To lose body fat, you must be in negative calorie balance (a calorie deficit). You can create a calorie deficit by increasing activity, by decreasing calories or with a combination of both. The most efficient approach to fat loss is to decrease your calories a little and increase your activity a lot.
The most commonly recommended guideline is to reduce your calories by 500 to 1000 less than your maintenance level. For example, if you are female and your calorie maintenance level is 2100 calories per day, then a 500 calorie deficit would put you at 1600 calories per day. If you're a male with a calorie maintenance level of 2900 calories per day, then a 500 calorie deficit would put you at 2400 calories per day.
A 500 calorie deficit over seven days is 3500 calories in one week. There are 3500 calories in a pound of fat, so (in theory), a 500 calorie per day deficit will result in a loss of one pound of body fat per week. It follows that a 750 calorie deficit would produce a loss of one and a half pounds per week and a 1000 calorie deficit would produce a two pound per week reduction.
Because of the way the weight regulating mechanism works, fat loss seldom follows these calculations precisely, so don't get caught up in them. An emphasis on exercise with a small reduction in calories is the best approach. 500 to 750 calorie deficit below your maintenance level is usually plenty. Add weight training and aerobics into the mix and this will produce as close to 100% fat loss as possible.
An alternate (and preferred) method is to set your calorie deficit as a percentage of your maintenance level. 15-20% is the recommended starting calorie reduction for fat loss. This is considered a small calorie deficit and a small calorie deficit is the key to losing fat while maintaining muscle.
With a 2100 calorie maintenance level, 20% would be a 420 calorie deficit, which would put you at 1680 calories per day. With a 2900 calorie maintenance level, a 20% deficit would be 580 calories. That would put you at 2380 calories per day.
The reason the percentage method is better is because using an absolute number like 500, 750 or 1000 calories as a deficit instead of a percentage deficit might drop your calories into the danger zone. For example, if you are a male with a 3500 calorie maintenance level, a 750 calorie deficit to 2750 calories per day is only a 21% drop (a small, safe and acceptable deficit.) However, if you are a female with an 1800 calorie per day maintenance level and you cut your calories by 750 per day to 1050 calories, that is a 41% cut. Using the percentage method is more individualized.
At times, an aggressive calorie deficit greater than 20% may be called for, but calorie cuts greater than 20% are much more likely to cause muscle loss and metabolic slowdown. If you do use a calorie deficit greater than 20%, then it's wise to raise calories at regular intervals using the "zig-zag method" you'll learn about in chapter six. This will "trick your body" and prevent your metabolism from slowing down when you have a large calorie deficit.
Always start with a small deficit. In other words, cut calories out slowly. It's better to start with a small deficit and then progressively increase towards your maximal deficit than to make a sudden drop in calories all at once. The body cannot be forced to lose fat - you must coax it.
Based on what you now know about the body's weight-regulating mechanism, the optimal amount to decrease your caloric intake for fat loss is as little as possible - as long as you're still losing body fat.
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