Fat synthesis and breakdown: from insulin to aerobic oxidation

1.2.4 Fat Synthesis and Decomposition

1. Synthesis is also part of energy metabolism.

Fat is synthesized from glycerol and fatty acids. The body's fatty acids come from two sources: the body's own fat metabolism and food supply.

Below is a graph showing the changes in blood sugar in a normal person: Blood sugar spikes within half an hour after breakfast, lunch, and dinner because the sugar from food is absorbed into the bloodstream, increasing the blood sugar level. In a normal person, blood sugar usually drops within half an hour to an hour; the sugar in the blood disappears, but it's not actually being used; it's being converted into fat.

Because insulin can convert glucose into fat, it is also known as the "fat converter." Insulin is secreted by the islets of Langerhans in the pancreas. Insulin acts like a key, allowing glucose from the blood to smoothly enter the cells of various organs and tissues to provide them with energy. Normally, insulin secretion increases after meals and decreases significantly during fasting. Therefore, although blood glucose levels fluctuate with meals, insulin regulates these fluctuations within a certain range. If insulin is lacking or malfunctioning, glucose cannot enter tissue cells to provide energy, leading to elevated blood sugar and diabetes.

The symptoms of diabetes are "three highs and two lows": high thirst, high hunger, and high urination; and low weight and low energy, i.e., fatigue and weakness. Therefore, once a person has diabetes, they will actually become thinner.

Whether diabetes is treated with drugs that increase cell sensitivity to pancreatic islets or by directly supplementing insulin, as long as there is enough insulin in the body, it can convert sugar into fat. Therefore, once treatment begins, diabetic patients start to gain weight.

All of the above tells us one thing: obesity is not only caused by eating too much oil; carbohydrates can also lead to obesity. 1g of carbohydrates contains 4kcal, and 1g of fat contains 9kcal. Although fat has more than twice the calories of carbohydrates, carbohydrate intake is generally twice that of oil. Therefore, people who do not eat oil can still be very fat because their stomachs empty very quickly without oil, so they always feel hungry and eat more.

Understanding where fat comes from will make weight loss easier. In short, weight loss is about metabolizing fat.

2. Conditions and results of fat catabolism

The body has a mechanism that converts fat into energy that cells can use through various chemical reactions, called ATP (adenosine triphosphate). Here's how it works: fat first becomes glycerol through chemical reactions, then glycerol becomes dihydroxyacetone phosphate, which in turn becomes pyruvate. If there's no oxygen, pyruvate becomes lactate. If oxygen is present, it becomes acetyl-CoA.

Fat metabolism produces acetyl-CoA, which then enters the tricarboxylic acid cycle (also known as the citric acid cycle). Acetyl-CoA is converted to citrate, citrate to alpha-ketoglutarate, alpha-ketoglutarate to ADP, and ADP to ATP. ATP, or adenosine triphosphate, is the energy source that cells can utilize. Regardless of the source of energy-whether it's carbohydrates, fats, or proteins-only after being converted into ATP can cells use it. Sometimes, not all the energy is converted to ATP; some remains and continues to undergo chemical reactions, cycling back through the cycle to convert the remaining energy into ADP, then back into ATP, until all the energy is released. This process is called the tricarboxylic acid cycle.

Through the tricarboxylic acid cycle and a series of chemical reactions, fat is eventually transformed into carbon dioxide, water, and ATP, thus eliminating fat. Carbon dioxide is expelled through the lungs, water through the kidneys, and ATP is supplied to cells for use. This is the aerobic oxidation of fat.

3. Fat breakdown requires 38 enzymes and coenzymes.

Lipid metabolism involves 38 chemical reactions. Chemical reactions occurring within a living organism are called biochemical reactions, and all chemical reactions occurring within a living organism fall under the category of organic chemistry. What is the difference between organic chemistry and inorganic chemistry?

Inorganic chemistry involves the spontaneous reaction of two substances without any external conditions, while organic chemistry requires specific conditions, namely, a catalyst. In living organisms, catalysts are called enzymes, which are involved in the 38 chemical reactions mentioned earlier. Each chemical reaction requires a corresponding enzyme to proceed within the body. Furthermore, chemical reactions in the human body require not only enzymes but also coenzymes. Only coenzymes can activate enzymes; therefore, both enzymes and coenzymes are essential.

Each chemical reaction in the body requires a different catalyst, and there are hundreds of thousands of enzymes in the body. Are all enzymes used to break down fat? Does fat synthesis also require enzymes? No. Therefore, some enzymes in the body may make people obese, while others can help people lose weight.