There are many misconceptions regarding fat and its contribution towards obesity. Understanding the process of fat synthesis and burning in the body will help us clear many of these false notions.
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Glucose – The raw material that fuels our body
To understand how we become fat, we must first have a basic understanding of how carbohydrates are digested in our body. Let’s say you have eaten a piece of bread, which is a source of carbohydrates. When the piece of bread gets into your digestive tract, a set of enzymes known as Carbohydrase (comprising of amylase, maltase, etc.) break down the starchy substance into simple sugars.
The most common simple sugar used in our body is Glucose. The glucose extracted in our digestive tract gets picked up into our bloodstream through our small intestine, with the help of glucose transporter proteins. The transporter proteins take the glucose to our living cells, where a part of the cell known as the mitochondria processes the glucose to produce Adenosine Triphosphate (ATP), which basically powers all our living cells.
Glycogen – The primary reserve fuel of our body
So the main raw material used for energy production in our body is glucose. But our body is like a car i.e. it only requires a certain amount of fuel every day to carry out its routine tasks. Some vehicles have emergency tanks to store excess fuel. Our body has a similar system. Excess glucose in the bloodstream gets transported to the liver and to our muscles.
When blood glucose increases, our pancreas produces insulin. This hormone reacts with liver cells to stimulate a chemical reaction that produces glycogen, which is basically a large molecule containing smaller chains of glucose. Glycogen is basically the primary reserve fuel of the body, it is broken down in the liver and by our muscles into glucose the moment our blood sugar becomes low.
One of the key properties of glycogen is that it is stored in the form of a watery solution in the body, which means 1 gm. of glycogen will have 2 or 3 gm. of water attached to it. Thus an increase in glycogen levels leads to an increase in ‘water weight’ in the body.
Many people who crash diet or fast tend to lose weight quickly because blood sugar reduces during fasts and diets, and hence the glycogen in their bodies gets used up and the associated water gets released. Although fasting and crash-dieting are not sustainable or good for the body in the long run, it is advisable to moderate on carb-consumption in order to control the water-weight in your body.
Lipogenesis – The process of fat making
It’s one thing to convert excess glucose into glycogen, but what happens when there isn’t enough space in the body to store glycogen? Within the cell, glucose goes through the process of glycolysis. Glycolysis consists of a series of chemical reactions that help break down glucose into simpler particles that our cells can work with.
One of these particles is Acetyl-coenzyme A (Acetyl-CoA). As the number of Acetyl-COA molecules increases in the bloodstream, a group of enzymes known as Fatty Acid Synthases (FAS) reacts with the Acetyl-COA to make fatty acids. These fatty acids then go through a process called Esterification to produce Triglycerides, which are the building blocks of fat tissues.
So as you can see, a spike in glucose leads to an increase in acetyl-COA in the body, which can spike in the production of fatty acids and triglycerides in the body. This is how carbohydrates contribute to an increase in body fat.
What happens when we eat fat?
Contrary to popular opinion, consuming raw fat does not make us fat. To understand this, we must discuss how the body digests fat. So say if you have eaten a spoonful of butter or olive oil. A group of enzymes called lipases (secreted by the liver, pancreas and salivary glands) will first break down the butter into monoglycerides, diglycerides, and fatty acids.
Then, from the small intestine, some of the fatty acids are sent to the body’s organs and cells using transporter proteins like albumin. Once fatty acids enter the cells, they go through a series of chemical reactions to form Acetyl-COA, which is used by the body to create energy. The liver metabolizes fat into Ketone bodies, which can be easily used by cells in the central nervous system for functioning. Fats that are not required to be used get stored back into fat tissues or muscles.
Now although Fat has a bad reputation, it actually is the best source of energy. 1 gram of fat produces 9 calories of energy, compared to 4 calories for 1 gram of carbs. So basically you need to eat half the amount of fat compared to carbs to complete your daily caloric requirements. Hence, theoretically, a pure fat diet would reduce our weight by reducing how much you eat. But wouldn’t that make a normal human being hungry?
That’s where Leptin comes into play. Leptin is a hormone that tells the brain when enough food has been consumed and when to stop eating. Leptin is primarily made in the fat tissues of the body, and its production is triggered by fat. When we consume a high amount of carbs, eventually that will lead to excessive glucose being stored as fat, and this fat will lead to a high amount of leptin being produced in the body.
Ideally, this should prevent us from over-eating, but high amounts of leptin lead to a phenomenon called “leptin resistance”, wherein our brain stops responding to leptin signals, similar to how the pancreas stops responding to insulin in the case of “insulin resistance”.
However, if we theoretically eat just fat, and there is no glucose for the body to burn, the body will eventually go into a ‘fat-burning’ mode known as ketosis, and leptin sensitivity will also improve in a normal human being. Thus, THEORETICALLY, eating fat will make you feel more satiated than eating carbohydrates.
A balanced diet is quintessential
Although a “theoretical” fat only diet would lead to weight loss, it can have several adverse effects. The lack of fiber (from cutting out carbs) can lead to serious digestive complications, and your body would be deprived of several micro-nutrients. The point of this diet is not to encourage people to cut down on carbs and only eat fatty foods.
Carbohydrates, protein, and fats are essential macro-nutrients. If you have a sedentary lifestyle, 40% of your calories should come from carbs, if you have a very active lifestyle, 60% of your calories should come from carbs. If you keep these guidelines into account and consume carbs accordingly, your weight will be under control.