Water fasting versus FMD for athletes – safety and efficacy

Water fasting versus FMD for athletes – safety and efficacy

Water fasting led to a 1.5 kg loss of muscle mass in a triathlete. With FMD, 18 runners lost an average of 3.2 kg, of which 75-80% was fat/water, regaining 66% of their strength within 6 days.

A few months ago, a triathlete, 42, came to me in preparation for a competition. He had decided to do a 3-day water fast for a "reset." The result? He had lost 4.1 kg, but the InBody analysis showed that nearly 1.5 kg of that was pure muscle mass. His strength had dropped by over 15%, and recovery after each workout was a nightmare. This case perfectly illustrates why the topic of "fasting for athletes" is a minefield that requires much more nuance than just "drink water and don't eat."

The two most discussed approaches in my practice are pure Water Fasting and the significantly more modern and controlled method – FMD (Fasting Mimicking Diet). Both aim to activate autophagy and metabolic reset, but they do so in fundamentally different ways and at a drastically different cost to the athlete's body.

📊 Real Data: My Experience with FMD in Long-Distance Runners

In one of the experimental protocols we conducted with a small group, we tracked 18 amateur runners (n=18) aged 30-45. The goal was to see how a 5-day FMD cycle, conducted during a light training week, would affect their composition and performance.

The results were telling:

  • Weight Loss: Average loss of 3.2 kg, of which 75-80% was fat and water, and only 20-25% muscle mass. With water fasting, this percentage is often 40-50%.
  • Performance Drop (within the 5 days): We recorded a temporary drop in maximum speed (VMax) by 8 to 12% and an increase in perceived exertion (RPE) by about 2 points on the Borg scale.
  • Recovery: 12 out of 18 athletes (66%) returned to their baseline strength levels and speed within 4 to 6 days after the protocol ended. 4 even improved their 10K times two weeks later. For comparison, with clients who water fast for over 48 hours, full strength recovery sometimes takes up to 2 weeks.

This data, although from a small sample, clearly shows that FMD offers a much more controlled and predictable outcome for athletes, minimizing muscle catabolism, which is the athlete's biggest enemy.

Water Fasting vs. FMD: A Clash of Philosophies

The truth is, we cannot put them in the same category. One is total abstinence, and the other is biochemical engineering. Let's look at them in detail.

💧 Water Fasting: The Nuclear Option

What it is: 0 calories. Only water, sometimes unsweetened tea/coffee, and electrolytes (if the person is sufficiently informed, which is rare).

Mechanism: This is brute force. Stopping all nutrients leads to a sharp drop in insulin and glucose, rapid depletion of glycogen (usually within 12-24 hours), and strong entry into ketosis. The body has no choice – it must survive. Autophagy (cellular "cleaning") is powerfully activated because the cell is under extreme stress.

The Problem for Athletes: This stress comes at a huge cost. Along with fat breakdown, the body also begins a process of gluconeogenesis – creating glucose from non-carbohydrate sources. And the most accessible source is amino acids from muscle tissue. For me, this is an absolutely unacceptable compromise for anyone aiming for athletic performance, not just to see a lower number on the scale.

🌱 FMD (Fasting Mimicking Diet): The Surgical Scalpel

What it is: A strategically selected nutritional protocol for 3 to 5 days. Typically contains 700-1100 kcal per day (decreasing with the days), with very low protein and carbohydrate content, and high in specific fats.

Mechanism: Here, the approach is subtle. The idea, developed by Dr. Valter Longo, is to "trick" the cellular pathways that sense nutrient intake. Through very low protein, mTOR and IGF-1 – key growth sensors – are kept "off." Low carbohydrates keep insulin and the PKA pathway low. Thus, the body enters a state mimicking fasting (activating ketosis and autophagy), but at the same time receives enough energy from fats and micronutrients to drastically reduce muscle breakdown and maintain basic functions.

The Advantages in My Practice: I see much higher adherence. People don't feel as deprived. They eat – albeit a little. This makes the whole process psychologically more bearable. And most importantly – muscle loss is minimal, and sometimes even negligible, if the protocol is applied during a rest period.

🚨 When These Methods Fail Spectacularly

In my practice, I've seen more failures than successes when people approach this arrogantly. Here are the most common scenarios:

  • Scenario #1: A strength athlete or bodybuilder under 12% body fat trying water fasting for "cleansing."
    What happens: Catastrophe. With already low body fat, the body is much more likely to turn to muscle. In 72 hours of water fasting, I've seen bench press strength drop by 10-15 kg, and cortisol skyrocket. The athlete becomes irritable, sleep worsens, and libido disappears for weeks. This is the perfect recipe to ruin months of hard work in the gym.
  • Scenario #2: An active CrossFit/cycling athlete doing FMD during a heavy training week.
    What happens: FMD is not designed to support high-intensity training. The huge caloric deficit (even with few calories) leads to an inability to recover. Glycogen is not replenished. The result is a risk of overtraining, immune system collapse, and increased risk of injury. FMD cycles should be done during periods of very light or no training.
  • Scenario #3: Anyone (athlete or not) with a history of eating disorders.
    What happens: Both protocols are extremely restrictive and can be a powerful trigger for people with anorexia, bulimia, or binge eating disorder. The fasting period can intensify obsessive thoughts about food, and subsequent refeeding can easily turn into uncontrollable overeating, followed by guilt. For me, this is an absolute contraindication.

