Why You Bonked Even Though You Fueled

If you’ve ever been cruising through a run and suddenly, out of the blue, you felt the energy drain from your body. Your legs became heavy, your brain foggy, your pace slowed to a crawl; then you experienced the bonk, aka hitting the wall.

What’s especially frustrating is that despite eating some gels, drinking a sports drink, and a few chews for good measure, you still fell apart. Does this sound familiar? If so, you are not alone. Let me explain.

Bonking isn’t just about whether you fueled, it’s about the how, when, and what you fueled with, plus how you’ve been eating and training in the days and weeks leading up to the run. Let’s break down the key reasons you may bonk even when you “did everything right,” and how to set yourself up for success in the future.

You didn’t follow a fueling strategy.

Taking gels and sips of sports drink on race day is not the same as having a strategic fueling plan. And you can’t assume that what worked in training will suffice on race day. Training runs are more forgiving—you can stop at intersections, take bathroom breaks, and pause to refill bottles. In a race, every pause is against the clock, compelling you to limit breaks and to maintain pace.  This shift in sustained race effort affects how your body uses fuel, accelerating the onset of a bonk, especially if you haven’t specifically trained for it.

In addition, many runners tend to grab random gels and chews during race week, even if they’ve never used them in training. Although your gut is trainable, just like your legs and lungs, it needs time to adapt to the amount, type, and timing of fuel at race intensity. Using unfamiliar sports fuel on race day is like going from couch to marathon without training: you may finish, but it won’t be pretty.

The fix:

Start formulating and test-driving your race fuel plan early in the training cycle in all race-paced, medium-distance, and long runs. Dial in how many grams of carbohydrate (carbs) and ounces of fluid per hour you need to perform your best. The goal is to practice your fueling and hydration plan multiple times a week for 6-8 weeks leading up to the race, so it becomes automatic. Remember the adage: “nothing new on race day.”

You waited too long to fuel

Your body can’t instantly use what you eat and drink. It takes time for fuel to empty your stomach, be digested and absorbed, and reach your muscles. If you only fuel based on how you feel, you would never take anything early in a race, yet that is exactly when you need to start. As your body tires, so does your gastrointestinal (GI) system, slowing digestion. To make matters worse, dehydration, which is common later in a race, further impairs this process. So, if you wait until you feel tired, foggy, or lightheaded, you are already behind.

The fix:

Start fueling early and often. For runs longer than ~60-75 minutes, begin fueling within the first 15-20 minutes and continue every 10-20 minutes, regardless of whether you feel you need it. You do.

Your fuel choice wasn’t sport-approved

Calories are not all equal when you’re running. During moderate to high intensity efforts (like racing), easy-to-digest carbs are your muscles and brain’s preferred energy source. If your fuel contains fat, protein, or fiber, you’re not getting the quick energy you need, even if the total calories are adequate. Poor race-fuel choices include snacks with nut butter, trail mix, and most sports or protein bars.

The fix:

Prioritize simple carbohydrates. Look for maltodextrin and fructose on the label. Common choices include:

Gels and chews designed for endurance sports
Properly formulated sport drinks that provide carbs and electrolytes, especially sodium.

You started with a poorly timed or planned pre-race meal.

When and what you eat matters. Foods high in fat, fiber, and protein slow gastric emptying and delay glucose delivery to your working muscles. If you eat a large, greasy, high-fiber meal too close to the start, it’s likely still in your stomach when you begin running – often leading to nausea, vomiting, cramps, bloating, and abdominal pain.

The fix:

Eat your pre-race meal 2-4 hours before the start.

Ideal meal composition:

High in easy-to-digest carbs (white rice, white bread/bagel, low fiber cereal, oatmeal, pancakes/waffles- light on the butter);
Moderate protein (eggs/egg whites, fat-free yogurt, small portions of lean meat)
Low-fat and low fiber (limit cheese, large portions of nut butter, bacon, sausage, hash browns, bran, and veggies).

