5th September 2017
Wipe your mind clear of all previous ideas about CVD. About as easy as standing in the corner and not thinking about a tiger. In reality, once you have read about, and talked about, and researched, and thought about anything, patterns are created in your mind. Familiar landscapes develop, and well-worn pathways become the comfortable and easy routes to travel down.
Say what you like about Ancel Keys (and I had better not, for I would end up swearing a lot), he created the tightly patrolled mental box for everyone. Diet and cholesterol and cardiovascular disease. These are the great beacons that mark out, the map of the mind, where all thinking and discussion must take place. They illuminate all, and beyond them is darkness.
Now, blow out the beacons. Move out into darkness. We shall create a new landscape of thought. We have control of the vertical, and the horizontal, you are entering the Outer Limits. [I suspect some people may not get that reference]. We are breaking free of the box. In fact, there is no box, it no longer exists.
In the distance, there is a glimmer of light… it is our first fact. At least we hope it is a fact. We approach the glittering light and scrape way the grime that has been obscuring it for many years, to reveal…
Atherosclerotic plaques only develop in larger arteries.
Quite close to it, almost hidden away, lies another fact.
Atherosclerotic plaques never develop in veins.
There are two exceptions to the second fact – well, there are more, but these are the most obvious. First, if you take a vein, and use it to create a coronary artery bypass graft, it will develop atherosclerosis very rapidly. Secondly, if you create an arteriovenous fistula AV-fistula (fusing an artery and vein together) for dialysis patients, the venous section will develop atherosclerotic plaques.
Setting aside these exceptions, these two facts were as close to inarguable as I have been able to find. Inevitably, they lead to my first question. Why do plaques develop in arteries, and not in veins? Right now, I can see you doing what everyone does, searching for a simple answer, with thoughts such as:
- There is less oxygen in veins, and oxygenation is damaging to arterial walls
- The pressure is less in veins
- The LDL level is lower in veins (it’s not, but I have heard a lot of people say this)
- Arteries and veins have a different structure (they do not).
And so on, and so forth. Isn’t the search for a quick and simple answer fun…?
After exploring almost every avenue that I believed could possibly be involved in CVD, I found myself returning more and more often to the difference in blood pressure in veins and arteries as the place where the answers were most likely to be found.
However, I knew pressure, by itself, is not going to cause anything, unless you succeed in ‘bursting’ an artery, or ‘bursting’ the lining of the artery. I mean, this can be done. You can develop an aneurysm (thinned and ballooned area) in an artery, which can then rupture – usually with catastrophic consequences.
But before that, what can pressure do? Force things carried within the artery into the artery wall behind? No, that does not make sense. For that would mean everything carried in the bloodstream would simply be blasted into all artery walls, everywhere. The smallest molecules would go first, molecules such H20 to start with. Does this happen…. No, of course not. Our arteries, and the endothelial cells that line our arteries (and veins), are not leaky.
In short, differences in pressure cannot provide any sort of an explanation.
However, there is a law of fluid dynamics which says – words to the effect – if the pressure in a tube is higher, the velocity of the fluid flowing through a tube will also be higher. Which means that blood is travelling far faster in an artery than a vein. A veritable white-water maelstrom, compared to a meandering river as it approaches the sea.
Thus, it is easy to imagine that anything lining an artery is going to be exposed to far greater ‘forces’ than anything lining a vein. These forces, which I shall call biomechanical stresses, will be particularly intense in certain places. For example, where arteries branch (bifurcate) e.g. where the carotid arteries, that supply blood to the brain, branch off (bifurcate) from the aorta.
Another place of extreme biomechanical stress is within the coronary arteries. These arteries are exposed to a unique stress, in that they are compressed with great force when the heart contracts. Some have likened this to stomping on a hose every second. Indeed, blood cannot flow in coronary arteries during systole (ventricular contraction) because they are squeezed shut.
In general, if you look at where atherosclerotic plaques develop, you find that they most often occur at maximum biomechanical stress. Where carotid arteries (main arteries supplying blood to the brain) branch from the aorta, and also where other arteries branch from the aorta, and within the coronary arteries. It seems, therefore, that biomechanical stress is required for plaques to develop. This is not the same as high blood pressure, but it is closely associated with high blood pressure.
In truth, this idea is not in any way contentious. This is a highly jargon filled section from a paper called ‘Biomechanics of Atherosclerotic Coronary Plaque: Site, Stability and In Vivo Elasticity Modelling.’
