Hello Runner,
Welcome back to your weekly moment of endurance nutrition!
Happy Weekend!
How are you?
I'm fine!
My week was hectic.
I checked off all my workouts and everything else I had to do. What a wonderful feeling, hehehe
There's less than a week to go until the Barcelona marathon, and I'm already excited. Even though I'm not running! But I love it because the route passes in front of the building where I live. So I can see everything from my balcony. Then I always go down to the finish line to cheer on the runners.
Today's topic is controversial. I'm going to talk about low-carb and ketogenic diets. According to strong scientific evidence, are they a good option for runners?
The low-carb and ketogenic movement has swept through the running world with impressive force, promising everything from sustained fat-burning energy to sharper mental clarity and leaner race-day bodies. Social media feeds are full of it. Maybe some of your training partners swear by it. And honestly, the idea is appealing: fewer carbs, fewer energy crashes, more fat burned. Sounds like a win, right?
But here's what the science, across decades of rigorous endurance sport research, keeps telling us: for most runners, cutting carbohydrates significantly doesn't just fail to improve performance. It can actively work against it. And in some cases, the risks extend well beyond the track. Your gut, your immune system, and even your ability to think clearly can all take a hit.
Let me break it all down with the evidence from the world's leading sports nutrition bodies, including the American College of Sports Medicine (ACSM), the International Society of Sports Nutrition (ISSN), and the International Olympic Committee (IOC). Let's talk fuel.
Three Diets, Three Different Realities
Before we talk performance, let's get clear on what we actually mean by each approach.
Ketogenic Diet
Carbs: <50g/day (~5-10%)
Extremely high in fat (70–80% of calories), very low in carbohydrates. Forces the body into ketosis (a metabolic state where fat-derived ketones replace glucose as the primary fuel source). Originally developed as a therapeutic diet for epilepsy.
→ For runners: profound limitations at moderate-to-high intensities.
Low-Carb
Carbs: 50-150g/day (~15-25%)
Reduces carbohydrate intake significantly below conventional recommendations but doesn't always reach full ketosis. Often adopted for weight loss or blood sugar management. Protein and fat fill the remaining caloric gap.
→ For runners: performance trade-offs, especially for high-intensity work.
Carb-Adequate Diet
Carbs: 5-12g/Kg/day (45-65%)
Carbohydrate intake calibrated to training load, per ACSM and IOC guidelines. Supports glycolytic energy systems, preserves muscle glycogen, and fuels both training quality and recovery, the approach has been consistently validated in endurance research.
✓ For runners: performance-optimized. The evidence-backed standard.
The IOC's 2016 Consensus Statement on Relative Energy Deficiency in Sport (REDs) and the ISSN's 2021 position stand both highlight that carbohydrate availability is a primary determinant of endurance performance. This isn't a preference, it's physiology.
What Happens to Your Performance on Each Diet?
Let's walk through what happens in your body when you head out the door under each nutritional approach.
On a keto or very low-carb diet, your body shifts away from glucose and relies heavily on fat oxidation and ketones. Fat is an abundant fuel source, which is great news for very low-intensity activity. But running economy research tells a different story at threshold pace and above. Carbohydrate is the only fuel that can be metabolized fast enough to power high-intensity muscle contractions. Without it, your pace suffers, your perceived effort climbs, and your top-end speed dissolves. A landmark 2017 study by Burke et al. (published in the Journal of Physiology) confirmed that elite race walkers on a ketogenic diet showed significantly impaired performance at race pace intensities (even after full keto-adaptation) compared to peers on high-carb diets.
On a carb-adequate diet, your muscles have access to glycogen, stored glucose in the muscle and liver, which is rapidly mobilized during tempo runs, interval sessions, and race efforts. The ACSM's position stand on nutrition and athletic performance is explicit: glycogen depletion is directly associated with fatigue, and carbohydrate intake before, during, and after endurance exercise is one of the most well-supported ergogenic strategies in sport science.
What the Research Confirms
The ISSN (2021) states that for endurance athletes, carbohydrates remain the dominant substrate during moderate-to-high intensity exercise, and that fat-adaptation strategies have not been shown to provide performance benefits beyond what carbohydrate periodization can achieve, while carrying greater risks.
The IOC's 2018 Dietary Supplement consensus further supports periodized carbohydrate availability as a key training strategy, particularly for athletes training more than 5 hours per week, which includes most recreational runners.
This doesn't mean fat plays no role. But the key is understanding when and how each fuel is used, not eliminating one.
