If like me you’re involved in sport you’ll be constantly looking for new ways to improve your performance. Over the next few blogs we’ll be looking at stored body fat and how we can better use this overlooked but critical energy source to improve exercise or sports performance at different intensities.
It’s true, fat or lipids often get overlooked in favour of their leaner fuel storage sibling glycogen, despite the fact that they are the body’s main energy reserve by some distance. The images above provide a visual representation of the differences. As an example, a lean male weighing 70kg carries about 15kg of body fat (based on 20% body fat) equivalent to 35,000 kcals or 20 days’ worth of energy (Frayn 2006). This is significantly greater than storage of glycogen, which at between 400-520g (1600-2080 kcals) represents about one day’s worth of energy (Frayn 2002). Further, muscle glycogen is restricted in use to the muscle group where it is stored so glycogen stored in your biceps can’t be used by your quads and visa versa. Fat energy in contrast has no such restrictions and is therefore a far more flexible energy source.
Given the size of our bodies lipid-stores it logically follows that they play a fundamental and critical role in fuelling many physiological processes and therefore any acute or chronic disruption in their supply will have an adverse impact on health and sports performance. So, let’s take a look at them in a little more detail.
Storage of Dietary Lipids
Dietary fats or lipids are stored in different tissues in the form of triacylglycerols (TG) or triglycerides as they are often referred to in the general medical literature. The TG molecule seen below is made up of a glycerol backbone and three fatty acids.
Fatty acids can either be saturated, unsaturated or polyunsaturated. The important thing about each of them is that they have to be split away from the glycerol backbone before they can be used for generating energy. When they are split away (‘de-esterified’) they are referred to as free fatty acids or to use the correct scientific term, Non-Esterified Fatty Acids (NEFA) – either term describes the fatty acid in a state where it can be oxidised, that is, used as a fuel.
NEFA are hydrophobic (ie., repel water) so they can’t circulate in blood alone; therefore, they are bound to a protein known as albumin and then transported to different tissues where they are used as fuel, stored away for later use or repackaged in larger lipid-carrying vessels known as lipoproteins. Most tissues use NEFA but the most important are liver cells (hepatocytes), muscle cells (myocytes) and specialist lipid storing cells known as adipocytes which are mainly located in adipose tissue surrounding the abdominal muscles. In the next blog we’ll be taking a closer look at how lipids are distributed in different tissues and how they are constantly recycled and ‘primed’ to meet the fluctuating energy demands of the body.
- Frayn KN, Arner P, Yki-Järvinen H (2006) Fatty acid metabolism in adipose tissue, muscle and liver in health and disease. Essays in Biochemistry; Chapter7:4289-103
- Frayn KN. Adipose tissue as a buffer for daily lipid flux. (2002) Diabetologia; 45:1201-1210