A 2014 study of endurance athletes found that more than 60% of non-elite endurance runners were unintentionally underfueling during their highest training weeks
More importantly, the researchers found that this happened not because they were dieting, but because they didn’t realize how dramatically their needs had changed.
If you’ve ever entered a spring training block feeling fit and motivated, only to end up flat, irritable, sore, or questioning your training halfway through, you’ve felt the impact of that gap firsthand.
The frustrating truth is that this isn’t always a fitness problem or a sign you need more recovery days, it’s often a fueling problem, and the consequences extend far beyond one bad workout.
A 2024 narrative review found that athletes who fail to meet energy intake requirements face compromised training adaptations, reduced muscle mass, decreased bone mineral density, and increased injury susceptibility.
The good news is that understanding the unique nutritional demands of heavy training blocks can transform how you feel, recover, and ultimately perform on race day.
That’s why in today’s email I am going to share some unique insights on metabolism I learned from registered dietician, Amber Velasquez, lead coach at MetPro who specializes in working with endurance athletes.
She’s going to also help you…
Understand why your metabolism responds differently during peak training and what happens when you don’t keep up
- Break down the specific macronutrient targets research shows you need during heavy training
- Give you practical timing strategies for before, during, and after your runs that you can implement immediately
Why Underfueling Is So Common and So Damaging
Here’s a scenario that plays out constantly among runners in heavy training blocks:
You’re logging more miles than ever, eating what feels like a reasonable amount, and yet you feel flat, fatigued, and like you’re getting slower instead of faster.
This is metabolic adaptation in action and it’s one of the most misunderstood concepts in runner nutrition.
When your body consistently receives less energy than it needs for daily functions and training demands, it doesn’t just power through the deficit.
Instead, it compensates by becoming more “efficient”. This means slowing metabolic rate, reducing energy expenditure, and prioritizing survival over performance.
“Your metabolism starts to slow down because you’re not eating enough, and it’s in fight-or-flight mode at that point. The body doesn’t know you’re training for a spring marathon, it just knows it’s not getting enough fuel for the demands being placed on it.”
Your body essentially enters a protective state, conserving resources rather than adapting to training stress.
Workouts feel harder, recovery takes longer, and the fitness gains you’re working so hard for simply don’t materialize.
And one of the biggest reasons this happens to so many runners is that the nutrition advice you see for the general population simply does not apply to you during a heavy training block.
With that understanding, let’s dig into what the nutritional differences and how to prevent metabolic adaptation from sabotaging your training.
Carbohydrates: Your Primary Training Fuel
Research shows that endurance athletes need 7-10 grams of carbohydrates per kilogram of body weight daily during heavy training to adequately replenish glycogen stores.
For a 150-pound (68 kg) runner, that’s 476-680 grams per day, which is far more than most runners actually consume.
This lack of carbohydrate intake happens for three reasons.
- Most runners don’t know they need to consume this many carbohydrates, so it’s not something they pay attention to.
- Eating this many carbohydrates isn’t something that usually happens naturally and this it’s something most runners have to pay attention to.
- A lot of runners have body composition goals alongside performance goals, so they reduce their carbohydrate intake (especially given the demonification of carbs on social)
But, the research is almost universally clear that higher carbohydrate diets increase endurance performance and recovery.
We’ll get into the specifics of how to do this later in the email, but keep in mind that carbohydrates are critical to your training and race day performance.
Protein: More Important Than You Think
While protein is certainly having its moment in the public dialogue about nutrition, there are still a lot of misconceptions.
The uncomfortable truth is that most runners dramatically undervalue protein.
But protein is by far the most critical macronutrient when it comes to fueling of the recovery process after running.
In fact, a systematic review found that protein supplementation during marathon preparation dramatically improved the speed of recovery and reduced both metabolic and muscular damage.
Your target should be 1.8 to 2.0 grams of protein per kilogram of body weight (.73 to 1 gram of protein per pound of bodyweight).during intense training.
This may sound like a lot, but if you want to support your training properly it can be easy to fit in high-quality proteins when you focus on the right fuel choices.
Fats: The Overlooked Recovery Nutrient
Of all the macronutrient mistakes Velasquez encounters, the war on dietary fat might be the most damaging.
“I see a lot of runners take their diet to the extreme of being low-fat,” she says. “Cutting fat might help with some body recomposition, but recovery is not going to be as great as it could be.”
Dietary fat supports joint health and transports fat-soluble vitamins (A, D, E, and K) to where they’re needed.
Without fat, most if your fat soluble vitamins, especially vitamin, is basically just being excreted.
You should target 20-35% of total calories from fat, prioritizing fatty fish, avocados, nuts, seeds, and olive oil.
Practical Implementation: Making It Work in Real Life
Theory and numbers are useless without execution.
If you want a do-it-yourself option, check out our free runner’s calorie calculator.
Just input your details and then details of your run or workout for the day and you’ll have a good breakdown of the carbohydrates, proteins and fats you need to support your training.
I highly recommend you consider tracking your macros for a week (temporarily).
In my experience, most runners overestimate how much they’re eating, and even just a week of honest tracking reveals the gap between perceived intake what you actually need.
If you want something more tailored to your exact metabolic profile, unique needs and help and support throughout your training, I highly recommend you check out MetPro.
Amber is just one of the amazing coaches on their team who specialize in working with endurance athletes.
I consider myself pretty educated in nutrition space, but I learned so much and made progress much faster when I worked with them last spring. The data they were able to provide was so insightful and something I still use today.
Regardless of how you approach it, make sure you’re not underfueling this spring training season.
