The Complete Guide to Fasting for Athletes: What 3 Hours with a Sports Nutritionist Taught Me

I'll be honest with you - before sitting down with Dr. Jules Strauss for three hours to discuss fasting, I thought I had a pretty good handle on the topic. I'd experimented with it myself, seen the social media hype, and watched athletes around me try everything from 16:8 protocols to extreme multi-day fasts.

But after three comprehensive podcast episodes with Jules - who has a PhD in exercise metabolism and over 10 years of research experience - I realised how much misinformation is floating around, especially when it comes to endurance athletes.

If you're curious about fasting, considering trying it, or wondering whether it fits with your training goals, this guide distills everything we covered into the essential insights you need to know.

The Evolution Story: Why Your Body Already Knows How to Fast

Here's something that might surprise you: you're already an expert faster. Every night, you go 8-12 hours without food, and your body handles it perfectly. This isn't coincidence - it's evolution.

"We all typically will fast overnight," Jules explained, "and ultimately, that overnight fast will mean that we lean slightly more towards fat metabolism."

Our hunter-gatherer ancestors regularly went 16+ hours between meals, not by choice, but because that's how survival worked. Your body developed incredible backup systems for this - the ability to switch from burning glucose to burning fat, and eventually to produce ketones to fuel your brain.

The problem? Modern life keeps us running on the main fuel tank 24/7. Many folks eat from the moment they wake up until they go to bed, never accessing those backup systems our bodies spent millions of years perfecting.

What Actually Happens When You Fast: The Hour-by-Hour Breakdown

Understanding what happens in your body during fasting is crucial for making informed decisions. Here's the timeline:

0-12 Hours (Your Normal Overnight Fast)​
Insulin drops naturally, and your body starts shifting from burning sugar to burning fat. Most people never extend beyond this point, and that's perfectly fine.

12-24 Hours (Extended Territory)​
Your liver's sugar stores start depleting significantly, so your body begins making fuel from other sources like lactate and glycerol. You might feel sharper or more tired - both are normal responses.

24+ Hours (Survival Mode)​
Your body starts producing ketones to fuel your brain and activates protein-sparing mechanisms. But here's the critical point Jules emphasised: "By that point, we're really in this kind of adaptive state for survival, essentially, and it's certainly not something that we'd be having conversations with athletes about." Athletes should aim to thrive, not survive!

The Methods: What Works vs. What's Just Clever Marketing

Not all fasting approaches are created equal. After reviewing the research, Jules identified a clear hierarchy:

Time-Restricted Eating (16:8 or 14:10) gets the green light as the most manageable approach. Early eating windows (7am-5pm) show slightly better results for insulin sensitivity than late windows.

Intermittent Fasting (5:2 approach) - five normal eating days with two very low-calorie days - can work but is challenging for athletes to manage around training.

The Red Flags include anything with a fancy name designed to grab attention. The "Warrior Diet" (20:4 eating window), "Lean Gains" with complex carb cycling rules, and other extreme protocols often promote disordered eating behaviours.

Jules was particularly clear about this: "When we layer on all these additional rules and restrictions, that's the point at which this becomes incredibly challenging for anyone to really implement... and actually lean more towards promoting and supporting people to engage with more kind of disordered eating behaviours."

Who Should Avoid Fasting Entirely

This is where Jules' expertise in female-specific nutrition and eating disorder awareness became invaluable. Certain groups should steer clear of fasting protocols:

• Anyone with a history of disordered eating
• Youth athletes (still growing and developing)
• Pregnant or breastfeeding women
• Older athletes with higher protein needs
• Those with recurrent injury histories
• People already dealing with high stress levels

The reason? Fasting is physiological stress. Add it to training stress plus life stress, and you might tip over the edge.

The Athlete's Dilemma: Performance vs. Body Composition

Here's the reality check every athlete needs: "This isn't going to be a performance enhancer," Jules stated clearly. "This isn't an approach that an athlete should pursue from an improved performance perspective."

Fasting can be managed without hurting performance, but it requires careful planning and adds complexity to an already complex lifestyle. Jules uses a brilliant framework - the performance-health-body composition triangle. You're always trying to balance these three elements, and you can't optimise all simultaneously.

When it might work for athletes:

• Off-season periods with lighter training loads.
• When you have time to experiment safely.
• With professional nutritional guidance.
• When other fundamentals are already dialled in.

When to absolutely avoid it:

• Build phases and competition preparation.
• Peak race weeks and tapering periods.
• During high-stress life periods.
• When you're already struggling with energy or recovery.

The Monitoring Reality: How to Know If It's Working (Or Hurting)

If you do decide to experiment with fasting, monitoring becomes crucial. Jules emphasised tracking multiple markers:

Training metrics: Are you maintaining power/pace? Does the same effort feel harder? Can you complete sessions as prescribed?

Health markers: Sleep quality, mood stability, immune function, energy levels throughout the day.

For female athletes: Menstrual cycle regularity becomes a critical indicator of whether energy availability is adequate.

"Monitor some of these symptoms and how you feel," Jules advised, "and ensure that you feel as you did before, essentially."

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The Complexity Trap

One insight that really struck me was Jules' point about complexity. Fasting might look simple on the surface - just eat less frequently, right? But the reality is different.

"Going to two meals might seem more simple because we've only got two meals, rather than three or five," she explained. "But to really get those two meals right for that individual's needs, that's where the complexity comes in."

As athletes, we need less complexity in our lives, not more. Sometimes the "boring" approach of regular, balanced meals is actually the smartest choice.

The Bottom Line: Foundations First

After three hours of discussion, the message was clear: get the basics right first. "Having strong foundations and getting the basics right, it might be boring, but they're ultimately the actions that are really going to move us on," Jules concluded.

Before considering any fasting protocol, ensure you have:

• Consistent training and recovery patterns
• Solid sleep habits
• Basic nutrition fundamentals in place
• Manageable stress levels
• Professional guidance if needed

Fasting isn't magic, and it's not necessary for achieving your goals. It's simply another tool that works for some people in specific circumstances.

The question isn't whether fasting is good or bad - it's whether it's right for you, at this point in your life, with your current goals and circumstances.

Have you tried fasting? What approach did you choose and how did it work out for you? I’d love to hear your story. Feel free to share on my new Facebook page Battle Ready after 50​

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