‘Although the coronary and peripheral systems in their entirety are exposed to the same atherogenic cells and molecules in the plasma, atherosclerotic lesions form at specific regions of the arterial tree. Such lesions appear in the vicinity of branch points, the outer wall of bifurcations and the inner wall of curves. Pathologic studies, have shown that healed plaque ruptures are predominantly in the proximal portions of the left anterior descending (LAD), right coronary (RCA), left circumflex (LCx) and left main (LM) arteries. Investigations over the last decade have elucidated both fluid mechanical and most recently structural biomechanical factors that mediated the site of plaque formation.’1
Which is all fine and sensible. However, this very same paper states the following:
‘Plaque formation is now recognized as an inflammatory process triggered by high levels of serum LDL that enter the coronary wall, encounter oxygen reactive species, and become oxidized. The oxidization, in turn, stimulates the recruitment of monocytes that convert to macrophages to phagocytize oxidized LDLs. This forms a necrotic core with recruitment of smooth muscle cells from the media to seal over the fatty core.’
That is the official party line as to how CVD starts, and develops. But if you believe that, you immediately face a conundrum. How can you reconcile the hypothesis that raised LDL entering the artery wall initiates plaque development, with the observation that atherosclerotic lesions form at specific regions of the arterial tree? It is surely one, or the other, but it cannot be both. Sorry, but at this point I need to take you back into the landscape of raised LDL and CVD.
You may think, in fact you probably are already thinking: “Well, biomechanical stress damages the endothelial cells, allowing LDL to enter.” Now, that could be true. However, if that is true, then you have (if you believe in the cholesterol hypothesis), just made a move that will result in checkmate against you.
The argument goes like this:
If LDL can only leak into the artery wall at an area where the endothelial layer is damaged, and this is where plaques develop, this means it cannot leak through in areas where the endothelium is not damaged. Ergo, the first step in the development of plaques cannot be LDL ‘leaking’ into the artery wall past the endothelium, it is damage to the endothelium. Ergo, a raised LDL level is not the primary cause of CVD. Checkmate.
You don’t like that logic? If you prefer a few more facts, using a different approach.
If you think LDL is capable of, simply, transporting itself past the endothelium, then you need to define a mechanism. Is it simply osmotic pressure, with LDL travelling down a concentration gradient from the bloodstream into the artery wall? Is it actively transported through endothelial cells? Does it leak between the endothelial cells? These are the mechanisms that I have seen most commonly proposed – although they are often presented with so much surrounding jargon that it is almost impossible to work out what is being said.
In truth, I have spent years and years trying to establish if LDL can, or cannot, move into the arterial wall, past the healthy, undamaged, endothelium. If I had been organised enough, I could have gathered together ten thousand papers saying that it can, and another ten thousand saying that it cannot.
Having torn up twenty thousand papers, on the basis of complete uselessness, I began with, what may seem a simple question, a thought experiment if you like. Why would endothelial cells allow LDL to pass through them, to then allow LDL to be oxidised in the arterial wall behind? This process serves no physiological purpose, other than to kill you from cardiovascular disease!
The idea that endothelial cells simply cannot prevent this from happening is, frankly bonkers. Cells can quite easily control the passage of single atoms/ions through their cell membranes Indeed, this is one way that all cells function. To give one example, they can pump individual sodium ions out, and individual potassium ions in, to maintain an electrical action potential. They only lose the ability to control their own internal environment, within very tight parameters, when they die.
Therefore, the idea that an endothelial cell cannot prevent a relatively massive LDL molecule from entering the side facing the bloodstream, then passing straight though, then ejecting itself out the other side, is complete nonsense. Complete… nonsense.
Indeed, it has been well established that the only way LDL can enter a cell, is for that that cell to manufacture an LDL receptor, wave it about it the bloodstream to lock onto an LDL molecule, before dragging the receptor and the LDL back inside. Ergo, LDL does not get into an endothelial cell, unless the cell wants LDL to enter. It activates complex processes to allow this to happen.
The reason why some people have very high LDL levels is because they cannot manufacture enough LDL receptors, or the LDL receptors they manufacture are faulty. A lack of LDL receptors, or faulty receptors is, of course, the underlying problem in Familial Hypercholesterolaemia(FH). Proof, if proof were truly needed, that LDL cannot force its way into cells – no matter what the concentration in the bloodstream.
In short, even a superficial understanding of how cells control the passage of atoms and molecules, leads to the inescapable conclusion that LDL cannot possibly travel straight through an endothelial cell, without the activation of complex and highly controlled cellular process.
This problem has been duly noted by those who support the LDL/cholesterol hypothesis. So, the current thinking, although I have never seen it expressed clearly, is that there must be gaps between endothelial cells, wide enough for LDL to leak past.
Again, no. The fact is that, in a healthy artery wall, with healthy endothelium, there simply are no gaps between endothelial cells. Here, from a paper entitled. ‘Endothelial Cell Junctional Adhesion Molecules.’ [jargon alert].
