If you got into a conversation about cholesterol with someone off the street, they would most likely start talking about hypercholesterolemia, or high cholesterol. The conversation would no doubt touch upon topics such as low-cholesterol diets, statin drugs, and perhaps even exercise. But what about the unspoken other side to the cholesterol story? Nobody ever tells us just how important this 27 Carbon beauty is. Cholesterol is not an evil molecule the gods have cursed us with, but rather, the unsung hero in much of human physiology.
Though we talk an awful lot about cholesterol itself, this molecule is far from the end of the road in this physiological map. Quite the contrary, actually. Cholesterol is the mother molecule to many familiar substances in the body such as the sex hormones Estrogen, Testosterone and Progesterone, Vitamin D, the adrenal hormone Cortisol, and Bile Acids. Cortisol is the famous "stress hormone", but also plays a crucial role in the circadian rhythm and keeps your blood sugar stable while you sleep. Bile acids are the main constitute of bile, which is released made in the liver, but is ultimately released from the gallbladder to aid in fat digestion and absorption. Cholesterol also plays a vital role in keeping our cell membranes flexible, and making our skin water-proof(1). Of course, having cholesterol that is too low can negatively affect these pathways. For this reason, most functional medicine doctors (more on that later) like to see cholesterol that is somewhere between 150-200, while most conventionally trained doctors will tell you "the lower the better" in regards to your cholesterol.
As with most things that are this vital to our health, our bodies can make their own cholesterol. Actually, the vast majority of cholesterol is made by your body, rather than ingested(1). The ingestion of dietary cholesterol has little, if any affect on one's total cholesterol levels. There are two primary reasons for this: 1. The majority of exogenous (ie. what you eat) cholesterol is esterified, or in a form that is relatively difficult for our bodies to absorb and 2. Rising levels of cholesterol provide a "negative feedback loop" to the enzyme that is chiefly in charge of cholesterol synthesis (HMG-CoA Reductase). In other words, your body knows better than to keep making cholesterol when you already have enough, so there is a signal to that enzyme that says "enough". This then brings up the question, how is it that anyone has high cholesterol, then? Something must be pushing that enzyme to make more cholesterol despite the negative feed-back loop...
In enters insulin. Insulin is a hormone that is secreted from the pancreas in response to a rise in blood sugar (glucose). This hormone has many roles in the body, but it's most famous (and probably most important) is to bring glucose into the cells. As your blood sugar levels rise, insulin levels are not far behind. The amount of insulin your pancreas secretes is directly related to the amount of sugar in your blood, which of course is directly proportional to the amount of sugar (and to a lesser extent, carbohydrates) in the food you eat. More sugar consumed --> More insulin secreted. It has now been shown that insulin up-regulates the rate-limiting enzyme in cholesterol synthesis (HMG-CoA Reductase)(2). Thus, high blood sugar (ie. diabetes and insulin resistance) directly leads to increased synthesis of cholesterol. Add to this the fact that the per-cursor molecules that make cholesterol can be made from sugar (Acetyl-CoA), and one can easily see why diabetes and high cholesterol often go hand in hand.
The trouble is, most conventionally trained doctors and nutritionists put their patients on a low-fat, low-cholesterol diet when they see high cholesterol in a lipid panel. While this doesn't seem like a bad idea in and of itself, low-fat diets are often laced with unbelievably high amounts of sugar. One prime example of this is yogurt. While brands like Yoplait boast their fat-free and low-fat labels with pride, these supposedly healthy snacks often have upwards of 28-30 grams of sugar per serving! Ever since the low-fat campaign of the 1980s, countless low-fat and fat-free snacks have appeared on grocery store shelves, but for what? Americans are more unhealthy than we were in the 1980s, and many experts trace this back to the initiation of the low-fat campaign. I, for one, am among those that say sugar is far worse for you than fat ever thought of being- weather that be so-called "natural sugars" like maple syrup and honey, or artificial sugars. The two exceptions to this are saturated fats and trans fats, which are found in greasy animal products and hydrogenated oils like margarine, respectively.... But we'll go more into detail on the low-fat versus low-carb debate in a later post.
So go ahead, eat those egg yolks. There's other things on that breakfast table that are scarier than eggs, anyway...
(1) Guyton and Hall Textbook of Medical Physiology pp 847-848
(2) Dietschy JM "Effects of alterations of the specific activity of the intracellular acetyl CoA pool apparent rates of hepatic cholesterogenesis" J Lipid Res 1974;15:508-516