In the modern endurance world, we are living through a curious cultural shift. We have—rightfully—moved away from the “thin at all costs” era of distance running, replacing it with a “food is fuel” mantra that prioritizes performance and body neutrality. But in our haste to dismantle a toxic diet culture, we’ve created a new taboo: admitting that we actually want to be leaner.
“We’ve kind of swung the pendulum a little bit too far the other way,” says Alex Larson, a Registered Dietitian and Ironman finisher. “Athletes come to me and they almost downplay their interest in getting leaner. They’re almost embarrassed to tell me. I have to ask them two or three times if they have body composition goals before they admit, ‘Well, if I lost five or ten pounds, I’d be really happy.’”
For the marathoner, the desire for a leaner physique isn’t just about the mirror—it’s about the physics of the sport. Every pound carried is a pound that must be fueled and propelled over 26.2 miles. The challenge lies in reaching that “racing weight” without the wheels falling off.
Body Composition vs Weight Loss for Runners
The Science of “Better,” Not “Smaller”
The first step in a successful transformation is a shift in vocabulary. Larson prefers the term body composition over weight loss. The goal for a runner is rarely to simply see a smaller number on the scale; it is to decrease excess body fat while aggressively protecting—or even building—lean muscle mass.
“It’s not about the goal; it’s about how you pursue it. We have to do it the right way so we don’t deregulate hormones or put you at risk for stress fractures and injuries. For me, it’s about visualizing where the gaps are in an athlete’s nutrition.”
Can you actually get faster while eating in a deficit? Larson insists you can, citing a recent client who lost ten pounds while simultaneously setting personal records (PRs) in the 5K, 10K, and half-marathon. “It’s the combination of her being just as strong, if not stronger, and not having those eight to ten pounds to carry. It is certainly possible, but it requires strategy and close monitoring.”
Front-Loading Calories for Marathon Performance
The “Front-Loading” Revolution
Most runners follow a “back-loaded” eating pattern: a small breakfast, a rushed lunch, and a “binge raccoon episode” in the pantry at 8:00 PM because they are biologically starving. Larson’s primary intervention is to flip this script entirely.
“Breakfast is a magical meal,” Larson explains. “It kickstarts your energy and regulates your appetite so you don’t have that constant, nagging hunger as your mileage ramps up.”
The 40-Gram Rule: Larson suggests aiming for 35 to 40 grams of protein at breakfast. For those who “don’t do” morning food, she suggests a “gateway food” like a high-quality protein shake to wake up the metabolism. “If you can focus on this for two weeks, your appetite usually comes back. Your metabolism starts getting more regulated during the day.”
The Fueling Firewall: Crucially, this front-loading strategy includes a strict rule: Never diet during your runs. “If you’re going to take anything from this, it’s: do not diet during your workouts,” Larson says. “That is not the place to save calories. It will come back and bite you in the butt.” Any run over 60 minutes should be fueled properly to ensure you hit your training marks and avoid entering a massive calorie deficit that triggers overeating later.
Common Nutrition Mistakes When Trying to Lean Out
The “Fat Trap” and the Peanut Butter Paradox
One of the most common pitfalls for runners trying to lean out is the “Peanut Butter Paradox.” In an effort to eat “cleaner” and hit protein goals, athletes often over-rely on nuts, seeds, and avocados.
“People say, ‘I’m going to eat more protein, so I’ll eat more peanut butter.’ Well, peanut butter is pretty much fat,” Larson warns. “It’s not that fat is bad, but it is calorically dense. If your weight is creeping up despite ‘eating clean,’ look at your portions of nuts, oils, and cheeses. You might need to swap for leaner protein options—like Greek yogurt or powdered peanut butter—to keep the calories in check while supporting recovery.”
Sleep, Stress, and Body Composition for Endurance Athletes
The Performance Multiplier: Sleep and Stress
A surprising byproduct of fueling better during the day is the impact on recovery. Larson notes that athletes who fuel properly early on often report “exponentially better” sleep.
“When athletes are training day after day and they’re under-fueled, their body is always in this stress state and they don’t sleep well,” Larson says. By front-loading, you enter the evening in a less stressed state. This hormonal balance is the “secret sauce” for body composition changes. When the body isn’t in a state of high-cortisol panic, it is much more willing to release excess body fat.
Essential Blood Markers Runners Should Monitor
The Internal Audit: Beyond the Calories
You cannot manage what you don’t measure, but Larson isn’t just talking about macros. For an athlete in a fat-loss phase, internal health markers are the “check engine” lights of the body. She recommends all marathoners track two specific metrics through annual blood work:
Ferritin (Iron Stores): “Even if the clinic says 15 is ‘normal,’ for endurance athletes, we want to see that above 50,” says Larson. Low ferritin leads to chronic exhaustion that no amount of training or caffeine can fix.
Vitamin D: Essential for bone density, immune function, and muscle performance.
Supplements That Support Performance While Leaning Out
Strategic Supplementation
When calories are slightly lower, Larson is a staunch advocate for two specific supplements to maintain power:
Creatine (3–5g daily): “I am totally Team Creatine,” she says. “It helps the muscle mass you have be stronger, helps with recovery, and even provides mental clarity. The benefits far outweigh the minor risk of initial water weight gain.”
Whey Protein: A convenient tool to hit those high-protein targets at breakfast or post-workout without the added fats found in whole-food protein sources like steak or full-fat dairy.
Sustainable Fat Loss for Marathon Runners
The Long Game
The journey to a leaner body composition is not a six-week “shred.” It is a series of “sneaky little small changes” that respect the body’s psychology. “Our body hates change. It will push back if you’re too drastic,” Larson concludes. “But if you make the changes minimal and sustainable, you won’t just see the body composition improvements—you’ll actually be able to maintain them while you’re standing on the start line of your next PR.”
Alex Larson is a registered dietitian specializing in endurance sports nutrition. She works with runners and triathletes through her private practice, Alex Larson Nutrition, and hosts the Endurance Eats podcast. More information can be found at alexlarsonnutrition.com.
