Chronicles

Data from the European Commission show that almost half of the European population never exercise. Portugal is at the bottom of the list with 68% of Portuguese people reporting that they never exercise. Why is that? Recent technological advancements have allowed us to almost completely eliminate physical activity from our daily life. From the agricultural revolution, to the industrial revolution and to the digital age, the amount of physical work necessary for daily activities has substantially decreased. We are less physically active due to the nature of our jobs, increased urbanization and the availability of mechanized transportation. We also consume foods that are rich in fat and sugar that highly surpass the energy requirements for our sedentary lifestyles. So, we consume more energy than we need but we do not have effective means of expending this excess.

This 21^st century world of sedentary and overfeeding lifestyles is blamed for increased prevalences of a wide spectrum of metabolic disorders. As described in previous chronicles, when we eat fat and sugar-rich foods the liver converts much of the excess sugar into fat. For healthy individuals this, alongside dietary fat, is stored in adipose tissue. 

However, if this excess of nutrients is recurrent, the capacity of adipose tissue for storage is overwhelmed, and fat spills over into other tissues such as liver, pancreas and skeletal muscle. In liver cells, the fat collects in the cytoplasm resulting in hepatic steatosis or “fatty liver” and is the first stage of a disease called Non-Alcoholic Fatty Liver Disease (NAFLD).

One in every fourth person in the world is affected by NAFLD, which has now become the first cause of chronic liver disease and is considered a major global health problem. The presence of fat droplets inside liver cells seriously jeopardizes normal cell function. Among the most affected cellular components are mitochondria, organelles with a critical function in energy production in the cells. Mitochondria are present in almost all cells of the human body and are in charge of oxidizing sugars and fats to produce energy for the cells (and our body) to function.

At first it would seem that the more fat available in liver cells, the better, since the mitochondria could produce even more energy. However, at some point, the mitochondrial system of energy production cannot process all fats with good efficiency and begins to fail. In this process, mitochondria release products that trigger an inflammatory reaction that may eventually kill the cells. This liver inflammation is called steatohepatitis, a more advanced stage of NAFLD. If there is no change in lifestyle habits, the liver tissue develops extensive scarring, that further compromise its function. This final stage is called cirrhosis and irreversibly impairs the function of the liver. Sedentary lifestyle and overconsumption of unhealthy food are considered the main culprits for the development of NAFLD.

There is evidence that patients diagnosed with NAFLD have low levels of physical activity. Therefore exercise, in combination with a healthy diet, is highly recommended as strategy for preventing NAFLD. The chances of developing NAFLD are markedly reduced if people exercise more than three times a week. These benefits of exercise on the liver are seen following a relatively modest reduction in body weight (7 to 9%).

To exercise, we need energy for our muscles to contract and our heart and lungs to work faster. To do this, the mitochondria in those organs need to use fats and sugars to produce energy, which increases the energy expenditure and reduces excess fat depots in liver and other tissues. Also, regular physical activity helps us preserve mitochondrial structure and function, which can be seriously compromised by NAFLD.

Researchers are now establishing which exercise pattern is best to prevent or halt the progression of NAFLD. The most recommended type of exercise for the prevention and treatment of NAFLD is moderate to vigorous aerobic exercise. This way of exercising, also known as endurance training, needs to be executed for long periods of time at a moderate or vigorous intensity, which allows for the mitochondria of the muscle to increase oxidation of both fats and sugars. Endurance or aerobic training involves the mobilization of large groups of muscles, the formation of more mitochondria and a better function of the cardiorespiratory system, for which it is considered a metabolic-like stimulus. On the other hand, strength training, such as weight lifting, is generally considered a more contractile-related challenge and promotes an increase in the number and size of muscular fibers restricted to the  group of muscles involved. Both training modalities, despite generally via different biological mechanisms promote several health benefits in many body systems, including the skeletal, muscular, cardiovascular and respiratory, nervous and endocrine.

A moderate and regular amount of endurance training, for instance walking, running, swimming, running or biking for at least 45 minutes, usually 3-4 times a week, is the most time-efficient and effective exercise modality to reduce the content of liver fat and thus, to treat NAFLD. Overall, staying active - preferably with endurance training - will help keep your liver healthy.

Authors: Emilio Molina and Jelena Stevanović are early stage researchers of the FOIE GRAS project. Emilio Molina is doing his research at the Università degli studi di Bari Aldo Moro (UNIBA), in Bari (Italy), at the Center for Neuroscience and Cell Biology, at the University of Coimbra (Portugal),and at the company microBiolytics (Germany). Jelena Stevanović is doing his research at the Universidade do Porto, at the UNIBA, in Bari (Italy), and at the microBiolytics (Germany).

The project: This chronicle results from the collaboration between the Center for Neuroscience and Cell Biology (CNC) of the University of Coimbra, the European Training Network FOIE GRAS (http://www.projectfoiegras.eu), the Erasmus+ Program and the Academic Sports Federation University (FADU) in the scope of the European University Games Coimbra 2018.

Coordination: Anabela Marisa Azul, João Ramalho-Santos, Mireia Alemany i Pagès, Paulo Oliveira and Sara Varela Amaral

Revision of the text: Mireia Alemany i Pagès, Anabela Marisa Azul, John Jones, João Ramalho-Santos, Piero Portincasa, Martin Rossmeisl, José Magalhães, Fernanda Borges and Paulo Jorge Oliveira

Illustration: Rui Tavares

This chronicle reflects only the authors’ views and the Commission is not responsible for any use that may be made of the information it contains.