The Hidden Truth: Why is Carb Loading Bad for Some Athletes?

October 28, 2025 •

4 min read

Studies have shown that while carb loading can boost endurance performance by 2-3% for activities over 90 minutes, it is not a universally beneficial strategy and carries several risks. This raises an important question for many: why is carb loading bad, or at least inadvisable, for a significant portion of the athletic community?

Quick Summary

Carb loading, when done improperly or for unsuitable events, can lead to negative side effects like gastrointestinal distress, temporary weight gain, and insulin spikes. The strategy is only recommended for intense endurance activities exceeding 90 minutes, and alternatives exist for other athletes.

Key Points

Digestive Risks: Improper carb loading often involves excessive fiber and fat, causing bloating, gas, and cramps, which are harmful on race day.
Unintended Weight Gain: The strategy causes temporary water retention, leading to a few pounds of weight gain that can make some athletes feel sluggish.
Not for Short Events: Carb loading is generally unnecessary and potentially counterproductive for activities lasting less than 90 minutes, such as 5ks or weightlifting.
Health Concerns: Individuals with conditions like diabetes or insulin resistance face significant health risks from the blood sugar spikes caused by excessive carbohydrates.
Better Alternatives Exist: For most athletes, a balanced daily diet combined with strategic, easy-to-digest carbs during exercise is a safer and more effective fueling strategy.
The Importance of Practicing: Any carb-loading plan, even for appropriate events, should be practiced during training to ensure the athlete's body tolerates it well.

The Core Risks of Misguided Carb Loading

For decades, carb loading has been a nutritional rite of passage for endurance athletes. The principle is simple: maximize muscle glycogen stores before a long event to delay fatigue. However, modern sports nutrition has a more nuanced perspective, highlighting the potential pitfalls when this method is misapplied. The risks largely stem from improper execution, unsuitable scenarios, and neglecting individual physiology.

Gastrointestinal Distress and Discomfort

One of the most immediate and common negative side effects of carb loading is stomach upset. This often happens because many high-carb foods are also rich in fiber, and suddenly increasing this intake can overwhelm the digestive system. The discomfort can manifest as bloating, gas, stomach cramps, or even diarrhea—all of which are highly undesirable on race day.

To combat this, nutritionists recommend switching to lower-fiber, refined carbohydrate sources in the final days before an event, such as white bread, plain pasta, and refined cereals. This reduces the bulk in the digestive tract, minimizing the risk of a mid-race emergency. Unfortunately, many athletes new to the practice consume large, fibrous meals, leading to unnecessary suffering.

Unintended Weight Gain

Effective carb loading results in temporary weight gain, which can be mentally or physically challenging for some athletes. For every gram of glycogen the body stores, it also stores approximately 3 grams of water. This can result in a short-term gain of 2-4 lbs. For a competitive athlete focused on feeling light and fast, this sudden bulkiness can feel disconcerting, even if it's a sign that the loading is working correctly.

Moreover, if carb loading is undertaken by an athlete not performing long-duration, high-intensity exercise, the caloric surplus can lead to actual, unwanted fat gain. Recreational athletes often fall into this trap, mistaking the strategy as an excuse to gorge on pasta, which is completely counterproductive to their fitness goals.

Increased Risk for Specific Health Conditions

For individuals with pre-existing conditions, carb loading can be genuinely harmful. Athletes with diabetes or insulin resistance, for example, must approach increased carbohydrate intake with extreme caution. A sudden, large increase in carbohydrates can cause a significant spike in blood sugar levels, which is dangerous for those with impaired glucose regulation. Consulting a physician or sports dietitian is crucial for these individuals before attempting any carb-loading regimen.

Who Should Avoid Carb Loading?

Carb loading is not a one-size-fits-all strategy. It is specifically designed for endurance events lasting over 90 minutes, such as marathons, triathlons, and long-distance cycling. Athletes engaging in shorter or less intense activities should avoid it.

