There has been some news recently about trans fats. The FDA has given industry three years to remove artificial fats from our food supply. New York Times foodie superstar Mark Bittman wrote an op-ed column recently in which he opined that “butter is better” [than Crisco.] In the popular hierarchy it goes like this: fat is bad; saturated fat is worse and trans fats are baddest. And don’t forget cholesterol. I started to wonder as I was reading Mark’s column, what exactly are these things and what makes them so bad? Looking into it was pretty fascinating.
Maybe you know that in addition to their obvious role as just pure food – i.e., stuff to be digested and used as fuel – fats play a role in the structures and chemicals in our bodies. One of their most important functions is formation of the so-called “lipid bilayer,” a primary constituent of all cell membranes. Just like everything else, all living things are governed by energetics. Molecules take the easiest way to do anything (well, unless they are poked or prodded – but that is another topic.) So a lot of (particularly animal) biology is based on self-assembly – molecules arranging themselves in a way that is easiest to maintain. They just do not want to go to any trouble. In the body, fats exist in a form called fatty acids. That means that they have an organic acid (COOH or COO-) group at one end, a long hydrocarbon chain in the middle and a methyl group (CH3) at the other end. As you probably know – oil and water do not like each other and as you also probably know, we humans are pretty much made out of water. So the fatty acids turn their methyl end away from water and their acid end towards the water. The result of this is the membrane or lipid bilayer that looks like this:
These bilayers are not at all stiff – they are at body temperature (37OC) – about the consistency of olive oil. The chemical that bodies use to stiffen these membranes is cholesterol – a substance manufactured in the liver – and which also comes from our food. It is stiffer than the fatty acids – sort of waxy. It has four rings in it, making it rigid and pretty flat.
Those rings are characteristic of steroids – along with the hydroxyl (OH) group on one end. In the same way that the fatty acids have a COO- end and a CH3 end, cholesterol has an OH end and a CH3 end. Both molecules are called “amphipathic” because they love water (hydrophilic) on one end and hate it (hydrophobic) on the other.
Because these molecules are essentially fats, even though they have a water loving bit at the end, they are not soluble in water and can’t move around in it without help. And because the body needs to move these molecules around, a transport system has evolved to take the lipids (fats) where they need to go. And that transport system has generated a lot of attention due to its relationship with disease.
When we eat fats they are typically ingested in the form of triglycerides which are three fatty acids hanging from a glycerol backbone. The molecule looks like this:
In the intestines, triglycerides are broken down by an enzyme from the pancreas called, appropriately, pancreatic lipase. Two of the fatty acids are clipped off and one remains on the glycerol, leaving a monoglyceride. This next bit is important – hope you are still with me. We have all heard of bile – that word has a lot of meanings. In pre-modern physiology, bile was associated with anger and gloominess – and the word is still used today to mean bad temper. For fat metabolism, bile is made in the liver and stored in the gall bladder. Bile is mostly water but the active ingredient is an amphipathic acid made from cholesterol called glycoholic acid. Here are two views of it showing the water attractive (blue) and water repellant (yellow) aspects.
The Glycoholic acid serves to emulsify the fats so that they might be suspended in the digesting food. With the peristaltic action of the intestines, the little blobs of fat and bile are eventually taken into the epithelial cells lining the intestines by diffusion across the cell membranes. The whole action of breaking down the triglycerides into fatty acids and monoglycerides is for this purpose – so that the fat can pass through the cell membranes. Once inside the cells, the fats and monoglycerides are reconstituted into triglycerides which can be readied for transport into the bloodstream for use by the body. This first step of transport is done by the cellular organelles. Triglyceriedes are boxed up with cholesterol, lipoproteins and other lipids into particles called chylomicrons.
Now we are at the real heart of the matter concerning fat and health.
