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This study, published in Proceedings of the Nation This study, published in Proceedings of the National Academy of Sciences (1984), investigates the effects of food restriction on aging, specifically aiming to disentangle the roles of reduced food intake and reduced adiposity on longevity and physiological aging markers in mice. The research focuses on genetically obese (ob/ob) and normal (C57BL/6J, or B6 +/+) female mice, examining how lifelong food restriction influences longevity, collagen aging, renal function, and immune responses. The key finding is that reduced food intake, rather than reduced adiposity, is the critical factor in extending lifespan and retarding certain aging processes.
Background and Objective
Food restriction (caloric restriction) is known to increase longevity in rodents, but the underlying mechanism remains unclear.
Previous studies suggested that reduced adiposity (body fat) might mediate the longevity effects. However, human epidemiological data show conflicting evidence: moderate obesity correlates with lower mortality, challenging the assumption that less fat is always beneficial.
Genetically obese ob/ob mice provide a model to separate effects because they maintain high adiposity even when food restricted.
The study aims to clarify whether reduced food intake or reduced adiposity is the primary driver of delayed aging and increased longevity.
Experimental Design
Subjects: Female mice of the C57BL/6J strain, both normal (+/+) and genetically obese (ob/ob).
Feeding Regimens:
Fed ad libitum (free access to food).
Restricted feeding: fixed ration daily, adjusted so restricted ob/ob mice weigh similarly to fed +/+ mice.
Food restriction started at weaning (4 weeks old) and continued lifelong.
Parameters measured:
Longevity (mean and maximum lifespan).
Body weight, adiposity (fat percentage), and food intake.
Collagen aging assessed by denaturation time of tail tendon collagen.
Renal function measured via urine-concentrating ability after dehydration.
Immune function evaluated by thymus-dependent responses: proliferative response to phytohemagglutinin (PHA) and plaque-forming cells in response to sheep erythrocytes (SRBC).
Key Quantitative Data
Group Food Intake (g/day) Body Weight (g) Body Fat (% of wt) Mean Longevity (days) Max Longevity (days) Immune Response to SRBC (% Young Control) Immune Response to PHA (% Young Control)
Fed ob/ob 4.2 ± 0.5 67 ± 5 ~66% 755 893 7 ± 7 13 ± 7
Fed +/+ 3.0* 30 ± 1* 22 ± 6 971 954 22 ± 11 49 ± 12
Restricted ob/ob 2.0* 28 ± 2 48 ± 1 823 1307 11 ± 7 8 ± 6
Restricted +/+ 2.0* 20 ± 2* 13 ± 3 810 1287 59 ± 30 50 ± 11
Note: Means not significantly different from each other are marked with an asterisk (*).
Detailed Findings
1. Body Weight, Food Intake, and Adiposity
Fed ob/ob mice consume the most food and have the highest body fat (~66% of body weight).
When food restricted, ob/ob mice consume about half as much food as when fed ad libitum but maintain a very high adiposity (~48%), nearly twice that of fed normal mice.
Restricted normal mice have the lowest fat percentage (~13%) despite eating the same amount of food as restricted ob/ob mice.
This demonstrates that food intake and adiposity can be experimentally dissociated in these genotypes.
2. Longevity
Food restriction increased mean lifespan of ob/ob mice by 56% and maximum lifespan by 46%.
In normal mice, food restriction had little effect on mean longevity but increased maximum lifespan by 32%.
Food-restricted ob/ob mice lived longer than fed normal mice, despite their greater adiposity.
These results strongly suggest that reduced food intake, not reduced adiposity, extends lifespan, even with high body fat levels.
3. Collagen Aging
Collagen denaturation time is a biomarker of aging, with shorter times indicating more advanced aging.
Collagen aging is accelerated in fed ob/ob mice compared to normal mice.
Food restriction greatly retards collagen aging in both genotypes.
Importantly, collagen aging rates were similar in restricted ob/ob and restricted +/+ mice, despite widely different body fat percentages.
Conclusion: Collagen aging correlates with food intake but not with adiposity.
4. Renal Function (Urine-Concentrating Ability)
Urine-concentrating ability declines with age in normal rodents.
Surprisingly, fed ob/ob mice did not show an age-related decline; their concentrating ability remained high into old age.
