Issue 66 / November - December 2008
The message to lower cholesterol to reduce the risk of heart disease has been widely circulated. However, recent findings beg the reevaluation of common knowledge about cholesterol. Among many, the following are some examples of questions that we might have: Is cholesterol level a good indicator of oneâ€™s risk of heart attack? Is it true that one can reduce his or her risk of coronary heart disease simply by lowering oneâ€™s cholesterol level? Is it really true that we should avoid eggs if we want to live longer?
The name cholesterol originates from the Greek chole- (bile) and stereos (solid), and the chemical suffix -ol for an alcohol, as researchers first identified cholesterol in solid form in gallstones in 1784.1 Cholesterol is a soft and waxy sterol (a combination steroid and alcohol) found among the lipids (fats) in the bloodstream of all animals and humans. Trace amount of cholesterol also exist in plants. Even though cholesterol can be found in almost every cell, it is mostly found in the brain, heart, bowels, and liver. It is an important part of a healthy body because it is used to form cell membranes and some hormones, and is needed for other functions. A small amount of cholesterol circulating in the blood is enough for the body to undertake all these important functions. But a high level of cholesterol in the blood-hypercholesterolemia-is accepted as a major risk factor for coronary heart disease, which may lead to heart attack. As cholesterol and other fats dissolve, in normal conditions, only in oil, they cannot dissolve in the blood. They have to be transported to and from the cells by special carriers called lipoproteins. There are several kinds of these, but the ones to focus on are low-density lipoprotein (LDL, so called bad cholesterol) and high-density lipoprotein (HDL, or the good cholesterol).
The American Heart Association suggests that the risk of coronary heart disease increases in adults with total cholesterol and LDL greater than 200 mg/dl and 130 mg/dl, respectively. It is also advised to keep HDL greater than 35 mg/dl to avoid cholesterol-related diseases. HDL level for young healthy adults is found to be, on average, around 55 and 45 mg/dl for women and men, respectively. Aging, diet, obesity, and menopause are among the factors which increase cholesterol, especially LDL, in addition to hereditary causes. While exercise helps in reducing LDL and increasing HDL, smoking, alcohol, and stress cause the opposite effect.
As you eat, cholesterol from food is absorbed by your digestive tract. It then makes its way into your liver and can circulate through your body in your bloodstream. That is one source. There is also a little-known second source of cholesterol-your body. Cholesterol from food is hard to get away from, even though you may be watching your diet. All foods of animal origin contain cholesterol, including eggs, red meat, and shrimp. Generally, it is also suggested to limit foods that are high in saturated fats or trans-fats. Egg is one of the most controversial foods in our fight against coronary heart disease. On average, one egg contains 66% water, 12% protein, 11% minerals, 10% fat, and 1% carbohydrate. The cholesterol level in one egg ranges from 180â€“210 mg based on its weight. It is found that each egg cause a temporary cholesterol spike of 3â€“4 mg/dl in the body, which tend to normalize after its digestion.
The American Heart Association recommends reducing daily intake of dietary cholesterol to reduce the risk of coronary heart problems. However, there are quite a few studies to suggest the contrary2â€“6. The â€śdietary cholesterol equals blood cholesterolâ€ť view is a standard of dietary recommendations, yet few consider whether the evidence justifies such restrictions.4 Over fifty years of cholesterol-feeding studies show that dietary cholesterol does have a small effect on plasma cholesterol concentrations. The 167 cholesterol feeding studies in over 3,500 subjects in the literature indicate that a 100 mg change in dietary cholesterol changes plasma total cholesterol by 2.2 mg/dL.4 In 1999, Frank B. Hu of the Harvard School of Public Health and his colleagues reported no increased risk of coronary heart disease or stroke in people who ate more than one egg per day. The analysis compared diet and cardiovascular risk among nearly 38,000 participants of two long-running epidemiologic studies.3, 5 In a separate study, Dr. Song and colleagues from the University of Michigan analyzed the diets and blood-cholesterol data of more than 27,000 people selected carefully to be a representative cross-section of the US population.5 They reported that cholesterol was lower in people who ate more than four eggs per week than among people who avoided eggs in their diet. However, a researcher cautioned, â€śthis study should not be used as a basis for recommending higher egg consumption for regulation of serum cholesterol.â€ť Nonetheless, until recently, there were no scientific findings to explain why it is so. Studies have shown that vitamin A, E, B, B6, B12 and folate, which can be found in egg, reduce the risk of coronary heart disease. But it was not until recently that we knew the scientific reasons behind the findings of all the earlier studies. Recent studies have shown that our body is quite intelligent in managing the extra cholesterol increase after eating eggs in a way that limits damage to the heart.
