In a study dating back to 1985, specialists from the University of Texas (USA) compared the metabolic rates of two groups of mice. The animals in the control group were fed a normal mouse diet, while the calorie content of the control group’s diet was reduced. The experiment lasted 4.5 months, during which time the scientists did not find any differences in the metabolic rates of the mice in both groups. In 1992, the same group of specialists repeated the experiment, slightly changing the conditions: now the scientists conducted lifelong observation of a group of mice whose calorie intake was reduced by 40% compared to the normal norm starting at six weeks of age. Metabolic rate was measured indirectly by analysing the gas composition in the cells. The metabolic rate in animals that were not restricted in calories was analysed in the same way. It was found that between 6 and 12 months of age, the rodents in the main group experienced a decrease in metabolic rate, but from 18 months onwards, the metabolic rate began to recover and by 24 months of age was the same as that of the mice in the control group. The results of the study show that under conditions of insufficient nutrition, metabolism gradually ‘adapts’ to new conditions of existence and compensatory mechanisms are activated, allowing metabolic processes to return to their previous level. Experts have suggested that it is this mechanism, rather than a slowing of metabolism, that may determine increased longevity in the case of prolonged calorie deficiency. In 2015, scientists from the University of Missouri (USA) conducted a meta-analysis of studies devoted to finding a link between a slowdown in metabolic rate, including that caused by calorie deficiency, and the lifespan of various animals. The experts noted that the level of energy metabolism can have different effects on the lifespan of animals with different life experiences and under different experimental conditions. However, to date, it has not been possible to identify a clear and reliable relationship between nutrition, energy expenditure and lifespan.

In 2003, scientists from the Pennington Biomedical Research Centre conducted a meta-analysis of studies on the relationship between diet, metabolic rate and longevity. Among the relevant studies, the authors cite research on the ‘Okinawa phenomenon’: this refers to a Japanese island known for its large number of centenarians. Experts found that the energy value of the food consumed by schoolchildren in Okinawa is 38% lower than the calorie content of the diet of students from other regions of Japan. Adult Okinawans consumed 20% fewer calories than other Japanese, while mortality from heart disease in Okinawa is 59% lower and from cancer 69% lower.

In 2018, this mechanism was studied in humans as part of the Calerie project – Comprehensive Assessment of Long-Term Effects of Reducing Intake of Energy. Fifty-three people aged 21 to 50 reduced their calorie intake by 25% and adhered to this diet for two years. A control group with a similar composition was also monitored: its participants adhered to a standard calorie intake corresponding to age norms. During the observation period, the average weight loss among volunteers was 9.4 kg, while the weight of participants in the control group remained unchanged. Participants from the main group and the control group were periodically placed in a metabolic chamber, where sensors measured the rate of metabolic processes in their bodies. It turned out that as people with a restricted calorie diet lost weight, their energy expenditure during sleep decreased by 10%. This indicates a slowdown in metabolism. Volunteers from the calorie-restricted group also showed a decrease in the levels of hormones responsible for metabolic rate. These included thyroid hormones (T3 and T4) and leptin: it had previously been established that a decrease in leptin concentration is a sign of a slowing metabolism during weight loss. There was also an increase in adiponectin, a hormone synthesised by adipose tissue. It is also one of the regulators of metabolic rate. In obesity, adiponectin concentration decreases, while the risk of developing inflammatory processes and atherosclerosis increases and cell sensitivity to insulin decreases. At the same time, scientists have established that a decrease in leptin levels and an increase in adiponectin are observed primarily during the period of active weight loss. Thus, these hormones are important for metabolic adaptation, i.e., they slow down metabolic processes in response to weight loss. During the weight stabilisation phase in the low-calorie group, leptin and adiponectin levels remained fairly stable, while the downward trend in T3 and T4 levels continued. A series of studies has shown that a decrease in thyroid hormone levels is one of the biomarkers of ageing. Thus, this study found that prolonged calorie restriction leads to a decrease in metabolic rate, accompanied by biochemical and hormonal changes. At the same time, changes during the weight loss phase are positive (decreased leptin concentration and increased adiponectin). At the same time, changes during the weight stabilisation phase, such as a decrease in thyroid hormone levels, are not considered beneficial for health and longevity.

To date, many data have been accumulated demonstrating that caloric restriction leads to a slowing of metabolism. Experiments involving animals have shown that a decrease in the rate of metabolic processes on the background of long-term caloric deficit has a positive effect on health and life expectancy. Similar studies in humans also make it possible to record a slowdown in metabolism during long-term calorie restriction. At the same time, not all these shifts are positive. Also today, there is no reliable data linking caloric deficit, the resulting decrease in metabolic rate and an increase in human longevity.