The Hidden Costs of Going Too Low
Beyond performance, significantly restricting carbohydrates carries real physiological risks that are often underreported in wellness circles. Here's what the evidence shows:
Gut Health & Microbiome Disruption
Dietary fiber, found almost exclusively in carbohydrate-rich foods like vegetables, legumes, fruits, and whole grains, is the primary fuel source for beneficial gut bacteria. Research shows that low-carb diets reduce gut microbial diversity, lower short-chain fatty acid production, and can compromise intestinal barrier integrity over time.
Immune System Suppression
Carbohydrates consumed during and after intense exercise reduce cortisol and inflammatory cytokine release (key markers of immune stress). The ACSM notes that training in low glycogen states (common on low-carb diets) increases the risk of upper respiratory tract infections. For runners doing consistent training blocks, immune resilience is non-negotiable.
Brain Function & Cognitive Performance
The brain is a glucose-preferring organ. While ketones can substitute during adaptation, research shows that sustained carbohydrate restriction can impair reaction time, decision-making, and mood regulation, particularly during high training loads. For runners managing work, family, and training, cognitive sharpness matters every day, not just on race day.
Bone Health & Hormonal Impact
Low carbohydrate availability is a hallmark of Relative Energy Deficiency in Sport (REDs), recognized by the IOC as a serious clinical concern. Chronically low energy and carbohydrate availability (especially in female runners) is associated with disrupted menstrual function, reduced bone mineral density, and increased stress fracture risk.
Muscle Protein Breakdown
When glycogen stores are chronically depleted, the body increasingly turns to muscle protein for gluconeogenesis (making new glucose). This undermines the lean muscle mass most runners work so hard to maintain, and can compromise running economy over time. The ISSN explicitly cautions against carbohydrate restriction in athletes seeking to maintain or build lean mass.
Sleep & Recovery Quality
Carbohydrates support serotonin and melatonin production, hormones central to sleep quality. Evidence suggests that athletes on very low-carb diets report poorer sleep quality and longer recovery times between sessions. For runners adapting to a training load, recovery IS training. Compromising it stalls progress.
Fat It's a Partner
Here's something worth saying clearly: a well-fueled runner's diet absolutely includes fat in meaningful amounts.
The goal isn't to eliminate dietary fat; it's to understand its role and pair it appropriately with carbohydrate availability.
The ACSM recommends that endurance athletes consume 20–35% of total calories from fat, with an emphasis on quality sources. Fat supports a range of critical functions for runners:
Cardiovascular Health
Omega-3 (EPA/DHA) reduces systemic inflammation and supports heart function, vital for high-volume training.
Hormone Production
Dietary fat is essential for estrogen, testosterone, and cortisol synthesis, hormones that regulate adaptation and recovery.
Fat-Soluble Vitamins
Vitamins A, D, E, and K require fat for absorption. Vitamin D in particular is critical for bone health and immune function in runners.
Low-Intensity Fuel
During easy and recovery runs (Zone 1–2), fat oxidation contributes significantly to energy production; there is no need to exclude it.
Brain & Neural Function
The brain is 60% fat. DHA is particularly important for cognitive performance, mood stability, and neuroprotection.
Satiety & Food Enjoyment
Fat slows gastric emptying and contributes to meal satisfaction, supporting a positive, sustainable relationship with food.
The best sources? Avocados, olive oil, fatty fish (salmon, sardines, mackerel, herring, anchovies, trout), nuts, seeds, and eggs.
→ These provide the quality fats your body needs without displacing the carbohydrates your muscles depend on.
Gif by miamifruit on Giphy
There is no credible evidence that low-carb or ketogenic diets improve performance in runners training at moderate-to-high intensities.
Carbohydrate periodization, adjusting intake based on training load, is the evidence-based standard for improving endurance, body composition, and long-term athlete health.
Product of the Week
This week's Product of the Week is the BAIA PROBIOTIC DREAMER.
This dietary supplement is very interesting because it works on the gut-brain-sleep axis.
It acts on the microbiota that helps regulate the sleep cycle.
The formula contains probiotics, fiber, melatonin, magnesium, and ashwagandha to relax the body and mind.
I thought it was VERY clever.
Track of the Week 🎧️
This week's Track of the Week is another slightly older song that I love. It's really exciting. Great for runs in zones 4 and 5 or for finishing a long race.
I’ve chosen Bullit by Watermat
Any questions or something you would like to share, drop me an email
Wishing you a great weekend and week ahead!
Here’s to health and good runs⚡️
Ana Paula Alonso