‘Endothelial cells line the lumen of all blood vessels and play a critical role in maintaining the barrier function of the vasculature. Sealing of the vessel wall between adjacent endothelial cells is facilitated by interactions involving junctionally expressed transmembrane proteins, including tight junctional molecules, such as members of the junctional adhesion molecule family, components of adherence junctions, such as VE-Cadherin, and other molecules, such as platelet endothelial cell adhesion molecule.’2
At the risk of simply repeating what this paper says, there are no gaps between endothelial cells. Instead, there is a highly complex structure of proteins and other molecules between each endothelial cell ensuring that nothing gets past – unless the endothelial cells are instructed to let them past. This happens with white blood cells, they can open the junctions between endothelial cells, and move into the artery wall – then out again. Clever stuff.
Of course, if most things travelling in the bloodstream had to overcome complex barriers to get past the endothelium you would die, as your blood would simply circulate round and round, struggling to exchanging nutrients back and forth with the underlying tissue. Which kind of negates the point of having a circulatory system in the first place.
Nature, in the way that nature does, noted this potential problem, and came up with a solution. As blood vessels get smaller, and smaller, the endothelium develops holes – called fenestrations. These fenestrations allow almost everything present in the blood to flow freely in and out of the surrounding tissues/organs. Red blood cells would be one exception.
Why, you could ask, would endothelial cells have fenestrations in them to allow the free passage of molecules in and out, if things can freely pass in and out of non-fenestrated, tightly bound, endothelium?
At this point, I am overwhelmed with the need to make a quick summary:
1: It is impossible for LDL to pass straight through a living endothelial cell
2: Endothelial cells are tightly bound together, and will not allow anything to pass between them.
In addition, here are a couple of other facts to consider.
The first of which is that, in the brain, the endothelium never becomes fenestrated. There are no holes, even in capillaries (the smallest blood vessels in the body). Which means nothing can move into, or be removed from the brain, that the endothelial cells do not grant passage. This barrier function is usually referred to as the blood brain barrier (BBB):
‘Cholesterol is a major constituent of the human brain, and the brain is the most cholesterol-rich organ. Numerous lipoprotein receptors and apolipoproteins are expressed in the brain. Cholesterol is tightly regulated between the major brain cells and is essential for normal brain development. The metabolism of brain cholesterol differs markedly from that of other tissues. Brain cholesterol is primarily derived by de novo synthesis and the blood brain barrier prevents the uptake of lipoprotein cholesterol from the circulation.’ 3
To put this another way, if LDL could pass the BBB, then the brain would not need to synthesize its own cholesterol, and the brain does synthesize cholesterol within specialised glial cells. Which is further confirmation that an intact, non-fenestrated endothelium, blocks the passage of LDL.
Now here is a final fact (a final fact in this blog at least) that I would like you to ponder. Which is that large blood vessels have their own blood vessels, known as vasa vasorum. Literally, ‘blood vessels of the blood vessels’. Vasa vasorum surround and penetrate large arteries, and veins, supplying them with the required nutrients.
They are, of course, fully fenestrated (full of holes). Thus LDL, or anything else, can simply leak out of the vasa vasorum and into the artery wall if it so wishes – yes, even down a concentration gradient, if you like to think of it in this way.
Which means that there is absolutely no need for LDL, or anything else, to be absorbed through the endothelium lining the arteries, as it can get in from ‘behind’, so to speak. Which takes me back to my first question here. Why would endothelial cells transport LDL past themselves, and into the artery wall behind – if LDL can perfectly easily get into the artery wall from the vasa vasorum? This truly would be an exercise in pointlessness.
I could go on, as I have only touched upon a small part of the complexity involved here. But I hope to have given you enough food for thought. Yes, you easily can make statements such as ‘Plaque formation is now recognized as an inflammatory process triggered by high levels of serum LDL that enter the coronary wall’. Certainly, if you say it fast enough, and do not think about it, such a statement can seem reasonable.
However, if you start looking at the actual process required for LDL to travel into the arterial wall, you begin to realise that it is (with a healthy and intact endothelium) simply not possible. Or, if it is possible, it should be happening everywhere, in all arteries and veins. Not at discrete points.
At which point, you begin to realise that the cholesterol hypothesis, whilst is sounds superficially reasonable, requires mechanisms of action that just do not exist.
LDL cannot enter the arterial wall, at least not from the lumen of the artery, unless the endothelium has been damaged in some way. If you damage the endothelium, all hell breaks loose – and then we have a completely different story on our hands. One where LDL may have a role in plaque formation, or it may not, but it most certainly cannot be the primary role.
This is a conclusion that I arrived at a long time ago. Not, initially, because I set out to debunk the hypothesis. I simply wanted to understand how a raised LDL could cause atherosclerosis. ‘Because it does’, has never ever been a reply that I am happy to accept. In fact, nowadays I would translate this particular ‘because it does’ into ‘because it must.’ It must, because if LDL cannot pass through, or past, a healthy endothelium, the cholesterol hypothesis is wrong. And it can’t, so it is.
Now I have got that out of the system, I shall move on to look at what happens when you damage the endothelium. For that, logically, must be the first step in plaque formation.