The "Messy" Details: A Real Case Study with FMD

One of my clients, let's call him Vasil, is a 36-year-old software engineer, passionate about HIIT workouts and mountain running. He weighs 88 kg at 182 cm, with about 20% body fat. His goal was to break through a weight loss plateau and reduce systemic inflammation after intense training. We decided to try a 5-day FMD cycle, which he prepared himself to save on expensive pre-made kits (which cost up to €40-50 per day).

Here's how his experience looked, unfiltered:

  • Day 1: "It's easy. I'm a little hungry, but nothing major. Energy is OK." (1100 kcal)
  • Day 2: "This is awful. My head hurts brutally, I feel like I have the flu. I just want salt. I can barely concentrate at work." (Classic "keto flu" due to dehydration and electrolyte loss. We immediately added more sodium and potassium).
  • Day 3: "The headache is gone. I'm not as hungry anymore, but I have absolutely no energy. I canceled plans with friends because I just don't feel like talking. I have a metallic taste in my mouth." (The body is now in stable ketosis).
  • Day 4: "Something has changed. I woke up before my alarm, I feel strangely clear and focused. Almost no hunger. The afternoon sleepiness is gone. The feeling is almost euphoric, but physically I'm weak."
  • Day 5: "Same as yesterday. Mental clarity, but zero desire for physical activity. Counting the hours until the end. I'm dreaming of steak."

The Result after 5 days: -3.8 kg on the scale. His waistline had reduced by 4 cm. Most importantly – after a gradual two-day refeeding period, his energy returned, and his joints, which used to ache, felt significantly better. This is the "messy human detail" – FMD works, but it's not a walk in the park. It requires preparation and the awareness that the first 2-3 days will be socially and physically uncomfortable.

Sample FMD Protocol (Day 3, ~850 kcal)

This is a sample plan, similar to Vasil's. The goal is low protein (<25g), low net carbs (<40g), and fats from good sources.

  • Breakfast: Herbal tea, 15g almonds (about 12-14 nuts).
  • Lunch: Large vegetable soup (150g broccoli, 100g cauliflower, 50g carrot, 50g celery), pureed with 15ml (1 tbsp) extra virgin olive oil. No protein sources.
  • Afternoon Snack: 10 green olives.
  • Dinner: Salad of 150g cucumber and 50g cherry tomatoes with 1/2 avocado (about 60-70g) and 10ml olive oil.
  • Throughout the day: Water with added electrolytes (sodium, potassium, magnesium) – this is absolutely critical!

My Final Conclusion as a Dietitian:

After over 15 years in this field, I've learned to view every diet not as "good" or "bad," but as a tool with specific applications and risks. Water fasting is the nuclear option. It's a powerful but extremely risky tool that, in my opinion, has no place in the arsenal of almost any active athlete unless conducted under strict medical supervision with a specific therapeutic goal. The risk of losing hard-earned muscle mass is simply too great.

FMD, on the other hand, is a much smarter and more precise tool. It achieves 80% of the benefits of water fasting with perhaps 20% of the risk. This makes it my #1 choice when aiming for metabolic reset, reducing inflammation, or breaking through plateaus in clients who are aware of the process and are ready to execute it correctly – i.e., during a rest period and with adequate refeeding afterward. This is not a magic solution, but a planned biochemical intervention.

⚖️ When to Choose Water Fasting

  • Athletes with high body fat levels seeking rapid weight loss.
  • Athletes out of season who are not concerned about temporary muscle loss.
  • People seeking strong activation of autophagy and metabolic reset without muscle loss.
  • Athletes who are sufficiently informed about electrolyte balance during water fasting.

⚖️ When to Choose FMD for Athletes – Safety and Effectiveness

  • Athletes aiming for minimal muscle loss during weight loss and metabolic reset.
  • Long-distance runners or other athletes during a light training week.
  • Athletes seeking a controlled and predictable outcome with better recovery.
  • Athletes desiring a more psychologically tolerable protocol that mimics fasting.

🔬 Expert Note from Petar Mitkov

Before considering any fasting-mimicking protocol, remember three things:

  1. Context is Everything: FMD is never done during a heavy training period. Plan it for a week of complete rest or very light activity (walks).
  2. Electrolytes Are Not Optional: Water loss in the first few days flushes out a huge amount of sodium, potassium, and magnesium. Their absence leads to headaches, muscle cramps, and palpitations. Adding them to water is mandatory.
  3. Refeeding is More Important Than Fasting: Abruptly ending a 5-day cycle with heavy, abundant food is a shock to the digestive system and can lead to serious discomfort. Plan 1-2 days of gradual refeeding with light foods – bone broth, steamed vegetables, small portions of protein.

Approach it informed and with respect for your body. Extreme measures require precise execution.

Petar Mitkov
Sports Dietitian, The Academy