Can eating less make you live longer? The latest large-scale mouse experiment tells us: It's not that simple

Produced by: Science Popularization China

Author: Li Juan (PhD in Biology)

Producer: China Science Expo

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As the pace of modern life accelerates, the incidence of metabolic diseases such as obesity and diabetes is increasing year by year, seriously threatening human health and life span. Therefore, finding effective dietary intervention measures to improve health and prolong life has become a research hotspot.

As a method to delay aging and prevent diseases by reducing calorie intake, caloric restriction (CR) has shown lifespan extension effects in multiple species (such as nematodes, fruit flies, and mice).

However, long-term strict calorie restriction (such as a 25% daily calorie restriction: assuming someone consumes 2,000 kcal per day, reduce calorie intake to 1,500 kcal) is often difficult for humans to adhere to. Therefore, intermittent fasting has gradually come into people's attention as a more flexible way of dietary restriction.

Intermittent fasting (IF) is a method of restricting food intake during a certain period of time, while eating freely at other times. For example, the 5:2 method restricts calorie intake for two days a week and eats normally for the other five days; or the 16/8 method restricts eating time to 8 hours a day and fasts for the other 16 hours, aiming to simulate the foraging pattern in nature, thereby stimulating the physiological adaptation mechanism of organisms. However, there is a lack of systematic research on the specific effects of calorie restriction and intermittent fasting on healthy aging and longevity.

Dietary restriction can indeed extend the lifespan of mice

To further explore the effects of calorie restriction and intermittent fasting on health and lifespan, researchers at the Jackson Laboratory in the United States designed a large-scale, long-term mouse experiment, the results of which were published in the top academic journal Nature on October 9, 2024.

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This is the largest animal study ever conducted on dietary restriction.

The researchers selected 960 genetically diverse female mice as research subjects. These mice came from different genetic backgrounds and could better reflect genetic diversity, making the experimental results more universal.

Some readers may be curious about why female mice were chosen. This is mainly because the behavior and physiological state of female mice are more stable under large-scale breeding conditions, and the experimental conditions are easier to control. Male mice often have aggressive behaviors when raised in groups, especially when fasting.

Experimental mice

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Mice were randomly assigned to five different dietary groups: unlimited access to food (Ad Libitum, AL), fasting one day per week (1D), fasting two consecutive days per week (2D), caloric restriction of 20% (20% CR), or 40% (40% CR) of baseline food intake.

In addition to measuring lifespan, the researchers performed several longitudinal assessments. They recorded weekly changes in the mice's weight, and assessed frailty, gripping ability, and body temperature every 6 months. Comprehensive annual checkups included metabolic cage analysis (to see how the mice used food and oxygen), body composition testing (ratio of muscle to fat), echocardiograms, wheel running tests (exercise), rotarod tests (balance), acoustic tests, bladder function tests, fasting glucose measurements, immune cell analysis, and whole blood analysis, frailty index determination, and disease indicators (such as inflammation, tumors, etc.).

Through comprehensive evaluation and in-depth analysis, the researchers found that both forms of dietary restriction - calorie restriction and intermittent fasting - can extend the lifespan of mice, and the effect is proportional to the degree of restriction: a 40% calorie reduction can extend the lifespan of mice longer than a 20% calorie reduction; the effect of calorie restriction on extending lifespan is more significant than that of intermittent fasting.

But! The effects of dietary restriction on mouse lifespan are complex and multifaceted

The mechanism of the effect of dietary restriction on life span is very complex, mainly reflected in the following aspects:

Weight and lifespan : The study found that two forms of dietary restriction had significant effects on the weight and body composition of mice. Whether it was calorie restriction or intermittent fasting, the mice lost weight in direct proportion to the degree of restriction. The greater the degree of dietary restriction, the greater the weight loss.

The results of the study on lifespan and weight showed that the early lifespan of mice was negatively correlated with their weight before intervention, that is, the higher the weight of mice, the shorter their lifespan. However, this correlation gradually weakened with age and turned into a positive correlation after two years, that is, the higher the weight of mice in later years, the longer their lifespan.

