Muscle is contractile protein tissue that stores glycogen and generates heat and movement. Adequate thyroid function and protein with carbohydrate build and maintain it; cortisol breaks it down under stress.

A self-reinforcing catabolic loop

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Muscle breakdown can itself drive further muscle breakdown: the tryptophan and cysteine released when muscle is catabolized suppress the thyroid and raise serotonin, which in turn raises cortisol:[1]

The formation of serotonin is increased by the excess tryptophan in muscle, and serotonin stimulates the formation of more cortisol, while the tryptophan itself, along with the excess muscle-derived cysteine, suppresses the thyroid function.

— Ray Peat[2]

Because this loop is triggered by the amino acid mix, not just the presence of stress itself, Peat's practical fix was balancing muscle-meat protein with gelatin (see Taurine and Carnitine for the specific quote and dosing note), which supplies the glycine that offsets tryptophan and cysteine's effects.

Amount and ageing

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MRI of 468 adults aged 18 to 88 found skeletal muscle makes up about 38% of body mass in men and 31% in women (33.0 kg versus 21.0 kg on average). Relative muscle mass starts to fall in the third decade, and absolute mass drops noticeably after the fifth, mostly from the legs.[3] Age-related loss of muscle is called sarcopenia.

Contraction and relaxation

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Free calcium inside the muscle cell is the trigger for contraction, magnesium is an important factor in relaxation, and hyperventilation and panting make fingers and toes cramp.[4]

Protein and training

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In a meta-analysis of randomised trials, protein supplementation during more than six weeks of resistance training added a small amount of extra fat-free mass in both young and older people.[5] The gain is small in absolute terms.

At equal calories and protein, 11 days of near-zero carbohydrate raised urinary nitrogen loss in six healthy men while plasma T3 fell.[6]

One moderate session of six machine lifts at 65%–75% of one-repetition maximum shortened Flanker and N-back response times versus a matched rest video in an acute randomized between-subjects trial (n = 121, ages 18–50).[7] See Brain and Bodybuilding & Weight loss.

Thyroid and weakness

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In a prospective study of newly diagnosed thyroid disease, 79% of hypothyroid patients had neuromuscular complaints and 38% had measurable weakness in at least one muscle group. After a year of treatment 13% still had weakness, which the authors read as a true myopathy, whereas the weakness in overactive thyroid resolved fully within about 3.6 months.[8]

Thyroid is the basic anabolic hormone: it activates protein synthesis, lowers cortisol and protein breakdown, and opposes aromatase.[9] Severe hypothyroidism can cause myopathy, in which muscle proteins such as myoglobin leak into the blood, and growing pains near puberty can come from rising estrogen interfering with thyroid function.[10] Very high creatine phosphokinase, a muscle breakdown enzyme, together with high TSH can reflect hypothyroidism rather than autoimmune disease.[11]

Practice

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Train and eat adequately, thyroid and food together, not exercise alone, build and maintain muscle. Balance muscle-meat protein with gelatin rather than eating unbalanced lean meat as the sole protein source: for a half-pound of steak, roughly 20 grams of gelatin at the same meal is a reasonable starting ratio, with a higher gelatin-to-meat ratio often preferable.[12] See Protein, Gelatin, Thyroid, Cortisol, Glycogen.

Body composition

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Body composition (fat mass vs muscle) sits on top of metabolic rate: thyroid, sugar oxidation, lower PUFA, sleep, and training load. Cold starvation and stimulant cuts are the wrong default in this frame. See Satiety for cravings, stress eating, and distraction effects on intake.

Fat loss

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Fat loss while staying warm beats chronic cold and under-eating. Lower PUFA, keep oxidation up, and track temperature / pulse. See Obesity, How to heal your metabolism, Roadmap/08 - Bodybuilding & Weight loss.

Muscle gain

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Needs progressive load, surplus energy, protein, and recovery sleep. Not built from restriction alone.

Body recomposition

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More muscle and less fat together is slow. It needs training and enough food, not permanent deficit. A 4-week randomized trial tested this directly in 40 young men on a 40% energy deficit doing 6 days/week of resistance plus high-intensity training: those eating 2.4 g protein/kg/day gained 1.2 kg lean mass while losing 4.8 kg fat mass, versus only 0.1 kg lean mass gained and 3.5 kg fat mass lost on 1.2 g/kg/day, showing higher protein, not more restriction, was what let both happen together.[13]

Body fat percentage

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Estimates fat mass share; methods vary. Trends beat single numbers.

