Web edition: May 2, 2006
Adults are continually bombarded with messages about how eating foods rich in cholesterol can elevate an individual's risks of atherosclerosis and heart attacks. Many such warnings have focused on eggs because their yolks are a major dietary source of cholesterol.
However, eggs may be getting a bum rap, suggest the findings of a study of middle-aged and elderly volunteers. Researchers from the University of Connecticut reported the work in early April at the Experimental Biology 2006 meeting in San Francisco.
Cholesterol moves through blood within capsulelike structures known as lipoproteins. Yes, ingestion of several eggs a day does tend to increase blood concentrations of cholesterol, particularly the amount circulating in low-density lipoproteins (LDLs)the so-called bad cholesterol. However, the new study showed, eating eggs can also increase the amount of cholesterol in high-density lipoproteins (HDLs)the good cholesterol.
Moreover, the new study showed that 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's 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. Similarly, larger HDLs are more robust than smaller ones at hauling cholesterol out of the bloodstream and, ultimately, out of the body, notes the lead researcher for the new study, Christine M. Greene.
In fact, she notes, her team's accumulating data indicate that most people's bodies handle the cholesterol from eggs in a way that is least likely to harm the heart.
Cholesterol warnings have especially scared elderly people away from eggs, says Greene. And that's a shame, she adds, because eggs are an affordable and easy-to-eat source of high-quality protein for this population. The new findings, Greene says, contribute to a growing body of data suggesting that eggs shouldn't be construed "as a dietary evil."
Cholesterol, a soft, waxy substance, isn't found only in the blood but also in all cell membranes and the material that sheathes nerves. It also plays an integral role in the production of steroid hormones, such as estrogen, and of bile acids that take part in fat digestion in the gut.
In most cases, the body can synthesize all the cholesterol it needs. Any dietary contribution of cholesterol is unwelcome, says the American Heart Association, since an excess in the blood will foster the development of the fatty plaques that can eventually clog arteries and provoke a heart attack.
"In order to keep your LDL and your risk for heart disease low," the National Heart, Lung, and Blood Institute (NHLBI) argues, "you should start on [a] heart-healthy diet" that includes fewer than 300 milligrams (mg) of cholesterol per day from all sources. Because the yolk of a single large egg contains slightly more than 200 mg of cholesterol, the institute's Web site recommends that a heart-healthy diet should strive to limit intake to "no more than 4 yolks per week [including] the egg yolks in baked goods and processed foods." Egg whites are cholesterol-free, so NHLBI recommends substituting the whites from two eggs for a single whole egg in breakfast items and baked foods.
The American Heart Association acknowledges that it's possible "to fit an egg a day into a healthy diet," but only by limiting dietary cholesterol from other sources, including baked goods. Moreover, the group cautions, for people with existing "coronary heart disease, diabetes, high-LDL cholesterol or other cardiovascular disease, your daily cholesterol limit is less than 200 mg."
Greene says that such recommendations make sense, because excess LDLs are bad, but she points out that guidelines to date have considered all LDLs the same, while research is revealing differences. Indeed, a number of studies have shown that especially small, dense LDLs confer the greatest risk (SN: 9/21/96, p. 182). Several recent studies have shown that people with diabetes or heart disease tend to package relatively more of their cholesterol in these tiny LDLs than do healthier people. Last year, a group of Korean researchers argued that the differential effects of small and large LDL lipoprotein particles are already established well enough that LDL size "could be used as a marker for coronary heart disease risk."
Not all people respond similarly to cholesterol. Studies by Greene's group and others have shown that 30 to 40 percent of any given population is made up of "hyperresponders." In these people, blood-cholesterol concentrations spike disproportionately in response to dietary cholesterol. Her team decided to investigate whether such people put an egg's cholesterol into different-sized lipoproteins than most other people do.
So, the team recruited 29 postmenopausal women and 13 elderly men to take part in a dietary trial. None was taking cholesterol-lowering medicine at the time of the study, Greene notes, which means that for a population of middle-aged-to-elderly people, the group was relatively heart healthy.
