Caloric Restriction in Humans Reveals Regulator of Metabolism, Immunity, and Longevity
While moderately decreased food intake that does not cause malnutrition (caloric restriction) has beneficial effects on lab animals' healthspan and lifespan, it’s not exactly easy to cut calories by 40%. What’s more, the extension of life span driven by 40% caloric restriction (CR) in rodents causes trade-offs in growth, reproduction, and immune defense that make it challenging to identify therapeutically relevant CR-mimetic targets.
In an article published in Science, Spadaro and colleagues found that 14% CR in humans over two years showed improved thymus function. This organ sits in the chest between the lungs and makes white blood cells (T lymphocytes), which are part of the immune system and help fight infection. Also, CR led to gene activity changes in fat (adipose) tissue implicated pathways regulating mitochondrial bioenergetics, anti-inflammatory responses, and longevity. In particular, the platelet activating factor acetylhydrolase (PLA2G7) gene activity was decreased in humans undergoing caloric restriction, suggesting the PLA2G7 might mediate some beneficial effects of CR. The inactivation of the PLA2G7 gene confirmed this speculation in mice, which decreased inflammation and improved markers of thymic function and some metabolic functions in aging mice.
“These findings demonstrate that PLA2G7 is one of the drivers of the effects of calorie restriction,” said senior author Vishwa Deep Dixit, who is also director of the Yale Center for Research on Aging. “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.”
Caloric restriction of mice and men
CR-driven lifespan extension in rodents is associated with increased mortality from viral and parasitic infections, including sepsis. In addition to host defense, the resident immune system in every organ aids in integrating cellular metabolism, tissue repair, and function. So, identifying clinically relevant beneficial compounds mimicking CR may be challenging. Moreover, forced extreme CR in animals may cause severe stress responses, which can further compromise the immune system by causing the thymus to become dysfunctional.
“As we get older, we begin to feel the absence of new T cells because the ones we have left aren’t great at fighting new pathogens,” said Dixit. “That’s one of the reasons why elderly people are at greater risk for illness.”
To address CR’s relevance in human health, researchers conducted the Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy (CALERIE) clinical trial. This study was designed to test the long-term effects of 2 years of moderate CR on physiology, aging biomarkers, and predictors of healthspan and longevity in healthy volunteers. The CALERIE study established that humans achieve ~14% sustained CR for two years. This level of voluntary CR in humans in free-living conditions doesn’t reach the forced 25 to 40% restriction of calories in laboratory animals and may engage unique mechanisms.
“There’s so much debate about what type of diet is better — low carbohydrates or fat, increased protein, intermittent fasting, etc. — and I think time will tell which of these are important,” said Dixit. “But CALERIE is a very well-controlled study that shows a simple reduction in calories, and no specific diet has a remarkable effect in terms of biology and shifting the immuno-metabolic state in a direction that’s protective of human health. So from a public health standpoint, I think it gives hope.”
Making thymus with caloric restriction
Aging of the thymus precedes aging of other organs, a process characterized by increased thymic lipid (fat) accumulation and loss of production of immune cells called T cell lymphocytes. Mice on life-long 40% CR maintain the thymic production of lymphocytes (lymphopoiesis) into late life and have a diverse T cell receptor (TCR) diversity, whereas 14% CR in humans reduces the number of circulating lymphocytes and proinflammatory cytokines in the blood.
“Because we know that chronic low-grade inflammation in humans is a major trigger of many chronic diseases and, therefore, has a negative effect on life span,” said Dixit. “Here we’re asking: What is calorie restriction doing to the immune and metabolic systems and if it is indeed beneficial, how can we harness the endogenous pathways that mimic its effects in humans?”
Spadaro and colleagues investigated thymic function at baseline and after two years of sustained CR in middle-aged healthy CALERIE participants. CR maintained for two years significantly increased thymic mass and total thymic volume in study participants. The participants with unaltered diets did not show a significant change in thymic volume from baseline to year two. Compared with baseline, two years of CR significantly increased the recent thymic emigrant T cells in the majority of the study participants. These data may indicate that 14% CR in healthy humans activates a tissue-protective immuno-metabolic program that can enhance thymic function without altering the transcriptome of CD4 T cells.
“The fact that [the thymus] can be rejuvenated is, in my view, stunning because there is very little evidence of that happening in humans,” said Dixit. “That this is even possible is very exciting.”
Fighting fat with caloric restriction
Reduction in caloric intake induces a decrease in carbohydrate (glucose) utilization and a switch to fat-based metabolism. Accordingly, the participants in the CALERIE study experienced a reduction in fat mass. Given that adipose tissue also contains a resident immune system that controls inflammation, Spadaro and colleagues measured the impact of CR on gene activity in the fat (adipose) tissue biopsies. Compared with baseline, one year of CR altered the gene activity in adipose tissue, and this difference was maintained after two years of CR. The top 20 up- and down-regulated genes in human adipose tissue after 14% CR identified gene activity previously not highlighted in rodent studies. Changes in gene activity in adipose tissue after CR were similar to those observed after bariatric surgery.
“We found remarkable changes in the gene [activity] of adipose tissue after one year that were sustained through year two,” said Dixit. “This revealed some genes that were implicated in extending life in animals but also unique calorie restriction-mimicking targets that may improve metabolic and anti-inflammatory response in humans.”
PLA2G7, a mediator of caloric restriction benefits
Since CR is known to lower inflammation, Spadaro and colleagues investigated the impact of CR on gene activity in the primary immune cells of the inflammatory response, myeloid cells. Several proinflammatory-like genes and many genes of unknown function in myeloid cells were specifically inhibited after CR, and there was an overlap with the top six genes inhibited at both 1 and 2 years of CR. One such gene was PLA2G7, secreted by immune cells called macrophages that eat up pathogens and degrade compounds important for inflammation.
To test whether the depletion of PLA2G7 influences inflammation, Spadaro and colleagues deleted the PLA2G7 gene in mice, specifically in macrophages. The mice lacking PLA2G7 were partially protected from hepatic steatosis (scarring) as well as weight gain and increased fat mass when put on a high-fat diet. The reduced weight gain in Pla2g7-depleted mice corresponded with increased energy expenditure and fat metabolism.
“We found that reducing PLA2G7 in mice yielded benefits that were similar to what we saw with calorie restriction in humans,” said Olga Spadaro, a former research scientist at the Yale School of Medicine and lead author of the study. Specifically, the thymus glands of these mice were functional for a longer time, the mice were protected from diet-induced weight gain, and they were protected from age-related inflammation.
The data from this human study are also relevant regarding controversies emerging from animal studies that questioned CR’s effects on health and inflammation. Collectively, these findings demonstrate that sustained CR in humans activates a core gene activity program that promotes immune function, reduces inflammation, and reveals PLA2G7 as one of the potential mechanisms to mimic the beneficial effects of CR.
“Spadaro et al. provide new insight into the biology of PLA2G7 and its important role in immunometabolic regulation and systemic homeostasis,” write Timothy Rhoads and Rozalyn Anderson in a related Perspective. “This work is a good example of what can be gained by focusing on the mechanisms underlying the beneficial effects of CR.”
Spadaro O, Youm Y, Shchukina I, et al. Caloric restriction in humans reveals immunometabolic regulators of health span. Science. 2022;375(6581):671-677. doi:10.1126/science.abg7292