GAINESVILLE, Fla. — North American wolf packs can feast on bison or elk and then go two weeks before the next kill. But the wolves’ bodies and brains still perform at high levels as they hunt for their next meal, researchers say.
Humans, too, appear well-adapted to periods of hunger. This is likely a trait evolved from early human hunter-gatherers who, like wolves, alternated between periods of feast and famine.
The answer as to how the body manages this, University of Florida researchers said, provides insight into an increasingly popular dieting strategy — intermittent fasting.
UF Health researchers said in a recent study published in the journal Obesity that emerging findings in scientific literature show intermittent fasting can be a reliable means of weight loss and may optimize physiological functioning, enhance performance and slow the aging and disease process.
To understand why, researchers said, one must look to how the body essentially flips a “metabolic switch” during fasting. This means the body moves from burning glucose, or sugar, for energy to fatty acids and their byproduct, ketones.
During fasting, the body converts fat into fatty acids, which can be absorbed by the blood.
Stephen Anton, Ph.D., the division chief of clinical research for the UF College of Medicine’s department of aging and geriatric research and the paper’s lead author, said research indicates ketones are the preferred fuel for the brain and body during periods of fasting and extended exercise.
Ketones, he said, are a cleaner source of energy than glucose, protein or carbohydrates, the body’s other sources of energy. That is because ketones produce fewer “metabolic disrupters,” or molecules that can harm cells.
“This switch can happen after a certain period of time fasting,” said Anton, who also is affiliated with the UF Institute on Aging. “It’s a gradation in which your metabolism over time shifts to use higher and higher amounts of ketones for energy.”
Typically, he said, after eight to 12 hours of fasting, the levels of ketones in the blood significantly increase.
Anton’s paper looked at two popular forms of intermittent fasting. The first is time-restricted feeding, when the dieter eats during discrete windows during the day. For example, they might fast up to 16 hours a day, eating during the other eight hours.
And in that window, the dieter isn’t restricted to what they eat. “Of course, we recommend healthy food,” Anton said.
The second approach is called alternate-day fasting. In the more common model of this fasting regimen, people limit their meals on one day, usually 500 calories. On the next day, they can eat anything at all. “It can be truly called a feasting day,” Anton said.
A second version of this fasting method is to eat nothing at all on one day while feasting the next.
In a review of scientific literature, Anton and his colleagues found that people lost significant body fat in 10 of 10 clinical trials involving alternate-day fasts. “So in my mind, it’s not a question of whether it works for producing fat loss,” Anton said.
Three of four time-restricted feeding studies demonstrated significant fat loss.
In most studies, participants did not lose significant lean tissue, which is tissue other than fat that includes the body’s organs, muscles and skeleton. An adequate amount of lean tissue, Anton said, is necessary to help maintain good physiological function as people age, among other health benefits.
Other forms of dieting, however, often lead to a reduction in both fat and lean tissue, he said.
Anton said research in rodents and other nonhuman species points to the possibility that food restriction, and the turning of that metabolic switch, can lengthen lifespan, improve metabolic health, cognitive and physical performance, lower inflammation and lead to superior cardiovascular health.
“An important takeaway is that we all have the ability to switch our metabolism from glucose to ketone utilization,” said Anton. “And that switch has the potential to have profound health benefits for us, in addition to the positive changes in body composition.”
Anton urges anyone considering a new diet to consult with a physician, noting fasting may not be the right for everyone.
Co-authors of the paper include William T. Donahoo, M.D., an associate professor in the UF College of Medicine’s division of endocrinology, diabetes and metabolism; Stephanie A. Lee, a doctoral student in the UF College of Medicine’s department of aging and geriatric research and the department of clinical and health psychology in the UF College of Public Health and Health Professions; Arch G. Mainous III, Ph.D., the Florida Blue Endowed Professor of Health Administration and chair of health services research, management and policy in the UF College of Public Health and Health Professions; Christiaan Leeuwenburgh, Ph.D., a professor and chief of the UF College of Medicine’s division of biology of aging who also is affiliated with the UF Institute on Aging; and researchers from The Johns Hopkins University and the National Institute on Aging Intramural Research Program, both in Baltimore.