On Science: Step away from the Big Mac - Part 1
This article first appeared in the St. Louis Beacon, Dec. 9, 2009 -My diet is rich in fats. There is nothing I like more than a Big Mac and fries, except maybe a plump porterhouse. My wife tries mightily to interest me in salads and the odd chicken, but I am a bovinitarian at heart. Nor am I alone in my love of fat-rich foods. Far from it. About 20 percent of the total calories the average American consumes are fat. Most health experts agree with my wife in feeling that number to be far too high, and argue that Americans should reduce their fat consumption to less than 7 percent of total calories, a bland but healthy future. Why? What is there about fat molecules that make them bad for my health, and yours?
To answer this question, you need to focus for a moment on cholesterol, a kind of fat molecule called a lipid. A lipid with a bad reputation among health advisers, cholesterol plays a key role in determining how flexible a membrane is and is a major component of the plasma membranes that encase our body's cells. Its structural role is absolutely essential. Your body can no more do without cholesterol than the steel frame of a skyscraper can do without strengthening rivets.
But however essential, cholesterol can also be dangerous. This is because cholesterol circulating in your blood tends to adhere to cells lining your blood vessels, forming deposits called plaques. A narrowing of the vessels results leading to high blood pressure and heart disease. I have discussed heart disease in some detail in past Beacon columns, columns that focused on the role of inflammation in causing heart disease and the potential of C-reactive protein as an early indicator of potential heart attacks. Underlying all that was discussed in those columns, and in everything we have learned about heart disease, one essential lesson stands out clearly: excessive cholesterol damages health.
So if you want to continue to wolf down hamburgers and fries, it's worth taking a closer look at cholesterol, and asking WHY it damages health. As it turns out, we humans did not evolve to be hamburger eaters, certainly not as a way to gain the cholesterol our membranes require. Actually, you don't need to eat much cholesterol at all. Your body manufactures almost all the cholesterol it needs in your liver. This cholesterol then circulates in the bloodstream to the many cells of your body, which put it to use as they assemble cell membranes.
There is a problem with all this, though, that must be solved before the cholesterol leaves the liver. Because cholesterol is a lipid, it is not soluble in water (any more than the oil in salad dressings), and your blood is mostly water. Your liver solves this problem by attaching the cholesterol to a lipoprotein (that is, a protein with a lipid stuck to it) carrier molecule, forming a packet that is called a low-density lipoprotein, or LDL (proteins are more dense than lipid, so a packet with more lipid than protein is called "low-density"). LDL packets are quite soluble in blood, and are readily taken up by cells throughout your body.
Now this is where the Big Mac and fries come in. Problems arise when additional cholesterol is introduced into your body in the food you eat. About 80 percent of the cholesterol in your body is manufactured by your liver; the rest is contributed by the food you eat. A Big Mac and fries eating experience adds much more than this!
To deal with this excess, your body has an efficient scouring system. Excess circulating LDL packets pass their cholesterol to a special recycling lipoprotein carrier. Because these scavenger packets of lipoprotein contain more protein than lipid, they are called "high density" lipoprotein packets, or HDL. The HDL packets go on to circulate in the blood throughout your body. Your liver cells, the only body cells with the receptors necessary for HDL uptake, absorb the HDL packets from the blood and metabolize the excessive cholesterol. In this way, your body ensures that excess dietary cholesterol is cleansed from its blood before it can damage the blood vessels.
Now we can answer the question posed above: What is it about fat in your diet that is so bad for your health? As you might expect from this discussion, fat is bad for your health when it interferes with your body's ability to control cholesterol levels. Anything that raises levels of LDL cholesterol above the levels that HDL can scour from the system is going to be harmful.
Five kinds of fat need to be discussed:
Cholesterol itself. Clearly, foods rich in cholesterol, like eggs and richly marbled beef, will tend to raise the dietary portion of cholesterol, and so excess should be avoided.
Unsaturated fats. Not all fats are the same in their influence on LDL levels. Every fat molecule has the same basic structure, chains of carbon atoms each carrying two hydrogen atoms like backpacks and linked to adjacent carbon atoms in the chain by simple single covalent bonds. Sometime one or more carbon atoms shed one of its backpacks, instead forming two covalent bonds with an adjacent carbon atom in the chain. Fats with such "double" bonds (said to be unsaturated) leave LDL levels unchanged, and so are not a significant health hazard. These sorts of fats are often found in plants. You consume them when you eat olive oil, peanut butter and avocados.
Omega-3 fats. The far trip of the hydrocarbon chain of a fat molecule is called the omega end. When a carbon double bond is located three carbons from the omega end, the chain is called an omega-3 fatty acid. Omega-3 unsaturated fats tend to reduce both LDL and HDL levels in the bloodstream, reducing the formation of fatty plaques that clog up heart arteries. Omega-3 fats are typically found in fatty fish, including sardines, mackerel, tuna and salmon.
Saturated fats. Saturated fats are fats without any double bonds on their fatty acid chains. Because saturated fats raise LDL cholesterol levels, they tend to be bad for your health, raising your LDL levels above the capacity of your HDLs to accommodate the additional load of cholesterol. With a few exceptions, like coconut oil, saturated fat is animal fat. You consume it primarily in fatty meat and dairy products.
Trans fats. In some commercial foods, the unsaturated fats are deliberately converted by their manufacturer into hard saturated fats to change the consistency or texture of the food. Food manufacturers do this by bubbling hydrogen gas through the oil. This so-called "hydrogenation" turns an unsaturated fat into a saturated one, and is how oily natural peanut butter is converted into the firm, smooth product we expect to find on grocery shelves. Not only does saturating the fat produce a less healthy food, there is another chemical consequence: Some carbon double bonds aren't converted, but the orientation of hydrogen atoms at these bonds becomes shifted from the normal cis form (the two hydrogen atoms are on same side of the carbon atom chain) to the trans form (the two hydrogen atoms are on opposite sides of the chain.
Trans fats produced in this way inflict a double whammy on your health: They increase levels of LDL while at the same time decreasing levels of HDL. They intensify the problem while reducing your ability to cope with it. Starting in 2006 the "nutrition facts" labels that food producers are required to place on food packages have been required to list the level of trans fats.
So that's the scoop. High-fat diets are dangerous to your health for reasons we understand quite clearly. There is no fuzziness about this, no wiggle room. And yet, fully appreciating this, I cannot be trusted near a Big Mac. If knowing the right thing to do was all there were to it, life would be a snap and we'd all go to heaven. Sadly, or wonderfully, things are not that simple.
So I try to keep my fat-hunger under control, listen to my wife as much as I can, and hold my fat-consumption sinning to a minimum. I swear, it makes the odd Big Mac I do manage to sneak by taste even more delicious!
George B. Johnson's "On Science" column looks at scientific issues and explains them in an accessible manner.
Johnson, Ph.D., professor emeritus of Biology at Washington University, has taught biology and genetics to undergraduates for more than 30 years. Also professor of genetics at Washington University’s School of Medicine, Johnson is a student of population genetics and evolution, renowned for his pioneering studies of genetic variability. He has authored more than 50 scientific publications and seven texts.
As the founding director of The Living World, the education center at the St Louis Zoo, from 1987 to 1990, he was responsible for developing innovative high-tech exhibits and new educational programs.
Copyright George Johnson