Restricting calorie intake in species such as mice, rhesus monkeys, and fruit flies has been shown to extend their lifespans. In some cases, these animals not only live longer, but are also free of disease. But when pushed too far, calorie restriction can have negative impacts. Mice that undergo a 40% reduction in calorie intake, for example, are more susceptible to infections, less likely to reproduce, and experience stunted growth.
Scientists have wondered whether there is a way to reap the longevity benefits of calorie restriction in humans without its negative repercussions. And in a new study published in Nature Aging, they found a potential answer in an immune-related protein called complement component 3 (C3).
Yale researchers have previously shown that people who undergo moderate calorie restriction—a 14% reduction in calorie intake—for two years developed better immune defense without any growth or reproductive trade-offs.
“This concept demonstrates that aging is actually malleable and a process that can be targeted,” says senior author Vishwa Deep Dixit, Ph.D., Waldemar Von Zedtwitz Professor of Pathology, professor of immunobiology and of comparative medicine, and director of the Yale Center for Research on Aging (Y-Age) at Yale School of Medicine.
Calorie restriction reduces inflammation-related protein
In the new study, Dixit and his colleagues at YSM analyzed the plasma samples of 42 individuals who took part in a two-year trial called the Comprehensive Assessment of Long-Term Effects of Reducing Intake of Energy or CALERIE.
“It’s the only trial of its kind that has been done with such rigor and control and demonstrates relevance to human physiology,” Dixit says. During the trial, participants were able to reduce their calorie intake by 11 to 14% without feeling deprived.
In their analysis, the researchers detected more than 7,000 proteins in the longitudinal plasma samples. Among them was an immune-related protein called C3 that was significantly reduced following calorie restriction. C3 was of particular interest to the scientists as prior studies have suggested that activation of the complement system—a network of proteins involved in the defense against pathogens—could drive chronic inflammation, a major hallmark of aging and age-associated diseases.
“But the causal effects of C3 in aging and chronic inflammation have not been identified. So, we were very excited to find that in our study,” says Hee-Hoon Kim, Ph.D., a postdoctoral associate in the Dixit lab and a co-first author of the paper.
A target to slow aging
When comparing the protein levels before and after two years of calorie restriction, the researchers identified white adipose tissue—the main type of fat tissue in mammals—as the primary site affected by calorie restriction.
The researchers confirmed their findings in animals. As with the human plasma, they found that C3 expression increased with age in mice. Further biochemical analyses showed that visceral white adipose tissue was responsible for an increase in C3 during aging.
“We were not expecting that because these proteins are mainly synthesized in the liver,” says Manish Mishra, Ph.D., a postdoctoral associate in the Dixit lab and a co-first author of the study.
Single-cell RNA sequencing further revealed that the protein is produced by age-associated macrophages—essential white blood cells—within the adipose tissues.
“This whole process was unknown in the beginning,” Mishra says. “Just to narrow it down to the subtypes of macrophages responsible for this complement protein production was very challenging.”
Macrophages are the body’s first line of immune defense, mostly known for their role in engulfing pathogens. These immune cells also help maintain the balance of tissue functions, Dixit adds.
The question is whether the benefits gained from a reduction in C3 can be achieved without weight loss.
The researchers initially suspected that the shedding of adipose tissue or body fat due to weight loss may have stalled C3 production and slowed down the aging process. After all, most of the study participants lost about 18 pounds after two years of moderate calorie restriction.
However, when the researchers analyzed the body mass index of the study participants, they did not observe any correlation between weight loss and a decrease in complement proteins.
“This suggests that calorie restriction has a beneficial effect that is unique to adipose tissues and is likely independent of weight loss,” Kim says.
Further, when the researchers inhibited C3 activation using a drug to mimic the effect of calorie restriction, the mice experienced less age-related inflammation.
The finding demonstrates that what is beneficial early on in life can be detrimental later on, Dixit says. This theory, known as antagonistic pleiotropy, was first proposed by biologist Peter Medawar in 1952 to describe the aging process. A prime example of this theory is growth hormone production, which is essential in early development but could also drive cancer later in life.
Proteins like C3 are evolutionarily designed to protect us from infections, but as humans live much longer than their ancestors, these molecules can come back to harm us. Lowering the level of C3 proteins may be the key to enhancing health span, Dixit says.
The researchers are now investigating whether they could hold back C3 production to slow down aging in humans using FDA-approved inhibitor drugs. “The idea is not to remove complement systems that are required for us to fight infections,” Dixit says. “Instead, the goal is to restore the balance.”