Here is a representation of chylomicrons and other lipid transporting structures:
These four structures all ferry fat and cholesterol around the body. The chylomicrons carry it first from the intestinal mucosa via the thoracic lymphatic duct into the plasma. In the capillaries, circulating chylomicrons release the triglycerides and cholesterol at fat cell and muscle cell sites for storage or use as energy. Lipoprotein lipase, an enzyme on the surface of capillary cells, breaks the triglycerides back down into fatty acids and monoglyceride. Having all these fats in the bloodstream creates effects because biochemical systems frequently work using feedback inhibition. Such inhibition controls serum lipid levels – more or less – but the system is delicate and can go awry – too much can collect. In addition to the chylomicrons, there are three other significant lipid transport structures. They are named according to their ratio of lipids to proteins. Chylomicron is the largest and least dense, then Very Low Density Lipoprotein (VLDL), Low Density Lipoprotein (LDL) and High Density Lipoprotein (HDL).
The way that things are supposed to work is that the liver packages up fatty acids and cholesterol in protein packets (the LDL) and ships them off around the body to be incorporated into membranes and converted to other chemicals – e.g., steroids. The High Density Lipoproteins (HDL) circulate through the blood gathering up stray molecules of fatty acids and cholesterol and return them to the liver. But the whole system balance is susceptible to those well know items: diet and exercise.
Let’s take a minute to talk about different types of fatty acids.
How the carbons in the fatty tail are linked together is what determines the health consequences of the fatty acid. Unsaturated wild type (natural) fatty acids are typically bent and won’t lie flat together. That gives them a lower melting point – like an oil. Saturated fatty acids are flat and lie together to form a more solid structure at a similar (say, room) temperature, like lard. This is the reason why a cold fish is flexible and a cold pork chop is stiff. Fish fats are unsaturated and meat fats are more saturated. When they talk in advertising about Omega Three oils – that means that there is a double (unsaturated) bond at the third carbon from the methyl end of the fatty acid chain. More saturation simply means more hydrogen.
Additionally, unsaturated bonds can be of two types. Most natural fats contain “cis” bonds. As shown in the illustration above, what that means is that in the hydrocarbon chain the carbons next to the carbons in the double bond are on the same side of the bond. Atoms can rotate freely about single bonds but they are constrained by a double bond. Cis bonds cause a bend in the molecule and a lower melting point. Trans bond lie flat – much the same as saturated chains causing a higher melting point.
So let’s review for a minute. We have two things happening here – a transport system for fatty acids and cholesterol and a variety of possible fatty acid configurations. In the transport system, LDL brings fats from the liver to the body tissues for use. HDL returns fats to the liver for recycling. Because of this, LDL is known as “bad” cholesterol and HDL is known as “good” cholesterol. For reasons that are currently unknown, unnatural trans fatty acids cause an increase in serum LDL.
Here’s the connection to health: elevated levels of LDL cause arterial inflammation. If a “Mediterranean” style diet is followed then normal arterial endothelium does not support adhesion of molecules from the blood stream. But if the diet is rich in trans fats or even saturated fats then elevated levels of LDL occur and the cells lining the interior of the arteries begin to express adhesion molecules as a result of inflammation. This can most typically happen at arterial branches. What follows is a cascade of degradation. White blood cells (leucocytes) are attracted to substances secreted by the cells and adhere. (They are called there to fight the inflammation.) The leucocytes in turn secrete chemicals that promote the migration and proliferation of smooth muscle cells (SMCs). SMCs express specialized enzymes that can degrade the vascular elastin and collagen (muscle proteins) in response to inflammatory stimulation. Once resident in the arterial wall, the blood-derived inflammatory cells participate in and perpetuate a local inflammatory response – including the production of foam cells which are lipid engorged vesicles. The ultimate result of this cascade is development of a dense matrix of cells which is an atherosclerotic lesion or, as commonly known, plaque.
Plaques rich in soft extracellular lipids are the most rupture prone. When plaque deposits do rupture, platelets rush to the site forming a clot and a stroke or heart attack or deep vein thrombosis can occur.
Platelets shown gathering into a ball at the site of a plaque rupture.
One’s diet is definitely worth watching.
Dishes that conform to the Mediterranean Diet available at Sara’s Table include fish tacos, white fish platter, Peasant’s Meal and many others. Ask your waitress.