Understanding Lipids and Fat

The word fat has a lot of negative connotations due to the rising number of overweight and obese persons in the United States. However, fat is essential to human health just like protein and carbohydrates. In fact, you cannot live without fat in your body which some find surprising. Understanding the role of lipids and fats in particular is important to diabetes management.

What is A Lipid/Fat?

A lipid is a biological compound found in the body that is soluble in a number of organic solvents like ether and alcohol, but insoluble in water.[1] Lipids are classified according to their solubility and there is a wide range of lipids that have varying chemical structures. Simple lipids include fatty acids, oils, waxes, triglycerides and sterols. The complex lipids include phospholipids and glycolipids to name two.

Fat is a simple lipid and chemically includes triglycerides, fatty acids and triesters of glycerol. Fat is generally used to refer to fat that is solid at room temperature while oil is fat that is liquid at room temperature. Lipids refer to both forms of fat – solid and liquid. So fat is actually just a class of lipids.

Besides fat, you should be familiar with triglycerides, cholesterol, phospholipids and glycolipids. Triglycerides are composed of 3 fatty acid groups and glycerol. They make up a significant portion of the fat in our diets. Cholesterol is also a type of waxy fat that is produced in the intestines and liver. Phospholipids are a component of cell membranes, while glycolipids are cell membrane components bonded to monosaccharides or polysaccharides.

Together with other substances and the metabolism, lipids play a significant role in nutrition, health and disease. They are particularly important because cholesterol, glycolipids and phospholipids form the cell membrane which is a barrier that separates a cell from the rest of the external environment.[2]

The Characteristics and Functions of Fat

Fat is insoluble in water. It is, therefore, difficult to transport it in a liquid substance like blood. What makes it possible to mix blood and fat are lipoproteins which are made of lipids and protein and are water soluble.

Fat has many functions in the body in that it:

  • Provides energy storage
  • Composes the myelin sheath of the central nervous system (CNS) and lines the peripheral nerves
  • Forms the cell membranes
  • Forms an integral part of the prostaglandin (fatty acid substances) mechanism in inflammation
  • Contributes to the overall immune response

People tend to think of fat only in negative terms, but it plays an essential role in a number of ways. It acts as a key source of energy for the body by serving as a reserve supply rather than an instantaneous energy source.  This is one of its main differences from glucose. As a reserve resource of energy, fat is the largest energy warehouse in the body. In the average male body weighing 155 pounds (70 kg), approximately 20 percent is fat – that is 31 (14 kg) pounds of fat. This small amount fat is capable of providing 131,600 kcal of energy.

Besides being a key dependable source of energy, lipids partner or work with various other substances in the body. This means they participate in many structural and protective mechanisms. For example, without fat, there would be no protective cell membrane or myelin sheath – the white matter of the central nervous system and lining the peripheral nerves. Lipids are a component of the prostaglandin mechanism in inflammation and therefore part of the body’s immune response system.

The Mechanism of Lipids: Transportation and Beyond

Yet, fat cannot move by itself. It needs to be transported throughout the body if it is to be useful to the body. When dietary fat is ingested, it moves through the digestive or gastrointestinal tract. When it reaches the small intestine, it is absorbed into the intestinal lining. Once absorbed it is transported by chylomicrons, the lowest density lipoprotein.[3] Chylomicrons do not, like broken down carbohydrates, enter the blood vessels of the membrane. Instead, chylomicrons enter the lymphatic drainage system by means of a lymphatic vessel. In this manner, dietary fat reaches the liver.[4]

The liver is the basic factory of the human body. It manufactures the very low density lipoproteins (VLDL). This is how insoluble lipids can be transported in a soluble solution such as blood to reach and to be deposited in the adipocytes (fat cells). In fact, it is crucial that lipids be able to be transported through the blood system as well as elsewhere to be metabolized into energy.

The chemical solution in the body that makes this happen is by associating the least soluble lipids with those that are more soluble. As a result, the least soluble lipids combine with the phospholids. After this process, they merge with a protein. The result is a water soluble lipoprotein complex. This complex of lipoproteins is grouped according to their density. Since fat is lighter than water, a low density lipoprotein consists of more fat while a high density lipoprotein means it consists of a larger amount of protein. These range from the very low density lipoprotein (VLDL) to low density (LDL) to the high density lipoprotein (HDL).

The LDL is required by the body in the formation of cell membranes. However, too much LDL cholesterol can be deposited along the inside of the blood vessels. If it blocks the blood vessel it can lead to heart disease, heart attack or stroke.

By contrast, the HDL is called the “good cholesterol.”  It’s called “good” because this lipoprotein scoops up any excess cholesterol in the bloodstream and carries it back to the liver. Here, it is broken down by the liver cells and then excreted as part of the waste products of the bile.

Lipids, Cholesterol and Diabetes

There is a connection between lipids, cholesterol and diabetes. It extends beyond the issue of obesity. Those who suffer from diabetes are at high risk for coronary heart disease (CHD). CHD is the result of several factors including high cholesterol levels. The lipid targets for diabetes are identical to those of CHD individuals.[5]  In both instances, control and management are the key issues. This is the reason it is so important to have a fasting lipid profile done at least annually. Cardiovascular disease is one of the leading causes of death in those diagnosed with diabetes.

The lipid profile measures HDL, LDL and triglyceride levels. Diabetic paitents have certain ideal levels of each of these three measurements in the lipid panel. The HDL should be greater than 40. The LDL should be less than 100, but some doctors advocate less than 70. Finally, triglycerides should be less than 150. The lipid profile can also indicate a condition called dyslipidemia. Dyslipidemia is common in people with type 2 diabetes. It is a condition in which you have low HDL cholesterol levels and high triglyceride levels.

References

[1] Marcovitch, H (2006). Black’s Medical Dictionary 41st edition. Lantham, Maryland: Scarecrow Press.

[2] Walker, S and McMahon, D (2008). Biochemistry DeMYSTiFieD. New York: McGraw Hill.

[3] Murray, R.K;  Bender, DA; Botham, KM; Kennelly, PJ; Rodwell, VW; and Weil, PA (2009). Harper’s Illustrated Biochemistry 28th ed. New York: Lange McGraw Hill.

[4] Guyton, AC; and Hall, JE (2011). Textbook of Medical Physiology, 12th ed. Philadelphia: Saunders.

[5] Solano, MP; and Goldberg, RB (2006). “Lipid Management in Type 2 Diabetes.” Clinical Diabetes, 24 (1): 27-32.

This article was originally published July 12, 2012 and last revision and update of it was 9/10/2015.