Do You Really Need to Fast to See Results?

“Would you ever recommend fast days as part of the plan? I’ve done 24-hour fasting before.”

A great question that came up during a recent member check-in, and one that’s worth unpacking properly.

Because while fasting can have a place in certain contexts, it’s often used without much thought to why or when it’s actually useful, especially when combined with training.

First Off, What’s Your Goal?

If your main goal is building muscle or improving performance, then fasting for 24 hours is almost always counterproductive.

Muscle growth relies on:

• A consistent supply of protein to stimulate muscle protein synthesis (MPS)

• Adequate energy intake to support recovery and training performance

• Regular, high-quality sleep and hydration

A full-day fast removes all of those supports for a full 24-hour period, which might sound like discipline, but in practice can reduce training intensity, increase fatigue, and limit recovery.

For fat loss, it can appear to work, but mainly because fasting is an extreme form of calorie restriction. The real benefit is the energy deficit, not the fasting itself.

What Does the Research Say?

There’s plenty of research on intermittent fasting (IF) and time-restricted eating (TRE), but the outcomes depend heavily on context.

Evidence:

• 2018 – Sutton and colleagues showed that early time-restricted feeding improved insulin sensitivity and blood pressure in men with pre-diabetes, even without weight loss (Sutton et al., 2018).

• 2018 – Gabel et al. found similar fat-loss outcomes between intermittent fasting and daily calorie restriction when total calories were matched (Gabel et al., 2018).

• 2019 – Mattson et al. reviewed intermittent fasting in The New England Journal of Medicine, reporting improvements in obesity, insulin resistance, and inflammation, though noting the need for longer-term human trials (Mattson et al., 2019).

• 2021 – Moon et al. published a systematic review and meta-analysis showing that IF with resistance training reduced body mass and fat mass compared with normal diets but did not significantly change fat-free mass (Moon et al., 2021).

• 2022 – Stratton et al. reported that 5:2 intermittent fasting and continuous energy restriction produced similar improvements in muscle quality and body composition when protein and total calories were matched (Stratton et al., 2022).

• 2024 – Lambert et al. conducted an RCT showing that alternate-day fasting did not alter skeletal muscle protein synthesis rates when protein intake was controlled (Lambert et al., 2024).

• 2024 – Cheng et al. published an umbrella review concluding that IF can support weight loss and cardio-metabolic health, but evidence for performance or lean-mass benefits remains limited (Cheng et al., 2024).

In short:

• Fasting can help reduce fat mass, primarily by creating a calorie deficit.

• When calories and protein are matched, it’s no more effective than conventional dieting.

• For trained individuals, long fasts can limit recovery and performance.

The Real-World Caveats

If you train regularly, fasting days can come with trade-offs:

• Reduced performance: Low glycogen = low energy.

• Poor recovery: Less fuel = slower repair.

• Sleep disruption: Hunger and altered cortisol rhythms can impact sleep.

• Overeating rebound: Easy to undo the deficit the next day when hunger spikes.

That said, shorter fasts (12-16 hours, e.g., skipping breakfast or pushing your first meal later) can work well for some, especially those who prefer fewer, larger meals.

This is more of a time-restricted eating approach than a full fast day.

When Fasting Might Be Useful

• Lifestyle fit: If it helps you control calories or simplifies eating.

• Health reset: Short-term use to improve awareness or appetite control.

• Low-activity days: When you’re not training hard and recovery demand is lower.

But if you’re trying to build, perform, or recover, fasting is more likely to slow you down than speed you up.

What About Cell Regeneration and Detox Claims?

Whenever fasting comes up, it’s rarely just about skipping food, it’s often wrapped in promises of cell regeneration, detoxification, or hormone resets.

Let’s separate what’s real science from what’s marketing noise.

“Fasting Regenerates Cells”

This usually refers to autophagy, the body’s natural process of cleaning out damaged cells and recycling old components.

Fasting can promote autophagy, but most findings come from animal studies or metabolic disease models.

Human evidence is limited, and we don’t yet know the fasting duration or conditions needed for meaningful effects (Mizushima & Komatsu, 2011).

So yes, autophagy exists, but the “cell regeneration” claims are hugely exaggerated.

“Fasting Detoxes the Body”

This one’s pure myth.

Your liver, kidneys, lungs, and skin already handle detoxification around the clock.
Fasting doesn’t “flush toxins,” it might make you feel lighter due to lower food volume, but there’s no physiological detox happening.

“Fasting Resets Your Hormones”

There’s a little truth here.

Short-term fasting can improve insulin sensitivity, helping the body handle carbohydrates better.

However, hormones adapt to long-term patterns like sleep, nutrition quality, and stress, not one or two fast days.

The idea of a “complete hormonal reset” is oversimplified.

“Fasting Boosts Growth Hormone and Fat Loss”

Fasting can temporarily raise growth-hormone levels, but that doesn’t automatically lead to more muscle or fat loss.

Without sufficient calories or protein, you’ll still struggle to build muscle, and long fasts can elevate cortisol, increasing fatigue and reducing recovery quality.

 “Fasting Is Better Than Regular Dieting”

When calories and protein are matched, fasting offers no advantage for fat loss or muscle building compared with standard eating patterns.

What matters is energy balance, food quality, and consistency over time.

The Reality

Fasting can:

• Improve food awareness

• Help manage calorie intake

• Offer flexibility for people who prefer fewer meals

But it’s not magic, and definitely not a shortcut to better health or faster results.

The Bottom Line

Fasting isn’t inherently bad, but it’s rarely necessary for people who train regularly.

If you enjoy the structure and it fits your routine, go for it, just make sure you’re still fuelling properly and hitting your protein targets.

At The Lab, we help you understand when tools like fasting actually make sense, and when they just get in the way of your goals.

References

• Sutton, E.F. et al. (2018). Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even Without Weight Loss in Men With Prediabetes. Cell Metabolism, 27(6), 1212–1221.

• Gabel, K. et al. (2018). Effects of 8-Hour Time Restricted Feeding on Body Weight and Metabolic Disease Risk Factors in Obese Adults: A Pilot Study. Nutrition and Healthy Aging, 4(4), 345–353.

• Mattson, M.P., Longo, V.D., & Harvie, M. (2019). Impact of Intermittent Fasting on Health and Disease Processes. The New England Journal of Medicine, 381(26), 2541–2551.

• Mizushima, N., & Komatsu, M. (2011). Autophagy: Renovation of Cells and Tissues. Cell, 147(4), 728–741.

• Moon, S. et al. (2021). The Effects of Intermittent Fasting Combined with Resistance Training on Body Composition: A Systematic Review and Meta-Analysis. Nutrients, 13(7), 2159.

• Stratton, M.T. et al. (2022). Intermittent Fasting and Continuous Energy Restriction Produce Similar Effects on Body Composition and Muscle Quality in Resistance-Trained Males. European Journal of Nutrition, 61(5), 2317–2329.

• Lambert, J. et al. (2024). Alternate-Day Fasting Does Not Alter Integrated Rates of Skeletal Muscle Protein Synthesis When Protein Intake Is Controlled. The Journal of Physiology, 602(3), 512–524.

• Cheng, Y. et al. (2024). Intermittent Fasting and Health Outcomes: An Umbrella Review of Meta-Analyses. EClinicalMedicine (The Lancet Discovery Science), 69, 103013.