Losing 30 pounds in 4 to 8 weeks is much too fast. If your average weight loss is more than 2 pounds a week, then you are not eating enough. The clinical guidelines for obesity recommend a loss of ½ to 2 pounds a week.  At first, most people lose a bucket of water weight and very obese people may lose faster, but that does not continue. If your BMI is > 30 or you have a medical condition that necessitates quick weight loss, you could consider a very low calorie diet (VLCD) with less than 800 calories a day.  VLCDs are administered by trained physicians who provide special monitoring and supplementation.