Did you lose muscle or fat by not eating for three days

Not eating for three days results in both fat and muscle loss, but in the initial stage, water and glycogen consumption are the main factors. Long term fasting can accelerate muscle breakdown and reduce fat metabolism efficiency.

The human body will preferentially consume glycogen stored in the liver and muscles during short-term hunger. Each gram of glycogen binds a large amount of water, which results in weight loss in the first 24 hours mainly due to water loss. As glycogen reserves are depleted, the body begins to break down fat for energy, but fat metabolism requires sufficient time to adapt. At this point, protein breakdown also occurs synchronously, and amino acids in muscle tissue are converted into glucose to maintain blood sugar stability. Not eating for three consecutive days will cause a decrease in basal metabolic rate, and the body will activate protective mechanisms to reduce energy expenditure. Although ketone bodies produced by fat breakdown can partially replace glucose function, organs such as the brain still require carbohydrate energy supply. In this case, the proportion of muscle loss will significantly increase, especially for people who lack exercise. Long term dieting can also lead to thyroid suppression, which in turn reduces the efficiency of subsequent weight loss.

Healthy weight loss should ensure a moderate daily calorie deficit, and it is recommended to maintain muscle mass through a high protein diet combined with resistance training. Moderate supplementation of branched chain amino acids can help reduce muscle breakdown, and regular eating can maintain metabolic activity. If a light fasting is required, it is recommended to use a 16:8 timed feeding method and ensure sufficient water and electrolyte supplementation during the fasting period.