What does it actually mean to “burn” fat? This is when the fat stored in our fat cells is removed and “burned” for energy.
3 Major Steps of Fat Metabolism:
Mobilization: To mobilize body fat, we must first break down the triglyceride (body fat is stored triglyceride) into free fatty acids. A limiting step within this first process is an enzyme called Hormone Sensitive Lipase (HSL). The two major hormones that affect HSL are insulin and the catecholamines. Insulin is the main inactivator of HSL – small amounts can have a detrimental effect. Small increases from insulin (from food intake) can also inactivate HSL. Also, the mere presences of fat in the blood will inhibit HSL.
The main activator of HSL are the catecholamines – with adrenaline being the primarily one. The adrenaline in our body is released from the adrenal cortex, traveling through the bloodstream. Blood flow will have an impact on how much adrenaline will reach our fat cells. Catecholamines have their own receptors called adrenoceptors. The major ones are; beta and alpha, they are throughout our bodies. Alpha-2 receptors and beta 1 +2 receptors are the main receptors in the fat cells, these actively bind to the catecholamines. When catecholamines bind to beta receptors, fat breakdown will increase. The opposite happens when they bind to alpha receptors (fat breakdown decreases). So depending on the binding site, catecholamines can either help increase fat breakdown or decrease it.
Due to females having about 9 times as many alpha receptors than beta in their lower body, they in turn have a much harder time getting rid of stubborn fat areas. For the best possible results and consistent progress we are going to want to make sure insulin levels are low and catecholamine levels are high!
We can reduce carbohydrate intake to decrease insulin levels and increase are catecholamines by simply exercising!
Transport: So now we will break down stored body fat into free fatty acids (FFAs) which will enter our bloodstream. We cannot burn FFAs in our blood stream so they are then transported away from the fat cell (this will depend on blood flow to and from the fat cell). Sections of the body will have higher blood flow than others, this will result in faster fat loss in these areas. We have less blood flow in our abdominal region which makes it more stubborn than visceral fat for example (has a higher blood flow rate). However, in response to eating a meal, blood flow to stubborn fat increases steadily, which in turn can help with getting rid of fat in those areas. It is also easier for our body to store calories in stubborn fat areas than to get it back out.
So the takeaway is that poor blood flow to stubborn fat regions is another reason why getting lean can be difficult. We can increase blood flow by reducing your carbohydrate intake, which will result in lowering our insulin levels as well!
Oxidation (burning): This is when FFAs (free fatty acids) will run into tissue in our body (muscle) which will then use FFAs for fuel. To be used for fuel. The FFAs need to be transported into the mitochondria (you may remember this from your freshman biology class as being the powerhouse of the cell) by carnitine. Now carnitine activity is controlled by a couple different factors, including our glycogen levels (carbs stored). When glycogen levels are very high, carnitine and fat burning will be low and vice versa. By depleting (decreasing) our muscle glycogen (carbs stored in muscles) then we can increase carnitine activity, this will then allow you to burn off the fat at a much faster rate.
The takeaway from this step is that we can ramp up fat metabolism very effectively with the combination of lowered carbohydrate intake and intensive exercise.