Can cutting calories lengthen your life?
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Research indicates that both calorie restriction and intermittent fasting may prolong life in animals, according to experts in aging. What implications might this have for humans?
Studies show that when laboratory mice have their daily calorie intake reduced by 30 to 40 percent, they tend to live approximately 30 percent longer on average. This strategy, known as calorie restriction, must be balanced carefully to avoid malnutrition, yet be substantial enough to initiate significant biological responses.
This life-extending effect was first observed in the 1930s, and subsequent research over the last 90 years has confirmed these findings in a variety of species, from worms to monkeys. Moreover, these studies often report that calorie-restricted animals are less prone to cancer and other chronic ailments associated with aging.
However, despite extensive animal research, many questions remain. Scientists continue to debate the mechanisms behind this phenomenon and whether the reduction in calorie intake or the timing of consumption (as seen in intermittent fasting) plays a more critical role.
The real challenge lies in determining if these benefits can be replicated in humans. The field of aging research includes many experts who test various dietary approaches on themselves, yet robust longevity studies on humans are scarce and challenging to conduct due to their lengthy nature.
As we examine the key findings from landmark animal studies, here's what they could potentially mean for human health and longevity.
Why might fewer calories lead to longer lives?
The question of how calorie restriction may prolong life is complex, with many theories suggesting evolutionary roots. In nature, animals, including early humans, underwent cycles of plenty and scarcity. Thus, their biological systems adapted to not only survive but thrive under both conditions.
One hypothesis posits that calorie restriction enhances cellular resilience against stress. James Nelson, a professor at the University of Texas Health Science Center at San Antonio, notes that mice on restricted diets demonstrate a higher tolerance to toxins and recover more quickly from injuries.
Another theory centers on metabolism. Consuming fewer calories typically slows down metabolic processes. Dr. Kim Huffman, an associate professor at Duke University School of Medicine, explains, "By reducing metabolic activity, the body's systems endure less wear and tear, potentially extending lifespan—similar to how slowing a car might preserve its tires."
Furthermore, calorie restriction shifts the body's energy reliance away from glucose. This change is believed to improve metabolic health and promote longevity. The process of autophagy, where cells digest and repurpose their faulty parts for energy, plays a significant role here, enhancing cell function and reducing the onset of age-related illnesses.
Dr. Richard Miller from the University of Michigan emphasizes that the longevity observed in calorie-restricted mice largely stems from their delayed or absent onset of diseases typically associated with aging.
However, the effects of calorie restriction on lifespan are not universally positive. A notable counterpoint was a 2010 study by Dr. Nelson on genetically diverse mice, revealing that while some lived longer on a reduced-calorie diet, others actually had reduced lifespans. This finding challenged the conventional wisdom that calorie restriction universally extends life. Dr. Nelson remarked on the ubiquity of such assumptions in scientific literature, highlighting the controversy this study sparked.
Critics of Dr. Nelson's study argue that it should not overshadow decades of robust evidence supporting the benefits of calorie restriction. Dr. Miller contends that focusing on this one study ignores a vast body of research demonstrating consistent positive effects.
Moreover, long-term studies on monkeys have produced mixed results. Research spanning over 20 years, published in 2009 and 2012, showed that while both studies observed health improvements associated with calorie restriction, only one group experienced a significant extension in lifespan along with reduced incidence of age-related diseases like cardiovascular disease and diabetes.
This ongoing debate underscores the complexity of translating findings from animal models to humans and the need for more nuanced understandings of how calorie restriction might influence aging and longevity.
What role does intermittent fasting play in longevity?
Amid the varying outcomes of calorie restriction studies, some scientists are exploring whether the timing of meals could be as influential, or even more so, than the quantity of calories consumed. This aspect of dietary restriction, known as intermittent fasting, is gaining attention for its potential to significantly impact lifespan.
One notable difference in two prominent monkey studies highlights this theory. In the 2009 study at the University of Wisconsin, calorie-restricted monkeys were given a single daily meal and any leftovers were removed by late afternoon, resulting in an enforced fasting period of about 16 hours. Conversely, in the 2012 study by the National Institute on Aging, monkeys were fed twice daily and food was available overnight. Notably, the Wisconsin monkeys, who experienced longer fasting periods, showed greater longevity.
This link between meal timing and lifespan was further examined in a more recent mouse study, where the effects of calorie restriction were tested with varying access to food. Some mice were restricted to eating within a two-hour window, others had a 12-hour window, and a third group had continuous 24-hour access to their low-calorie diet. Compared to a control group with unrestricted access to a full-calorie diet, the mice with 24-hour access to a low-calorie diet lived 10 percent longer. However, those restricted to specific eating windows saw life span increases of up to 35 percent.
Rafael de Cabo, a senior investigator at the N.I.A., who contributed to the 2012 monkey study, now believes that the fasting duration is potentially as crucial as calorie restriction itself for promoting longevity. This suggests that the patterns of eating and fasting might hold significant implications not just for animal models but potentially for human health and longevity as well.
Exploring the potential longevity benefits of calorie restriction and intermittent fasting in humans
Determining whether intermittent fasting, calorie restriction, or both can extend human life remains a complex question. According to Dr. Nelson, "We have no evidence that it extends life span in humans," which doesn't rule out the possibility but highlights the difficulty in obtaining such data, given it requires a lifetime to collect.
One notable effort to explore this question was the Calerie study, which investigated the effects of a 25% calorie reduction over two years in over 100 healthy adults. The participants received guidance on meal planning and ongoing support to meet their dietary targets. However, the challenge of maintaining such a strict diet meant they only achieved an average reduction of about 11%.
Despite this, compared to a control group, those who reduced their caloric intake saw improvements in several markers of cardio-metabolic health, such as blood pressure and insulin sensitivity, and displayed lower inflammation levels.
The study also looked at "biological age" through various blood tests at the start and end of the two years. While two tests showed no significant changes, a third indicated that calorie restriction slowed the aging process, though it did not reverse it. "Calorie restriction didn’t make people younger, but it slowed how quickly they aged," noted Dr. Huffman, who was involved in the trial.
Dr. Miller points out that the study reveals the unrealistic nature of achieving 25 to 40 percent calorie reduction in humans—a common range for benefits in animal studies—even with substantial support, participants fell short.
Conversely, Dr. de Cabo found the results encouraging, emphasizing that even the modest 11% reduction had beneficial effects.
Further studies have examined intermittent fasting across various body mass indices, showing promising results in metabolic health and inflammation reduction in short-term trials. However, a specific study involving 116 overweight or obese individuals found no benefits for those who ate within an eight-hour window without reducing calorie intake, compared to controls.
Adding complexity, some research suggests that being slightly overweight might actually be protective later in life. Studies consistently show that individuals classified as overweight have a lower mortality risk compared to those who are normal or underweight. This could be due to older or chronically ill individuals having lower body mass, or perhaps because those with higher BMIs have more muscle mass. Dr. Huffman speculated that a greater body mass might offer protection in later years.
Despite decades of research, the translation of longevity benefits from animal studies to humans remains uncertain. While there are indications that calorie restriction and intermittent fasting may contribute to longer life and certainly offer short-term health benefits, particularly for heart and metabolic health, the possibility remains that these dietary strategies might only leave one feeling hungrier, with minimal impact on overall lifespan.
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