Notably, calorie restriction extended lifespan to the same extent regardless of the mice's weight, but intermittent fasting had no effect on extending lifespan in mice that were higher in early life.

Metabolic health and lifespan : Both forms of dietary restriction reduced fasting blood sugar levels, energy expenditure and body temperature in mice. However, there was no clear correlation between these metabolic indices and lifespan, suggesting that dietary restriction may extend lifespan through other pathways rather than simply improving metabolic indices.

Several health indicators of mice were also affected by dietary restriction, such as frailty index, tumor incidence, abdominal distension, glucose homeostasis, energy expenditure, body temperature and metabolic flexibility, but only the frailty index was closely related to lifespan . The frailty index is a measure of age-related health defects, such as spinal curvature or difficulty in movement. This is consistent with the phenomenon observed in the human population. The study found that dietary restriction can reduce the incidence of these defects, thereby improving health and prolonging lifespan.

The two types of dietary restriction had different effects on the health of the mice. Although a 40% calorie restriction had the strongest life-extending effect, it also may have led to weight loss and changes in the immune system that increased the risk of infection—suggesting the potential negative effects of extreme dieting on humans. In contrast, intermittent fasting had more modest effects on the health of mice, but not all mice benefited from it, particularly those that were heavier before the intervention.

Blood indicators and life expectancy:

Immune cells: As they age, mice experience a series of changes in immune cell subsets . Specifically, the relative proportions of B cells, effector T cells, and inflammatory monocytes gradually increase, while the total proportions of lymphocytes, mature natural killer (NK) cells, and eosinophils decrease. These changes are similar to those in humans, suggesting that changes in these immune cells may be a common phenomenon during aging.

However, when mice were subjected to varying degrees of dietary restriction (such as a 40% calorie restriction), their immune cell frequencies changed significantly. In particular, the frequencies of mature NK cells, eosinophils, circulating B cells, and inflammatory monocytes were profoundly affected. These changes suggest that dietary restriction may affect the lifespan of an organism by regulating the function and number of immune cells.

Lymphocytes: The percentage of total circulating lymphocytes was positively correlated with lifespan , meaning that mice with a higher number of lymphocytes tended to have a longer lifespan. Further analysis also showed that cells in a physiologically quiescent state, such as CD4+ and CD8+ naive T cells and immature NK cells, were also positively correlated with lifespan.

These cells are usually in an unactivated or immature state, with stronger proliferation and differentiation potential, and thus may play a more important role in responding to external challenges; on the contrary, immune cells showing activated or mature phenotypes (such as CD4+ and CD8+ effector T cells and CD11+ memory B cells) are usually associated with a shortened lifespan. These cells have undergone activation and differentiation processes, and their functions and numbers are limited, thus reducing the organism's stress resilience and resistance.

Red blood cells: With aging, the number of red blood cells, hemoglobin content, and hematocrit of mice gradually decrease, while the red blood cell distribution width (RDW) and hemoglobin distribution width (HDW) increase . These changes reflect the increased heterogeneity of red blood cell morphology and function, which may be related to physiological changes during aging. These changes are also present in aging humans, where anemia is a common problem.

Red blood cells

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It is worth noting that dietary restriction also had a significant effect on red blood cell characteristics. In particular, 40% of calorie-restricted mice and intermittent fasting mice showed significant changes in RDW. As an indicator of the coefficient of variation of red blood cell volume, its increase may be related to red blood cell aging, destruction or abnormal production.

With so many factors and results, what are the connections among them?

The findings above show that physical traits associated with longevity, such as weight, metabolic rate, and energy expenditure, are all affected by dietary restriction. To understand how dietary restriction affects lifespan, the researchers used a method called multivariate network analysis to construct a large network of relationships to reveal the complex connections between these traits.

They found that dietary restriction has multifaceted effects on lifespan, both direct and indirect, and that these effects are sometimes opposite (there are pathways that extend lifespan as well as pathways that shorten it).

