Role of Mitochondria, Insulin and Glucagon in Metabolism: Brown and White Fat

In our body, we do have 2 types of fat. Brown fat and white fat. Brown fat has higher concentration of mitochondria which makes the color of the fat cells brownish color. White fat on the other hand has less mitochondria and makes the color of the fat cell whiter.

Why are these fats important? It helps us understand how we can improve our metabolism and utilize fat more efficiently.

Let’s look at key players called mitochondria, insulin and glucagon before discussing what brown and white fats are.

Physiology of Mitochondrial Respiration

Mitochondria is an organelle that lives in our cells, and its major function is production of energy. Whether you sleep, sit, exercise, read books, and so on, mitochondria are consistently at work providing us with energy that we are required to complete the tasks such as physical activity, work, or recovery.

Mitochondrial respiration refers to how mitochondria produce energy from fat and glucose for our daily performance.

We have two energy sources that can be converted into energy, fatty acid, and glucose. Glucose can also be converted into two molecules called pyruvate and lactate.

Ideal primary energy source is fat. Stored fat or triglyceride can break up into 3 fatty acids and glycerol by the enzyme called hormone sensitive lipase (HSL). Fatty acid, then, with the help of the amino acid called carnitine, it enters mitochondria through the CPT1 transporter. Once fatty acid enters the mitochondria, the process called Beta-Oxidation starts. This is the process where mitochondria further break down into acetyl-CoA. After the breakdown into acetyl-CoA then enters the last phase called Kreb cycle then to electron transport. Through this process mitochondria can generate 129 ATP from fatty acids.

Glucose has 2 pathways. First pathway is glucose breakdown to pyruvate through a process called glycolysis, then enter mitochondria convert into acetyl-CoA then enter Kreb cycle and electron transport like fatty acids. This process creates 32 ATP.

ATP stands for Adenosine Triphosphate. Think of it as currency for the cells. By consuming meat, vegetables, fruits, rice, the body does not know how to use it, so they need to convert them into ATP so that body knows what to do with it. Just like when you travel to foreign countries. You cannot use US dollars in Japan so you will need to exchange the dollar to yen to make any purchases.

As we exercise, regardless of how fit you are, energy demand will increase. We do normally produce some amount of lactate, however, as energy demand increases, we create more lactate through glucose. Therefore, the second pathway for glucose is after broken down to pyruvate with decrease in cellular oxygen, then convert into lactate. If there’s enough MCT1 transporters in mitochondria, the lactate enters mitochondria then convert back to pyruvate.

Pyruvate converts into acetyl-CoA and enters Kreb cycle and electron transport. In this process 32 ATP are created. You may see 2 ATP is created; however, studies have shown that lactate pathways can generate 32 ATP.

Insulin and Glucagon

Insulin and glucagon are both secreted by same organ called pancreas. Although they are both secreted by same organ, however, they have completely opposite characteristics. When insulin is working hard, glucagon doesn’t want to do anything. When glucagon is working insulin is not working at all.

Insulin is a hormone that is widely known. It is to transport hormones that are secreted when the food is consumed. Purpose is to transport nutrients to cells. On the other hand, glucagon is not so well known, but it is secreted when insulin is not working due to fasting or low carbohydrate diet. Simple way to remember is insulin likes to store and glucagon likes to spend.

When you want to get more nutrients and gain more mass (fat or muscles) you want insulin to work. However, if you want to exhaust your fat and focus on cleaning up your system, then you want glucagon to be working.

Brown fat and White Fat

If you have kids, you may have witness this. After taking a bath infants do not shiver whereas adults do and feel cold with the temperature difference between shower and outer environment. This is because infants have more brown fat than adults. Brown fat as mentioned earlier has more mitochondria which means it can produce more energy or heat. As we get older, unfortunately, those brown fats get smaller. White fat on the other hand is just a storage and by itself does not produce any heat or energy.

Brown fat, even with adults, is set around chest cavities where heart, lungs and other vital organs sit. Whereas white fat usually sits in subcutaneous and visceral areas.

Although they have different structures, depending on insulin or glucagon, they can act like one or the other.

For example, when insulin is working brown fat will work like white fat and stay calm and watch the white fat accumulates. When glucagon is at work, white fat will start working like brown fat and start emitting heat and being able to burn more energy.

When Insulin is High

Insulin's job is to store. Therefore, when insulin level is high, all our fat cells will act like white cells. In fact, our metabolic rate decreases, which makes total sense. When you want to store something, you do not want to waste or get rid of anything. For our body, when we are eating food, all the nutrients should not be wasted. For this reason, our body does not want to increase metabolic rate, where it can have the chance of wasting the important nutrients.

So, while the insulin is high, even mitochondria rich brown fat will act like white fat and entire body will act to retain food in the form of nutrients and fat.

It is recorded that when the insulin level is high, whether from diet or insulin treatment, one can increase their fat mass by 20lb within 6-month period.

When Glucagon is High

Glucagon's job is to spend. Therefore, when glucagon level is high, all the fat cells will start acting like brown fat. Metabolic rate during this time is about 10% higher, which also makes sense. When you are either fasted or lowered in carbohydrate intake, the body needs more energy to live. So, the body will be using stored energy called fat as energy source and will be spending like crazy.

Ways to Increase Glucagon and Brown Fat

There are several ways to activate glucagon and brown fat.

1. Low-carb and high fat diet - Many are doing low carb but not high fat. Important thing about low carb diet is to elevate the amount of fat intake. Benefit is to teach the body to use more fat as energy and glucagon to be more active.

2. Fasting - This may be an easier process than low carb diet. Give yourself 12, 16, 24 hours fasting, which will keep the insulin level low. However, if you are diabetic or have insulin resistance, it may take a while to bring the residual insulin level down.

3. Fasted Workout - This forces the glycogen stored in liver and muscle and forces the fat to be the primary energy source.

4. Building muscular strength and mass - When muscle is stimulated, insulin sensitivity of the cells will be improved and able to lower the blood sugar level along with insulin level.

5. Merging with cold - This is bit of challenging. When we merge ourselves with a colder environment (cold enough to shiver), the body will utilize fat to act like brown fat to heat up the body. Sleeping with less cloth, taking cold shower, exercising in cold environment can all increase the activity of brown fat. It is not recommended by everyone.

   
   

Conclusion

Throughout the week and depending on your goals, you should be mindful about which hormone and fat to activate. When building strength and muscles, you may want to increase the insulin secretion. Yet, when you are focusing on your figure, you may want to be friend with glucagon by suppressing insulin level. If you are not metabolically flexible, meaning not able to switch between fat energy and food source energy, you will need to first be friend with glucagon to promote more fat burning.

Kota Shimada