Question

A low-fat diet of pasta, bread, beer, and soda can easily lead to an increase in body mass. The increase is stored triacylglycerols in adipocytes. Explain the increase in mass and why the excess carbohydrate is stored as fat.

Short answer

Excess carbs are converted to fat (triglycerides) and stored in adipocytes, increasing body mass.

Step-by-step solution

Understand the Essentials of Metabolism

When you consume more calories than your body needs, the excess energy gets stored for future use. Carbohydrates, fats, and proteins can all provide energy, but carbohydrates are typically the primary energy source in everyday diets.

Carbohydrate Conversion to Fat

Excess carbohydrates are converted into acetyl-CoA, a molecule that enters the biochemical pathway known as the citric acid cycle (Krebs cycle). If energy demands are low, acetyl-CoA is instead used to produce fatty acids, which are then stored as triglycerides in adipocytes (fat cells).

Fat Storage in Adipocytes

Triglycerides, which are triacylglycerols, are stored in adipocytes. Adipocytes expand to accommodate the excess triglycerides, resulting in an increase in body mass, specifically body fat.

Key concepts

Carbohydrate Metabolism

When we talk about carbohydrate metabolism, we are referring to the process by which our bodies break down carbohydrates to produce energy. Carbohydrates are our body's preferred source for energy. They are broken down into simple sugars, mainly glucose. Once broken down, glucose is absorbed into the bloodstream and used as fuel by our cells.
Carbohydrates that are not immediately needed for energy are stored for later. They are converted to glycogen and stored in the liver and muscles. However, there's a limit to how much glycogen can be stored. When glycogen stores are full, and excess carbohydrates still remain, the body initiates a conversion process into fat. This process sets the stage for the next step, which is how these carbohydrates are stored when not needed for immediate energy.

Triacylglycerol Storage

Triacylglycerol, also known as triglyceride, is the fat storage form in our body. Once excess carbohydrates are converted into fatty acids through lipogenesis, they are assembled into triglycerides. These molecules consist of one glycerol unit combined with three fatty acid chains.
The storage of these triglycerides takes place predominantly in fat cells, or adipocytes, scattered throughout the body. When the body senses it has more energy from carbohydrates than it can use immediately or store as glycogen, it directs the conversion of carbohydrates into fatty acids, ultimately storing them as triacylglycerols. This efficient storage mechanism ensures we have backup energy reserves, which can be used later when energy from food is not readily available.

Adipocytes

Adipocytes, or fat cells, play a crucial role in energy storage. They have the unique ability to expand and store large amounts of energy in the form of triglycerides. These cells are found beneath the skin, around internal organs, and in specific areas designed for fat storage.
As they fill up with triglycerides, adipocytes increase in size, which leads to an increase in body mass. Although this capacity to store energy was beneficial in times when food was scarce, it can lead to weight gain if the intake of calories consistently exceeds the body's energy needs. Hence, understanding how adipocytes function is key to adopting a balanced diet and healthy lifestyle.

Citric Acid Cycle

The citric acid cycle, also known as the Krebs cycle, is a fundamental metabolic pathway that plays a pivotal role in cellular respiration. It takes place in the mitochondria, the powerhouse of the cell.
When carbohydrates are consumed in excess, they are first broken down into acetyl-CoA, a central metabolite in the cycle. However, if the body does not need more energy at that moment, the acetyl-CoA is diverted from the cycle. Instead of continuing to generate ATP, the energy currency of the cell, acetyl-CoA becomes a building block for synthesizing fatty acids. These fatty acids are then used to form triglycerides, which are stored in adipocytes. This pathway demonstrates how excess carbohydrate intake can ultimately lead to increased fat storage.