“Sweat is your body's natural cooling system, not a measure of fat loss. How much you sweat depends on: Temperature, Humidity, Genetics, Hydration, Fitness level. Sweat cools your body. It doesn't tell you how much fat you've burned." Plus the personal account: eating "barely 1200 calories" with HIIT and home workouts caused weight loss…”

The core physiological statement is consistent with established exercise physiology. Sweating is an evaporative heat loss mechanism. A review of individual characteristics affecting sweating found that improvement in aerobic fitness and heat acclimation promote sweating, that males exhibited higher sweat rates than females when metabolic heat production was high, and that surface area to mass ratio also influences the response. On the determinants question, the strongest primary source adds an important nuance the post omits. Cramer and Jay noted that individual variation in thermoregulatory responses to exercise is large, and that although aerobic fitness (VO2max) and body fatness are traditionally considered important predictors of core temperature and sweating responses, recent evidence indicates important and independent roles for biophysical factors. Using stepwise regression across 69 trials in 28 males, they examined how much individual variability in rectal temperature change, whole body sweat loss, and steady state local sweat rate was independently described by biophysical factors associated with metabolic heat production and evaporative heat balance requirements relative to body size, versus factors related to VO2max and body fatness. The practical implication is that the dominant driver of how much you sweat is how much metabolic heat you produce relative to your body size and the environment's capacity to evaporate it, with fitness playing a smaller independent role than popular fitness content assumes. On hydration, the claim is supported. Hypohydration increases heat storage by reducing sweating rate and skin blood flow responses for a given core temperature, with both hypertonicity and hypovolemia contributing to reduced heat loss. Institutional reference material states that hypohydration is associated with reduced or unchanged sweating rates at a given metabolic rate during exercise in the heat, and that sweating rate is lower for a given core temperature under hypohydration, reducing the potential for heat dissipation through sweat evaporation. On environment, temperature and humidity are established determinants. High ambient temperature increases heat load and forces a higher sweat rate to compensate, while high humidity decreases cooling effectiveness because evaporation is limited in moisture saturated air. Modern prediction work confirms these variables are modelled together. Whole body sweat rate prediction equations were developed and validated using 112 and 171 trials respectively for cycling and treadmill exercise, across dry bulb temperatures of 20C to 40C, relative humidity of 14% to 60%, and intensities of roughly 40% to 85% of peak aerobic power. On the second, anecdotal claim: the underlying physiology is well supported. A randomized trial found that lean body mass increased in a higher protein group during an energy deficit with intense exercise (1.2 kg) and to a greater extent than a control group (0.1 kg), while fat mass loss was also greater in the higher protein group. This is consistent with the idea that low intake without adequate protein and resistance training tends to produce less favourable body composition change, though the author's own body composition was never measured.