Metabolic Syndrome is a plague of modern society and would better be named Erectile Dysfunction Syndrome or Castration Syndrome. I cover Metabolic Syndrome several times on this site because it is so critical to the health of your heart and your penis. Metabolic Syndrome, or Erectile Dysfunction Syndrome as I like to call it, is a set of symptoms that includes
Many researchers would also include increased Inflammation and blood clotting as well. Some of you well-read health fanatics may even remember when this syndrome was called Syndrome X or Insulin Resistance Syndrome. Whatever the name, it leads to nasty stuff for us males!
If you have two or more of the above symptoms, then you need to consider yourself as having Erectile Dysfunction Syndrome and you need to take action immediately for the sake of your sex life (and health). Metabolic Syndrome is a full frontal assault on your penis. First of all, it is associated with lower levels of testosterone and increased levels of erectile dysfunction. 
This relationship was clearly established in a study that showed strong correlations between one of the hallmarks of Metabolic Syndrome, insulin resistance, and testosterone levels.  In these senior men the greater the insulin resistance, the lower the testosterone. Other researchers have concluded that "while it is clear that disease, and in the context of this contribution, in particular the metabolic syndrome can suppress circulating testosterone levels, it has also been documented that low testosterone levels induce the metabolic syndrome ". 
NOTE: Interested in the hormonal link? See my page on Testosterone, Insulin and Insulin Resistance for more information.
To put it more clearly, Metabolic Syndrome likely causes low testosterone and low testosterone likely causes Metabolic Syndrome.  Again, it's the Kiss of Death to your sexual life and all you hold dear as a male.
Just as bad, this Erectile Dysfunction Syndrome has another little known symptom: it increases levels of ADMA, an amino acid that mimics Nitric Oxide! In other words, your body pushes that most precious of all chemicals, Nitric Oxide, out of the way. That means the Little Guy stays little. (For more details, read here.)
While Erectile Dysfunction Syndrome is annhilating your testosterone and decreasing your Nitric Oxide levels, it also is taking out your cardiovascular system. That means that it is clogging up your veins and arteries, including the all important ones that supply blood to your penis! This, in turn, leads to high blood pressure, which is also strongly associated with Erectile Dysfunction.
Consider also what researchers found when they did an analysis of the Third National Health and Nutrition Examination Survey (NHANES III), a large national survey of the U.S. population ages 20 to 89 taken between 1988 and 1994. They found that each of the aspects of Metabolic Syndrome raises your chances of a heart attack by the following percentages:
This metabolic disorder doesn't stop with decreased erectile and cardiovascular function: it is also associated with "brain damage". One large study of 999 men found that "cross-sectional measurements at age 70 showed that high 24-hour BP, nondipping, insulin resistance, and diabetes all were related to low cognitive function".  None of this is surprising considering that Erectile Dysfunction Syndrome and insulin resistance are associated with systemic Inflammation.  Inflammation ravages both the body and mind as informed Peak Testosterone readers already know.
Recent research shows that insulin resitance may be the deadliest aspect of this syndrome and, adding to the problem, is the fact that insulin resistance can be tricky to diagnose. One warning sign, regardless of your weight, is if you have higher triglycerides and lower HDL scores.  That ratio can be a warning sign even for someone who is relatively thin.
In fact, it should be pointed out that only 60% of those classified with Erectile Dysfunction Syndrome are actually overweight. And the explanation for that may be the fact that the 40% may be insulin resitant even though they are not overweight. This can result from, for example, being thin with a lot of visceral fat or being thin and very metabolically active.  Again, watch the triglyceride to HDL ratio for possible clues.
NOTE: The Triglyceride to HDL-C ratio is certainly not a perfect measure of insulin resistance and has been found to be only a little better than the Total Cholesterol to HDL ratio for predicting cardiovascular outcomes.  Again, keep in mind that total cholesterol less than 150, and not having resulted from disease or medical condition of course, is probably the best insurance against heart disease. I cover this in the following link on a Low Fat Diet, which is the easiest way to get your cholesterol at or near the 150 mark. Interesting enought, a Low Fat Diet can actually raise Triglycerides and lower HDL and yet it still provides excellent heart protection assuming one's cholesterol is near 150.
Researchers have also discovered that where fat is located is all-important when it comes to insulin resistance for example. One study pointed out that there are no sumo wrestlers or NFL players who are insulin resistant even though many are significantly overweight. However, when they retire they almost always become insulin resistant because "their fat moves from the subcutaneous compartment to the visceral compartment".  Visceral fat is considered to be an actual organ and effects many part of our metabolism. A good measure is basically the ratio of a measurement around your belly button to that of your waist.
