I’ve been fascinated for years by the idea that what we eat can shift not just our weight and health, but how our bodies perform during long, grueling runs. The low-carb ketogenic (keto) approach promises better fat-burning and steady energy — appealing for marathoners and ultra-runners who face hours of sustained effort. But can a low-carb keto strategy actually boost endurance performance for long-distance runners without sacrificing recovery? I dug into the science, tested practical strategies, and spoke with coaches and athletes to bring you a realistic view.

Why runners consider low-carb keto

At its core, keto forces the body to rely more on fat for fuel by severely restricting carbohydrates (usually <50 g/day). For long-distance runners, the appeal is clear:

  • Huge energy stores: Even lean athletes carry far more energy as fat than as glycogen — theoretically providing an almost endless fuel tank.
  • Stable blood sugar: Fewer carb spikes can mean steadier energy and less “bonking” from insulin swings.
  • Metabolic efficiency: After adaptation, some runners report improved ability to use fat at higher intensities.

    • These ideas sound great on paper, but translating them into faster race times and better recovery is complex.

    What the evidence says about performance

    Short answer: it depends on the event intensity and how long you’ve been adapted.

    Research consistently shows that well-trained athletes on ketogenic diets increase fat oxidation dramatically. That means at low-to-moderate intensities, they can sustain effort using fat rather than burning through limited glycogen. However, high-intensity bursts — think surges, hill repeats, or the final kick in a marathon — rely primarily on carbohydrate metabolism. Multiple studies have found that keto-adapted athletes often have reduced peak power, reduced high-intensity speed, or slower times in events with repeated high-intensity efforts.

    For steady-state ultra-distance events done at low intensity (ultras run at easy paces), keto can be workable or even beneficial for some athletes. For marathon or half-marathon runners who need to sustain faster paces or execute speed changes, the reduced glycolytic capacity can be a limiting factor.

    Recovery concerns: can you keep recovery intact?

    Recovery after long runs depends heavily on replenishing glycogen, repairing muscle with amino acids, controlling inflammation, and restoring hydration and electrolytes. Here’s how keto affects each:

  • Glycogen replenishment: On strict keto, glycogen stores are chronically lower. For many training sessions this may be acceptable, but repeated hard sessions without sufficient carbohydrate refeeding can impair performance and adaptation.
  • Muscle repair: Protein needs remain high. Keto diets often include more dietary protein and fat, which can support repair; however, timing of protein intake post-run still matters.
  • Inflammation and immune function: Some athletes report reduced inflammation on keto, but evidence is mixed. Low-carb diets can sometimes elevate stress hormones (cortisol) if energy availability is low, which hurts recovery.
  • Hydration & electrolytes: Keto causes water and sodium losses initially. If not managed, this can cause cramping and poor recovery.

    • So maintaining recovery on keto is possible, but it requires deliberate planning: enough protein, attention to electrolytes, and strategic carbohydrate timing if needed.

    Practical strategies that work for runners

    There’s no one-size-fits-all. Below are practical approaches people use to try to get the best of both worlds.

  • LCHF (low-carb, high-fat) with periodized carbs: Keep daily carbs low during base endurance weeks, then introduce targeted carbs around harder sessions. This can preserve fat adaptation while supporting hard training.
  • Targeted ketogenic diet (TKD): Consume 20–40 g of fast carbs 30–60 minutes before high-intensity workouts to provide immediate glucose without fully disrupting ketosis for the rest of the day.
  • Cyclical ketogenic diet (CKD): Follow keto most days but include one or two higher-carb refeed days per week to replenish glycogen for longer sessions.
  • Protein prioritization: Keep protein at ~1.6–2.2 g/kg body weight to support muscle repair. Brands like Myprotein or Optimum Nutrition make convenient whey products if you need quick post-run protein.
  • Electrolytes & hydration: Use sodium, potassium, magnesium supplements (e.g., Nuun, LMNT) especially during the first weeks of adaptation.

    • How long does adaptation take?

    People often say “three weeks,” but adaptation is progressive. The first few days to two weeks you’ll feel sluggish, lower power, and poor recovery. By 3–6 weeks many markers of fat oxidation improve, but full restoration of performance at higher intensities may take months — and in some athletes, never fully returns to pre-keto levels.

    Practical meal ideas for training and recovery

    Session Pre-run Post-run
    Easy 60–90 min Black coffee or bulletproof coffee (MCT oil) and a small handful of nuts Omelette with spinach, salmon or chicken; avocado; electrolyte drink
    Interval or tempo (higher intensity) 20–40 g fast carbs (banana or sports gel) 30–45 min before Whey protein shake + 30–50 g carbs (rice cake or fruit) if not keto-strict; otherwise protein + starchy veg on refeed days
    Long run (3+ hrs) Small carb snack if doing TKD (rice porridge or sports bar) Protein + carbs to refill glycogen (chicken + sweet potato) on non-strict days; otherwise protein, fat, and electrolytes

    Common questions runners ask

    Will I lose speed on keto? Many runners do experience reduced top-end speed, especially during the first weeks. If your training relies on repeated high-intensity efforts, keto may hinder those sessions unless you use targeted carbs.

    Can I race fast on keto? For low-intensity ultras, yes—some athletes have raced successfully and felt more stable. For marathons or events requiring higher sustained pace or tactical surges, most evidence favors some carbohydrate availability.

    How do I avoid muscle loss? Prioritize protein, ensure adequate total calories, and include resistance work. A leucine-rich protein source post-exercise helps trigger muscle protein synthesis.

    Are supplements necessary? Not required, but helpful. Electrolyte supplements (LMNT, Nuun), MCT oil for quick ketone production, and branched-chain amino acids (BCAAs) for intra-workout use can be useful tools.

    How I approach it with clients and myself

    I rarely recommend an all-or-nothing keto for runners who want both speed and recovery. Instead, I favor a flexible, evidence-based approach:

  • I test targeted carbs before hard sessions to preserve quality training while maintaining overall low-carb habits.
  • I emphasize post-run protein timing and weekly refeed windows to support glycogen and recovery.
  • I monitor performance objectively — pace, power (if using a power meter), and subjective recovery scores — and adjust carbs based on what the athlete needs for their specific event.

    • Ultimately, the right strategy depends on your goals: if you’re chasing ultras at an easy pace or seeking metabolic health benefits, a low-carb keto path can work. If you’re aiming for a fast marathon or frequent high-intensity sessions, a carbohydrate-inclusive, periodized approach is usually the safer bet to protect both performance and recovery.