If you don't eat for 2 days, several physiological changes occur as your body adapts to the lack of food:
- Energy source shift: Within the first 24 hours, your body uses up the glucose from your last meal and then starts converting glycogen stored in your liver and muscles into glucose for energy. By the second day, glycogen stores are depleted, so your body begins breaking down muscle tissue for energy temporarily while it shifts metabolism
- Ketosis begins: To prevent excessive muscle loss, your body starts relying on fat stores, converting fat into ketone bodies to use as an alternative energy source. This process is called ketosis and usually starts within 48 to 72 hours of fasting
- Symptoms: Early symptoms of not eating include fatigue, difficulty concentrating, nausea, shakiness, headaches, irritability, and mood changes like anxiety or anger. You may also experience hunger pangs and increased irritability ("hangry"). As the fast continues, symptoms like dizziness, weakness, and cognitive impairment can occur
- Metabolic slowdown: Your metabolism slows down to conserve energy, reducing the rate at which your body burns calories
- Weight loss: Significant weight loss occurs, mostly from water loss initially, but also from fat and some muscle breakdown. In the first 5 days, weight loss can be 1–2 kg (2.2–4.4 lbs) per day, mostly from dehydration and electrolyte imbalance
- Mental effects: Prolonged fasting can affect your mental state, causing mood swings, irritability, and difficulty concentrating
Overall, after 2 days without food, your body is in a state of ketosis, using fat for energy, with depleted glucose and glycogen stores, accompanied by symptoms like fatigue, irritability, and cognitive difficulties. While short- term fasting can be survived by healthy adults, continuing beyond this point without food leads to more severe physical and mental effects
