Fasted Training, Fat Oxidation, and Low-Carb, High-Fat Diets

Welcome to the Uphill Athlete Podcast! Today, we’re diving into some hot topics in the endurance world—specifically, fat oxidation, fasted training, and low-carb high-fat (LCHF) diets. I’m Alyssa Clark, your host, joined by registered dietitian Alyssa Lieb. Alyssa, thank you for being here!

Thanks for having me, Alyssa! These are big topics in mountain sports and definitely have a long history with Uphill Athlete, so I’m excited to provide some clarity.

Let’s start by touching on Uphill Athlete’s previous stance on fasted training. Historically, we supported fasted training, especially for activities like mountaineering and trail running. But recent research shows that it doesn’t offer the benefits we once thought. In fact, fasted training can be harmful to both training and overall health. We want you to be athletes well into your later years, so we’re officially moving away from recommending fasted training.

Alyssa, could you break down fat oxidation and explain why it’s such a big focus for endurance athletes?

Absolutely! Fat oxidation is the body’s process of breaking down fat for energy, which is particularly relevant for endurance activities. While our bodies burn both fat and carbs, the proportion shifts based on exercise intensity. During lower-intensity, long-duration activities, we burn more fat. For ultra-endurance sports, improving fat oxidation is appealing because it helps sustain energy for long efforts, like multi-hour runs or climbs.

So fat adaptation sounds useful for ultra-endurance. But can you explain metabolic flexibility and its role in endurance sports?

Sure. Metabolic flexibility is the body’s ability to switch between burning fat and carbs based on energy needs. For example, in mountain sports, you might need a sudden energy boost to navigate a difficult section. Fasted training, or aerobic fat metabolism, alone won’t support that. Instead, training your body to efficiently shift between fat and carbs is the goal.

What common mistakes do you see among athletes trying to boost fat oxidation?

One of the biggest mistakes is assuming that a low-carb, high-fat (LCHF) diet or fasted training is the only way to increase fat utilization. Athletes need carbs for high-intensity efforts. Without enough carbs, you risk low energy availability, poor recovery, and injury. Another issue is intermittent fasting, which many assume promotes fat oxidation but often leads to inadequate recovery. Remember, consistent aerobic training—without restrictive diets—is enough to improve fat oxidation.

Can you explain low energy availability (LEA) and how it relates to fasted training?

LEA happens when you aren’t eating enough for your energy output, a common result of fasted training. Skipping meals before and after training can lead to chronic deficits, impacting recovery and performance. Fasted training combined with intermittent fasting can worsen LEA, increasing risks of low bone density, menstrual dysfunction, immune issues, and mental health impacts. These risks are even higher for athletes practicing fasted endurance training or restrictive diets.

How important are carbs in endurance nutrition strategies?

Extremely important! Carbs are essential for both performance and recovery, especially in ultra-endurance sports. When you’re pushing the limits of endurance, your body needs easily accessible energy, which carbs provide. High-performing athletes consistently fuel with carbs and set course records because of it. In contrast, a low-carb high-fat diet can restrict this performance edge.

What do you say to athletes who believe fasted training works well for them?

Anecdotal experiences are valid, but they don’t capture the full picture. For instance, while someone may feel okay on a fasted training regimen, they could be missing out on better results with proper fueling. In fact, we’re seeing athletes who prioritize carbs break records and improve performance consistently. For those in endurance sports, balanced fueling with carbs is the new standard.

Could you explain what a low-carb high-fat (LCHF) diet is and why someone might follow it?

The LCHF diet limits carbs and emphasizes fat intake. While it can increase fat oxidation, studies show limited endurance performance gains. Athletes on LCHF diets can struggle with high-intensity efforts due to reduced carb availability. Studies also show increased health risks at high altitudes, which is a concern for mountain athletes. For ultra-endurance, aerobic training alone is enough to enhance fat metabolism without the downsides of an LCHF diet.

What are your top nutrition recommendations for athletes focusing on fat oxidation, fasted training, and endurance?

For fat adaptation, I recommend consistent aerobic training, which naturally improves fat utilization. Fueling for endurance performance means consuming adequate carbs daily and during workouts. Aim for up to 80–90 grams of carbs per hour in long sessions to support energy and recovery. Finally, avoid training in calorie deficits, as nutrition for weight loss and performance don’t align. Prioritize rest, balanced meals, and stress management over hacks like fasted training.

Thank you, Alyssa! This discussion reinforces that performance isn’t about shortcuts or restrictive diets. Proper fueling, consistent training, and focusing on the basics are what truly support long-term success in endurance sports.

Absolutely. Fasted training and low-carb diets may seem appealing, but the basics—carb intake, balanced meals, and proper recovery—are the real keys to sustained performance.

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