Here is a list of individuals and scenarios where carb loading is likely inappropriate:

Casual exercisers: Those who train for general health and fitness, rather than preparing for a specific endurance event.
Short-distance runners: Competing in events under 90 minutes, such as 5k or 10k races, does not deplete glycogen stores enough to warrant a loading phase.
Strength and power athletes: Activities like weightlifting or sprinting rely on different energy systems, making carb loading largely unnecessary and potentially detrimental.
Team sport athletes: While some team sports involve extended play, the stop-and-go nature means high-volume glycogen depletion is less likely than in continuous endurance events.

Modern Alternatives and Strategic Fueling

Instead of a risky, blanket carb-loading approach, many modern sports nutrition strategies emphasize consistent, daily fueling and strategic carbohydrate intake during exercise. For events lasting between 60 and 90 minutes, simply fueling properly during the race with sports gels or drinks can suffice. The goal is to provide a steady supply of energy rather than relying on maxed-out stores.

Comparison of Carb-Loading Methods


Aspect	Improper Carb Loading (Risky Approach)	Proper Carb Loading (For Endurance)	Strategic Fueling (For Shorter Events)
Application	Used by casual athletes or for short-duration events.	Used by endurance athletes (90+ min events).	Used for events under 90 minutes.
Food Choices	Consuming high-fiber and high-fat carbs; often junk food.	Consuming low-fiber, high-carb sources like refined grains.	Normal balanced diet, focusing on simple carbs pre-event.
Risks	High risk of bloating, cramps, diarrhea, and fat gain.	Low risk of digestive issues if done correctly.	Minimal risks; focuses on maintaining normal function.
Timing	Often a week-long, unstructured feast.	Tapering exercise and increasing carbs for 1-3 days before the event.	Small, high-carb snack 1-4 hours before the race.
Outcome	Potential for GI distress and hindered performance.	Optimized muscle glycogen for delayed fatigue.	Sustained energy without digestive disruption.

Conclusion: Fuel Your Body Wisely

So, why is carb loading bad? It's not inherently bad, but a flawed understanding of its purpose and application is. It is a highly specific tool for elite-level endurance athletes, not a magic bullet for all sports. For most people, the risks of digestive issues, temporary weight gain, and blood sugar fluctuations far outweigh any potential performance gains for shorter activities. By understanding who benefits and who doesn't, and focusing on consistent, smart fueling, athletes can avoid the pitfalls and optimize their performance safely. Proper sports nutrition is always an individual journey that requires careful planning, not a last-minute dietary binge.

For more detailed nutritional guidance from experts, consider visiting resources like the reputable sports nutrition blog on TrainingPeaks.

Frequently Asked Questions

Yes, improper carb loading can cause significant stomach problems, including bloating, gas, cramps, and diarrhea, especially if you consume too much fiber or fat in the days leading up to an event.

Carb loading typically causes a temporary weight gain of a few pounds due to water retention, as the body stores water alongside glycogen. For athletes not engaging in intense endurance exercise, this can turn into unwanted fat gain.

Individuals engaging in activities shorter than 90 minutes (like sprints or weightlifting), casual exercisers, and those with certain health conditions like diabetes should avoid carb loading.

If done incorrectly, you risk digestive distress, feeling heavy and sluggish on race day, and even hindering your performance instead of improving it. It can also lead to unintended fat gain if exercise is insufficient.

For most athletes, maintaining a consistently balanced diet and strategically consuming carbohydrates during their event via sports drinks, gels, or chews is a safer and more effective fueling method.

No, carb loading is not necessary for a 10k race, as it typically takes less than 90 minutes to complete. Your body's existing glycogen stores are usually sufficient for this duration.

Bloating during carb loading is often caused by a combination of increased water retention associated with glycogen storage and the high fiber content of some carbohydrate-rich foods that can lead to gas and digestive discomfort.