Monday, July 22, 2013

What is the difference between cholesterol and fat?

Q. I know that fat is broken down into fatty acids and glycerol and that the fatty acids are transported by lipoproteins but isn't cholesterol also transported by them too. They're both lipids but where do they differ?

A. They are both lipids, but "lipid" is a pretty broad term that includes a variety of hydrophobic organic molecules produced by living organisms. Members of the other major classes of biological moleculesâproteins, nucleic acids and carbohydratesâare united by similar chemical structures, but that is not the case for lipids. Fats are either fatty acids or fatty acid derivatives, like triglycerides, and are characterized by long hydrocarbon chains, which give them their hydrophobic quality, and a polar carboxylic acid "head" group which can react to form esters (like with glycerol) and other structures. In general, "fat" refers to triglycerides (fatty acid chains + glycerol), but "fat" could also refer to individual fatty acids. Fatty acids are a group of molecules that differ in the length of the hydrocarbon chain and the presence/absence and position of double bonds. Cholesterol, on the other hand, is a single molecule with a very different chemical structure from the fatty acids. Cholesterol's main structural feature us four hydrocarbon rings. Take a look at these drawings of the molecular structure of cholesterol and myristic acid, a fatty acid:
Cholesterol -
Myristic acid -
Even if you are not familiar with the way that chemists represent molecules and exactly what these drawings mean, you can see that they are very different.

The biological functions of cholesterol and fats are also different. Fats are primarily used for energy storage and metabolism. When you eat more calories than you need, fats are efficient molecules for storing this excess energy for the future. Fats are broken down and energy is released when our bodies need more power, although our bodies preferentially burn carbohydrates for energy before dipping into our fat stores. Fatty acids do play many other roles in biology, but fats and oils (usually) specifically refer to glycerol esters of fatty acids, which are basically fatty acids linked to glycerol. The main role of fats and oils is in energy metabolism and nutrition.

Cholesterol has many different biological roles. It is a component of cell membranes, where it gives them increased rigidity; in other words, cholesterol helps maintain the shape of the cell membrane, making it less flexible. Phospholipids, the main component of membranes, also contain fatty acid chains, and these are relatively long and flexible molecules, but cholesterol is much less flexible because of its ring structure, and the more cholesterol molecules you stick in the cell membrane, the better able it is to hold its shape. Organisms that live in very high temperatures often have more cholesterol and similar molecules in their cell membranes, to prevent them from melting and spilling open the contents of the cell. Cholesterol is also part of bile, where it aids in the breakdown of fats, and is a precursor to many important biological molecules like vitamin D and testosterone.

You are correct that cholesterol and fatty acids are both transported by lipoproteins, but because "lipid" is such a broad category of molecules, so is "lipoprotein." Lipoproteins are proteins that transport lipids, but lipids are diverse and even the same lipid have multiple functions, and different proteins may carry the same lipid to different parts of the body for different purposes.

For more detail, go to your biology textbook first. A general/introductory biology text may not have a lot of detail on cholesterol, but it should describe the general structure and function of fats (triglycerides) and fatty acids. The Wikipedia articles on lipid, fat, oil, cholesterol and lipoprotein also have more info, and a web search for these words will also provide a lot more information.

How does cholesterol effect low thyroid function?
Q. What is the relationship between the two?
I can't find any clear answers on this?

A. Some years ago, and before being diagnosed with Hypothyroidism, I had been tested for cholesterol and was within the 'safe' zone. I did a re-test about a year ago (after diagnosis) and was shocked to discover that my cholesterol levels were above the accepted levels even though I lead a healthy lifestyle and eat very little red meat or fat. I started to do some research on the internet and discovered, mainly on American sites, that next to a bad diet, thyroid problems are the next biggest cause of raised cholesterol levels.

I can't explain why, medically, but it would just seem that there is a clear link between the two since I was fine before my thyroid condition and now have a raised level even though I have not changed my diet or lifestyle.

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