WHY ENDURANCE ATHLETES SHOULD AVOID FAD DIETS

By Jennifer A. Kurtz, PhD, CSCS, CISSN, EP-C

As endurance athletes, our bodies are finely tuned machines that require proper fuel to perform at their best. One of the mistakes that I see some athletes make is that in the quest for peak performance or ideal body composition, they are tempted to try fad diets promising quick weight loss. While the promises of the quick weight loss and other health benefits promised by these diets can be tempting, they pose risks due to their restrictive or imbalanced approaches. For athletes, in particular, they may fail to provide adequate energy or nutrients to support training and recovery. Therefore, rather than these diets being a shortcut to lower weight and faster times, they can significantly hinder performance, recovery, and overall health, due to one of more of the following shortcomings, all of which are backed by recent research: 

Fad Diets Often Lead to Nutrient Deficiencies

Many fad diets drastically restrict certain food groups or macronutrients, leading to imbalances and nutrient deficiencies. For endurance athletes, this is particularly concerning. 

For example, the low carbohydrate availability often seen in the ketogenic diet not only impairs endurance performance, but also significantly increases the perception of effort during exercise (Harvey et al., 2019; Medica, 2018). Carbohydrates serve as the primary energy source for high-intensity and prolonged endurance activities, via glycogen stores in the muscles and liver (Mata et al., 2019). When carbohydrate intake is insufficient, glycogen levels become depleted, forcing the body to rely on fat and protein for energy. This metabolic shift is less efficient and leads to an earlier onset of fatigue, reduced exercise capacity, and diminished athletic performance (Harvey et al., 2019). Furthermore, low carbohydrate availability has been shown to compromise immune function, delay recovery, and increase the risk of overtraining, highlighting the critical role that carbohydrates play in sustaining both performance and overall athlete health during demanding endurance activities.

Beyond carbohydrates, fad diets often lack essential vitamins and minerals for recovery and performance. For example, inadequate calcium and vitamin D intake can weaken bone density and integrity, increasing the risk of stress fractures and other skeletal injuries, particularly in endurance athletes who experience repetitive mechanical loading. This issue is common in diets that eliminate or severely restrict dairy products, such as paleo or vegan diets, without proper supplementation or fortified alternatives. Athletes following these diets must ensure adequate calcium and vitamin D intake through fortified plant-based milks, leafy greens, or supplements to maintain bone health and prevent injury (Tenforde et al., 2013).

Rapid Weight Loss Can Reduce Muscle Mass

Many diets that promise quick weight loss tend to do so at the expense of lean muscle mass. Given that muscle is essential for maintaining strength and efficiency during endurance activities, as well as being crucial to long-term athletic performance (Kaufman et al., 2023), athletes who follow such diets might experience decreased capacity, as their bodies lack the necessary nutrients to maintain lean mass. While caloric restriction is sometimes necessary for weight loss, athletes need to balance this with adequate protein intake to preserve muscle mass and support recovery (Cava et al., 2017). This, combined with a balanced, nutrient-dense diet ensures muscle preservation, especially during periods of caloric restriction. 

Negative Impacts on Energy Levels and Recovery

Endurance training demands a consistent and adequate energy supply. Fad diets—especially those that severely restrict calories—often fail to provide the necessary energy, leaving athletes feeling fatigued, and compromising their recovery. Low energy availability (LEA) is a significant concern for athletes, as it can negatively impact both performance and long-term health. LEA occurs when an athlete’s energy intake fails to meet the demands of exercise, leading to a caloric deficit that impairs recovery, increases the risk of injury, and reduces endurance. Athletes in this state often experience slower recovery times and diminished performance due to insufficient energy for tissue repair and muscle maintenance (Loucks, 2004). Fad diets, which often promote extreme caloric restriction or eliminating entire food groups, can lead to LEA by failing to provide the necessary energy intake for athletes to meet the physical demands of their training.

In addition to these physical impairments, LEA disrupts hormonal balance, with serious consequences for both male and female athletes. In men, LEA can lead to decreased testosterone levels, which affects muscle mass, strength, and recovery (Logue et al., 2018; Logue et al., 2020). For female athletes, LEA is associated with menstrual irregularities, including amenorrhea or oligomenorrhea, which can result in low bone mineral density, increasing the risk of stress fractures and osteoporosis (Logue et al., 2020; Skarakis et al., 2021). These hormonal disruptions not only affect athletic performance, but also compromise long-term bone health.

