Sari Harrar
If couch-potato lab mice had beach-body dreams and if they could speak, they might tell you they’re thrilled by advances in the science of exercise and calorie-restriction (CR) mimetics.
In recent studies conducted at research centers across the United States, mice have chowed down, fattened up, exercised only if they felt like it, and still managed to lose body fat, improve their blood lipids, increase muscle power, avoid blood sugar problems, and boost heart function.
How did these mice get so lucky? They were given mimetics, experimental drugs that “mimic” the effects of exercise and calorie reduction in the body without the need to break a sweat or eat less.
“The mice looked like they’d done endurance training,” said Thomas Burris, PhD, a University of Florida pharmacodynamics professor and coauthor of a September 2023 study of the exercise mimetic SLU-PP-332, published in the Journal of Pharmacology and Experimental Therapeutics.
photo of White Mice Team Exercising in Running Wheel
“The mice looked like they’d done endurance training.”
Meanwhile, the CR mimetic mannoheptulose (MH) “was incredibly effective at stopping the negative effects of a high-fat diet in mice,” said Donald K. Ingram, PhD, an adjunct professor at Louisiana State University’s Pennington Biomedical Research Center, who began studying CR mimetics at the National Institute on Aging in the 1980s. In a 2022 study published in Nutrients, MH also increased insulin sensitivity.
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These “have your cake and eat it, too” drugs aren’t on the market for human use — but they’re edging closer. Several have moved into human trials with encouraging results. The National Institutes of Health and the pharmaceutical industry are taking notice, anteing up big research dollars. At the earliest, one could win US Food and Drug Administration (FDA) approval in 4-5 years, Burris said.
The medical appeal is clear: Mimetics could one day prevent and treat serious conditions such as age- and disease-related muscle loss, diabetes, heart failure, and even neurodegenerative disorders like Parkinson’s disease and Alzheimer’s disease, said the scientists studying them.
The commercial appeal is unavoidable: Mimetics have the potential to help non-dieters avoid weight gain and allow dieters to build and/or preserve more calorie-burning muscle — a boon because losing weight can reduce muscle, especially with rapid loss.
How do these drugs work? What’s their downside? Like the “miracle” glucagon-like peptide 1 (GLP-1) weight-loss drugs that are now ubiquitous, are mimetics an effective pharmaceutical way to replicate two of society’s biggest lifestyle sticking points — diet and exercise?
It’s possible…
CR Mimetics: The Healthspan Drug?
CR mimetics, despite the easy assumption to make, aren’t really for weight loss. Not to muscle in on the GLP-1 turf, the CR drugs’ wheelhouse appears to be extending healthspan.
From nematodes and fruit flies to yeast, Labrador Retrievers, and people, plenty of research shows that reducing calorie intake may improve health and prolong life. By how much? Cutting calories by 25% for 2 years slowed the pace of aging 2%-3% in the landmark CALERIE study of 197 adults, according to a 2023 study in Nature Aging. Sounds small, but the researchers said that equals a 10%-15% lower risk for an early death — on par with the longevity bonus you’d get from quitting smoking.
Trouble is low-cal living isn’t easy. “Diets work,” said George Roth, PhD, of GeroScience, Inc., in Pylesville, MD, who began studying CR at the National Institute on Aging in the 1980s with Ingram. “But it’s hard to sustain.”
That’s where CR mimetics come in. They activate the same health-promoting genes switched on by dieting, fasting, and extended periods of hunger, Roth said. The end result isn’t big weight loss. Instead, CR mimetics may keep us healthier and younger as we age. “Calorie restriction shifts metabolic processes in the body to protect against damage and stress,” he said.
Roth and Ingram are currently focused on the CR mimetic mannoheptulose (MH), a sugar found in unripe avocados. “It works at the first step in carbohydrate metabolism in cells throughout the body, so less energy goes through that pathway,” he said. “Glucose metabolism is reduced by 10%-15%. It’s the closest thing to actually eating less food.”
Their 2022 study found that while mice on an all-you-can-eat high-fat diet gained weight and body fat and saw blood lipids increase while insulin sensitivity decreased, mice that also got MH avoided these problems. A 2023 human study in Nutrients coauthored by Roth and Ingram found that a group consuming freeze-dried avocado had lower insulin levels than a placebo group.
Other researchers are looking at ways to stimulate the CR target nicotinamide adenine dinucleotide (NAD+). NAD+ assists sirtuins — a group of seven enzymes central to the beneficial effects of CR on aging — but levels drop with age. University of Colorado researchers are studying the effects of nicotinamide riboside (NR), an NAD+ precursor, in older adults with a $2.5 million National Institute on Aging grant. Small, preliminary human studies have found the compound reduced indicators of insulin resistance in the brain, in a January 2023 study in Aging Cell, and reduced blood pressure and arterial stiffness in a 2018 study published in Nature Communications.
