Low Carb Diets are incredibly popular right now. They're a fast way to lose weight - mostly because of the shedding of water during the initial glycogen depletion - and attractive to the typical man used to a high fat Western Diet. Given a choice between cutting back on meat or fruit and whole grains, most (American) guys will toss out the fruit and whole grains.
But what do these diets do to the heart and arteries? That's the all-important question, because cardiovascular disease is the #1 killer of men and will likely remain so for years to come. The answer to that question is not as simple as one might expect. For example, according to traditional thinking, low fat diets tend to regress plaque and higher fat diets the opposite, righf? Again, the answer is not as simple as we might like. One study, in particular, has demonstrated that fat levels are likely irrelevant in reducing atheroclerosis if significant weight loss is involved.
This study looked 322 middle-aged and early senior aged participants
with BMI greater than 27. They were then randomized into three different diets that included low carb, Mediterranean and
low fat. In other word, the authors wished to cover a wide spectrum of fat and carb levels in the diet. 
Of course, if you've poked around the site much, you know I am a follower of Drs. Gould and Esselstyn who use a low fat diet as their starting point for plaque maintenance and reversal. So the results of this study took me by surprise: they found that ALL the diets regressed plaque according to a volumetric ultrasound technique. The two that showed the most plaqure regression were the "low fat" and low carb diets, but low carb was actually the winner of the two.
NOTE: The authors found that the most important predictor of plaque loss was the drop in systolic blood pressure. Again, this is yet more proof how important control of blood pressure is. See my page on High Blood Pressure and Erectile Dysfunction for natural ways to significant lower your blood pressure. Also, one other important side note is that this was a fairly diseased population: 26% of them where on anti-hypertensive and 20% on statins for example. Someone with less plaque and a less plaque-building lifestyle will probably experience less regression.
Now does this mean that low carb regresses plaque at the same rate as a low carb diet? Well, not necessarily and the reason is simple: this was not really a low fat diet. Their low fat diet consisted of 26% of calories as fat and this is way above what you and I would call a low fat diet, which usually clocks in at around 10-15% fat. This is nothing surprising by the way: researchers call a 25-35% diet "low fat" and a 10-15% diet "very low fat."
Secondly, this is under weight loss conditions. The real test of a diet is when it is consumed for a decade or two and I discuss the evidence that a low fat diet, if done correctly, has the best research behind it in my page called The Evidence for Long Term Arterial Plaque Reduction Using Low Fat Diets.
Nevertheless, it appears from this study that a lower carb diet, as long as you are losing weight, can do a nice job of reversing your plaque and thus should be good for arterial health.
CAUTION: Not all men do well on a low carb diet, due to gut issues, decreased arterial blood flow and lowered nitric oxide and elevated stress hormones, much of which I cover in this page on Low Carb Diets and Inflammation. My impression from running the forum is that the older the man, the less he can withstand some of the negative effects of a low carb diet, and lately I have had a number of middle-aged and beyond men on the retreat from higher fat and lower carb dietary regimens. Chris Kresser has a number of tactful articles that describe how he sees a significant percentage of men that do well on a low carb diet for a few months, but then have to stop.
Okay, so a low carb diet can reduce plaque if you are losing weight. But what if you are not losing weight? Well, a number of things should give any low carber pause and reason to reflect. Let me give you just a few examples:
1. Very High LDL and LDL Particle Counts. Many of you probably know who Jimmy Moore is: he is one of the leaders of probably the biggest low carb site. He posted his lipid numbers recently and please check them out:
"So, what were the results? Here were my numbers:
Of course, these are very high numbers. Not all low carb dieters have numbers this high, so I'm not quite sure what is going on here.
But I have seen it many times on the forum: men on classic Paleo and low carb have medium high or high LDL and LDL particle counts, but low triglycerides. Basically, they are hoping that their low triglycerides will protect them their high LDL. In fact, Jimmy Moore says this in so many words. Essentially he argues that his small LDL particle size will protect him from atherosclerosis.
In my opinion, that is risky. There is substantial evidence that even larger LDL particles can cause plaque, something I cover in my page on Beware of Large Particle LDL. Of course, for his sake and many other Low Carb folks, I hope that it works out that way. However, I can only say that all the plaque regressors that I know of, require low LDL as well as good triglyceride and HDL numbers. And one of them, Dr. Davis, even dislikes low fat diets. See my page on HDL, LDL and Triglyceride Requirements to Regress Plaque. Plus, when you stop losing weight, low carb dieters will probably add in some additional carbs and then this is where it gets dangerous in my opinion - more on that below.
2. During Maintenance Mode. What if you are not losing weight? Well, an animal study shows a low carb dieting accelerating arterial plaque when the animals put on weight. The animals on the low carb diet developed 15% more plaque as they gained weight. The lead researcher actually stopped his own low carb diet:
"Lead researcher Anthony Rosenzweig said the findings were so concerning to him that he decided to come off the low-carb diet he was following." 
3. Middle-Aged And Senior Men. Young guys can withstand just about anything, but, as you age, you struggle to handle the increased stress of things like a low carb diet. (And this is no exagerration - low carb diets work by increasing stress hormones.) Based on a recent animal study, low carb diets look like they may be a bad idea for many men who are middle or senior aged: the researchers found that older animals experienced actual heart alterations and damage: "these metabolic disturbances were responsible for cardiovascular damages only in adult mice, with decreased aortic distensibility and left ventricle dysfunction."  The ventricles are the all-important lower chambers of the heart that receive and pump blood.
So, in older animal, low carb diets can negatively alter the heart itself. What about in other circumstances? Recent research has shown that elevated ketones can induce arrhymias that, in some men, could be very serious, something you can read about in my page on Low Carb Diets and Arrhythmias. Yet another recent study showed that a low carb diet may actually leave your heart vulnerable to stressors such as a heart attack. For example, an animal study showed that obese animals experienced the following:
The head of this study was quoted as saying: "If I had heart disease or I was predisposed to having a heart attack, I would think carefully before starting this type of diet." Now the author of the study was careful to say that his study does not show that low carb diets cause a heart attack, but we already showed you above that in maintence mode, low carb diets may accelerate plaque, so the sword may cut both ways for some men. In other words, if you are not losing weight or are older per the above research, then you may be vulnerable to both getting a heart attack and worse outcomes afterward (if you survive). Cardiovascular disease is the #1 killer of men and I cannot see any reason to tilt the pendulum in the wrong direction.
The reasons for these poor outcomes after a heart attack are probably multiple and include the fact that ketogenic diets may increase free radical damage or that there is insufficient glucose for recovery. This is the same issue that higher intensity athletes have trying to make it with a ketogenic diet: they lose performance. So be careful...
1) Circulation, "Dietary Intervention to Reverse Carotid Atherosclerosis"
2) Jimmy Moore, "LipoScience's NMR LipoProfile Test: A Revolutionary, More Accurate Lipid Profile Particle Size Screening"
4) Am J Physiol Heart Circ Physiol, 2014 Sep 1, 307(5):H649-57, "High-protein-low-carbohydrate diet: deleterious metabolic and cardiovascular effects depend on age"