It's one of the hottest weight loss trends you'll find right now, and also one of the most hotly debated. Proponents of intermittent fasting, or IF—which requires people to perform short-term fasts at regular intervals, such as drastically limiting calorie consumption on certain days of the week, or during specific windows of the day—will tell you that IF will help you feel better, lose weight, and even help you stave off conditions such as heart disease. Several studies agree.
But the overall bulk of hard science on intermittent fasting remains slim, and doctors and weight-loss experts have warned that it's not for everyone. "Regarding intermittent fasting, I've seen very mixed results," writes Cynthia Sass, MPF, RD. "Many men, particularly those who struggle with excess weight… have reported positive results with this semi-fasting approach. But for many women I've counseled, any type of fasting—whether it be overnight for 16 hours every night, or capping calories at 500 two days a week—has seriously backfired."
Sass says that anyone who tries IF should prepare for side effects that include rebound overeating (you're fasting, after all), bad sleep, and muscle loss. According to a new study published in the journal Cell Reports, there could be another unwanted side effect to IF: your visceral fat—otherwise known as belly fat—may potentially not go away, and may even develop a resistance to fasting.
For the study, researchers in Australia examined various fat tissue types in mice to see how the tissue responds differently to every-other-day fasting, otherwise known as "alternate-day fasting." “Mouse physiology is similar to humans, but their metabolism is much faster, allowing us to observe changes more rapidly than in human trials, and examine tissues difficult to sample in humans," explained Dr. Mark Larance from The University of Sydney.
Larance and his colleagues found that when in a fasted state, the visceral fat essentially adapted to protect its energy stores. "During fasting, fat tissue provides energy to the rest of the body by releasing fatty acid molecules," says the study. "However, the researchers found visceral fat became resistant to this release of fatty acids during fasting. There were also signs that visceral and subcutaneous fat increased their ability to store energy as fat, likely to rapidly rebuild the fat store before the next fasting period."
In other words, if you've been trying every-other-day fasting and you haven't been seeing results, it could be that you have highly adaptable visceral fat.
Of course, it bears repeating that the study was performed on mice and only focused on every-other-day fasting. "Dr. Larance says it should be noted that findings from the intermittent study may not apply to different diet regimes such as the 5:2 diet (fasting 2 days out of 7) or calorie restriction, which is common in people wanting to lose weight," says the study.
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