Keto Fit Pro The fastest way to lose weight, without a doubt, is fasting. In a 2002 experiment conducted by resident scientists from Rowett Research Institute in Aberdeen, Scotland, it was revealed that a 36 hour deprivation of food can result in a loss of 1% to 2% of the subject's body weight. As an example, if you weighed 300 lbs., then you could expect to lose 3 to 6 lbs. after only 36 hours of fasting.