What is visceral fat? You may have read about "apples" versus "pears", referring to the general shape of a person's body. Basically, this is referring to whether or not any excess weight on your body tends to go to your belly button area or to your butt. If it goes to your belly button area, you are more quickly building up "organ fat", which is linked to type 2 diabetes, Erectile Dysfunction Syndrome and cardiovascular disease. So even if you are a Skinny Bastard, like myself, you can have a relatively high level of visceral fat and its ensuing negative health consequences.
Another factor that may lead to insulin resistance is a High Fat Diet. Researchers found that, at least in animals, a High Fat Diet led to mitochondrial function genes being down-regulated which mimics what is seen in insulin resistance.  This has been verified in multiple animal studies and scientists have just discovered in the last few years the pathway by which this occurs.  Originally, it was thought that free fatty acids might compete with glucose-like substances for use in the cell, but research is pointing in a different direction now. The important thing to remember is this: the more fat, the less the mitochondrial function, the more likely you are to be insulin resitant and receive all of the nasty symptoms that come with it.
Keeping and raising mitochondrial function is also related to its sister disorder: type II diabetes. The lead of one recent study  on the subject wrote "There is a strong relationship between lipid [fat] content in the muscle and insulin resistance in skeletal muscle". He then stated something ultra-critical: "Insulin resistance is the best predictor for whether someone will eventually develop type 2 diabetes". In other words, type II diabetes is really not the sister of Erectile Dysfunction Syndrome, it is the offspring!pring!
Another important aspect of Metabolic Syndrome to remember is that it generally leads to weight gain, due to the insulin resistance as Japanese scientists have just discovered that high insulin levels keep your body from breaking down fat. An insulin resitant individual will have high insulin levels and high glucose concentrations, a toxic combination that causes damage in different ways. The high insulin levels are deadly because the block the breakdown of fats by adrenaline and lipase. 
So is there hope in the battle against Erectile Dysfunction Syndrome or is it simply an inevitable part of aging? As always, science has uncovered many practical solutions, which I have compiled in my link How to Defeat Metabolic Syndrome.
NOTE ON APNEA: Of course, the easy weight gain that comes from insulin resistance is deadly in many ways. I document the many Dangers of Being Overweight. But one I would like to highlight is that weight gain is associated with apnea.  Apnea is a sleep disorder where the airway is temporarily restricted leading to extremely restless and interrupted sleep and it whacks testosterone, makes Erectile Dysfunction Syndrome even worse and can lead to a host of nasty distorders.
1) Jour Urol, 2007 Feb,177(2):651-4; Jour Clin Endocrin Metab 1990 Oct,71(4):929-31
3) Obesity Reviews,Jul 2008,9(4)340-354(15)
4) Am J Clin Nutr,2003 Nov,78(5):965-71
5) J of Mens Health, Sep 2008, 5(1):S40-S45
6) Med Sci Sports Exerc, Oct 2003, 35(10):1703-9
7) Tex Heart Inst J, 2005, 32(3): 387ï¿½389
8) Am J Cardiol, 2008 Feb 15,101(4):497-501
9) Diabetes Care, 2005, 28:1636-1642
10) Kobe J Med Sciences, 2007, 53:99-106
11) Diabetes, 2005, 54:1926-1933
12) J Clin Invest, 2007, 117(712) J Clin Invest, 2007, 117(7): 1995-2003
13) Yale University (2004, February 23). Young People Prone To Type 2 Diabetes Exhibit Alterations In Mitochondrial Activity
14) J of Clin Hypertension, May 9 2007, 9(4):249-255
15) Circulation, 2005, 111(11):1448-1454; Lupus, 2005, 14(9):760-764
17) Henriksen EJ, et al. Chronic thioctic acid treatment increases insulin stimulated glucose transport activity in skeletal muscle of obese zucker rats. Diabetes (Suppl.) (1994) 1:122A
18) Diabetes Res Clin Pract, 2003, 62:139-48
19) Physiologica Scandinavica, 1992, 146:505ï¿½510
20) Science, 1987, 237(4817):885-888
21) J. Nutr, Nov 2005, 135:2634-2638
22) European Heart Journal, Advance Access published online on January 23, 2008, "Work stress and coronary heart disease: what are the mechanisms?", Received 1 August 2007; revised 14 November 2007; accepted 22 November 2007.
23) "Alpha-Glucosidase Inhibitors for Type 2 Diabetes." Chen, H., Journal of Food Science, published online June 30, 2009.
24) Obesity Reviews,Jul 2008,9(4)340-354(15)
25) J of Clin Endocrin & Metab, Nedeltcheva, A.V. , published online ahead of print, June 30, 2009
26) Amer J of Clin Nutr, March 2005, 81(3):611-614, "Short-term administration of dark chocolate is followed by a significant increase in insulin sensitivity and a decrease in blood pressure in healthy persons"
27) Perez-Pozo S, et al "Excessive fructose intake raises blood pressure in humans" AHA BPRC 2009; Abstract P127
28) J Endocrin Invest, Jun 2007,30(6):451-8; Journal Andrology, 2009, 30:10-22