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RIVERSIDE, Calif. — For scientists, mice often serve as stand-ins for humans. So, what happens when these tiny test subjects are bred to be Olympic-level athletes? Researchers from the University of California-Riverside are shedding light on how exceptionally active animals respond to food and calorie restrictions, with surprising implications for our understanding of diet, exercise, and metabolism.
For over six decades, scientists have been breeding “High Runner” (HR) mice – the rodent equivalent of elite athletes – that voluntarily run nearly three times as much as normal mice. When researchers decided to test how these super-fit mice would respond to severe food restrictions, the results, published in Physiology & Behavior, were nothing short of astonishing.
These mice maintained their incredible running regimens and body weights even when their food was cut by 20%, and they only showed modest changes when food was slashed by 40%. This resilience in the face of drastic calorie reduction flies in the face of conventional wisdom about energy balance and weight loss.
“People often lose about 4% of their body mass when they’re dieting. That’s in the same range as these mice,” says UCR biologist and corresponding study author Theodore Garland, Jr., in a media release.
While we can’t directly equate mouse biology to human physiology, this study raises intriguing questions about our own capacity to adapt to reduced calorie intake, especially for highly active individuals. Could some humans, particularly those who are very physically active, have similar abilities to maintain their weight and performance even when drastically cutting calories?
Moreover, these findings challenge the simplistic “calories in, calories out” model of weight management that has dominated public health messaging for decades. The mice in this study seemed to have developed remarkable metabolic efficiency, suggesting that our bodies might be far more adaptable to changes in food intake than we previously thought.
For the average person trying to lose weight or improve their fitness, this study offers a fresh perspective. However, it also underscores the complexity of metabolism and the dangers of one-size-fits-all approaches to nutrition and fitness.
For athletes and fitness enthusiasts, this study might offer hope that their bodies can adapt to periods of reduced calorie intake without necessarily sacrificing performance. However, it’s crucial to remember that the mice in this study were the product of generations of selective breeding for high activity levels, so their responses may not be typical.
The findings also contribute to our understanding of eating disorders, particularly those involving high levels of exercise combined with food restriction. The ability of these mice to maintain their weight despite severe calorie restriction could shed light on why some individuals with eating disorders struggle to lose weight even when severely limiting their food intake.
This study opens up a world of possibilities for future research. What if we could unlock the genetic secrets that allow these mice to maintain their performance under such extreme conditions? Could this lead to new treatments for metabolic disorders or more effective strategies for weight management? As scientists continue to explore these questions, we may be on the cusp of a new era in our understanding of human health and fitness.
“We don’t want people on diets to say, ‘I don’t have enough energy, so I’ll make up for it by not exercising.’ That would be counterproductive, and now we know, it doesn’t have to be this way,” Garland concludes.
Paper Summary
Methodology
The researchers used 99 female mice from the 65th generation of a selective breeding experiment for high voluntary wheel-running behavior. The mice were divided into two groups: the High Runner (HR) mice and the Control (C) mice. All mice were given access to running wheels and food ad libitum for 19 days to establish baseline running distances. Then, their food intake was measured for three days. Following this, the mice underwent two phases of food restriction: a 20% reduction for 7 days, followed by a 40% reduction for another 7 days. Throughout the experiment, the researchers measured the mice’s body weight and recorded their wheel-running activity continuously.
Key Results
The study found that HR mice consistently ran about three times more than C mice throughout the experiment. During the 20% food restriction, neither group significantly reduced their running. With 40% restriction, C mice slightly increased their running, while HR mice showed a small decrease. Surprisingly, both groups maintained their body weight during the 20% restriction and only lost about 2-3% of their weight during the 40% restriction. The components of running behavior (duration and speed) were affected differently in HR and C mice during food restriction.
Study Limitations
The study was conducted only on female mice, so the results may not apply to males. The food restriction period was relatively short (two weeks total), so long-term effects weren’t assessed. The mice used were the product of many generations of selective breeding, so their responses may not be typical of regular mice or translatable to humans. The study didn’t investigate the physiological mechanisms behind the observed resilience to food restriction.
Discussion & Takeaways
The researchers were surprised by the mice’s ability to maintain high levels of activity and body weight despite severe food restrictions. This challenges assumptions about how animals respond to calorie restriction and suggests complex metabolic adaptations, especially in the HR mice. The study has potential implications for understanding human metabolism, eating disorders, and the relationship between genetics and exercise. However, the researchers caution against directly applying these findings to humans without further study.
Funding & Disclosures
The study was supported by NSF grants to Theodore Garland and a fellowship from Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – CAPES/BRAZIL to Ivana A.T. Fonseca. The authors declared no conflicts of interest.
