efforts produce some great positive adaptations, but compared with the aerobic work, don’t improve fat burning. Build the aerobic first then add on the intensity.  Or, as Lydiard put it, intensity is “the icing on the cake.”

If you reduce carbs and go long, you might feel a bit sluggish on these early no-gas runs (relying on the “electric” fat-burning), but you will soon be making your own internal fuel from your fat. If you’re not overloading your system with fast-acting carbs, and insulin is low, the body can generate glucose from non-carbohydrate substrates—which is a good thing if you want consistent sustainable energy. You do not need to eat glucose to maintain blood glucose, your body will make it, if you train it to.

How long does the fat adaptation take? Dr. Stephen Phinney showed with fit athletes it can happen in four weeks.  With older, more insulin-resistant athletes it often takes longer. Everyone adapts differently to training stimuli.

Months and even years of this healthy, sustainable training increases mitochondrial density, mitochondrial enzymes, and capillaries to deliver oxygen, and lymphatics to “take out the garbage.” By avoiding “fast carbs” and keeping insulin response low, we are able to exercise longer and make it through the day and night with fewer calories, reducing the need to consume sugars and increasing utilization of our internal body fat.

For runners who have a medical issue such as Type 1 or Type 2 diabetes, the ability to use fat as fuel is critical for health not just performance.  For more on this topic see Mark Cucuzzella’s recent post on the use of continuous glucose monitors for self assessment.
Mark Cucuzzella is the author of Run For Your Life:How to Run, Walk, and Move Without Pain or Injury and Achieve a Sense of Well-Being and Joy.