Most Runners Underfuel by 800 to 1,000 Calories Daily. Here’s How It’s Quietly Destroying Your Metabolism (And What to Do Instead)

Registered dietitian and endurance sports nutrition coach Alex Larson has spent 15 years helping everyday runners, cyclists, and triathletes navigate one of training’s trickiest challenges: how to fuel performance without compromising body composition or metabolic health.

In a recent conversation on the Endurance Lab podcast, Larson pulled back the curtain on fueling strategies that work in real life—not just in controlled lab settings.

Her approach centers on a simple but often overlooked principle: fuel your workouts hard, manage your calories elsewhere.

What follows is a practical roadmap for midlife runners trying to get leaner, faster, or both—without burning out or breaking down.

The Hidden Cost of Massive Calorie Deficits

Larson sees it all the time: athletes running 800 to 1,000-calorie deficits most days of the week.

They believe drastic restriction equals faster fat loss. Instead, they’re tanking their metabolism.

Our body is very adaptable. So over time, if you’ve been in that massive calorie deficit, your metabolism is just going to drop significantly.

Larson advocates for a moderate deficit—300 to 500 calories per day—and only outside of workout windows. She stresses that dramatic changes don’t stick, and bodies resist abrupt shifts.

Progressive change beats shock-and-awe every time.

Fuel Workouts First, Manage Calories Second

Here’s where Larson’s philosophy gets interesting.

She doesn’t count workout carbs toward daily calorie targets. Anything consumed during training—gels, sports drinks, energy chews—stays off the books.

Anything that an athlete is consuming during their workout for carbs and calories, we won’t count that towards their baseline daily macronutrients and calories. And it works really well.

Why? Because endurance athletes are Type A. They see themselves exceed carb targets after a long run and panic—even when those carbs powered the session.

Separating workout fuel from daily nutrition eliminates that mental friction. Athletes fuel sessions properly without guilt, then focus on consistency the rest of the day.

Protein: The Underrated Recovery Tool

Larson recommends 1.5 to 2 grams of protein per kilogram of body weight daily.

Most athletes she works with fall short. Protein supports muscle recovery, regulates appetite, and preserves lean mass during calorie deficits.

For body composition purposes, she typically aims for around 25% of total calories from protein, though this can range from 21% to 30% depending on training load and goals.

Fat usually sits around 30%, with carbs filling the remainder—often 40% to 55%.

What Good Metabolic Health Actually Looks Like

Forget complicated protocols. Larson’s vision of metabolic health is refreshingly straightforward.

Wake up. Eat a real breakfast within an hour—30 to 40 grams of protein, quality carbs, fruits or vegetables. Not a granola bar and coffee.

Three to four hours later, eat again. Another solid meal with similar macros.

Mid-afternoon snack if dinner’s still hours away. Then a satisfying dinner that leaves you comfortably full—not stuffed, not starving.

When I think of good metabolic health, I think of someone that’s got a really nice structure and routine and they’re eating enough to support their needs and their body’s in this really good functional state.

No bedtime snacking required if you’ve eaten well throughout the day. Quality sleep follows naturally.

Signs You’re Chronically Underfueling

Short-term, underfueling shows up fast: workouts feel harder, recovery drags, appetite swings wildly.

Athletes often report ferocious hunger after training, then binge-eat at night “like a raccoon in the pantry,” Larson says.

Long-term underfueling is sneakier. Hunger disappears entirely. Athletes lose their appetite during the day, only feeling hungry in the evening.

Other red flags include:

• Chronic fatigue after long runs, needing naps or couch time the rest of the day
• Irritability with family or coworkers post-workout
• Recurring injuries—stress fractures, muscle pulls, nagging pains
• Stalled performance despite consistent training

If Saturday long runs leave you immobilized on the sofa, unable to engage with family or basic chores, nutrition needs immediate attention.

Why Fasted Training Isn’t Worth It

Larson isn’t a fan of fasted training, especially for midlife athletes trying to preserve muscle mass.

Yes, fasted sessions increase fat oxidation. They also accelerate muscle breakdown.

For athletes in their late 30s, 40s, 50s, 60s, preserving muscle mass should be your top priority. It is the key for longevity in your sport.

