Sports
Why Undereating Sabotages Your Running, with Dani Jodar
The sports dietitian explains the risks of severe food restriction, weight loss medications, and gut health.
Training hard while in a severe calorie deficit compromises performance, increases injury risk, and impairs metabolism. Sports dietitian Dani Jodar explains why carbohydrates and gut health are essential pillars for runners.
The pursuit of progress in running often runs into a silent trap: trying to balance intense training with overly restrictive diets. Sports dietitian Dani Jodar explains that the fuel provided by food is the foundation not only for performance on the road, but also for long-term physical and hormonal health.
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Performance versus calorie deficit: doing the math
One of the main challenges seen in clinical practice is the tension between wanting to lose weight and aiming for peak running performance. While performance requires an optimal intake of energy substrate to meet high expenditures and facilitate cellular recovery, weight loss depends on a calorie deficit. When restriction is too severe, the body struggles and cannot sustain heavy training volumes.
According to Dani Jodar, many amateur athletes attempt to cut portions or delay mid-run fueling during long runs out of fear of gaining weight, ultimately leading to pacing crashes, stagnation, and even stress fractures. When nutrient intake is planned properly around training demands, the body uses energy with peak efficiency without leading to unwanted fat gain.
The irreplaceable role of carbohydrates
Despite recurring low-carb diet trends and food fads, carbohydrates remain the primary and most efficient source of quick energy for endurance runners. Fats and proteins digest significantly slower, making them unable to provide the immediate glucose required by muscle contractions at moderate and high intensities.
Strategic intake of complex carbohydrates in daily meals—such as root vegetables, rice, and fruit—alongside easily digestible supplements during workouts (like energy gels) ensures muscle glycogen stores remain replenished. Fasted running or training without adequate carb support raises perceived exertion and impairs metabolic signaling.
Weight loss medications, muscle loss, and the risk of RED-S
Another critical warning from the specialist involves the unsupervised use of weight loss medications by individuals engaging in high-impact physical training. By suppressing appetite and slowing gastric emptying, these drugs often result in drastically insufficient calorie and protein intake.
The direct consequence of this lack of fuel includes accelerated muscle loss, weakened immunity, menstrual irregularities, and hormonal disruption. This state of chronic low energy availability is known as RED-S (Relative Energy Deficiency in Sport), whose metabolic and bone health impacts can persist for years even after weight is restored.
Gut training and gastrointestinal distress on the run
Gastrointestinal symptoms like bloating, acid reflux, and runner's diarrhea are common complaints among road runners, but they should not be normalized. The mechanical motion of running accelerates digestive transit, making it essential to train the gut progressively to tolerate race-day nutrition.
Dani Jodar points out that foods high in fermentable compounds (FODMAPs)—such as onions, garlic, beans, and certain raw greens—can exacerbate gastric distress when eaten close to long runs. The best strategy is to test fueling options during training, stay properly hydrated, and choose carbohydrate sources that individual runners tolerate best.
Making peace with food to build lasting consistency
The key to a sustainable athletic lifestyle lies in demystifying food and ditching guilt around eating. Viewing nutrition as a tool for health, recovery, and performance frees runners from chasing unrealistic aesthetic ideals and ensures long-term training consistency.
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