For 30 days, each volunteer received a liquid-egg product or a fat-and-cholesterol-free, protein-rich egg substitute in portions comparable to three large eggs per day. The real-egg ration delivered some 640 mg of cholesterol; the egg substitute contained no cholesterol. None of the participants knew which food he or she was getting until the end of the study. The researchers also supplied recipes for items such as drinks and vegetable frittatas that volunteers could turn to if they got bored with scrambled eggsthe easiest dish to prepare from the products.
Using liquid eggs instead of eggs in their shells made it possible for the researchers to present products that looked and handled identically, Greene explains. Being homogenized and pasteurized, these products could also be used without cookingfor instance as a protein boost to a fruit shake. At least as important, using a commercially prepared product ensured that each daily egg dose contained precisely the average cholesterol content of three large eggs. That's not a trivial issue, since eggs vary in their actual cholesterol content according to their size and the laying chicken's diet (see Cholesterol Medicine for Eggs?).
After a month on the first diet, all volunteers took a 3-week breather and then resumed participation. For the second phase, each person received the alternative to the product he or she had initially received: real eggs or the cholesterol-free egg substitute. At the beginning and end of each phase of the trial, Greene's group took blood samples from each participant to measure lipoproteins and more.
Throughout both phases of the trial, the amount of both HDL and LDL lipoproteins remained unchanged. However, the 15 hyperresponders among the volunteers had much higher amounts of cholesterol circulating with their lipoprotein particles while they were eating real eggs. Greene told Science News Online that "all of the increase went into large [lipoprotein] particles."
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.
Not only did the two groups handle the eggs' cholesterol differently, Greene notes, but the hyperresponders handled the excess that showed up in their blood "in the most anti-atherogenic way"by depositing it in the largest lipoproteins. The take-home message, Greene concludes, is that an LDL-cholesterol reading that ignores lipoprotein size may exaggerate the heart risks posed by eggs' cholesterol.
Greene's team also looked at blood concentrations of two beneficial blood components derived from food: lutein and zeaxanthin. Diets rich in these yellow-orange pigments, which are responsible for much of an egg yolk's color, appear to diminish an individual's risk of macular degeneration. That disease is the leading cause of blindness in people over age 65. Moreover, lutein appears to inhibit processes that jumpstart the development of atherosclerosis.
During the egg phase of the new study, blood concentrations of both the pigments, which are types of carotenoids, increased. In the egg-substitute phase, concentrations of another yellow carotenoidbeta-caroteneincreased in the participants' blood, Greene notes. This pigment had been used to mimic the yolks' color.
How much the lutein and zeaxanthin increased during egg supplementation varied between individuals, Greene says, but generally reflected the proportionate increase in the size of a volunteer's LDL and HDL lipoproteins. That makes sense, she adds, because these carotenoids tend to be carried on the surface of lipoproteins in blood: As the particles got bigger, so did their surface area that was available to carotenoids.
Indeed, a group of Greene's University of Connecticut colleagues, headed by Richard M. Clark, reported data in the March Journal of Nutrition indicating a similar relationship between carotenoids and people's response to cholesterol. "The bottom line," Clark says, is that this might be "a good-news/bad-news type of story." Although few people would wish for the genes that render them hyper-responders to dietary cholesterol, that trait "may decrease your risk for blindness from macular degeneration" by increasing lutein's circulation in blood.
Altogether, the findings should please the American Egg Board, which funded Greene's study. However, until studies independent of industry financing confirm the new data, the jury is still out on how many eggs most people can safely eat.
The new study's findings do dovetail with large studies by other groups having no industrial financing. For instance, 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 men or women 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.
A Michigan State University analysis, reported a year later, analyzed the diets and blood-cholesterol data for more than 27,000 peoplea representative cross-section of the U.S. population. It found that cholesterol was lower in people who ate more than four eggs per week than among people who eschewed eggs. However, the researchers cautioned, "this study should not be used as a basis for recommending higher egg consumption for regulation of serum cholesterol."
American Egg Board
1460 Renaissance Drive
Park Ridge, IL 60068
Web site: [Go to]
American Heart Association
7272 Greenville Avenue
Dallas, TX 75231
Web site: [Go to]
Christine M. Greene
3624 Horsebarn Hill Road
University of Connecticut
Storrs, CT 06269
National Heart, Lung, and Blood Institute
Health Information Center
P.O. Box 30105
Bethesda, MD 20824-0105
Web site: [Go to]
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