A new study found that the benefits of fasting may depend heavily on what happens when eating resumes.
Researchers discovered that shutting down fat-burning processes after a fast appeared to be important for lifespan extension in laboratory worms.
The findings could help scientists better understand the biology of aging, though the research has not yet been proven in humans.
Fasting has long been studied for its potential effects on health and longevity. Scientists have known for years that calorie restriction and intermittent fasting can extend lifespan in a variety of organisms, but exactly why that happens has remained something of a mystery.
Now, researchers from UT Southwestern Medical Center say they may have uncovered an important piece of the puzzle.
Their findings, published in Nature Communications, suggest that the life-extending benefits associated with fasting may not come from the fasting period alone. Instead, a critical factor could be how the body responds when food becomes available again after a fast.
“Our discoveries shift the focus toward a neglected side of the metabolic coin – the refeeding phase. Our data suggest that the health-promoting effects of intermittent fasting are not merely a product of the fast itself, but are dependent on how the metabolic machinery recalibrates during the subsequent transition back to a fed state,” study leader Peter Douglas, Ph.D., said in a news release.
The study
The study was conducted in Caenorhabditis elegans, a tiny roundworm frequently used in aging research because many of its biological pathways are similar to those found in other animals.
While the results don't directly translate to humans, they provide new clues about the cellular processes that may influence aging.
During fasting, the body shifts from using readily available glucose for energy to breaking down stored fat. The researchers focused on a protein called NHR-49, which helps activate this fat-burning process when nutrients are scarce.
The results
Initially, scientists expected that this fat-burning activity would be responsible for the lifespan benefits linked to fasting. However, experiments revealed something unexpected.
Even when the worms lacked NHR-49, fasting still increased lifespan and helped older worms maintain more youthful movement.
The more significant finding emerged after the fasting period ended. Researchers discovered that when worms were refed, a separate biological mechanism switched off NHR-49 and slowed fat breakdown. When scientists interfered with this shutdown process and kept fat-burning pathways active after refeeding, the lifespan benefits of fasting disappeared.
In other words, the ability to transition efficiently from a fasting state back to a fed state appeared to be crucial.
What this means for consumers
The findings add to a growing body of research suggesting that metabolic flexibility — the body's ability to adapt to changing nutrient conditions — may play an important role in healthy aging. Rather than focusing solely on what happens during fasting, scientists may need to pay closer attention to the biological changes that occur when eating resumes.
It's important to note that this study was conducted in laboratory worms, not people. The researchers say much more work is needed before these discoveries could lead to recommendations for human health or aging.
Still, the research offers an intriguing new perspective. If future studies confirm similar mechanisms in humans, scientists may eventually be able to develop treatments that mimic some of fasting's benefits without requiring people to follow strict fasting regimens.
For now, the study highlights just how important the body's recovery and rebuilding phase may be after periods without food.