The amount of weight one can lose eating 300 to 500 calories daily for a week varies based on activity level, height and weight. However, eating such a small number of calories can lead to nausea, diarrhea, fatigue, constipation and gallstone formation, and can make the body hold onto fat.
According to the U.S. Department of Health and Human Services, the lowest number of calories a woman should consume each day is 1,000 calories, and men should not restrict their caloric intake below 1,200 calories daily. Eating too few calories can cause the body to go into starvation mode, which means that no weight loss occurs. Slow and steady weight loss is the safest and healthiest approach with an aim of losing 1 to 2 pounds a week.