Restricted mice (both genotypes) showed a slower decline than fed normal mice.
This suggests obesity does not necessarily impair this aspect of renal function, and food restriction preserves it.
5. Immune Function
Immune responses (to PHA and SRBC) decline with age, more severely in fed ob/ob mice (only ~10% of young normal levels at old age).
Food restriction did not improve immune responses in ob/ob mice, even though their lifespans were extended.
In restricted normal mice, immune responses showed slight improvement compared to fed normal mice.
The spleens of restricted ob/ob mice were smaller, which might contribute to low immune responses measured per spleen.
These results suggest immune aging may be independent from longevity effects of food restriction, especially in genetically obese mice.
The more rapid decline in immune function with higher adiposity aligns with previous reports that increased dietary fat accelerates autoimmunity and immune decline.
Interpretation and Conclusions
The study disentangles two factors often conflated in aging research: food intake and adiposity.
Reduced food intake is the primary factor in extending lifespan and slowing collagen aging, not the reduction of body fat.
Genetically obese mice restricted in food intake live longer than normal mice allowed to eat freely, despite retaining high body fat levels.
Aging appears to involve multiple independent processes (collagen aging, immune decline, renal function), each affected differently by genetic obesity and food restriction.
The study also highlights that immune function decline is not necessarily mitigated by food restriction in obese mice, suggesting complexities in how different physiological systems age.
Findings challenge the assumption that less fat is always beneficial, offering a potential explanation for human studies showing moderate obesity correlates with lower mortality.
The results support the idea that reducing food consumption can be beneficial even in individuals with high adiposity, with implications for aging and metabolic disease research.
Implications for Human Aging and Obesity
The study cautions against equating adiposity directly with aging rate or mortality risk without considering food intake.
It suggests that caloric restriction may improve longevity even when body fat remains high, which may help reconcile conflicting human epidemiological data.
The authors note that micronutrient supplementation along with food restriction could further optimize longevity outcomes, based on related studies.
Core Concepts
Food Restriction (Caloric Restriction): Limiting food intake without malnutrition.
Adiposity: The proportion of body weight composed of fat.
ob/ob Mice: Genetically obese mice with a mutation causing defective leptin production, leading to obesity.
Longevity: Length of lifespan.
Collagen Aging: Changes in collagen denaturation time indicating tissue aging.
Immune Senescence: Decline in immune function with age.
Renal Function: Kidney’s ability to concentrate urine, an indicator of aging-related physiological decline.
References to Experimental Methods
Collagen aging measured by denaturation times of tail tendon collagen in urea.
Urine osmolality measured by vapor pressure osmometer after dehydration.
Immune function assessed by PHA-induced splenic lymphocyte proliferation in vitro and plaque-forming cell responses to SRBC in vivo.
Body fat measured chemically via solvent extraction of dehydrated tissue samples.
Summary Table of Aging Markers by Group
Marker Fed ob/ob Fed +/+ Restricted ob/ob Restricted +/+ Interpretation
Body Fat (%) ~66 22 ~48 13 Ob/ob mice retain high fat even restricted
Mean Lifespan (days) 755 971 823 810 Food restriction increases lifespan in ob/ob mice
Max Lifespan (days) 893 954 1307 1287 Max lifespan improved by restriction
Collagen Aging Rate Fast (accelerated) Normal Slow (retarded) Slow (retarded) Related to food intake, not adiposity
Urine Concentrating Ability High, no decline with age Declines with age Declines slowly Declines slowly Obesity does not impair this function
Immune Response Severely reduced (~10%) Moderately reduced Severely reduced (~10%) Slightly improved Immune aging not improved by restriction in obese mice
Key Insights
Longevity extension by food restriction is independent of adiposity levels.
Collagen aging is directly related to food consumption, not fat content.
Obesity does not necessarily impair certain renal functions during aging.
Immune function decline with age is exacerbated by obesity but is not rescued by food restriction in obese mice.
Aging is a multifactorial process with independent physiological components.
Final Remarks
This comprehensive study provides compelling evidence that lifespan extension by food restriction is primarily driven by the reduction in caloric intake rather than by decreased fat mass. It highlights the complexity of aging, showing that different physiological systems age at different rates and respond differently to genetic and environmental factors. The findings have significant implications for understanding obesity, aging, and dietary interventions in mammals, including humans.
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