Furthermore, chronic LEA has been linked to other serious health issues, such as impaired immune function and delayed recovery (Jeppesen et al., 2024; McKay et al., 2022). Female athletes with persistent LEA are also at a higher risk of developing osteopenia or osteoporosis, which can further compromise their athletic capabilities and overall well-being (Tenforde et al., 2013). Addressing LEA through proper nutrition and energy balance is crucial for supporting both performance and long-term health in athletes.

Fad Diets Can Increase Risk of Gastrointestinal Issues

Diets high in fat or low in fiber, common in many weight-loss fads, can significantly impact gastrointestinal (GI) health, particularly in endurance athletes. The consumption of high-fat foods, often found in fad diets like the ketogenic diet, can exacerbate gastrointestinal distress, leading to symptoms such as cramping, bloating, and diarrhea during training or competition (Devrim-Lanpir et al., 2021). High-fat diets can also increase gut permeability and promote inflammation, which can disrupt normal digestive processes (Ribichini et al., 2023). This is particularly concerning for endurance athletes, as gastrointestinal comfort is crucial for performance during long events. If the gut is inflamed or permeable, athletes may experience disrupted nutrient absorption, leading to reduced energy availability and performance declines (O’Brien et al., 2022).

In addition to the effects of high-fat diets, low-fiber diets—often promoted in weight-loss fads—can lead to further GI issues. Fiber is essential for maintaining healthy gut motility and function, as it supports regular bowel movements and helps prevent constipation. Low-fiber diets can lead to slower digestion and increased discomfort, especially during high-intensity physical activity (Joyner & Coyle, 2008). Furthermore, a lack of dietary fiber can affect the gut microbiome, potentially reducing the diversity of beneficial gut bacteria, which plays a key role in nutrient absorption and overall digestive health (Clark & Mach, 2017; Fu et al., 2022; Miranda-Comas et al., 2022). 

These dietary issues can be particularly problematic for athletes training for endurance events, where energy and fluid intake must be carefully managed. GI distress not only compromises performance, but can also increase the risk of dehydration and nutrient deficiencies, further impairing athletic performance (de Oliveira et al., 2014; Ribichini et al., 2023). Maintaining a balanced diet with adequate fiber, moderate fat intake, and proper hydration is essential for optimizing GI health and endurance, ensuring that athletes can perform at their best during long-duration competitions.

Fad Diets Are Unsustainable (Especially for Athletes)

One of the primary concerns with fad diets is their inability to provide sustainable, long-term solutions for athletes. These diets often impose extreme restrictions on calories or macronutrient intake, and are inherently difficult to maintain long term, due to their unrealistic and unenjoyable nature. Restrictive dieting commonly leads to a cycle of weight loss followed by weight regain, with most individuals gaining back the weight within a year (Blomain et al., 2013; Tomiyama et al., 2013). These fluctuations in body weight and energy availability can also affect metabolic efficiency, which in turn impairs an athlete’s ability to maintain optimal performance levels, especially during long-duration activities where steady fueling is essential. 

Moreover, fad diets focus on rapid weight loss or short-term improvements, which are understandably enticing, but fail to address the specific needs of endurance athletes, who require more unique fueling to support their training and performance. Extreme dietary restrictions can impair endurance performance by limiting essential nutrients like the carbohydrates, fats, and protein necessary for sustained energy and muscle repair (Jeukendrup, 2013). A sustainable, well-rounded nutrition plan that aligns with an athlete's training demands is essential for achieving optimal performance and maintaining overall health.

What are Some Examples of Modern Fad Diets?

Now that we have a basis for the shortcomings of fad diets in general, let’s take a look at some of the most popular fad diets and how they could be detrimental to endurance performance:

Keto Diet (Ketogenic Diet): The ketogenic diet, which is a very low-carbohydrate, high-fat diet aimed at inducing ketosis, can be unsustainable due to its restrictive nature. Common nutrient deficiencies include calcium, potassium, magnesium, and fiber, as the diet limits fruits, vegetables, and grains. Additionally, gastrointestinal issues such as constipation, bloating, and diarrhea are often reported, due to the low fiber intake and sudden shift in macronutrient composition.

Paleo Diet: The paleo diet focuses on eating foods presumed to be available to humans during the Paleolithic era, such as meat, fish, fruits, and vegetables, while avoiding grains, dairy, and processed foods. Despite its focus on whole foods, it can lead to nutrient imbalances or deficiencies in calcium, vitamin D, fiber, B vitamins (particularly B12), magnesium, potassium, and omega-3 fatty acids, due to the exclusion of dairy, grains, legumes, and certain plant-based foods. These nutrients are essential for bone health, digestive function, energy production, and overall well-being.

