Research has suggested that extreme calorie restriction may extend people's lifespan or improve their health outcomes,1 but scientists are still uncovering its long-term risks and benefits.
Recently, a team of Yale researchers found that limiting calorie intake moderately could enhance the immune system.
The team used data from the Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy (CALERIE) clinical trial, the first controlled study of calorie restriction in healthy humans. The researchers then asked some of the 200 study participants to limit their calorie intake by 14% over a two-year period.2
The participants who restricted their calories had a healthier thymus gland, an organ located in the upper chest, compared to control group. The thymus gland is responsible for creating T cells, a type of white blood cells that helps the body fight off infections. It's also one of the fastest-aging organs, which makes it especially important for longevity studies.
“The fact that this organ can be rejuvenated is, in my view, stunning because there is very little evidence of that happening in humans,” Vishwa Deep Dixit, DVM, PhD, director of the Yale Center for Research on Aging and senior author of the report said in a press release.
Yale researchers also observed a decrease in gene expression for the protein platelet-activating factor acetylhydrolase (PLA2G7) in the group that restricted calories for two years.3 Previous research has shown that lowering PLA2G7 protected older mice from inflammation,4 the researchers said.
"These findings demonstrate that PLA2G7 is one of the drivers of the effects of calorie restriction,” Dixit said. “Identifying these drivers helps us understand how the metabolic system and the immune system talk to each other, which can point us to potential targets that can improve immune function, reduce inflammation, and potentially even enhance healthy lifespan.”
The researchers said they hope that the PLA2G7 protein could be harnessed in the future for its benefits without requiring people to go through diet changes.3
"The idea that we might somehow be able to distill the complex effects of this behavioral intervention which is caloric restriction into a pharmacologic treatment has been a holy grail of a certain kind of field of aging science for quite some time," Daniel Belsky, PhD, an assistant professor of epidemiology in the Columbia Aging Center, Mailman School of Public Health, told Verywell.
What Is Caloric Restriction?
Caloric restriction is a sustained lifestyle change where people consume a lower amount of calories while still getting enough vitamins and minerals to meet their nutritional needs.5
"Caloric restriction is not starvation, it’s a reduction of macronutrient intake with a maintenance of micronutrient sufficiency," Belsky said.
But there's not enough evidence to warrant a broad recommendation for caloric restriction as a longevity aid.5 Johna Burdeos, RD, a clinical dietitian, said that while the new study focused on caloric restriction, there's limited information about the participants' diet quality and what they ate before adopting a more restrictive diet.
Limiting calories is most often associated with weight loss. But a study conducted by Belskey that used the same dataset from the CALERIE trial showed that weight loss did not contribute to the association between caloric restriction and biological aging.6
"Humans are not machines. Our metabolism and our energy needs can differ from day to day and through different stages in life," Burdeos said.
Scientists say that genetics, environmental factors, gut microbiome, age, and activity level can all impact how our bodies use calories.5 Importantly, more than 70% of the participants in the CALERIE trial are White and most of them are women. Additional studies are needed to determine if the observed effects of caloric restriction can apply to other demographics.7
Belsky also pointed out that caloric restriction might not tell the full story.
"In many experiments, caloric restriction co-occurs with restriction of the timing of feeding," Belsky said, adding that there has been greater research interest in the impact of time-restricted feeding and intermittent fasting.
Diabetes and Intermittent Fasting
What This Means For You
Calorie restriction and intermittent fasting are not safe or appropriate for everyone. Before starting a new restrictive eating pattern, talk to your healthcare provider or nutritionist. Together, you can develop a plan that is right for your unique goals and lifestyle.
Improving Both Healthspan And Lifespan
Belsky noted that the way scientists think about aging has changed over the past few decades.
It has gone "from being a description of something that happens to us to a set of biological changes that occur in our bodies and actually drive our risk for a range of different disease, disability, and death," he said.
This relatively new way of thinking about and studying aging is called "geroscience."8
"Under the umbrella of what is called the geroscience hypothesis, researchers have been pursuing interventions that slow or reverse these molecular changes that are the biological substance of aging with the goal of delaying or even preventing the progressive loss of system integrity that occurs as we grow older and drives risk of disease," Belsky shared.
The Yale study added to a growing body of geroscience research, the study of the biology and mechanisms of aging.4 However, experts have cautioned that geroscience is still an emerging field, and that more studies are needed before treatments, or even dietary recommendations, could be made based on these findings.5
Burdeos pointed out that there are many different markers for longevity and health besides calorie intake. Sleep, stress, hydration, movement, and diet quality can all affect these markers. "If you were to look at this study and try to apply it, there's more to it than just the calorie reduction," she said. "What are your other goals besides living longer?"