It is well known that the consumption of several eggs a day does tend to increase blood concentrations of cholesterol, particularly the amount circulating in LDLs. However, a new study from the University of Connecticut showed that eating eggs can also increase the amount of cholesterol in HDLs.2 When people ate three or more eggs per day their bodies made bigger LDL- and HDL-lipoprotein particles than when they ate no eggs. That is important because other recent studies have suggested that larger LDLs are less likely than small ones to enter artery walls and contribute their cholesterol load to artery-clogging plaque.3, 4, 6 Similarly, larger HDLs are more effective than smaller ones at moving cholesterol out of the bloodstream and, ultimately, out of the body. In addition, researchers from the University of Connecticut found that not all people respond similarly to cholesterol. Studies have shown that 30 to 40 percent of any given population is made up of â€śhyperresponders.â€ť In these people, blood-cholesterol concentrations increase disproportionately in response to dietary cholesterol. Surprisingly, such people are found to put an eggâ€™s cholesterol into larger-sized lipoproteins than most other people do. In contrast, among normal responders, only small increases in blood cholesterol occurred during the egg diet, and the size of LDL- and HDL-cholesterol particles covered the full range of lipoprotein sizes.2 These results suggest that any LDL-cholesterol reading that ignores lipoprotein size may exaggerate the heart risks posed by eggsâ€™ cholesterol. Several recent studies have also shown that healthier people tend to package relatively more of their cholesterol in these large LDLs than do people with diabetes or heart disease and the differential effects of small and large LDL lipoprotein particles could be used as a marker for coronary heart disease risk.4, 6
In conclusion, limiting eggs or other foods in our diet cannot be the solution to reduce cholesterol level and thus the risk of coronary heart diseases. Our fight against any disease requires a holistic approach, better understanding of our body and the world around us in addition to choosing a more balanced diet and stress-free lifestyle. Recent scientific findings allow us to put an important question: Is it too simplistic to believe that the One who placed the cholesterol in the egg yolk is the One who created a defense mechanism in the body in a way that limits its damage to the heart?
Bulent Aydogan, PhD, is a research fellow at the University of Chicago Department of Radiation and Cellular Oncology.<8em>
2. Greene CM, Waters D, Clark RM, et al. Plasma LDL and HDL characteristics and carotenoid content are positively influenced by egg consumption in an elderly population1. Nutr Metab (Lond) 2006;3:6.
3. Hu FB, Stampfer MJ, Rimm EB, et al. A prospective study of egg consumption and risk of cardiovascular disease in men and women. Jama 1999;281:1387â€“1394.
4. McNamara DJ. The impact of egg limitations on coronary heart disease risk: do the numbers add up? J Am Coll Nutr 2000;19:540â€“548.
5. Song WO, Kerver JM. Nutritional contribution of eggs to American diets. J Am Coll Nutr 2000;19:556â€“562.
6.Yeomin Yoon JS, Hyung Doo Park, Kyoung-Un Park and Jin Q. Kim. Significance of small dense low-density lipoproteins as coronary risk factor in diabetic and non-diabetic Korean populations. Clinical Chemistry and Laboratory Medicine 2005;43:431â€“437.