What Happens to Your Body During a 7-Day Water Fast?

Benefits, Drawbacks, and What Science Says

Fasting is increasingly popular in health and fitness circles, but many wonder: What really happens to your body and exercise performance during prolonged fasting? Recent cutting-edge research shines a light on this, revealing fascinating adaptations in muscle strength, endurance, metabolism, and overall physical capacity during a seven-day water-only fast.

This blog breaks down the science behind these changes, helping you make informed decisions about fasting as part of your health or fitness routine.

Muscle Strength: Surprisingly Well Preserved

One of the most striking findings from a 2025 study by Kolnes et al. (2025) in Nature Communications is that maximal leg muscle strength remains unchanged after seven days of water-only fasting despite a significant loss of lean body mass (about 4.6 kg on average) (Kolnes et al., 2025). This suggests that the body selectively preserves contractile muscle proteins critical for strength, even while breaking down other protein stores.

This preservation of strength contrasts with other conditions like bed rest, where strength declines rapidly even without large lean mass loss (Marusic et al., 2021).

Aerobic Endurance: Noticeable Decline

While strength is maintained, aerobic endurance capacity decreases significantly during prolonged fasting. The same study reported a 13% reduction in peak oxygen uptake (VO₂peak) and a 16% decrease in peak power output during exercise (Kolnes et al., 2025).

This reduction is linked to a metabolic shift where the body limits carbohydrate oxidation to conserve blood glucose, essential for brain function and survival during starvation. Specifically, the enzyme pyruvate dehydrogenase kinase 4 (PDK4), which inhibits carbohydrate use, increased 13-fold in muscle tissue, reducing the muscles’ ability to burn carbs during exercise (Kolnes et al., 2025; Sugden & Holness, 2003).

Metabolic Shift: From Carbohydrates to Fat and Ketones

During fasting, your body switches fuel sources:

• Fat oxidation nearly doubles, with fat providing up to 73% of energy at rest after fasting, compared to 37% before (Kolnes et al., 2025).

• Muscle glycogen stores are halved, but enough glycogen remains to maintain anaerobic capacity for short bursts of intense activity (Kolnes et al., 2025).

• Ketone bodies (like β-hydroxybutyrate) rise dramatically, reaching ~4 mM at rest, and are used as alternative fuel by muscles and brain (Kolnes et al., 2025; Owen et al., 1967).

This fuel flexibility is a key survival adaptation, allowing humans to endure periods without food while maintaining critical functions.

Muscle Protein Breakdown and AMPK Activity

Contrary to some expectations, AMP-activated protein kinase (AMPK), a regulator thought to promote muscle protein breakdown during energy stress, did not show increased activity during the fast (Kolnes et al., 2025). This challenges the idea that AMPK is a major driver of muscle loss in fasting.

Weight Loss and Body Composition Changes

A related study by Wilkinson et al. (2023) involving a 7-day water fast found:

• Average weight loss of around 5.7 kg, comprising both fat and lean mass.

• Lean mass loss was almost fully regained after three days of refeeding, while fat loss was sustained (Wilkinson et al., 2023).

• Changes in over 200 plasma proteins during fasting suggested potential benefits for conditions like arthritis and heart disease, indicating fasting’s systemic effects beyond weight loss (Wilkinson et al., 2023).

Benefits of a 7-Day Fast

• Preservation of muscle strength despite lean mass loss, enabling continued physical function (Kolnes et al., 2025).

• Enhanced fat metabolism and metabolic flexibility, which can support weight loss and improved metabolic health (Kolnes et al., 2025; Cahill, 2006).

• Potential systemic health benefits through changes in plasma proteins linked to inflammation and cardiovascular health (Wilkinson et al., 2023).

Drawbacks and Cautions

• Reduced high-intensity and aerobic endurance may impair performance in endurance sports or demanding physical work during fasting (Kolnes et al., 2025).

• Lean mass loss, though partially reversible, still occurs and may impact long-term muscle health if fasting is prolonged or frequent without adequate nutrition (Kolnes et al., 2025; Dai et al., 2022).

• Potential nutrient deficiencies and energy deficits require careful monitoring, particularly for vulnerable individuals (Dai et al., 2022).

• Not suitable for everyone, especially those with medical conditions, pregnant or breastfeeding women, or those with eating disorders.

What About Intermittent Fasting and Other Fasting Types?

Systematic reviews show mixed effects of intermittent fasting (IF) on exercise performance:

• Ramadan fasting often causes a small, temporary drop in aerobic capacity (VO₂max), likely due to dehydration and glycogen depletion (Tinsley & La Bounty, 2015).

• Time-restricted feeding (a form of IF) may improve VO₂max and fat oxidation when hydration and nutrition are managed well (Tinsley & La Bounty, 2015; Moro et al., 2016).

Practical Recommendations

• If you want to try a prolonged fast, consult a healthcare professional and ensure you’re healthy to do so.

• Expect muscle strength to remain stable in the short term, but anticipate a drop in endurance performance.

• Use fasting strategically for fat loss and metabolic health, but refeed properly to restore lean mass.

• Avoid high-intensity training during prolonged fasting and focus on low-to-moderate intensity activities.

• Stay hydrated and monitor for any adverse symptoms.

References

Cahill, G.F. Jr (2006) ‘Fuel metabolism in starvation’, Annual Review of Nutrition, 26, pp. 1–22. .

Dai, Z. et al. (2022) ‘Effects of 10-day complete fasting on physiological homeostasis, nutrition and health markers in male adults’, Nutrients, 14(18), 3860.

Kolnes, K.J., Nilsen, E.T.F., Brufladt, S. et al. (2025) ‘Effects of seven days’ fasting on physical performance and metabolic adaptation during exercise in humans’, Nature Communications, 16, Article 122.

Marusic, U., Narici, M., Simunic, B., Pisot, R. & Ritzmann, R. (2021) ‘Nonuniform loss of muscle strength and atrophy during bed rest: a systematic review’, Journal of Applied Physiology, 131, pp. 194–206.

Owen, O.E., Smalley, K.J., D’Alessio, D.A., Mozzoli, M.A. & Dawson, E.K. (1967) ‘Brain metabolism during fasting’, Journal of Clinical Investigation, 46(10), pp. 1589–1595.

Sugden, M.C. & Holness, M.J. (2003) ‘Recent advances in mechanisms regulating glucose oxidation at the level of the pyruvate dehydrogenase complex by PDKs’, American Journal of Physiology-Endocrinology and Metabolism, 284(5), pp. E855–862.

Tinsley, G.M. & La Bounty, P.M. (2015) ‘Effects of intermittent fasting on body composition and clinical health markers in humans’, Nutrition Reviews, 73(10), pp. 661–674.

Wilkinson, M.J. et al. (2023) ‘Proteomic and physiological responses to 7-day water-only fasting in humans’, Nature Metabolism, [online]