Dietary restriction directly affects lifespan, for example by reducing body weight and red blood cell distribution width (RDW), accounting for 61%, while it also indirectly affects lifespan by affecting physiological processes such as metabolic traits and the immune system, accounting for 39%.

The researchers sequenced the genomes of nearly a thousand mice and studied the effects of genes and diet on lifespan. Although dietary restriction had a significant effect on the lifespan and health of mice, the researchers found that genetics had a greater impact on lifespan than dietary intervention.

The results showed that genes explained about 23.6% of the lifespan effect, while diet only explained 7.4%. Even when the same dietary restriction intervention was implemented in mice, there were significant differences in the response of mice with different genetic backgrounds to the intervention. Interestingly, as age increases, the effect of genes on lifespan gradually weakens, while the effect of diet gradually increases. This may be because the environment and lifestyle have a greater impact on health as we age.

In addition, the researchers also discovered a gene locus associated with lifespan, located on mouse chromosome 18. This gene locus explains about 4.34% of the genetic lifespan effect, equivalent to 23.4% of the total genetic effect. As much as 75% of the genetic effect remains unexplained, but this provides important clues for our understanding of the genetic basis of lifespan.

Interestingly, red blood cell distribution width, a measure of the variability in red blood cell size, is also affected by the above gene loci and is also associated with anemia risk, suggesting that red blood cell health may be related to lifespan.

These results indicate that genetic factors play a crucial role in determining lifespan, and although dietary intervention can prolong lifespan to a certain extent, its effect is still constrained by genetic background.

Please note that improved health does not equate to longer life

These findings provide a new perspective for evaluating the effects of dietary restriction, which has a complex relationship with lifespan that is reflected in the following aspects:

First, the weight loss and fat content reduction caused by dietary restriction in mice are not always beneficial to lifespan, which indicates that dietary restriction is not enough to offset the negative effects of obesity. Studies have shown that in some cases, maintaining a certain level of weight and fat content is associated with a longer lifespan, which contradicts the traditional concept that "the thinner the better", revealing the complexity of the relationship between dietary restriction and lifespan.

Second, dietary restriction had a profound effect on the metabolic characteristics of mice. Common physiological adaptations caused by dietary restriction, including lower fasting blood glucose, reduced energy expenditure, and oscillations in the respiratory quotient, however, these physiological adaptations could not accurately predict the lifespan of the dietary restriction group, indicating that changes in metabolic characteristics induced by dietary restriction may be beneficial to health, but may not directly lead to a significant extension of lifespan.

The association between health and lifespan was not significant, suggesting that improving health is not equivalent to extending lifespan. In contrast, some characteristics related to immunity and hematology were closely associated with lifespan. For example, indicators such as high lymphocyte ratio and low red blood cell distribution width were more common in long-lived mice. This finding has important implications for the selection of biomarkers in human dietary intervention studies focusing on metabolic function.

Therefore, physiological characteristics closely related to life span, such as maintaining stable body weight, high lymphocyte ratio, low red blood cell distribution width, and low obesity rate in late life, are positively correlated with extended life span. These characteristics are called "physiological resilience" by researchers - when mice face the stress of food restriction, the stronger the body's ability to adapt to stress and recover, the longer their life span. These indicators can be regarded as the main biomarkers of longevity.

What can the mouse experiments teach us about humans?

The results of this study suggest that human responses to dietary restrictions may be highly personalized and greatly influenced by genetic background. This means that different people may have different responses to dietary restrictions, and a personalized diet plan needs to be developed based on individual circumstances - this is also one of the research contents of the emerging discipline of precision nutrition.

It is still unclear whether intermittent fasting and calorie restriction can extend lifespan in humans. Due to differences in metabolic rates between humans and animals, the effects of these two forms of dietary restriction may be different in humans than in mice. Therefore, more research is needed to delve deeper into their complex physiological effects.

References:

1.Di Francesco, A., Deighan, AG, Litichevskiy, L. et al. Dietary restriction impacts health and lifespan of genetically diverse mice. Nature 634, 684–692 (2024).