She shared a personal story: before 30-minute social runs with a friend, she’d eat a simple applesauce pouch—about 15 grams of carbs. The last 10 minutes felt noticeably better than when she skipped it.

When she mentioned this on a podcast, comments flooded in criticizing her for “needing” fuel for such a short run. But she’d gone nearly 12 hours since her last meal. A tiny snack made sense.

Nutrition has become almost like religion, Larson notes. Judgment runs high, nuance gets lost.

Carb Loading Done Right

Guidelines suggest 8 to 12 grams of carbs per kilogram of body weight for carb loading. Larson adjusts based on race distance and athlete experience.

Races under 2.5 hours? One-day carb load may suffice. Three-plus hours? Two days. Ultra distances? Three days.

Carb loading isn’t about stuffing yourself with pasta. It’s about high-carb, low-fat foods with moderate protein to stabilize blood sugar and prevent that strange “empty but not hungry” feeling.

Larson emphasizes variety—mix sweet and savory to avoid palate fatigue. Chicken noodle soup in a bread bowl, sushi with extra rice, anything that keeps carbs interesting over multiple days.

Think of carb loading as insurance. If race-day fueling goes sideways—dropped bottles, lost gels—those glycogen stores become your safety net.

How to Fuel Runs by Duration

Larson breaks fueling into simple tiers based on effort duration:

• Under 75 minutes: 0 to 30 grams of carbs per hour (or none if pre-fueled well)
• 90 minutes to 2.5 hours: 30 to 60 grams per hour
• Over 2.5 hours: 60 to 90+ grams per hour

She encourages front-loading nutrition in longer efforts—start fueling around 20 minutes in rather than waiting an hour. Staying ahead prevents the dreaded energy deficit spiral.

Elite athletes now push well beyond 90 grams per hour without GI distress, but Larson cautions recreational runners against chasing those numbers unless training intensity truly demands it.

Glucose-to-Fructose Ratios Matter

Not all gels are created equal. Glucose and fructose absorb via different pathways—glucose by “bus,” fructose by “train,” Larson explains.

Too much glucose (like pure maltodextrin) can overwhelm absorption, causing bloating and abdominal distension during long races.

She recounts two athletes—one cyclist, one triathlete—who experienced severe bloating around the five- to six-hour mark. Both were using maltodextrin-heavy products.

Switching to gels with higher fructose ratios solved the issue. The key: don’t overload one pathway.

Exogenous Ketones: Skip the Hype

Larson interviewed a ketone researcher for her podcast. His conclusion? Not worth the money.

Ketones are expensive, taste terrible, and research shows no performance benefit—and in some cases, performance declined.

Worse, ketones spike insulin, which shuts down fat oxidation. If you’re low on glycogen and can’t burn fat, you’re setting yourself up to bonk.

You’re going to get so much more benefit out of just eating well and fueling well in your day-to-day.

Save your money. Buy nice running shoes instead.

Racing Weight: Timing Matters More Than the Number

Athletes often panic weeks before a race, trying to drop three to four pounds fast. Larson pushes back hard.

Body composition work should happen months in advance—gradual, sustainable fat loss at one to two pounds per week.

Restricting calories close to race day tanks glycogen stores and compromises performance far more than a few extra pounds would.

Don’t weigh yourself. Don’t worry about that in the last couple weeks. Just focus on going into the race healthy and feeling really great.

Water weight fluctuations make the scale meaningless in taper weeks anyway. Prioritize fueling well, sleeping deeply, and arriving at the start line energized.

The Real Win: Energy for Life Beyond Training

Larson’s favorite success stories aren’t PRs. They’re athletes who finish long runs and still have energy to play with their kids, tackle yard work, or simply not snap at their spouse.

I did my long run this morning and then I played with my kids. I didn’t snap at them once the entire day. Those are my favorite wins.

Training should enhance life, not drain it. Proper fueling makes that possible.

Whether chasing a PR or simply trying to feel better in your 40s, 50s, or 60s, Larson’s message remains consistent: eat enough, fuel workouts well, recover intentionally, and trust the process.

Progressive beats drastic. Always.