How do dietary restrictions affect metabolism and life expectancy?

Can caloric restriction help slow down metabolism and increase life expectancy? To answer this question, scientists have conducted loads of experiments involving both animals and humans. 

Experiments on animals

In a study dated 1985, specialists from the University of Texas (USA) compared the speed of metabolism in two mouse groups. Animals from the control group fed normally, and caloric intake of rodents from the other group were reduced. The experiment lasted 4.5 months - during this time scientists did not reveal any differences in metabolism rates in mice from both groups. 

In 1992, the same group of specialists repeated the experiment, slightly changing the conditions of the experiment: now the scientists observed a group of mice for life, whose caloric value was reduced by 40% compared to the usual norm starting from six weeks. The rate of metabolism was measured indirectly by analyzing the gas composition in cells. The rate of metabolic processes in animals not limited in calories was analyzed in the same way. It was found out that in the period between 6 and 12 months of life of rodents from the main group a decrease in metabolism rate was observed, but from 18 months the rate of metabolic processes began to recover and by 24 months of age it was the same as in mice from the control group.

The results of the study show that inadequate nutrition provides a gradual "adjustment" of metabolism to new conditions and the inclusion of compensatory mechanisms bringing the rate of metabolism to the same level. Specialists suggested that this mechanism, rather than slowing down metabolism, may determine an increase in life expectancy under prolonged calorie deficiency. 

In 2015, scientists from the University of Missouri (USA) conducted a meta-analysis of studies looking for a link between slowing metabolism, including calorie deficiency, and the life expectancy of various animals. Experts noted that the level of energy metabolism may have different effects on the life expectancy of animals with different life experiences and under different experimental conditions. At the same time, it was not possible to identify a clear and reliable correlation between nutrition, energy costs, and life expectancy. 

Okinawan phenomenon

In 2003, scientists at the Pennington Center for Biomedical Research meta-analyzed studies on the correlation between nutrition, metabolic rate, and life expectancy. Among the current studies, the authors cite the study of the "Okinawa phenomenon": the Japanese island, known for its large number of long-livers. Specialists have found that the energy value of food eaten by Okinawa school students is 38% lower than the caloric value of the diet of students from other regions of Japan. Adults in Okinawa consumed 20% fewer calories than other Japanese, with heart disease mortality in Okinawa 59% lower and cancer 69% lower. 

Project Calerie: detailing metabolism change in starvation

In 2018 this mechanism was studied in people as part of the Calerie project - Comprehensive Assessment of Long-Term Effects of Reducing Intake of Energy. 53 people aged 21 to 50 years reduced their calorie intake by 25% and maintained this style of diet for two years. A control group with a similar composition was also monitored: participants followed a standard calorie diet that was appropriate for their age. 

During the observation period, the average weight loss of volunteers was 9.4 kg and the weight of the control group participants remained unchanged. Participants from the main group and the control group were periodically placed in the metabolic chamber, where the rate of metabolism in their body was measured using sensors. It turned out that with weight loss in people with calorically restricted diets, energy consumption during sleep decreased by 10%. This indicates a slowdown in metabolism. 

Volunteers from the calorie-restricted group also showed a decrease in the hormone levels responsible for the metabolism rate. These hormones included thyroid hormones (T3 and T4) and leptin: previously, a decrease in the concentration of leptin was found to be a sign of slower metabolism during weight loss. There was also an increase in the level of adiponectin, a hormone that is synthesized by fat tissue. It is also one of the regulators of the rate of metabolic processes. In obesity, the concentration of adiponectin decreases, while the risk of inflammatory processes, atherosclerosis increases, and the sensitivity of cells to insulin decreases. 

At the same time, scientists have found: a decrease in the level of leptin, and an increase in adiponectin is observed primarily during active weight loss. Thus, these very hormones are important for metabolic adaptation, i.e. they slow down metabolic processes in response to the loss of kilos. At the stage of weight stabilization in the group with reduced calorie levels of leptin and adiponectin remained fairly stable, with a tendency to decrease the levels of T3 and T4. In a series of studies, it was proved that the reduction of thyroid hormone levels is one of the biomarkers of aging. 

Thus, in this study, it was found that a prolonged limitation of calories leads to a decrease in the rate of metabolism, which is accompanied by biochemical and hormonal shifts. At the same time, changes in the stage of weight loss are among the positive ones (decrease of leptin concentration and increase of adiponectin). At the same time, shifts at the stage of weight stabilization - reduction of thyroid hormone levels - are not among the health and longevity favorable transformations. 

Conclusion

To date, there is a lot of data showing that caloric restriction leads to a slower metabolism. Experiments with animals have shown that reducing metabolic rates against a background of long-term calorie deficiency has a positive effect on health and life expectancy. Similar studies in humans also show slowing down metabolism with prolonged caloric restriction. At the same time, not all these shifts are positive. There is also currently no reliable data available to link calorie deficiency, the resulting lower metabolic rate, and longer life expectancy. 

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