Fueling guidelines before a run:

• 1 hour before start – consume up to 1g carb per kilogram (kg) body weight
• 2 hours before start – consume up to 2g carb per kg
• 3 hours before start – consume up to 3g carb per kg*
• 4 hours before the start – consume up to 4g per kg*

*3-4 hours before a run: including some protein and a small amount of fat in your meal may be appropriate, but always experiment during training so that on race day, your pre-race meal is familiar and well-established.

2.2 lbs = 1kg

You were already in an energy deficit when you started.

Even textbook-perfect race fueling can’t make up for starting with low glycogen stores and an energy deficit. Runners who buy into the “thinner is faster” mindset, restrict carbs, and underfuel day after day are at high risk for chronic low energy availability. Intentional under-eating is dangerous and not to be taken lightly.  If your goal is to improve fitness and performance, consistently fueling your body during and after training is non-negotiable.

Common signs of chronic underfueling include:

• Slower recovery
• Reduced training adaptation and stalled progress
• Feeling tired even on rest days,
• Increase in mood swings and poor sleep
• Energy crashes mid-run, especially in longer sessions
• Frequent illness or injury
• Increased hunger and carb cravings during and after training

The fix:

Your priority is to eat enough to maintain energy balance. Many stress fractures and overuse injuries stem from chronic low-energy availability, not overtraining. A practical way to gauge energy balance is by monitoring body weight. Ongoing weight loss indicates negative energy balance, while gradual weight gain suggests a surplus.

Carbs are your friend, especially around training. They are to your body what gasoline is to a car. The body prefers carbs for fuel, but if fuel stores are insufficient, it turns to fat and muscle at a slower rate, which slows your pace. Breaking down muscle for energy due to inadequate fueling is reckless at best – nothing good comes from it. And, in the end, it’s a lose-lose proposition for both performance and health.

Key practices:

• Prioritize carbs during runs lasting longer than 60 minutes
• Prioritize a recovery snack rich in simple carbs and moderate protein within 60 minutes of finishing to replenish glycogen and support muscle repair.
• Aim for 1.0-1.2g carbs/kg and 0.4g protein/kg body weight.

Example: A 150 lb. (68kg) runner:

• Carbs: 68-82g
• Protein: 27-30g

You started too fast.

Just because you can doesn’t mean you should go out harder than your training and fitness support. Starting too fast can prematurely drain muscle glycogen, accelerate early fatigue, and leave you struggling to meet energy demands later in the race.

The fix:

If you haven’t successfully executed an aggressive start in race-simulated training sessions, the safer strategy is to start at a controlled, moderate effort and gradually build as the race progresses.

How to fuel for a race

Before the race

• 2-4 hours before:  2-4g carb/kg – (rice, oats, pasta, bread)
• 60–90 minutes before:  1-1.5g carb/kg – (sports drink, banana, white bread, rice cakes, saltine crackers, with jelly or honey as topping).
• 15-30 minutes before: ~20-30g simple carbs (gel, chews, sport drink), chase with water

Fueling during the race:

• Up to 2.5 hours – aim for 30-60g carbs per hour
• 2.5 hours or longer – aim for 60-90g carbs per hour.

Fueling strategy options:

• Option 1: Sip a sports drink from aid stations or from a hydration belt consistently throughout each hour to meet both fueling and hydration needs.
• Option 2: Take ~20-30g carbs (gel or chews), starting early and repeating every 20-30 minutes throughout.  Drink water (with electrolytes) at each aid station to hydrate and chase sport fuel.
• Option 3:  Use both a sports drink and gels or chews to boost carb intake and meet energy demands.

Fluid needs vary widely depending on individual sweat rate, conditions, and intensity. If fluid needs are low, lean on fuel (gels/chews) to meet carb targets. If the sweat rate is high, a sport drink may be adequate for both fluid and fuel needs.

A successful race-day fuel plan results from repeated testing, tweaking, and refining. Use your training to dial in what, when, and how you fuel both day to day and in training. If nothing else, remember these non-negotiables: nothing new on race day; start fueling and hydrating from the start; and stick to familiar low-fiber, low-fat foods before and during the race.