Introduction
To understand the mechanism of storing and burning body fat, we first need to get familiar with the basics of fat metabolism. The anabolic process is called lipogenesis, while the catabolic process is called lipolysis. In general, lipogenesis is activated by eating, while its reverse, lipolysis, is activated by fasting or physical activity. Most of the time, these two processes are in balance, but in the case when lipogenesis is active longer than lipolysis, fat accumulation occurs (and vice versa). In this blog post, we are going to do an overview of these two processes which will illuminate the basic concepts, mechanisms, and principles that dictate how fat is being metabolized. This will give us an understanding of future topics, where we will explore specific dietary interventions, that the principles described here, will lead to a particular metabolic process being more or less active and pronounced. As a bonus, we will also look at the mechanism of fat cell creation – adipogenesis.
Lipogenesis - Process of Storing Fat
Lipogenesis is the anabolic process where triglycerides (fat) are formed to be stored in our bodies. The process of lipogenesis starts with the molecule acetyl-CoA, which is derived from breaking down carbohydrates (glycolysis, fructolysis), fats (β-oxidation), and branch-chain amino acids – BCAAs [1]. Lipogenesis incorporates two sub-processes: Fatty-acid synthesis and triglyceride synthesis. The former presents the conversion from acetyl-CoA to free fatty acids and glycerol. The latter combines together those fatty acids and glycerol to form triglycerides and\or phospholipids – esterification. It’s the triglycerides that are stored as energy reserves, while the phospholipids are used as construction elements of the cell membranes.
While it is inevitable that acetyl-CoA will be formed no matter the type of food ingested, there is a pronounced difference between macronutrients and their effect on stimulating lipogenesis. The primary and the strongest nutrients upregulating lipogenesis are the carbohydrates [2]–[6]. The main site over which carbohydrates influence the lipogenic process is through the hormone insulin. Insulin is our main anabolic hormone and it is only with the presence of carbohydrates, where the lipogenic effect of insulin occurs [7]. In simple terms, this means that to control lipogenesis, one needs to primarily control the intake of carbohydrates [8].
Lipolysis - Process of Burning Fat
The antagonist of Lipogenesis is the process of fat burning i.e. Lipolysis. In this process, triglycerides are being broken down (via. hydrolysis) into smaller constituents: fatty acids and glycerol. The hydrolysis happens on-site where fat is required for energy. Because lipolysis is performed on-site, the triglycerides need to be transported from the adipocytes to the targeted tissue (skeletal muscles, heart,etc.). The demand for lipolysis is signaled through various (predominantly catabolic) hormones, such as glucagon, growth hormone, norepinephrine, to count a few [9]. On the other hand, insulin as the primary anabolic hormone blocks lipolytic activity [10]. When the aforementioned hormones are elevated (via fasting or physical activity) and insulin is low, adipocytes unlock the triglycerides stored inside, for transport. In the case of fat metabolism, triglycerides are transported via very low-density lipoproteins – VLDL. Once the lipoprotein arrives at the target site, triglycerides are exchanged from the VLDL to the bloodstream and then hydrolyzed. After hydrolyzation, the newly created fatty acids travel inside the cell to go through beta-oxidation where acetyl-CoA is formed. This can then be metabolized to produce energy and\or ketone bodies (in case of a low level of blood glucose and low glycogen).
Adipogenesis - Creation of New Fat cells
In relation to body fat mass, there is also the process of adipogenesis, which is responsible for the creation of adipocytes or fat cells – hyperplasia. This however needs to be distinguished from lipogenesis, where existing fat cells are filled up with triglycerides to increase their size – hypertrophy, but not the count of the cells [2], [11]. Once a fat cell is created, it can only change its size, but it can not vanish. The number of fat cells is established by adolescence, but the ability to proliferate still exists later on [12], [13]. The process of adipogenesis is however more prevalent in early childhood, while in later years, lipogenesis is much more pronounced. Both mechanisms of adipogenesis and lipogenesis seem to however require similar environments [14], [15]. Altogether, this would point towards a hypothesis that the ability to store body fat is determined in early childhood (and under influence of genetics), while later on, the size of those fat cells is controlled [16].
Conclusion
In this blog post, we have overviewed three processes that determine total body fat storage: lipogenesis, lipolysis and adipogenesis. Based on the research, it seems that the anabolic processes of lipogenesis and adipogenesis are related to the hormone insulin, while lipolysis functions as an inverse. This also makes sense, since we need to have counterbalancing processes to maintain equilibrium. However in today’s day and age, it seems that a large population is dominant in the anabolic processes compared to the catabolic ones.
References
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