Cleanses and Detox Diets: Diets that involve consuming juices, teas, or specific foods to “detoxify” the body often lack scientific support and can lead to nutrient deficiencies or energy imbalances.

Intermittent Fasting (when extreme): While some moderate approaches to fasting have evidence-based benefits, extreme forms (e.g., prolonged fasting or eating very few calories during fasting periods) can lead to energy deficits, especially for endurance athletes.

Carnivore Diet: A diet consisting entirely of animal products and excluding plant-based foods can lead to deficiencies in essential vitamins and minerals such as vitamin C, vitamin E, vitamin K1, folate, magnesium, and potassium, as well as a lack of fiber, which is vital for digestive health. Additionally, the carnivore diet misses out on important the polyphenols and antioxidants, such as flavonoids, resveratrol, and anthocyanins, found in plant foods, which help reduce oxidative stress, inflammation, and support overall immune and cardiovascular health.

The Whole30: The Whole30 is a 30-day restrictive diet that eliminates sugar, grains, dairy, legumes, and alcohol. While it emphasizes whole foods, its strict rules can make it challenging to sustain, even in short durations. Additionally, it can lead to the same types of nutrient deficiencies discussed with regard to the paleo diet.

Alkaline Diet: The alkaline diet claims that eating certain foods can alter the body’s pH to promote health. The body naturally regulates its pH, so these claims are not scientifically supported.

Vegan Diet: While a well-planned vegan diet can provide many health benefits, such as lower risks of chronic diseases, it may also lead to nutrient deficiencies if not carefully managed. Key nutrients like vitamin B12, iron, and omega-3 fatty acids can be more difficult to obtain from plant-based sources alone, making supplementation or thoughtful food choices necessary for optimal performance and health.

What Should Endurance Athletes Do Instead?

Focus on Nutrient-Dense Foods: Build your diet around whole grains, lean proteins, healthy fats, fruits, and vegetables to ensure you’re meeting your macronutrient and micronutrient needs, with an additional emphasis on adequate protein intake to support muscle repair and recovery, which is crucial for endurance athletes.

Prioritize Carbohydrates for Energy: Carbs are the cornerstone of endurance fueling. Aim to consume 6-10 g of carbohydrates per kg of body weight daily, as recommended by the International Society of Sports Nutrition.

Adopt a Sustainable Nutritious Approach: Work with a sports dietitian or nutritionist to develop a personalized, sustainable nutrition plan that supports longevity, training, and performance goals.

Maintain Energy Balance: To avoid LEA, ensure that your calorie intake matches your energy expenditure, especially during endurance training. Using tools like food diaries or nutrition apps can help monitor your intake, ensuring you're consuming enough nutrient-dense foods—particularly carbohydrates, protein, and fats—to support your performance and recovery needs.

Conclusion

Fad diets may promise quick results, but for endurance athletes, they can lead to serious performance, recovery, and health consequences. These diets are often unsustainable, and fail to meet the unique needs of endurance athletes, resulting in cycles of restriction and rebound weight gain. Prioritize balanced, evidence-based nutrition strategies to fuel your body for the long haul. Remember, success in endurance sports is not about quick fixes, it’s about consistent effort, both in training and in fueling. Your best performance starts with proper nutrition, so ditch the fads and fuel the right way.

References

Baranauskas, M., Kupčiūnaitė, I., & Stukas, R. (2022). The association between rapid weight loss and body composition in elite combat sports athletes. Healthcare, 

Blomain, E. S., Dirhan, D. A., Valentino, M. A., Kim, G. W., & Waldman, S. A. (2013). Mechanisms of weight regain following weight loss. International Scholarly Research Notices, 2013(1), 210524. 

Cava, E., Yeat, N. C., & Mittendorfer, B. (2017). Preserving healthy muscle during weight loss. Advances in nutrition, 8(3), 511-519. 

Clark, A., & Mach, N. (2017). The crosstalk between the gut microbiota and mitochondria during exercise. Frontiers in physiology, 319. 

de Oliveira, E. P., Burini, R. C., & Jeukendrup, A. (2014). Gastrointestinal complaints during exercise: prevalence, etiology, and nutritional recommendations. Sports medicine, 44, 79-85. 

Devrim-Lanpir, A., Hill, L., & Knechtle, B. (2021). Efficacy of popular diets applied by endurance athletes on sports performance: Beneficial or detrimental? A narrative review. Nutrients, 13(2), 491. 

Fu, J., Zheng, Y., Gao, Y., & Xu, W. (2022). Dietary fiber intake and gut microbiota in human health. Microorganisms, 10(12), 2507. 

