Fat Adaptation for Runners: Dr. Davitt's Science-Backed Guide to Fueling Endurance
When it comes to endurance running, few debates ignite more passion than the question of how to fuel. Should athletes constantly carbo-load? Is running fasted beneficial or harmful? And what about the highly effective strategy of metabolic efficiency—commonly known as "fat adaptation"? To cut through the noise, we spoke with an expert who lives, breathes, and studies human performance at the highest levels: Dr. Patrick Davitt.
A tenured associate professor at Saint Joseph’s University, Dr. Davitt leads the exercise physiology program and has spent more than a decade researching energy metabolism, fat oxidation, and endurance performance. His resume reads like a hybrid of scholar and athlete: a PhD in nutritional physiology and biochemistry, a postdoctoral fellowship at the University of Connecticut under Dr. Jeff Volek, six years as head sports nutritionist for Rutgers football, co-editor of a leading exercise physiology textbook—and an accomplished ultrarunner with a 100-mile finish and a newly registered 200-miler on deck.
“I don’t just read the science,” he says. “I practice what I preach.”
What is Fat Adaptation? (The Scientific Definition)
The concept of fat adaptation can be confusing, even for seasoned runners. At its core, Dr. Davitt defines it as the ability to efficiently burn fat as fuel as exercise intensity rises. While all humans burn mostly fat at rest, the shift toward carbohydrate during higher intensities is inevitable. But a fat-adapted athlete can maintain greater fat oxidation at faster paces.
For example, a runner who normally burns 50% fat and 50% carbohydrates at a seven-minute mile might—after fat-adaptation training—burn 60–65% fat at that same pace. This conserves carbohydrate use, reduces reliance on gels or sports drinks, and may help sustain performance longer.
Importantly, Dr. Davitt stresses the distinction between fat burning and fat loss. “Just because you’re burning fat during a run doesn’t mean you’re losing body fat,” he explains. Fat loss depends on overall daily or weekly energy balance—not what fuel you’re oxidizing moment to moment.
Why Runners Should Focus on Fat Oxidation for Endurance
Runners often hear that carbohydrates are the body’s preferred fuel. According to Dr. Davitt, that narrative needs context.
“Yes, carbs provide quick energy,” he says. “But fat is virtually unlimited. We store tens of thousands of calories’ worth of it.”
Carbohydrate stores, by contrast, are limited—about 400–600 grams of muscle glycogen, plus additional storage in the liver. Because endurance efforts eventually tap into these reserves, the ability to shift more energy demand to fat can help delay glycogen depletion and reduce the risk of bonking late in a race.
“And if you’re more fat adapted,” he adds, “you don’t have to consume as much carbohydrate during long efforts. That’s one of the biggest advantages.”
Carbohydrate Myths: Why Traditional Running Fueling is Misunderstood
The running world has long been taught to fuel before every run. Dr. Davitt believes this advice, though well-intentioned, often misses the science.
Several myths, he says, persist:
Myth 1: Eating carbs before a run spares muscle glycogen.
“Completely false,” he says. “Study after study shows that dietary carbohydrate does not spare muscle glycogen during exercise.”
Myth 2: You must eat before any workout for energy.
Not true. If you ate adequately the day before, your glycogen stores will be more than sufficient for runs up to an hour—often longer.
Myth 3: If you don’t eat before a run, your body will burn muscle.
“That’s one of the most incorrect—and widespread—claims,” he says. “Your body will not break down muscle during a 2- or 3-hour run simply because you didn’t eat breakfast.”
Myth 4: What works in competition should be applied to every training run.
“That’s the biggest mix-up,” he says. “Training and racing are not the same. You don’t need race-day fueling for a casual 45-minute run.”
Even Gatorade’s own guidelines, he notes, do not recommend carbohydrate intake for efforts under 45 minutes.
Elite Secrets: Training Low, Racing High (The Kenyan Method)
Observing elite Kenyan runners provides a clear example of strategic training. Despite world-class fitness, most run their daily hour-long runs without consuming any fuel beforehand.
Dr. Davitt wasn’t surprised.
“They eat high-carbohydrate diets, but they purposely train some runs in a fasted or low-carb state,” he explains. “It’s strategic. These workouts stimulate specific metabolic adaptations.”
Come race day, however? The tables turn. Elite athletes fuel aggressively, often grabbing bottles every 5K.
“Training is for adaptation,” he says. “Racing is for performance.”
Should You Do Intervals or Tempos Fasted?
The short answer: sometimes, but not always.
“If race day is within three weeks, I would not recommend fasted intervals or tempo sessions,” Dr. Davitt says. “You want to practice exactly what you’ll do during the marathon.”
But early in a training block—when key workouts are shorter than an hour—running fasted or low-carb can be beneficial.
Still, he warns: don’t do all workouts fasted. “You want your gut trained, too. If you don’t practice eating before or during workouts, then introduce it on race day, you’re setting yourself up for disaster.”
How Fat Adaptation Reduces Carb Dependence on Race Day
Two athletes of similar pace may require different fueling strategies based on their metabolic efficiency. If one is fat adapted and the other is highly carb-dependent, the carb-dependent runner will need more gels per hour at the same race pace.
Dr. Davitt is clear: fat adaptation doesn’t eliminate the need for carbs during a marathon, but it significantly reduces the reliance on them.
A fat-adapted runner may perform well on 30 grams of carbohydrate per hour, whereas another runner may require 45–60 grams to sustain the same pace.
“Taking more carbs than your body needs won’t make you more efficient,” he says. “It just increases the risk of GI distress.”
Who Should Experiment with Fat Adaptation?
Dr. Davitt’s view is refreshingly practical. “If you feel great, I probably won’t change anything,” he says. “But if you’ve hit plateaus—can’t break four hours, can’t get that Boston qualifier, or you crash late in every marathon—then you should absolutely experiment.”
Fat adaptation training may help:
Runners who frequently “bonk” late in races or long runs
Athletes sensitive to gels or sugary drinks
Those seeking more steady energy without highs and crashes
Runners who want to reduce supplement reliance and save money
Athletes stuck at a performance plateau
He emphasizes that experimentation should happen early in a training cycle—not three weeks before race day.
The Biggest Benefit of All
For Dr. Davitt, one advantage of metabolic flexibility stands above the rest.
“Once you’re fat adapted, you’re no longer dependent on constant sugar intake during the day or during every run,” he says. “You feel more stable—more energy, fewer crashes, fewer gut issues. And you’re not spending money on five different products just to get through a workout.”
His advice to runners: be open, be curious, and test what your body responds to best to unlock your maximum endurance potential.