by Emilie "Flo" Hester, CPT, CNS and Larry Smith, MD, CPT, CVT
The following is a brief description of how fats are used in the body.
Fats keep us healthy. They are important substrates for energy and hormone production. Once fats are absorbed by the small intestines they are carried in the blood to the liver or in the lymph fluid to the heart. In the liver some of the Fatty Acids (FA) are stored as triglycerides (TG) or passed on to the heart where they are mixed with the short and medium chain FAs. The FAs are pumped from the heart to the peripheral tissues carried by blood proteins and as chylomicrons. Transport proteins on cell surfaces recognize the circulating FAs and transport them into the various cells in the body. Muscle cells for example keep a small amount of intercellular FAs for quick access short term energy production. Hormone or Endocrine producing cells also transport FAs inside and make a variety of hormones necessary for health and homeostasis. Excess FAs are taken into fat cells where they are stored until needed for energy production, hormone synthesis and general cellular function.
Diagram 1. The transport and storage of Fatty Acids in the body.
The balance between the body's cells and the availability of fats is highly regulated. Interestingly, women are more effective at mobilizing and using fats than men but also store fats better than men. The fats we store can be mobilized and oxidized by the various cells in the body for energy production. This is called Beta-Oxidation. The production of energy from FAs by muscle cells occurs in the mitochondria. As noted, small amounts of FAs are stored in muscle cells and are the first source of on demand energy production. As exercise intensity increases, the concentration of circulating epinephrine (Epi) increases. Epi tells the cells to increase energy production which begins with the intracellular TGs being broken down to FAs and glycerin. The long chains of the FAs are then cleaved so that precursor molecules that enter the energy production cycle. The intracellular FAs however are limited and as exercise demand continues the body's fat cells respond to the demand by releasing their FAs into the blood stream. The FAs are carried to the heart, skeletal muscle and other cells involved in the exercise demand. These cells transport the FAs intracellular and oxidize them for energy.
Diagram 2. Beta Oxidation pathway for Fatty Acids.
Fatty acid oxidation (FAOx) is a well-regulated process. However, it can become more efficient if there is a constant demand placed on it in the form of exercise. Likewise, it can become less efficient if you are sedentary and let yourself become overweight. Consuming excess calories in the form of fat or carbohydrates will contribute to fat stores that create the conditions for all the health problems that develop with obesity.
So, how does all this relate to weight loss, exercise, diet and overall health? That will be Part Six and Seven of this series. In part Six we will discuss the role of exercise and diet in how fats are used and what factors influence the efficient use of FAs in FAOx.