Fat as a Fuel for Marathon Training

It Is More Efficient To Tap Into Your Unlimited Fat Supply

Migratory birds and whales rely on stored fat to fuel their long, strenuous journeys. Developing your fat engine will increase the amount of energy you can generate, reduce the amount of carbohydrates you use, and stretch out the glycogen supply during long runs. Added together, you have a more stable and enduring energy supply, better endurance, and faster finish times.

To illustrate, let’s consider Shane. Shane is a computer engineer in his late 30s who has stayed active over the years with yard work, occasional football games with his kids, and sporadic attempts to weight train. In short, he was not aerobically fit. Inspired by the fortitude and tenacity of his wife, who just ran her first marathon, he decided to train for a marathon.

He was determined to be informed and methodical about the process. Many of the books he read recommended training with a heart rate monitor. The books said that most people run marathons at 75 to 80 percent of maximum heart rate, so he decided to do a test. He consulted a chart to find his heart rate at a more manageable effort of 65 percent and set off running. After only 90 minutes on the road, he felt nauseated and fatigued. His legs felt like bricks, and finally he was forced to stop. In other words, he bonked well short of the distance he would need to cover to finish the marathon.

Due to his low level of fitness, most of Shane’s energy was coming from the limited carbohydrate stores in his liver and muscles. He simply ran to the end of his carbohydrate supply. Carbohydrates are necessary to maintain exercise at any intensity. An excessively high rate of usage combined with low carbohydrate stores reduced his endurance, even at relatively easy running speeds. Had he eaten GU or drunk Gatorade, he still would not have been able to continue for much longer. A training program that focused on switching to fat for fuel would change that.



Arthur Lydiard contended that the most important aspect of conditioning is volume. In the 1960s his training concepts were revolutionary. Even the track athletes whom he coached followed a marathon-based aerobic conditioning program in the initial phases of their training cycles. Considering the phenomenal success of athletes who trained under Lydiard’s tutelage, such as Peter Snell, John Davies, and Lorraine Moller, and other athletes who have followed his program principles, his theories were insightful. Subsequent research has shown that they also possess a sound physiological basis.

While many of America’s marathoners switched focus to quality (and reduced mileage) rather than quantity, coaches from Japan, Italy, Mexico, Germany, and China were incorporating Lydiard’s principles into highly successful training programs. Naoko Takahashi reportedly ran up to 80K (50 miles) per day in preparation to become the first woman marathoner in the world to dip under 2:20. She also claimed that it was the stomach juices of giant hornets that helped give her incredible endurance. Catherine Ndereba ran comparatively modest 100-mile weeks in the buildup to her world record of 2:18:47 at Chicago in 2001. Jerry Lawson, imitating the high-mileage successes of Bill Rodgers, Alberto Salazar, and Frank Shorter, ran up to 250 miles per week en route to his then American record of 2:09.

Metabolically, high-volume training makes sense. There are two main sources of fuel for exercise: carbohydrates and fats. The energy supply from carbohydrate and fat is inversely related. High rates of carbohydrate use reduce combustion of fat. Carbohydrates are used preferentially at very high efforts, such as a 5K race, or at low fitness levels when fat metabolism is underdeveloped.
Conversely, when you teach your body to rely on fat for fuel, your combustion of carbohydrates goes down, thus “sparing” carbohydrates. This benefits performance in endurance events. You become very fatigued when you run too low on carbohydrates. We store only a very limited amount of carbohydrate (glycogen) in our bodies. Compare this with a relatively unlimited supply of fat. Even an athlete with only 6 percent body fat will have enough fat to fuel exercise lasting for many hours. When you use more fat, you generate more energy and your carbohydrate supply lasts longer.
Follow the principle of specificity. If you want to teach your body to use more fat for fuel, then create training conditions that generate high fat metabolism. Your body will eventually learn to prefer fat.
Research conducted at the Karolinska Institute in Sweden during the 1980s showed that, within the leg muscles of highly trained endurance men, the activity of enzymes that break down fats was 100 percent higher than in the untrained subjects. As a result, during exercise they had a much higher ability to regenerate the ATP that fuels muscular contraction than those who had a greater reliance on carbohydrates.

These researchers found that the maximal oxygen consumption (or Vo2 max) was 50 percent greater in the trained men. Maximal oxygen consumption measures aerobic capacity: the efficiency of the lungs to transfer oxygen to the blood, the capacity of the blood to carry oxygen, the power of the heart and blood vessels to deliver large quantities of blood to the muscles, and the ability of the muscles to use the oxygen. Fats cannot be burned without oxygen. Not only did these men have more enzymes to combust the fat, but they also had more oxygen to feed the fire.

