In calorie-restricted mice, the body shifts to fasting metabolism 12 hours before the stomach empties, while equally hungry mice without that history respond only once the stomach is empty and accumulate fat in the liver
In calorie-restricted mice, the body shifts to fasting metabolism 12 hours before the stomach empties, while equally hungry mice without that history respond only once the stomach is empty and accumulate fat in the liver
The laboratory of Roman Kondratov (Cleveland State University) compared calorie restriction in mice with a single hunger-feeding-hunger cycle matched for the same total food intake and fasting duration. In the CR group, metabolism switched to fasting mode many hours before the stomach was empty; in the second group, the switch tracked stomach emptying in real time. Only CR produced the benefits: better glucose tolerance and less liver fat. The results were published on September 17 in Cell Reports.
Calorie restriction (CR) in rodents is set up simply: the mouse receives 70% of its normal ration as a single meal per day, eats it within about two hours, and then goes roughly 22 hours without food. The prolonged fast is thought to be part of what makes CR such a powerful intervention for lifespan. But what exactly does the work, the sheer length of the fast or something specific to the repeating pattern, remained unclear.
The same laboratory had previously found that CR does not extend lifespan in mice whose circadian clocks (the internal daily rhythm) are broken, and had already compared CR with a single unexpected fast. In that earlier experiment, however, the food amount and feeding onset were not matched precisely. To close that gap, mice with no CR history were given a single episode: food was removed for 22 hours, returned for two hours, and removed again for 22 hours. In total food intake and fasting duration the FRF group matched CR exactly, leaving one difference: months of habitual CR versus a single episode.
From there, the two groups diverge. In FRF, everything follows stomach status: the stomach empties quickly, nearly empty by six hours, and insulin drops, mTOR weakens (mTOR is the growth and protein synthesis pathway that is suppressed during fasting), and the body shifts from burning carbohydrates to burning fat. In CR, the stomach empties slowly and is not fully clear until hour 18, yet the metabolic changes engage earlier: insulin drops by two hours, and mTOR is suppressed by six hours, a full 12 hours before the stomach is empty.
The CR mice behave as though their bodies anticipate the coming fast and prepare for it before the food has physically run out.
The early suppression of mTOR appears to be more than coincidence: in early September, a separate study showed that the clock protein PER2 binds to mTOR and tunes its response to fasting, suggesting that mTOR acts as an effector of the circadian clock.
The difference is also visible at the gene level: in CR mice, the rhythmicity of liver clock genes increases, including genes involved in fat and carbohydrate metabolism, while in FRF mice on the same feeding schedule the rhythm breaks down. The same 2497 genes that are rhythmic under normal feeding and under CR lose their rhythmicity entirely under FRF. The 2017 Nobel Prize in Physiology or Medicine was awarded to Jeffrey Hall, Michael Rosbash, and Michael Young for deciphering the molecular machinery of circadian clocks.
In the CR group, blood sugar stays level throughout the cycle, and a glucose tolerance test (measuring how quickly the body clears sugar from the blood) shows a clear improvement. In FRF, blood sugar spikes sharply after feeding and does not come down during fasting; tolerance does not improve at all. The difference in liver fat is even more striking: CR mice have almost none at any time point, while FRF mice accumulate liver fat progressively during the fast, reaching a peak at hour 18.
A common view in the longevity community holds that any sufficiently long fast works like CR, since the body is equally hungry either way. This study contradicts that view: with hunger matched, a single hunger-feeding-hunger cycle performs worse than both CR and unrestricted feeding. The experiment was done in mice, comparing CR (one meal per day) with a laboratory fasting cycle, not with practices such as 16:8 time-restricted eating in humans.