CHAPTER 5
Metabolic Flexibility and Using More Energy If your cells are not already well-equipped to use fat for fuel, then in order to take full advantage of the fat burning benefits of intermittent fasting, you need to help your cells become more metabolically flexible. We touched on this concept in Chapter 3. Your body has many specialized cells, such as red blood cells, brain cells, and muscle cells, each of which need energy to do their cellular work. For example, liver cells work to detoxify substances that come into the body and kidney cells work to filter the blood and create urine. Most of the energy your body uses in a given day powers these functions, which are the minimum processes needed to keep your body alive and well. The sum total of all the energy needed for these minimum required tasks is called Resting Energy Expenditure, or REE. This energy comes from the macronutrients—carbohydrates, fats, and to a lesser extent, protein or amino acids (though, as discussed previously, protein contributes relatively little to the body’s energy needs because it is needed for other things).
What Is Metabolic Flexibility? For the purpose of providing energy to cells, carbs are broken down into glucose, and fats are broken down into fatty acids (and later ketones). Fat and glucose are the major fuel sources, and many cells can make use of either depending on what’s available and what is needed at the time. Muscle cells, for instance, want to use fats while at rest, and glucose when they’re contracting, such as during exercise. In a healthy person, these cells will be able to shift back and forth between the fuels as the situation demands, and get energy from either one without difficulty. But this is not always the case. With poor eating and sleep habits and lack of exercise, the cells can become metabolically inflexible—meaning they can’t use the different fuels very well, or switch back and forth between them. The classic case of this is someone with diabetes: they can’t use glucose very well, so it remains in their bloodstream and promotes weight gain due to persistently high insulin levels. What makes a healthy cell choose one fuel over another? It depends on a number of factors, including: Availability. Are you fasting and have only fat as an energy source? Or did you just eat a high-carb meal and deliver a lot of glucose to your bloodstream?
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