Weight cutting

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Weight cutting is the deliberate rapid loss of body mass, mostly water, to qualify for a lower weight class, followed by rapid regain after weigh-in. More than half of athletes in weight-class sports are estimated to have practiced it.[14] The research on performance is mixed. The physiology of the cost is clearer, and sodium and carbohydrate are the two variables an athlete can control.

Performance

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A meta-analysis of combat athletes found performance unchanged between before the cut and 3 to 36 hours after recovery and regain. Immediately after the cut, maximal strength (g = -0.29) and repeated high-intensity effort (g = -0.37) were modestly reduced.[15] A second meta-analysis found that losses of up to 5% of body mass in under 7 days did not affect performance in Olympic combat sports.[16] Reviews still recommend gradual, supervised weight loss, note that rule changes have done the most to reduce the practice in wrestling, and state that the long-term effects of frequent cuts are unknown.[14][17]

Fluid and sodium

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In fight week, sodium restriction and water loading induce polyuria and acute water loss. After weigh-in, oral rehydration solution at 1 to 1.5 L/h with 50 to 90 mmol/L of sodium comes first, followed by fast-acting carbohydrate at up to 60 g/h.[17]

Sodium is what makes the fluid stay. After about 2% body mass loss from sweat, drinks with 0 or 25 mmol/L sodium left subjects in negative sodium balance, and urine output was inversely related to sodium intake. Drinks at 50 mmol/L brought sodium balance close to even, and only 100 mmol/L left subjects in positive fluid balance after 6 hours.[18]

Salt restriction raises aldosterone, and more salt lowers it. Aldosterone lets the kidneys retain sodium while losing potassium and ammonium, and it raises vascular endothelial growth factor, which makes capillaries leaky.[19] Low blood sugar, low blood volume, and low sodium all drive the same response.[20] The low-sodium phase of a cut therefore adds an aldosterone response to the dehydration.

Stress hormones and energy

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Calorie restriction to 1,200 kcal/day for 3 weeks raised total cortisol output in 121 women.[21]

Carbohydrate protects thyroid output during a deficit. In obese subjects on 800 kcal diets, a zero-carbohydrate diet cut serum T3 by 47%, while isocaloric diets with at least 50 g of carbohydrate produced no significant change in T3 or reverse T3. Total fasting cut T3 by 53% and raised reverse T3 by 58%. The T3 decline on the zero-carbohydrate diet correlated with blood glucose and ketones.[22] Making ketones from scratch activates the stress system and signals that glucose or fruit intake is too low.[23]

Sauna and hot-water immersion are standard acute water-loss tools. Under supervision, they can produce about 2 to 4% of body mass loss within 24 hours of weigh-in.[17] Sauna and hyperthermic therapies are often used without regard to the actual internal temperature of the tissues or to serum glucose and free fatty acid levels.[24]

Brain

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In six healthy volunteers, MRI showed that dehydration reduced gray and white matter volume in several regions and enlarged the lateral, third, and fourth ventricles.[25]

Protocol limits

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The 2025 position stand sets these limits:[17]

  • Off-camp weight stays 12% to 15% above the weight division.
  • Suitable body mass loss is 6.7% at 72 hours, 5.7% at 48 hours, and 4.4% at 24 hours before weigh-in.
  • Macronutrients do not drop below 3.0 to 4.0 g/kg carbohydrate, 1.2 to 2.0 g/kg protein, and 0.5 to 1.0 g/kg fat per day.
  • Glycogen depletion through exercise and carbohydrate restriction can account for 1% to 2% of body mass.
  • After weigh-in, rehydration and refueling aim to regain at least 10% of body mass.

