The fat deposited within the body is stored, specifically, within particular cells called adipocytes or fat cells (Wertheimer, Shapiro, 1948; Rutkowski et al. 2015). Adipocytes, however, can be of three different types and not all of them are suitable for this task. It is known that the fewer adipocytes that are able to store fat, the less fat will accumulate. Similarly, and for the same reason, the size of fat cells can also influence body composition.

White, brown and beige adipocytes

  As mentioned, adipocytes can be of three different types: white, brown and beige (Cohen, Spiegelman, 2016). White adipocytes (so called because they appear light-coloured under the microscope) group the nucleus or other organelles of the cell at its periphery, while the fat is accumulated in the central part. Brown adipocytes (dark under the microscope), on the other hand, have many tiny fat droplets scattered about. Unlike the former, these are equipped with a significant amount of mitochondria, those particular organelles responsible for energy production. Their structure is therefore better suited to the latter task than to the storage of fat (energy source) (Gaudry et al. 2018; Ricquier, Bouilland, 2000; Lee et al. 2013). It is therefore obvious that the greater the quantity of white adipocytes and the smaller the quantity of brown adipocytes possessed, the more fat one tends to be. Conversely, a smaller amount of white adipocytes than brown adipocytes will result in less fat accumulation. Now, brown adipocytes are particularly represented in infants. These, over the years, tend to become, to a large extent, white adipocytes (Ponrartana et al. 2014). For some individuals, this process can be partially counteracted by engaging in regular physical exercise, as human adipose tissue retains significant plasticity (Dinas et al. 2017; Otero-Diaz et al. 2018). Not all studies, however, confirm this thesis (Martinez-Tellez et al. 2022; Tsilousis et al. 2018). Another way to counter this phenomenon is to avoid taking excessive countermeasures against the cold (Romu et al. 2016; Blondin et al. 2014; Anouk et al. 2013): the energy that is produced by brown adipocytes, in fact, being in the form of heat, is already enough to fight it. If one overdoes the practices of protection against low temperatures, the function of the brown adipocytes will have no way of being exercised and, in time, will fail, as they turn into white adipose cells. The third type of fat cells, the beige cells, has characteristics intermediate between those of white and brown cells (Wu et al. 2012; Wu et al. 2013). It is believed to be possible to change white adipocytes into beige cells even in adulthood. This phenomenon is mediated by a hormone called irisin, the production of which takes place in response to intense muscle contractions and shivering (cold again). Unfortunately, this phenomenon takes place only to a modest extent, especially in adulthood (Norheim et al. 2013; Dinas et al. 2017; Elsen et al. 2014).

The size of adipocytes

  The greater the need for fat storage (a function, the latter, of lifestyle and, more particularly, of the diet one follows), the more adipocytes will tend to grow in size (Drolet et al. 2007; Jo et al. 2009; Haczeyni et al. 2017). An increase in the size of adipose cells can in turn lead to a greater willingness of the body to store fat (Engin, 2024). It is therefore a vicious circle whereby the more fat you accumulate, the more fat you tend to accumulate. The process, however, can also take place in reverse (Look AHEAD Research group, 2007, 2013; Liu et al. 2022; Cabeza de Baca et al. 2024; Varela-Rodríguez et al. 2020; Varady et al. 2009; Murphy et al. 2017).

The adipocyte set point

  Both the distribution of the various types of fat cells and their size tend to stabilise after the childhood (Arner, 2018; Escofet et al. 1996; Ponrartana et al. 2014). So those who had a lifestyle as children that hindered the transformation of brown to white adipocytes and the growth of fat cells will find it easier to lose weight. Conversely, a fat child is unlikely to become a thin adult (Simmonds et al. 2015; Freedman et al. 2005). Even those who have been fat children can aspire to lose weight, but it can take a long time. Slimming does not simply mean emptying the adipocytes of the fat they contain, but also changing their tendency to accumulate more fat. This concept is called the adipocyte set point and is the most immediately practical translation of what we have seen so far about adipocyte types and size (the latter being relatively easier to change in adulthood). Stabilizing body composition at a new level following weight loss may take longer if there is a relatively long history of being overweight, especially if this began in childhood. (Sánchez-Cheinos et al. 2025; Hinte et al. 2024). Trying to lose weight quickly, without taking into account the adipocyte set point, may perhaps lead to success, but this will most likely be transient, to the extent that the weight loss could easily be followed by a new accumulation of fat..