Avoid eating for 16 or 18 hours daily, or abstain two days weekly.
These are common examples of intermittent fasting, adopted for weight reduction. The idea is that the body starts using fat stores during fasting, potentially leading to weight loss.
"There are many health benefits to fasting, but intermittent fasting itself does not lead to weight loss," says Philip Ruppert, a postdoctoral researcher at the Institute for Biochemistry and Molecular Biology, specializing in ketogenesis—the state where the body burns fat instead of carbohydrates.
Along with Sander Kersten, a professor at the Dutch Wageningen University and Research, Ruppert authored a review article summarizing existing research on metabolic mechanisms like ketogenesis and fatty acid oxidation during fasting.
This article is published in Trends in Endocrinology and Metabolism.
More research focuses on obesity than fasting.
"Even during intermittent fasting, the main rule is to consume fewer calories than we burn. You can't just eat whatever you want when not fasting. You also can't eat as usual if your normal diet exceeds your caloric burn. This basic physiology can't be altered by intermittent fasting," Ruppert explains.
Though intermittent fasting doesn't directly slim down, Ruppert advocates for this new fasting trend. The body's ability to store fat and calories during limited food access is an evolutionary result, ensuring survival in times without food storage and preservation like today.
"Today, there's a lot of research on what goes wrong in our metabolism when we become obese. But less attention is paid to what happens metabolically when we fast. I believe more research in this area could lead to more effective treatment strategies for obesity and its consequences," Ruppert says.
Though intermittent fasting doesn't inherently cause weight loss, Ruppert believes it offers several health benefits, including:
Learning how the body reacts to eating and fasting, thus understanding its needs.
Gaining more energy.
Syncing with natural circadian rhythms for better sleep and metabolic efficiency.
Other benefits mentioned in scientific literature include improved blood pressure, insulin sensitivity, blood sugar levels, skin health, and lower resting heart rate.
Long-term fasters can feel almost euphoric
Some people fast for extended periods, like several days or weeks. Research shows these fasters may experience a significant shift after 2-3 days of fasting.
"At first, they're hungry, but that disappears after 1-2 days, and they can become almost euphoric. It's similar to a runner's high, lasting until they start eating again," Ruppert says. He fasts for seven days annually, experiencing the disappearance of hunger and onset of well-being.
"One experiences a mental clarity that's hard to describe," he notes.
Science doesn't yet fully explain why fasting leads to mental clarity and possibly euphoria, but a theory is the formation of ketones during fasting. These special energy molecules are particularly needed by the brain.
"The brain is fed ketones during fasting. Perhaps that's why one experiences clarity," Ruppert suggests, noting this is still a hypothesis without definitive research.
Individual responses to fasting vary based on factors like age, gender, genetics, and lifestyle.
"But factors like season might also matter. For example, we get less vitamin D in winter than summer, and we know vitamin D is important for many metabolic processes. Hormones might also play a role, like whether a woman is menopausal or menstruating. All these factors can influence, and it's something I'd like to continue researching, so each individual can get solid, research-based recommendations tailored to them," Ruppert adds.
Fasting Phases:
Nutrients from food are digested, absorbed, and stored.
After nutrient absorption, the liver uses glycogen stores to maintain blood sugar, while fat tissue releases fatty acids.
Liver glycogen stores deplete, and the liver starts producing ketones for energy.
Ketone production in the liver ramps up, with the body primarily using energy from fat tissue. This phase can last several weeks.
When body fat is depleted, proteins in muscle mass are the last energy source to prevent death.
Depending on body fat percentage, a person can endure fasting from a few weeks to several months.
