After the fourth or fifth day of dieting the daily loss of weight begins to decrease to one pound or somewhat less per day, and there is a smaller urinary output. Men often continue to lose regularly at that rate, but women are more irregular in spite of faultless dieting. There may be no drop at all for two or three days and then a sudden loss which reestablishes the normal average. These fluctuations are entirely due to variations in the retention and elimination of water, which are more marked in women than in men.
The weight registered by the scale is determined by two processes not necessarily synchronized. Under the influence of HCG, fat is being extracted from the cells, in which it is stored in the fatty tissue. When these cells are empty and therefore serve no purpose, the body breaks down the cellular structure and absorbs it, but breaking up of useless cells, connective tissue, blood vessels, etc., may lag behind the process of fat-extraction. When this happens the body appears to replace some of the extracted fat with water which is retained for this purpose. As water is heavier than fat the scales may show no loss of weight, although sufficient fat has actually been consumed to make up for the deficit in the 500-Calorie diet. When then such tissue is finally broken down, the water is liberated and there is a sudden flood of urine and a marked loss of weight. This simple interpretation of what is really an extremely complex mechanism is the one we give those patients who want to know why it is that on certain days they do not lose, though they have committed no dietary error.
Patients who have previously regularly used diuretics as a method of reducing, lose fat during the first two or three weeks of treatment which shows in their measurements, but the scale may show little or no loss because they are replacing the normal water content of their body which has been dehydrated. Diuretics should never be used for reducing.
Excerpt source: Pounds and Inches by Dr. Simeons