Researchers have since demonstrated that, after a 12-week six-day-per-week program of 45 minutes of running and cycling at a high intensity, fat combustion increased by 41 percent. This was accompanied by reduced reliance on carbohydrates.



The enzymes of fat metabolism are located in structures within the muscle cells called mitochondria. Fats are transported into the mitochondria where, in the presence of oxygen, they are broken down to generate energy. More mitochondria means more fat metabolism, more ATP, and more energy.

High-volume training increases the amount and size of mitochondria. Longer exercise bouts produce the greatest gains in mitochondrial content. A 90-minute run provides a better stimulus than a 60-minute run. It is common for runners to do “two-a-day” workouts to get in the necessary mileage. However, this research indicates that a runner will receive much more benefit from running one 90-minute workout than two 45-minute workouts. There is, however, a point of diminishing returns. A three-hour run is better at nudging the mitochondria content upward than a 90-minute run, but the gains are offset by the necessity of a longer recovery time between workouts.

During the base phase of building miles, it is the daily consistency of training over a period of weeks and months that will boost fat metabolism.

After the base phase and basic fat metabolism have been established, training time should be shifted into very prolonged runs of three or more hours, depending on your event. Very long runs are important in preparation for the marathon and longer events. After two to three hours of running, the leg muscles run low on glycogen. Hormonal adjustments to the low glycogen levels shift fat metabolism into an even higher gear.

Miles may make champions, but those miles should be carefully developed, monitored, and arranged to get the maximum effect. In his buildup program, Lydiard recommends alternating longer 90-minute to two-hour runs with 60-minute runs on other days, aiming for a total of 10 to 11 hours of weekly running.

Give yourself plenty of time to build up to these levels. Jon Sinclair, former world-class runner turned coach, cautions that it is not practical or even possible for most people with full-time jobs and families to build up to running 10 hours per week in a mere three months. The amount of mileage you will be able to run depends on your lifestyle, physical capabilities, and prior training history. He advises his less-experienced athletes to build up mileage over a period of many months or even years. His associate, Kent Oglesby, took four years to prepare a 3:15 marathoner for the rigors of running 100 miles per week. The result was a 2:46 marathon, which earned her a spot at the U.S. Olympic Marathon Trials.



My speed in long races had been declining since I had become a masters runner. For a number of years I had been running LSD (long, slow distance) type training. In the process of researching and writing about fat metabolism, I read Lydiard’s book Running the Lydiard Way. Lydiard’s formula advocates not just high-volume training but high volume at speeds near the “maximum steady state.”

In other words, most training should be conducted close to the highest speed that you can run without going anaerobic. This is the speed where fat metabolism is at its highest. For experienced runners, the maximum steady state equals an intensity of 70 to 75 percent of maximum heart rate. For those just launching their running careers, it will be closer to 60 to 65 percent of maximum heart rate. Studies have confirmed his theories. Volume and intensity interact to produce even greater gains in mitochondria development. Daily runs of 90 minutes at 70 percent max will boost mitochondria 30 percent higher than equivalent time spent at an easier 50 percent effort.

After purchasing a heart rate monitor and calculating my target heart rates, I was surprised to find that my LSD training intensity had been substantially below my target training intensity of 70 percent. Initially I had a hard time running more than 60 minutes at that effort. However, after only six weeks of faster training, I was easily able to maintain that pace for a full two hours. Although LSD training will increase fat metabolism and endurance, it will limit your endurance at marathon paces. Long, slow running will only teach you to run slowly for long periods.

On the other hand, you can run too fast on your daily runs. At faster paces, oxygen demand exceeds supply. You are now anaerobic. Fuel reliance switches predominantly to carbohydrates, and the result is the accumulation of lactic acid. Lactic acid inhibits the enzymes that break down fat and therefore reduces fat metabolism. If you go out for a 45-minute run at 10K race pace, you will be burning less fat and generating more waste products than if you ran those 45 minutes at only a 60 percent effort. Daily hard efforts will result in accumulation of waste products and decreased recovery, and lead to declining performances. It’s better to run a little too slow than a little too fast.



