Ask the experts
When relying on body fat percentage as a measure of fitness goals, how rapidly should one's body fat composition change with diet and exercise? What is a normal/healthy rate of decrease in body fat percentage?
You can expect, on average, to lose 1% to 3% of your body fat per month, but the range of loss varies widely between individuals because there are so many variables that affect body composition, including age, gender, amount of body fat and muscle mass that you start with, and a myriad of hormones that control how efficient your body is at storing fat and metabolizing it. Unlike body weight, where no more than two pounds per week of loss is recommended, there are no standards or guidelines for how quickly to lose body fat or gain muscle because there hasn't been enough research to determine what a healthy body fat percentage is.
You should also know that when you lose weight as much as 25% of the lost weight could be muscle. That is, you could lose 25 pounds of muscle if you lose 100 pounds. The principle problem in terms of weight loss with losing so much muscle is that muscle is the metabolically active "organ" in your body that burns most of the calories, and so losing it could slow down your weight- and fat-loss efforts. In fact, losing muscle is one of the factors involved in weight-loss plateaus. The good news is that studies show that aerobic and resistance exercise can prevent muscle loss by as much as 8% and 20%, respectively, if you do it consistently three or more times per week.
Measuring body composition during weight loss is objective data that could prove as important as measuring weight, and so I recommend using a bioelectric impedance (BIA) scale to do that. BIA scales estimate body fat by sending a very weak (and safe) electrical signal through your body and then measuring the resistance to the signal created by your body fat. Fat is a poor conductor of electricity, and so the more fat you have the more resistance there is. There is a 3% to 5% error possible with BIA that depends on your hydration status, time of last meal, last bout of exercise, amount of alcohol in your system, and some medications (for example, diuretics). The error can be even higher than 5%, so be sure to follow the manufacturer's guidelines to obtain the most reliable estimate. You can find BIA scales online for under $50. Tanita and Omron are two of the reputable companies selling them. I recommend measuring body fat once a month since it changes in small increments.
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