Triacylglycerols, more commonly known as triglycerides, are the most frequent type of fat in the body, obtained from foods like oils and butter. They represent the body’s primary method of energy storage. When you consume more calories than needed, the body converts them into triacylglycerols and stores them in fat cells for later use.
Chemical Composition of Triacylglycerol
The structure of a triacylglycerol molecule resembles a capital “E.” The vertical backbone is a three-carbon molecule called glycerol, and attached to it are three long fatty acid chains. These fatty acids connect to the glycerol through an ester linkage, which is formed by a dehydration reaction where water is removed.
The specific properties of a triacylglycerol are determined by the types of fatty acids attached. Since fatty acids vary in length and structure, many different triacylglycerol molecules can be formed. This structural diversity is responsible for the different characteristics observed in fats and oils.
A key distinction among fatty acids is their chemical bonds. Saturated fatty acids have no double bonds in their carbon chains, which allows them to be straight. This structure lets them pack together tightly, which is why triacylglycerols rich in saturated fats, like butter, are solid at room temperature.
In contrast, unsaturated fatty acids contain one or more double bonds that create “kinks” in the chain, preventing them from packing closely. Triacylglycerols high in unsaturated fatty acids, such as most vegetable oils, remain liquid at room temperature. A single triacylglycerol molecule can contain both saturated and unsaturated fatty acids.
Biological Roles in the Body
The primary role of triacylglycerol is as a long-term energy reserve stored in adipose tissue, or body fat. These molecules are an efficient energy source, providing more than double the energy per gram compared to carbohydrates or proteins. When other energy sources like glucose are low, the body breaks down these stored fats for fuel.
Adipose tissue also provides thermal insulation. The layer of fat beneath the skin helps shield the body from cold and maintain a stable internal temperature. This is because triacylglycerols have low thermal conductivity, meaning they do not easily transfer heat.
Triacylglycerols also play a protective role. Deposits of adipose tissue surround organs like the kidneys and heart, acting as a shock absorber. This padding cushions them from physical impact and injury during daily movement.
How the Body Processes Triacylglycerol
Dietary triacylglycerol processing begins in the small intestine. Since fats are hydrophobic and do not mix with water, they arrive as large globules. The liver produces bile, which acts as an emulsifying agent, breaking the large globules into smaller droplets and increasing the surface area for digestive enzymes.
Once emulsified, enzymes called lipases begin digestion. Pancreatic lipase is the primary enzyme that breaks down triacylglycerols into free fatty acids and monoglycerides. These smaller components can then be absorbed by the cells lining the intestinal wall.
Inside the intestinal cells, the absorbed components are reassembled back into triacylglycerols. These are then packaged into large lipoprotein particles called chylomicrons. This packaging is necessary because fats cannot travel freely in the bloodstream.
Chylomicrons are released from the intestinal cells into the lymphatic system, eventually reaching the bloodstream. As they circulate, they deliver triacylglycerols to tissues like adipose for storage or muscle for energy. When the body needs energy, a process called lipolysis breaks down stored triacylglycerols, releasing fatty acids into the blood for fuel.
Health Implications of High Levels
While triacylglycerols are important, persistently high levels in the blood can lead to health problems. This condition, hypertriglyceridemia, is diagnosed through a blood test, often as part of a lipid panel. A normal level is below 150 milligrams per deciliter (mg/dL), while 200 mg/dL or higher is considered high.
Elevated triacylglycerol levels are a known risk factor for atherosclerosis, a condition where plaque builds up inside the arteries. This plaque, made of fats, cholesterol, and other materials, can harden and narrow the arteries. This restriction of blood flow increases the risk of a heart attack or stroke.
When triglyceride levels become extremely high, above 500 mg/dL, there is a risk of acute pancreatitis. This is a sudden and painful inflammation of the pancreas. The condition is caused by the breakdown of excess triglycerides within the pancreas, which leads to cellular damage.
Several lifestyle factors and medical conditions contribute to high triglyceride levels. A primary cause is consuming more calories than the body burns, especially from sugar and refined carbohydrates. Other factors include:
- Excessive alcohol consumption
- Cigarette smoking
- Being overweight or having obesity
- Poorly controlled type 2 diabetes
- Kidney disease