Let’s return to Shane after 24 weeks on his Lydiard-based training program. His fat metabolism is augmented, there is a substantially reduced reliance on glycogen, and his glycogen stores are larger. He again decides to test his ability to run at 65 percent of his maximum. Before the test he makes sure to get plenty of carbohydrates in his diet so that his leg muscles and liver are loaded with glycogen. This time he was able to continue for three hours.

His skeletal, connective, and muscle tissues; his metabolism; and his cardiovascular, nervous, and endocrine systems are now prepared for some faster training. His next step is to focus on increasing his endurance running speed and reducing his lactate production.

Endurance races are aerobic races. Marathons tend to be run at just below the level where you start to accumulate lactic acid, which is known as the anaerobic threshold (AT). How many times have you started a race too fast and gone anaerobic, only to suffer later and run slower than you planned or even had to drop out?

With a higher AT, you will be able to sustain faster marathon and ultramarathon paces. Elite world-class marathoners often have such a highly developed fat-burning engine that they can run marathons at 85 percent or higher of their maximum. For the rest of us, 75 to 80 percent is a realistic goal.

Anaerobic threshold training augments the basic fat metabolism you have spent so much time developing. The result is faster running speeds over the long haul. A measured dose of faster, anaerobic training will teach your muscles and blood to metabolize and buffer lactic acid. The goal is to generate a manageable quantity of lactic acid that your muscles can dispose of easily and permit a sufficiently long training session and quick recovery. Venturing too far into the anaerobic zone will generate too much lactic acid, reduce the amount of work you can do within your training session, and risk lasting fatigue and overreaching. Marathoners don’t derive much benefit from 400-meter repeats.

Faster, sustained running at 80 to 85 percent and mile repeats are good methods to increase lactate tolerance. Oglesby recommends tempo runs of 10 to 12 miles at 15 to 30 seconds per mile faster than goal marathon race pace. An added benefit of these tempo runs is that the marathon pace feels easier and more manageable.

A recent study examined the effect of high-intensity interval sessions on fat and carbohydrate metabolism and lactate concentrations in cyclists who had been training two to three hours per day for years. They replaced some of their endurance miles with two weekly sessions of 6-9 x 5-minute intervals with 1 minute of recovery between. After six weeks, the percentage of energy coming from fat during a one-hour trial had increased from 6 percent to 13 percent. How well this applies to a race lasting more than two hours is unclear.

Because of the results from studies on interval training such as these, many runners have opted out of the extended base-building phase citing “quality over quantity” as the rationale. I would like to emphasize that high-intensity training builds on the increased strength, resilience, and fat metabolism developed during those long, high-quality aerobic miles. Jumping into AT training before your body is sufficiently prepared will not produce the desired results: fast marathons.



It is best to start an exercise session with stable, fasting blood glucose levels and higher blood fat levels. Glucose is a powerful regulator of fat metabolism. The higher the glucose content of the blood, the lower the fat metabolism. High blood glucose levels are generated from dietary carbohydrates.

This effect is associated with insulin. High blood glucose stimulates the hormone insulin to be released from the pancreas. Insulin is a storage and growth hormone. Its main job is to reduce blood glucose but it also acts to store fat and protein. In the process, insulin directly blocks removal of fat from fat deposits. These deposits are an important source of fat for exercising muscle. Insulin also reduces fat burning within the muscle. Therefore, increased insulin is considered to be antagonistic to fat combustion during exercise.

In an interesting piece of research, investigators at the University of Limburg in the Netherlands and at the University of Texas collaborated to determine whether high blood glucose and high insulin levels reduce the amount of fat burned during moderate-level exercise. A group of endurance-trained men cycled for 40 minutes at an aerobic 50 percent of maximum after an overnight fast. On another day, they ingested a drink containing 100 grams of glucose at 60 minutes before and then again at 10 minutes prior to the exercise test. This is a carbohydrate equivalent of drinking one and one-half liters of Gatorade an hour before a race and again 10 minutes before the start. While this may not mimic real-life situations, what the researchers found was telling. Fat metabolism was substantially reduced for the full 40 minutes of the exercise after the carbohydrate load.

While most people would not eat that much carbohydrate before a run, it is common for people to eat a sports bar, bagel, or banana in the hour prior to training. Try to avoid eating for at least two hours before a run.

It takes as little as 20 grams of ingested carbohydrate to raise insulin and reduce fat as fuel. If you have nutrition awareness or read the nutrition labels on foods, you will know that a couple of slices of bread, a banana, a sports bar, or a soda each delivers more than 20 grams of carbohydrate.

