On the 75 percent figure. This is well supported. Direct biopsy analysis of human rectus abdominis muscle found average skeletal muscle water content was 76.3 ± 3.3% in normal-weight individuals and 65.7 ± 5.8% in obese subjects . That same study exists specifically to challenge the assumption that this value is fixed: intramuscular water content is assumed to be constant in humans independent of anthropometric characteristics, and the researchers measured intramuscular water, proteins, glycogen and triglycerides in 51 muscle samples from 16 lean and 35 overweight and obese subjects to test whether that assumption is correct . So the number is real but is a compositional average, not a constant.

On strength and the 2 percent threshold. The evidence here is real but weaker and more mixed than the post implies. The most cited meta-analysis found that hypohydration impaired overall muscle endurance by 8.3 ± 2.3%, with no significant difference between upper body and lower body , and muscle strength fell by 5.5 ± 1.0%, muscle endurance by 8.3%, and anaerobic power by 5.8%, with anaerobic capacity and vertical jump height affected less, at around 3.5% . The same meta-analysis also found that trained individuals demonstrated a 3.3 ± 1.7% lesser decrease in muscle performance with hypohydration than untrained individuals , and its authors concluded cautiously that hypohydration, or factors associated with dehydration, are "likely to" impair muscle performance .

Critically, the experimental study that tested a deficit closest to the post's stated 2 percent threshold found no strength effect at that level: body mass decreased 2.4 ± 0.4% and 4.8 ± 0.4% in the two hypohydration trials, and no significant differences existed among trials in vertical jump height, peak lower-body power assessed via jump squat, or peak lower-body force assessed via isometric back squat . A separate line of evidence found that isometric strength and endurance were unaffected 3.5 hours after dehydration of approximately 4% body mass . And a more recent study reported that body mass was reduced by 3.2 ± 1.1% immediately after dehydration while finding, per its own title, impaired endurance without modulation of neuromuscular function.

The 2 percent figure itself is genuine but is anchored to a different outcome. Cheuvront and Kenefick (2014) reported that dehydration of 2 percent or greater impaired endurance exercise performance mediated by body water volume loss . For strength specifically, the review estimate is small: hypohydration appears to consistently attenuate strength by approximately 2 percent, power by approximately 3 percent, and high-intensity endurance by approximately 10 percent . Note that this is a roughly 2 percent decrement in strength, not a decrement occurring at 2 percent dehydration. Those are two different numbers that are easily conflated. Other summaries place the strength threshold higher: in the case of fluid loss of more than 2 to 4 percent of body weight, impairments to strength and endurance are to be expected .

On protein synthesis. This is the weakest element and I could not locate direct human evidence for it. What exists is indirect:

• A hormonal study, not a protein synthesis study: hypohydration significantly increased circulating concentrations of cortisol and norepinephrine, attenuated the testosterone response to exercise, and altered carbohydrate and lipid metabolism, suggesting hypohydration can modify the hormonal and metabolic response to resistance exercise . The effects were observed at meaningful deficits: participants completed six sets of back squats at 80% of 1RM in three hydration states (euhydrated, 2.5% dehydrated, 5% dehydrated), and dehydration of 4.8% increased markers of catabolism (cortisol, epinephrine, norepinephrine) and decreased markers of anabolism (testosterone) . Hormone concentrations are not the same measurement as muscle protein synthesis rate.

• The cell-volume mechanism originates in liver, not muscle. Cell volume changes were identified as important mediators of proteolysis control by amino acids and hormones in the liver, and the cellular hydration state was suggested to be an important determinant of protein catabolism in health and disease . The underlying experiments include cell swelling inhibiting proteolysis in perfused rat liver and progressive cellular dehydration and proteolysis in critically ill patients .

• The specific claim that muscle protein synthesis drops at 2 percent dehydration appears to circulate through commercial blogs citing a source that does not appear to be a dehydration study. One such blog states that muscle protein synthesis rates are reduced when participants were dehydrated by just 2% body mass (Tipton et al., 2001) . The Tipton 2001 papers I could locate concern something entirely different: Tipton et al. (2001) reported that a mixture of 6 g essential amino acids and 35 g sucrose ingested prior to exercise was more effective in stimulating muscle protein synthesis than ingesting the same mixture immediately after exercise , and Tipton and Wolfe 2001 is a reference point in the controversy over whether acute measurements of muscle protein synthesis predict chronic changes in muscle mass with resistance training . Neither involves hydration status.

On the source chain of the post itself. The wording is not original. Near-identical phrasing appears on commercial health content pages, for example: "Muscle tissue is roughly 75% water... Even mild dehydration (1–2% bodyweight loss in fluids) can impair strength, endurance, and protein synthesis—undermining your mass-gain goals" and "Muscle is roughly 75% water, so it is no surprise that muscle function is impacted by dehydration" . This indicates the post is downstream aggregation of existing fitness-content boilerplate rather than independent reading of the literature.