High Fat vs. High-Carb: What Is Best For Aging Endurance Athletes?

Large crowd of runners in a city marathon — Photo: Max Herman / Shutterstock

Runners over 40 keep hearing two opposite promises: burn fat like a furnace, or load up on carbs and never bonk. Science says both camps are half right, and the real answer depends on the athlete’s age, event, and recovery needs.

Quick Take

Short-term high-fat, low-carb eating can boost fat burning in as little as five to six days, but studies find little proof it makes athletes faster.
Coaches now describe fat adaptation as a spectrum built through training, not a strict keto requirement, even for elite marathoners.
Aging athletes, especially those 55 and older, are told to chase metabolic flexibility and muscle preservation over pure fat-burning.
Carbohydrate and protein within 45 minutes after a race still beats water alone for restoring endurance in masters athletes.

The Fat-Adaptation Promise And Its Limits

Researchers led by sports nutrition expert Louise Burke found that elite athletes who switched to a low-carb, high-fat diet for just five to six days saw fat-burning rates jump to levels once thought to need months of strict eating. That’s a real, measurable change in how the body uses fuel. But raising fat oxidation is not the same as running faster, and that gap is where a lot of amateur fueling advice falls apart.

Follow-up work from the same research group found something uncomfortable for keto believers. Athletes who stayed on a strict low-carb, high-fat diet long-term actually got worse at converting oxygen into speed, a measure called exercise economy. Their bodies burned more fat, sure, but they lost some of the efficiency gains from training. Another widely cited review called it a “nail in the coffin” for the idea that fat-loading alone boosts race-day performance.

What Coaches Actually Tell Runners Over 40

Sports scientists at The Endurance Lab now push back on the idea that fat adaptation requires giving up carbs at all. They point to a 2:33 marathoner who ate normally, with plenty of carbohydrates, yet still showed strong fat-burning during testing. Their conclusion: fat adaptation is a spectrum built mostly through consistent training, not a strict diet label.

Fitness coach and nutritionist Debbie Potts, who works with masters athletes, warns against ultra-low-carb, high-fat plans for a different reason. She cites Burke’s research showing those diets can hurt performance right when it matters most, during race-pace efforts. Her advice for older athletes leans away from diet extremes entirely and toward flexibility, meaning the ability to use both fat and carbs efficiently depending on the moment.

Why Muscle And Recovery Matter More After 55

Potts also raises a point rarely discussed in the fat-versus-carb debate: muscle loss. Athletes past 55 face something called anabolic resistance, where the body struggles to build and keep muscle even with normal effort. Her fix isn’t a dietary ideology at all. She recommends resistance training two to four times a week plus steady protein intake, arguing that skipping this step costs more speed than any fueling strategy gains.

Recovery nutrition backs up that muscle-first thinking. A study on masters-class endurance athletes tested water and electrolytes against carbohydrate drinks and carbohydrate-protein combinations after hard efforts. Plain water and electrolytes failed to restore endurance capacity for a second bout of exercise. Both the carbohydrate and carbohydrate-protein drinks worked better. Dietitian Alex Larson echoes this, recommending racers get carbs and protein into their system within 30 to 45 minutes of finishing, roughly half a gram to seven-tenths of a gram of carbohydrate per pound of body weight.

The Bigger Picture For Race-Day Fueling

Broader nutrition guidelines still lean carbohydrate-heavy for a reason. A widely cited review calls a high-carb, moderate-protein, low-fat diet the “evidence based choice” most commonly recommended for endurance athletes. During races lasting one to three hours, guidelines suggest 30 to 60 grams of carbohydrate per hour, climbing to 60 to 90 grams per hour for anything longer.

None of this means fat adaptation is a myth or that carb-loading is foolproof. It means the loud, either-or marketing around both camps oversells certainty the research doesn’t support. For athletes over 40, the smarter move looks less like picking a tribe and more like matching fuel to the demands of the day, protecting muscle along the way, and refueling properly once the race is done.

An elite ultrarunner burned 16,104 calories in 14 hours but only consumed 6,720. The 9,384-calorie deficit reveals what your body does when extreme performance meets impossible fueling demands.

Researchers at Loughborough University published the first comprehensive… pic.twitter.com/wJ7X63bzNJ

— Daniel Tawfik (@dantawfik) September 14, 2026

Extreme diets that sound simple rarely survive contact with a marathon course or a masters athlete’s recovering joints. The athletes who age well tend to be the ones who train consistently, eat enough protein, and treat fueling as a tool, not a religion.