The quickest way to lose weight is to starve yourself. Fasting is the quickest weight loss strategy because it causes a loss of muscle mass, glycogen, water, bone density and body fat, and this provides rapid weight loss. However, the weight loss although rapid initially, does not continue for long because the hypothalamus soon initiates countermeasures to increase the efficiency of energy metabolism. These countermeasures include a huge increase in appetite, a down regulation of resting metabolic rate and an inhibition for the motivation and fuels for physical activity. The loss of bone and muscle mass also makes long term fasting dangerous and unhealthy, because losses of lean mass are associated with mortality. After fasting, the next quickest way to lose weight is a very low calorie diet, and the the principles at work are the same as with fasting, but the weight loss is not quite so extreme. Both fasting and very low calorie diets are poor strategies for long term weight loss as they cause permanent damage to energy metabolism that increases the risk of future weight gain significantly.
So fasting and very low calorie diets are detrimental to the health. Perhaps the question should therefore be rephrased. Perhaps instead we should ask, not what is the quickest way to lose weight, but instead what is the best way to lose weight? In this regard many people assume that calorie restriction, often in combination to aerobic exercise, the the best way to lose weight. However, evidence from the literature does not support this contention. In fact, calorie counting although not as severe, still leads to the same ill health as fasting and very low calories diets, but with the former, the journey to the grave takes longer. In other word, the weight loss includes muscle and bone loss and this induced permanent damage to the resting metabolic rate and causes significant health problems. High quality diets address the problems of calorie restriction because they address the cause of the weight gain, which is insulin resistance caused by low quality foods. High quality diets lower insulin levels, and as insulin is a brake on fat oxidation, they increase body fat oxidation, ultimately leading to effortless fat loss, without lean tissue losses.