Low-Carb Diets and Fasting: Why Rapid Weight Loss Isn't Sustainable

Fasting and low-carb diets: risks and sustainability

Starvation or a significant reduction in carbohydrates, especially when combined with physical exercise, is not recommended for sustainable and healthy weight loss.

Studies show that within the first 3–7 days of starting a low-carb diet, such as the keto diet, many people experience rapid weight loss of 1 to 5 kg. However, this rapid weight loss is not due to burning fat, but rather to reducing water retention in the body. When carbohydrates are reduced, glycogen levels, which store water in the muscles and liver, also decrease. The process of glycogen breakdown leads to the elimination of water from the body, which is the main reason for the initial rapid weight loss.

Scientific evidence

One of the most interesting studies on this subject, published in The American Journal of Clinical Nutrition, shows that the initial weight loss on low-carb diets is almost entirely water, not fat. This is the result of a metabolic shift in which the body begins to use fat for energy, but initially relies on glycogen, which is bound to water. After adaptation, water levels stabilize.

If carbohydrates are reintroduced into the diet, glycogen stores are restored, and with them water weight. Studies by experts at Harvard Medical School show that initial weight loss is not permanent and is not the result of fat burning.

Yo-yo effect and long-term approaches

These scientific facts emphasize the importance of long-term and balanced approaches to weight loss. Short-term diets that promise quick results often lead to yo-yo effect - rapid weight regain after returning to normal eating. For sustainable weight loss, it is important to build healthy habits with a varied diet and regular physical activity.

Fasting, training and muscle health

Training without proper nutrition, especially during low-carb diets or fasting, can lead to serious problems:

• Reduced muscle mass – lack of enough carbohydrates and protein for muscle recovery.
• Impaired recovery – prolonged regeneration period after training.
• Increased risk of injuries – muscles do not function optimally.

Studies in The Journal of Physiology show that a lack of glycogen leads to faster fatigue and lower exercise efficiency. Not enough protein also leads to catabolism—the breakdown of muscle tissue for energy, which is detrimental to long-term health.

Symptoms of nutritional deficiency in active exercisers

• Exhaustion and fatigue – reduced energy for daily activities and workouts.
• Muscle weakness and reduced endurance – muscles cannot work at full capacity.
• Slow recovery – slowing down recovery processes after training.
• Injury risks – greater susceptibility to injuries during high-intensity training.
• Hormonal imbalances – decreased testosterone in men, menstrual disorders in women.

Electrolyte imbalance and risk of dehydration

One of the main risks of low-carb diets or fasting, especially with intense training, is electrolyte imbalance and risk of dehydration.

• Muscle cramps: A deficiency in sodium, potassium, and magnesium can lead to muscle cramps.
• Fatigue: Disturbed electrolytes → reduced energy and endurance.
• Headache and dizziness: Lack of water and electrolytes → reduced blood supply to the brain.
• Hypertension: Imbalance of salts and fluids → increased blood pressure.
• Hypoglycemia: Decrease in glycogen → drop in blood sugar, tremors, irritability.
• Muscle loss (catabolism): The body breaks down muscle for energy.
• Delayed recovery: Insufficient electrolytes → extended recovery time.
• Delayed anabolic response: Lack of nutrients → delayed muscle protein synthesis.

Is this approach sustainable?

The answer is no. Low-carb diets and fasting may produce short-term results, but they are not sustainable for long-term health.

1. Yo-yo effect: Rapid weight loss and regain → weight gain/loss cycle.
2. Reduced muscle mass: Insufficient nutrition → muscle breakdown and decreased strength.
3. Dehydration and electrolyte imbalance: Prolonged loss of water and salts → fatigue, cramps, injuries.

In conclusion, radical dietary restrictions, such as fasting or low-carb diets, may produce short-term results, but they carry serious health risks. For sustainable results, it is best to follow a balanced approach that includes all macronutrients and regular exercise.