Tip: Fasting Diets & Belly Fat Rebound

Two groups of mice were studied here. They were fed the same amount of food (after a short three-day dieting period). One group was given a day's worth of food and could nibble on it as they wished – the human equivalent of several small meals or eating instinctively.

The other group was given the same amount of food but were forced to eat it in "one meal" or about a 4-hour feeding window – similar to some intermittent fasting plans for people.

The fasting mice developed the mouse version of a beer belly – excess abdominal fat. Their bodyweights ended up the being the same as the control mice who nibbled during the day, but they greatly increased their intra-abdominal fat stores.

They also developed gorging or binge-eating behaviors, developed insulin resistance in their livers, and "a gene expression profile favoring lipid deposition." Basically, their genes started "preferring" to store more fat, especially in the belly.

In short, the intermittent fasting mice displayed a host of metabolic and behavioral abnormalities, including inflammation. The control mice who ate several meals per day did not.

Why? The researchers found that glucose lingered in the blood of mice that gorged and fasted, meaning the liver wasn't getting the insulin message.

"Under conditions when the liver is not stimulated by insulin, increased glucose output from the liver means the liver isn't responding to signals telling it to shut down glucose production," Dr. Martha Belury said.

"These mice don't have type 2 diabetes yet, but they're not responding to insulin anymore and that state of insulin resistance is referred to as prediabetes. If you're pumping out more sugar into the blood, adipose is happy to pick up glucose and store it."

Sure, it was an animal study, but it should give you pause if you're considering a fasting-based diet that limits your eating window for the day or encourages one big meal per day.

The study also shows us that (once again), body composition is about a lot more than "calories in, calories out." The study subjects ate the same amount of calories, but the "eating window" mice got fatter and unhealthier.

Human fasting-based diets are appealing to many, and they can be convenient and "fun," at least during the feeding window. But if people eventually develop the same issues as the mice, these plans could lead to fat loss followed by belly fat gain caused by various "metabolic miscues," as the researchers described them.

But there are some smarter ways to go about it. Most of them are more like semi-fasts and involve protein pulsing. Check out The 5/2 Diet for Lifters.

1. Kliewer KL et al. Short-term food restriction followed by controlled refeeding promotes gorging behavior, enhances fat deposition, and diminishes insulin sensitivity in mice. J Nutr Biochem. 2015 Jul;26(7):721-8. PubMed.