What is the most important health test? Right at the top of the list should probably be postprandial (post-meal) blood glucose levels, because this number affects erections and hormone levels for literally hours after the meal. It also is a powerful predictor (like low SHBG) of your chances of developing diabetes. The tragedy is that so few men know about this health marker or how to test for it. Their erectile strength is weak; they feel lousy; they are suffering with fatigue; and yet they do not realize one simple and inexpensive test could help them troubleshoot a powerful root cause issue in many cases. I know I see it all the time on Peak Testosterone Forum.
What most men do not realize is that what you put on your plate sends a huge wave of lipids and blood glucose through your arteries and arterioles. The latter is something you can easily track and monitor at home and on this page we go into why it is so important and how you can get a powerful sense as to whether or not you are on track or not. Doctors will rarely pull or test this number unless you have already developed full-blown diabetes. However, long before you reach this point, you will have likely suffered for years with prediabetes and the softer erections and altered hormone levels that this syndrome brings with it.
I will go into some of the mechanics on another page, but realize that Walmart, Amazon and other large retailers sell electronic monitors that will give you a reading in seconds. You basically "lance" (prick) you finger, put a drop of blood on a test strip and insert the test strip into the monitor. However, first let's go over some of the very critical research that shows just Why Postprandial Glucose Levels Are So Important and What Is the Best Target Level for us men:
NOTE: If you have already developed type II (adult onset) diabetes, the condition can often be reversed with diet and exercise. See my pages on Reversing Diabetes with a Low Fat Diet (Dr. Bernard) and Reversing Diabetes Using Dr. Whitaker's Approach. You can also partially reverse diabetes often with HRT, something I discuss in my page on Testosterone and Diabetes.
Blood glucose levels may seem dry and unglamorous, but - trust me - they can hit you right where it counts:
1. Erections and Blood Flow. As insulin resistance mounts and insulin levels rise, endothelial dysfunction sets in and worsens. What this means is that nitric oxide decreases and your arteries start to have difficulties in dilating, which in turn means that blood flow is decreased. This will happen throughout your body, i.e. it will affect the arteries in your penis and can lead to erectile dysfunction and/or a decrease in hardness factor. Examples of this are studies in which diabetics are given insulin and endothelial function improves significantly 
2. Long Term Increase in Arterial Plaque. The same study mentioned in #1 documented that Repaglinide, an anti-diabetic drug," achieved regression of carotid atherosclerosis (intima-media thickness) in 52% of patients versus 18 % for glyburide over 1 year, although levels of HbAlc and CV risk factors were similar for both treatment groups." 
3. Increase in Cardiovascular Mortality. Elevated fasting insulin levels are known for microvessel damage and elevated postprandial glucose levels for macrovessel damage. Basically, this means that those post-meal blood glucose waves can damage your arteries and, unfortunately, increases our risk of an early and untimely death.
4. Decreased Testosterone. One study found that giving a man a significant dose of glucose will lower his post-meal testosterone levels by 25%. See my page Testosterone and Glucose for more information.
5. Fatigue. As you begin to lose your insulin sensitivity, fatigue is a common complaint. Blood glucose is your primary fuel and, once insulin resistance sets in, your body is struggling with managing it. Hypoglycemia can occur and result in alterations to the HPA axis. Diabetics, in paricular can really struggle with energy according to this research commentary:
"Fatigue is a common and distressing complaint among people with diabetes and likely to hinder the ability to perform daily diabetes self-management tasks." 
1. Healthy Subjects Postprandial Blood Glucose Less Than 120 mg/dl (Possibly 140). Researchers have pointed out that "in healthy, nondiabetic subjects, 2-hour postprandial blood glucose levels are usually <120 and rarely >140 mg/dl. Glucose levels peak at 1 h after the start of the meal and then return to preprandial levels within 2-3h.This rise and fall of postprandial glucose levels is mediated by the first-phase insulin response, in which large amounts of endogenous insulin are released, usually within 10 min, in response to nutrient intake. In individuals with type 2 diabetes, the first-phase insulin response is severely diminished or absent, resulting in persistently elevated postprandial glucose throughout most of the day." 
The obvious conclusion: if you want to be healthy, you need to never see your glucose rise over 120 - maybe 140 at the highest. If you are rising above that, then you have to work to do. I explain one of the reasons why this 140 number is so critical in #4 below.
2. Postprandial Well Below 157 mg/dl. The Honolulu Heart Study showed that those with postprandial glucose levels above 157 mg/dl had twice the risk of fatal coronary heart disease and, of course, heart disease is the #1 killer of men.  Of course, if 157 mg/dl causes this much death and destruction, one clearly wants to be well below that level! This corroborates well with #1 obviously. Also, note that the researchers were looking at non-diabetics, so this is relevant for the great majority of men reading this.
3. Postprandial (Two Hour) Below 140 mg/dl. This the current recommendation of the American College of Endocrinology and the American Diabetic Association, both of which based this advice on the fact that the European DECODE study found the best mortality rates for those with postprandial glucose peaks below 140 mg/dl.  This is the number that I see most often quoted as the beginning of the "danger zone" for damage and future medical issues.
4. Starling's Curve of the Pancreas. One powerful argument for maintaining glucose levels below 140 mg/dl is the fact that this is point at which the pancreas will no longer be able to produce enough insulin to control blood glucose levels. Basically, this is the point "when the dam breaks." One author wrote that "normally, fasting glucose maintenance mostly depends on glucose production by the liver. In the progression of type 2 diabetes, insulin output is able to increase with the increasing glucose level until the FBG reaches about 140 mg/dl, at which point the Î²-cell insulin output cannot keep pace with the increased glucose load, and the fasting insulin concentration decreases. This is sometimes referred to as Starling's Curve of the Pancreas. At this time, hepatic [liver] glucose production begins to increase because insufficient insulin is available for suppression." 
CONCLUSION: Purchase a blood glucose monitor and make sure your blood sugar never gets over 140 after a meal. And, ideally, it would never go over 120. To understand the details as to how to do this, see my page on How to Test for Insulin Resistance.
NOTE: Several studies have confirmed that postprandial blood glucose is an independent predictor of heart disease, even when things like fasting blood glucose and A1C are taken into account. In other words, it's a Big Gun and should be monitored pretty regularly by every man on planet earth over the age of 25. Furthermore, research supports that postprandial glucose peaks are actually a better predictor of insulin-related cardiovascular issues than fasting plasma blood glucose levels. 
1) Clinical Diabetes, Apr 2002, 20(2):71-76, "Is Postprandial Glucose Control Important? Is It Practical In Primary Care Settings?"
2) Diabetes, 1987 Jun,36(6):689-92, "Postchallenge glucose concentration and coronary heart disease in men of Japanese ancestry. Honolulu Heart Program"
3) Diabetes Metab, 2000 Sep, 26(4):282-6, "The DECODE study. Diabetes epidemiology: collaborative analysis of diagnostic criteria in Europe"
4) Diabetes Res Clin Pract, 1992 Aug, 17(2):111-23, "A comparison of the relationships of the glucose tolerance test and the glycated haemoglobin assay with diabetic vascular disease in the community. The Islington Diabetes Survey"
5) Clinical Diabetes, Oct 2004, 22(4):169-172, "Targeting Plasma Glucose: Preprandial Versus Postprandial"
6) Clinical Therapeutics, 2005, 27(Suppl 2):S42â€“S56, "Postprandial glucose regulation: New data and new implications"
7) Journal of Psychosomatic Research, Jul 2010, 69(1):33â€“41, "Fatigue in patients with diabetes: A review"