Four seasons
The calculation of energy consumption made it possible to divide human life into four periods
Polina Loseva, N+1
A person does not always spend as much energy on vital activity as is "supposed" with his body weight. This conclusion was reached by a group of scientists who analyzed the metabolic rate of more than six thousand people. Researchers have found that young children spend almost one and a half times more than their "norm", and older people, on the contrary, gradually reduce costs. However, it turned out that the periods of human life defined in this way are not at all similar to childhood, youth, maturity and old age in the everyday sense. The work was published in the journal Science (Pontzer et al., Daily energy expenditure through the human life course).
Today, metabolic disorders are considered an important component of many diseases. Obesity and overweight are risk factors for a variety of pathologies, from covid to atherosclerosis, and calorie restriction is suggested to be used even in cancer therapy and the fight against aging. But at the same time, we know quite little about how many calories the human body normally consumes and how this indicator changes during life. And this knowledge would be very useful if we really, for example, started limiting calorie intake to prolong life.
There are several ways to measure the energy consumption of the body. The easiest way is to measure the amount of carbon dioxide exhaled and recalculate it by the number of glucose molecules burned. But this method allows you to calculate only the basic level of metabolism – what a person spends on maintaining vital activity (blood circulation, respiratory muscles and brain, and so on) at rest. But as soon as we want to find out how much energy the body needs during the day – taking into account walks and sports – this method ceases to be applicable.
The alternative is the double labeled water method. Its essence is that a person drinks water with heavy isotopes of hydrogen and oxygen. Then, during the day, these isotopes gradually leave the body: oxygen – in the composition of water (with urine and sweat) and carbon dioxide (with exhaled air), and hydrogen – only in the composition of water. Therefore, if at the end of the day you measure the content of both isotopes in a person's blood, then you can calculate how much carbon dioxide he exhaled – and, again, convert it into calories spent.
The double-labeled water method (Garnotel et al. / J Appl Physiol, 2018).
The double-labeled water method is much more accurate, but also more expensive. Therefore, in order to get a lot of data on people's energy consumption, Herman Ponzer from Duke University had to join forces with colleagues from the USA, China, Europe and Africa. In total, the researchers collected information about 6,421 people aged 8 days to 95 years from 29 countries around the world. Of these, 2008 people managed to measure the basic metabolism (using the analysis of exhaled air), and to these data the authors of the work added the published results of studies on newborns and pregnant women.
Ponzer and colleagues suggested that the intensity of metabolism can vary greatly from person to person – depending on his body weight. Therefore, they normalized the level of energy consumption per body weight without taking into account adipose tissue. But it turned out to be difficult: researchers quickly discovered that energy consumption depends on mass non-linearly. Therefore, they built a logarithmic model, with the help of which they recalculated the body weight into the corresponding energy consumption. And then, for each person, the relative intensity of metabolism was calculated from the database. For example, 120 percent meant that a person spends 20 percent more calories than can be expected by his body weight.
Non–linear relationship between body weight and total energy consumption in different age groups (blue marks men, yellow – women, empty circles - the average for the sample). Figure from the article by Pontzer et al.
When the authors of the work constructed an average trajectory of how the relative intensity of energy exchange changes during life, they noticed that four periods can be distinguished in this curve. The first is newborns, up to a year old. They start life with about 100 percent (that is, they spend as much as the model expects from them), but by seven months they "accelerate" to almost 150 percent.
After that comes the second period, youth, and lasts from a year to 20 years. At this time, the overall metabolic rate is increasing, but its relative value is slowly falling – and after 20 years returns to 100 percent.
The third period, maturity, lasts up to almost 60 years, and represents a plateau – relative energy costs rest about 100 percent, regardless of gender and other conditions like pregnancy.
This is how the relative intensity of metabolism changes during a person's life (from the article by Pontzer et al.).
Then, at some point, the costs begin to decrease. The researchers could not determine the specific point of the curve break due to the small amount of data, but they suspect that it falls about 60 years (at least somewhere between 45 and 65 years). After that, the metabolism drops by about 0.7 percent per year, reaching -26 percent after 90 years.
Thus, the authors of the work divided a person's life into four periods according to the power of metabolism – and it turned out that these periods do not coincide with the boundaries we are familiar with. In this interpretation, childhood ends already in a year, there is no fracture in the puberty area, and "metabolic aging" begins only after 60 (while according to other signs, aging can begin even in the womb). Next, researchers will have to figure out what biological mechanisms are behind these transitions and how to influence them in order, for example, to "strengthen" childhood or postpone old age.
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