Fasting increases blood fat levels. Running after your overnight fast will increase fat burning. A cup of coffee beforehand may boost it even higher. Once exercise has started, eating carbohydrates does not generate a substantive insulin response. If you are starting a long run lasting two hours or more on an empty stomach, you may want to eat a sports gel or bar after 20 to 30 minutes throughout the run. Otherwise you will be faced with the nausea and fatigue of low blood sugar and have a poor training session. If you tend toward hypoglycemia when you get up in the morning, you may want to eat something in the minutes immediately before you head out the door. It takes 30 minutes for insulin levels to peak.

However, before a long race or run you will have more endurance and perform better if you eat a meal containing carbohydrate two to three hours before. Early in the morning, your liver glycogen stores, which supply blood glucose, have been depleted by the overnight fast. The brain and nervous system rely on blood glucose for energy. If you start a marathon without replenishing these stores, you will bonk. The two-hour time interval is sufficient to reduce blood glucose levels back to normal and restore fat metabolism.



There has been considerable research in the past decade on the effect of diet composition on endurance. Prior to now, endurance athletes usually followed a high-carbohydrate diet with the rationale that enhanced glycogen stores are known to fuel superior training and marathon race performances.

Most sports nutritionists recommend a diet that supplies 6 to 8 grams of carbohydrates per kilogram of body weight. These levels of dietary carbohydrate can easily reach 400 to 600 grams per day. This adds up to 1,600 to 2,400 calories of carbohydrate per day. This type of diet doesn’t leave room for adequate amounts of fat or protein.

The downside of a high-carbohydrate diet, especially a diet loaded with sugar, is reduced fat metabolism and fatigue. This is largely due to chronically stimulated insulin levels. The effects of insulin can last up to eight hours, especially after a big dose of carbohydrates, such as you might get from a big plate of spaghetti and rolls followed by a bowl of sorbet.

Initially, studies found that high-fat diets, where fats supply 60 percent or more of the calories, showed promise as a means to better endurance. Fat burning is increased on high-fat diets, even at rest. Exercise tests showed higher endurance in subjects who had been eating high-fat diets in comparison with high-carbohydrate diets.

At issue, however, was the intensity of exercise used for the tests. High-fat diets improved endurance at relatively low-intensity levels. When the intensity was increased to mirror race situations, the advantage disappeared. The higher- intensity exercise required more carbohydrate, and the subjects simply lacked adequate glycogen to continue for extended periods. The lesson is that you can reduce your reliance on carbohydrate, but you can’t eliminate it.

We now know that both high-carbohydrate and high-fat diets cause fatigue and poor performances. The best diet is probably somewhere in between: one that supplies enough fat to stimulate fat metabolism and maintain production of testosterone and estrogen and also supplies enough carbohydrate to keep the brain and nervous system happy and the glycogen stores filled. Many sports scientists are recommending a basic diet that supplies 50 percent carbohydrate, 30 percent fat, and 20 percent protein, with additional carbohydrates after hard or long-duration training.



There are still many unanswered questions regarding nutrition and endurance sports performance. Before a marathon or longer race, will fat loading in combination with glycogen loading boost performance? After hard or long training, should you also concentrate on replenishment of fat stores in the muscles? What type of fat, saturated or unsaturated, is burned for fuel? Will eating fat during races that last four hours or more benefit performance outcomes?

What profile of fats in the basic diet is best for an athlete? The skeletal muscle membrane is made of fat. The composition of this membrane directly reflects the profile of fats in the diet. A diet high in saturated fats will generate a more solid, less fluid membrane. A membrane that incorporates more unsaturated fats is more fluid, allowing a more efficient flux of oxygen, water, fat, and glucose. New theories hold that these membranes are more leaky and require more energy to maintain. Conceivably then, a diet too high in either saturated or unsaturated fats could be detrimental to endurance performance.

While there are new training methods being developed to enhance marathon performance, you will find substantial success with theories that are now 40 years old. In contrast, the field of sports nutrition research is currently experiencing great strides. In the early 1990s, the accepted dogma of a high-carbohydrate diet came under fire and was dismantled. Until we have more definitive information, it is wise to follow a moderate, low-sugar, common-sense diet with high nutritional quality.

With a training and nutrition regimen that coerces you to tap into your fat supplies, you can teach your body to use more fat during your migration through the marathon, and beyond.

To read some of the myths about fat burning click here