Another NAD+ precursor, nicotinamide mononucleotide, reduced low-density lipoprotein cholesterol, diastolic blood pressure, and body weight in a Harvard Medical School study of 30 midlife and older adults with overweight and obesity, published in August 2023 in The Journal of Clinical Endocrinology & Metabolism. And in an April 2022 study published in Hepatology of people with nonalcoholic fatty liver disease, a proprietary supplement that included NR didn’t reduce liver fat but had a significant (vs placebo) reduction in ceramide and the liver enzyme alanine aminotransferase, a marker of inflammation.
“I think it was a pretty interesting result,” said lead researcher Leonard Guarente, PhD, professor of biology at Massachusetts Institute of Technology and founder of the supplement company Elysium. “Fatty liver progressively damages the liver. This has the potential to slow that down.”
Exercise Mimetics: Fitness in a Pill?
Physical activity builds muscle and fitness, helps keeps bones strong, sharpens thinking and memory, guards against depression, and helps discourage a slew of health concerns from weight gain and high blood pressure to diabetes and heart disease. Muscle becomes more dense, more powerful and may even burn more calories, said Burris. The problem: That pesky part about actually moving. Fewer than half of American adults get recommended amounts of aerobic exercise and fewer than a quarter fit in strength training, according to the Centers for Disease Control and Prevention.
Enter the exercise mimetics. Unlike CR mimetics, exercise mimetics affect mitochondria — the tiny power plants in muscle and every other cell in the body. They switch on genes that encourage the growth of more mitochondria and encourage them to burn fatty acids, not just glucose, for fuel.
In mice, this can keep them from gaining weight, increase insulin sensitivity, and boost exercise endurance. “We can use a drug to activate the same networks that are activated by physical activity,” said Ronald Evans, PhD, professor and director of the Gene Expression Laboratory at the Salk Institute for Biological Studies in La Jolla, California.
Among notable mimetics moving into human studies is ASP0367, a drug in a class called PPAR delta modulators first developed in Evans’ lab. ASP0367 was licensed to the pharmaceutical company Mitobridge, later acquired by Astellas. Astellas is currently running a phase 2/3 human trial of the investigational drug in people with the rare genetic disorder primary mitochondrial myopathy.
“We can use a drug to activate the same networks that are activated by physical activity.”
At the University of Florida, Burris and team hope to soon move the exercise mimetic SLU-PP-332 into human studies. “It targets a receptor called ERR that I’ve been working on since the 1980s,” Burris said. “We knew from genetic studies that ERR has a role in exercise’s effects on mitochondrial function in muscle.” The calorie mimetics he’s studying also activate genes for making more mitochondria and driving them to burn fatty acids. “This generates a lot of energy,” he said. In a January 2024 study in Circulation, Burris found the drug restores heart function in mice experiencing heart failure. “Very little heart function was lost,” he said. It’s had no serious side effects.
The Future of Exercise and CR Pills
The field has hit some bumps. Some feel inevitable — such as otherwise healthy people misusing the drugs. GW1516, an early experimental exercise mimetic studied by Evans and abandoned because it triggered tumor growth in lab studies, is used illegally by elite athletes as a performance-enhancing drug despite warnings from the US Anti-Doping Agency. Burris worries that future CR mimetics could be misused the same way.
But he and others see plenty of benefits in future, FDA-approved drugs. Exercise mimetics like SLU-PP-332 might one day be given to people alongside weight-loss drugs, such as Mounjaro (tirzepatide) or Ozempic (semaglutide) to prevent muscle loss. “SLU-PP-332 doesn’t affect hunger or food intake the way those drugs do,” he said. “It changes muscle.”
Mimetics may one day help older adults and people with muscle disorders rebuild muscle even when they cannot exercise and to delay a range of age-related diseases without onerous dieting. “The chance to intervene and provide a longer healthspan and lifespan — that’s been the moon shot,” Roth said.
Guarente noted that CR mimetics may work best for people who aren’t carrying extra pounds but want the health benefits of slashing calories without sacrificing meals and snacks. “Fat is still going to be a problem for joints, cholesterol, inflammation,” he said. “Calorie mimetics are not a panacea for obesity but could help preserve overall health and vitality.”
And what about the billion-dollar question: What happens when these drugs become available to a general public that has issues with actual exercise and healthy diet?
Evans sees only positives. “Our environment is designed to keep people sitting down and consuming high-calorie foods,” he said. “In the absence of people getting motivated to exercise — and there’s no evidence the country is moving in that direction on its own — a pill is an important option to have.”
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