See also

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References

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  1. ↑ Ray Peat, "Gelatin, stress, longevity," raypeat.com.
  2. ↑ Gelatin, stress, longevity, Ray Peat newsletter
  3. ↑ Janssen I, Heymsfield SB, Wang ZM, Ross R. "Skeletal muscle mass and distribution in 468 men and women aged 18-88 yr," J Appl Physiol. 2000;89(1):81-88.
  4. ↑ Ray Peat, "Osteoporosis, harmful calcification, and nerve-muscle malfunctions," raypeat.com.
  5. ↑ 22 RCTs, 680 subjects; fat-free mass gain 0.69 kg. Cermak NM, Res PT, de Groot LC, Saris WHM, van Loon LJC. "Protein supplementation augments the adaptive response of skeletal muscle to resistance-type exercise training: a meta-analysis," Am J Clin Nutr. 2012;96(6):1454-1464.
  6. ↑ Isocaloric carbohydrate deprivation induces protein catabolism despite a low T3-syndrome in healthy men n=6 men, 11 days per diet, 15% protein, 85%, 44% or 2% carbohydrate at equal calories; plasma T3 1.78 ± 0.09, 1.71 ± 0.07 vs 1.33 ± 0.05 nmol/L (P < 0.01); urinary nitrogen 10.91 ± 0.67, 12.79 ± 1.14 vs 15.89 ± 1.10 g/24 h; reverse T3 and free T4 rose, TSH and resting energy expenditure unchanged. doi:10.1046/j.1365-2265.2001.01158.x. PMID 11167929.
  7. ↑ Baumgartner NW, Belbis MD, Noh K, Hirai DM, Amireault S, Kao SC. "Brawn and Brainpower: Acute Resistance Exercise Improves Behavioral and Neuroelectric Measures of Executive Function," Psychophysiology. 2025;62(11):e70171. doi:10.1111/psyp.70171. PMID 41168935. PMC12575885.
  8. ↑ Duyff RF, Van den Bosch J, Laman DM, van Loon BJ, Linssen WH. "Neuromuscular findings in thyroid dysfunction: a prospective clinical and electrodiagnostic study," J Neurol Neurosurg Psychiatry. 2000;68(6):750-755.
  9. ↑ Ray Peat, "Generative Energy #61 — Bodybuilding and Steroids", https://www.youtube.com/watch?v=3fVx5yk73t4, 38:23
  10. ↑ Ray Peat, "Heart and hormones," raypeat.com.
  11. ↑ Ray Peat, "'Ask the Herb Doctor: Brain "Barriers"'", https://www.toxinless.com/kmud-191018-brain-barriers.mp3, 24:53
  12. ↑ Ray Peat, "Tryptophan, serotonin, and aging," raypeat.com.
  13. ↑ Longland TM, Oikawa SY, Mitchell CJ, Devries MC, Phillips SM. "Higher compared with lower dietary protein during an energy deficit combined with intense exercise promotes greater lean mass gain and fat mass loss: a randomized trial,"Am J Clin Nutr. 2016;103(3):738-46. doi:10.3945/ajcn.115.119339.
  14. ↑ 14.0 14.1 Khodaee M, et al. "Rapid Weight Loss in Sports with Weight Classes", Curr Sports Med Rep. 2015. PMID 26561763.
  15. ↑ Brechney GC, et al. "Effects of Weight Cutting on Exercise Performance in Combat Athletes: A Meta-Analysis", Int J Sports Physiol Perform. 2022. PMID 35523423.
  16. ↑ Mauricio CA, et al. "Rapid Weight Loss of Up to Five Percent of the Body Mass in Less Than 7 Days Does Not Affect Physical Performance in Official Olympic Combat Athletes With Weight Classes: A Systematic Review With Meta-Analysis", Front Physiol. 2022. PMID 35492609.
  17. ↑ 17.0 17.1 17.2 17.3 Ricci AA, et al. "International society of sports nutrition position stand: nutrition and weight cut strategies for mixed martial arts and other combat sports", J Int Soc Sports Nutr. 2025. PMID 40059405. The abstract prints the sodium range as mmol/dL; mmol/L is meant.
  18. ↑ Shirreffs SM, et al. "Volume repletion after exercise-induced volume depletion in humans: replacement of water and sodium losses", Am J Physiol. 1998. PMID 9612323.
  19. ↑ Ray Peat, "When energy fails: Edema, heart failure, hypertension, sarcopenia, etc.", raypeat.com.
  20. ↑ Ray Peat, "Ask the Herb Doctor: Palpitations and Cardiac Events", KMUD, 2013-03-15.
  21. ↑ Tomiyama AJ, et al. "Low calorie dieting increases cortisol", Psychosom Med. 2010;72(4):357-64. PMID 20368473.
  22. ↑ Spaulding SW, et al. "Effect of caloric restriction and dietary composition of serum T3 and reverse T3 in man", J Clin Endocrinol Metab. 1976;42:197-200. PMID 1249190.
  23. ↑ Ray Peat, "Ask the Herb Doctor: Vaccination II", KMUD, 2014-07-18.
  24. ↑ Ray Peat, "Cumulative Damage, Degeneration, and Aging: Possibilities of Reversal", Ray Peat Newsletter.
  25. ↑ Streitbürger DP, et al. "Investigating structural brain changes of dehydration using voxel-based morphometry", PLoS One. 2012. PMID 22952926.