Harvey, K. L., Holcomb, L. E., & Kolwicz Jr, S. C. (2019). Ketogenic diets and exercise performance. Nutrients, 11(10), 2296. 

Jeppesen, J. S., Caldwell, H. G., Lossius, L. O., Melin, A. K., Gliemann, L., Bangsbo, J., & Hellsten, Y. (2024). Low energy availability increases immune cell formation of reactive oxygen species and impairs exercise performance in female endurance athletes. Redox biology, 103250. 

Jeukendrup, A. E. (2013). Nutrition for endurance sports: marathon, triathlon, and road cycling. Food, Nutrition and Sports Performance III, 91-99. 

Joyner, M. J., & Coyle, E. F. (2008). Endurance exercise performance: the physiology of champions. The Journal of physiology, 586(1), 35-44. 

Kaufman, M., Nguyen, C., Shetty, M., Oppezzo, M., Barrack, M., & Fredericson, M. (2023). Popular dietary trends’ impact on athletic performance: a critical analysis review. Nutrients, 15(16), 3511. 

Logue, D., Madigan, S. M., Delahunt, E., Heinen, M., Mc Donnell, S.-J., & Corish, C. A. (2018). Low energy availability in athletes: a review of prevalence, dietary patterns, physiological health, and sports performance. Sports medicine, 48, 73-96. 

Logue, D. M., Madigan, S. M., Melin, A., Delahunt, E., Heinen, M., Donnell, S.-J. M., & Corish, C. A. (2020). Low energy availability in athletes 2020: an updated narrative review of prevalence, risk, within-day energy balance, knowledge, and impact on sports performance. Nutrients, 12(3), 835. 

Loucks, A. B. (2004). Energy balance and body composition in sports and exercise. Journal of sports sciences, 22(1), 1-14. 

Martín-Rodríguez, A., Belinchón-deMiguel, P., Rubio-Zarapuz, A., Tornero-Aguilera, J. F., Martínez-Guardado, I., Villanueva-Tobaldo, C. V., & Clemente-Suárez, V. J. (2024). Advances in Understanding the Interplay between Dietary Practices, Body Composition, and Sports Performance in Athletes. Nutrients, 16(4), 571. 

Mata, F., Valenzuela, P. L., Gimenez, J., Tur, C., Ferreria, D., Domínguez, R., Sanchez-Oliver, A. J., & Martínez Sanz, J. M. (2019). Carbohydrate availability and physical performance: Physiological overview and practical recommendations. Nutrients, 11(5), 1084. 

McKay, A. K., Peeling, P., Pyne, D. B., Tee, N., Whitfield, J., Sharma, A. P., Heikura, I. A., & Burke, L. M. (2022). Six days of low carbohydrate, not energy availability, alters the iron and immune response to exercise in elite athletes. Medicine and science in sports and exercise, 54(3), 377-387. 

Medica, E. M. (2018). Low-carbohydrate, ketogenic diet impairs anaerobic exercise performance in exercise-trained women and men: a randomized-sequence crossover trial. The Journal of sports medicine and physical fitness. 

Miranda-Comas, G., Petering, R. C., Zaman, N., & Chang, R. (2022). Implications of the gut microbiome in sports. Sports health, 14(6), 894-898. 

O’Brien, M. T., O’Sullivan, O., Claesson, M. J., & Cotter, P. D. (2022). The athlete gut microbiome and its relevance to health and performance: a review. Sports medicine, 52(Suppl 1), 119-128. 

Ribichini, E., Scalese, G., Cesarini, A., Mocci, C., Pallotta, N., Severi, C., & Corazziari, E. S. (2023). Exercise-Induced Gastrointestinal Symptoms in Endurance Sports: A Review of Pathophysiology, Symptoms, and Nutritional Management. Dietetics, 2(3), 289-307. 

Skarakis, N. S., Mastorakos, G., Georgopoulos, N., & Goulis, D. G. (2021). Energy deficiency, menstrual disorders, and low bone mineral density in female athletes: a systematic review. Hormones, 20, 439-448. 

Tenforde, A. S., Sayres, L. C., McCurdy, M. L., Sainani, K. L., & Fredericson, M. (2013). Identifying sex-specific risk factors for stress fractures in adolescent runners. Medicine & Science in Sports & Exercise, 45(10), 1843-1851. 

Tomiyama, A. J., Ahlstrom, B., & Mann, T. (2013). Long‐term effects of dieting: Is weight loss related to health? Social and Personality Psychology Compass, 7(12), 861-877.