Energy expenditure is determined by body composition and sex, not just by body weight

Energy metabolism determines how efficiently the body uses the energy obtained from food, both at rest and during physical activity. In practice, this process is often described using general calculations and equations based on average values established in specific research samples. However, Rudīte Lagzdiņa’s study, Factors influencing resting and exercise energy metabolism in adults, shows that even among healthy adults these “average” indicators can be misleading. Individual differences are large enough that they should be taken into account in health prevention, as well as when planning physical activity and nutrition.

Rudīte Lagzdiņa’s interest in energy metabolism developed during her earlier studies in nutrition science and her work in human physiology. “We take many relationships for granted because they are described in the literature. It was important for me to see what they look like in real measurements in a healthy adult,” the researcher explains.

Focusing on metabolism without the influence of disease

The study involved 99 healthy adults aged 20 to 50 – 55 women and 44 men. This sample made it possible to focus on metabolic processes unaffected by chronic disease or medication. As the researcher points out, studies often concentrate on specific patient groups, yet there is still less knowledge about healthy people, especially women, than might be expected.

From everyday movement to laboratory measurements

Participants’ daily physical activity was assessed over several days (from three to seven), using both internationally recognised self-report questionnaires and wearable devices that recorded movement intensity and heart rate. In the laboratory, detailed body composition measurements were taken (fat mass, lean mass, total body water and its distribution), along with metabolic measurements at rest and during standardised exercise, using indirect calorimetry. In addition, certain gene variants linked in the literature to physical performance and energy use were analysed in some participants.

Body composition and sex explain calorie expenditure more accurately

The study’s central conclusion is clear: energy expenditure is significantly influenced by body composition and sex. Men have higher total energy expenditure at rest, but when recalculated per kilogram of body weight, substantial sex differences no longer appear. During physical exercise, however, the pattern changes: women expend more energy per kilogram of body weight, particularly while running.

Muscle uses more energy than fat tissue

One of the study’s most consistent findings was related to lean body mass. People whose body composition includes more muscle expend more energy both at rest and during exercise. This applies to both sexes. By contrast, a higher proportion of fat tissue is associated with lower energy expenditure per kilogram of body weight.

Everyday activity leaves a visible mark on the body

When daily physical activity was analysed, clear associations emerged between movement intensity and body composition. Participants who spent more time engaged in moderate to vigorous physical activity had less body fat, as well as a more favourable body mass index and body composition. These associations were particularly pronounced in men, but were also seen in women, including in relation to the amount of visceral fat. It should be noted that around 30% of the study participants had insufficient levels of physical activity, below the level recommended for health.

In addition, the study also analysed exerkines – biologically active substances released during physical exertion that may be linked both to energy use and to the regulation of inflammation. The results suggest that short, intense bouts of exercise typical of daily life, such as a brief run, do not trigger substantial activation of inflammatory processes in healthy people.

The study findings are important for specialists in healthcare, nutrition and physical activity planning. They emphasise that even in healthy people metabolism can vary considerably, and universal calculations do not always accurately reflect reality. An individual approach – taking into account sex, body composition and daily activity – may be an important step towards more effective health prevention.

A summary of Lagzdiņa’s doctoral thesis is available in the RSU e-resource repository.