According to the American Council on Exercise, essential body fat for men is 2 to 5 percent, and the “athlete” range is 6 to 13 percent. However, “although single digits seems enticing, low double-digits are just as impressive and easier to attain and maintain,” says Angel Aulet, an International Physique League Athletic Pro and personal trainer and group fitness instructor at serveral boutique gyms in NYC. “The average man can live his entire life with 12 or 13 percent and still have abs and look good—I’ve been there for the last 12 years.” (He’s not a complete freak of nature; he was in the more mortal 16 to 17 percent range before that.)

That said, if you’re starting at around 15 percent right now, you can work to get yourself down into the hallowed single digits in about two months, at least temporarily. Aulet does it before his shows and photo shoots, but he warns: It ain’t pretty. “Expect mood swings!” he jokes (kind of). He also points out, that as with anything, there are pros and cons. “You will have a fantastic-looking physique, but your face may look a little gaunt—expect dark undereye circles,” he says. “You may also feel weakness, possibly sudden, or lightheadedness because you’re not taking in enough carbs.” In which case, eat some, please! And you should also talk to your doc about your plans beforehand.

Finally, “consider your current profession and lifestyle when entering a program like this. If you’re not a fitness model or competitor, do you really need to go there?” asks Aulet. “During this time, you’re going to say no to social events, and casual eating out is going to be difficult—alcohol is not part of it.” To that end...