What is the worst thing about low testosterone? Well, that would be a tough choice, but I suspect most guys would pick an atrophied penis as #1. (See my link on Venous Leakage.) However, arguably the most insidious aspect of low testosterone probably has to do with its negative impact on insulin. Very few men know that the lower T levels are correlated with higher insulin levels.
This is very dangerous for middle and senior-aged urban-dwelling males, who are generally fighting elevated insulin levels from unhealthy lifestyle choices in the first place. Of course, the main job of insulin is to push blood sugar (glucose) into cells, so more insulin might seem like a good thing. In fact, bodybuilders love insulin and will often eat a high glycemic meal before and after lifting just to pump up insulin and drive nutrients into muscle cells.
Unfortunately, elevated insulin over time leads to insulin resistance, or prediabetes, and results generally from lack of exercise, a high fat diet, high glycemic load meals and/or being overweight. And, if prediabetes is allowed to run its full course, a man will find himself with either full-blown Metabolic Syndrome or Diabetes.
NOTE: Many men have a family history of diabetes or perhaps high triglycerides or Fasting Insulin. These can all be risk factors for developing diabetes down the road. See amy links on Testosterone and Diabetes and Diabetes Prevention for more information.
Research has clearly shown that low testosterone makes all of these insulin-related issues much worse, because low T significantly raises insulin levels. Studies of low testosterone men have clearly shown that hypogonadism, or even low normal testosterone, increases both insulin and blood glucose.  This translates to low T increasing insulin resistance and, for this reason, low testosterone is a risk factor for both diabetes AND Metabolic Syndrome. Metabolic Syndrome is a condition that I cover elsewhere on the site and is linked to heart disease, erectile dysfunction and Alzheimer s. Of course, Diabetes is even uglier and, with very careful blood glucose management, leads to rapidly accelerated aging of many tissues. Elevated glucose levels are VERY hard on the body.
NOTE: The situation is much different for the woman in your life. As women become insulin resistant, usually from being overweight, plasma insulin levels rise. This, in turn, actually triggers androgen release (testosterone and its analogs) from ovarian cells. You may temporarily enjoy a woman with very high libido, but usually it ends in PCOS (Polycystic Ovarian Syndrome) or some other disorder that can lead to sexual dysfunction and/or painful intercourse. In addition, the elevated androgen can lead to increased body hair, hair loss on the scalp and other masculinizing features that hurt her self esteem. In a kind way, try to get your woman exercising and losing weight for the sake of her health (and your sex life).
Another proof of testosterone s relationship to insulin is the fact that supplemental testosterone, i.e. testosterone therapy, can be used to treat and sometimes cure insulin-related issues in the obese, who are one of the toughest populations to treat.  Obese, type II diabetic men with low testosterone have also has good success on Hormone Replacement Therapy. 
However, there is a much more important relationship between low testosterone and insulin that likely exists. First of all, insulin resitance is primarily related to the condition where energy in exceeds energy out, i.e. we eat more calories than we burn in exercise. When this occurs, there is a very significant oxidative (free radical) load put on our mitochondria.
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In a rather involved metabolic adjustment, the body compensates for this increased oxidative load by adjusting a chemical pathway that results in insulin resistance.  In other words, insulin resistance simply results from our body's desire to protect our mitochondria from a modern sedentary, high caloric lifestyle.
Low testosterone, of course, only makes this worse, because we begin to lose muscle and this can easily lead to increased weight gain, i.e. a viscious cycle. However, even before this occurs, significant damage is likely occurring to our Leydig cells. Why? Because the same oxidative load and mitochondrial damage that is leading to insulin resistance has been shown to be responsible for accelerating andropause, the age-related loss in testosterone.  And, as our testosterone declines at an accelerated rate, we find ourselves more prone to insulin resistance.
Now there are many other ways to control insulin by the way. Here are just a few that you should be managing actively on a day-to-day basis:
1. Obesity. Yes, being overweight is strongly associated with insulin resistance and most middle-aged and senior men in the U.S. need to drop significant weight. Losing weight can also boost long term testosterone very significantly and greatly improve erectile strength as well.
2. Exercise. Nothing improves insulin resistance like exercise. 30 minutes of exercise once a day if you have a desk job just "ain't gonna cut it". You've got to keep moving to lower your insulin resistance throughout the day. If you've got a desk job, get up and walk somehow, someway and make sure you are on a regular exercise regimen (without Overtraining).
3. Low or Medium Fat Diets. Researchers use higher fat diets to induce insulin resistance in lab animals. Modern higher fat diets will do the same thing to you. The highest you want to go generally are the fat levels in a standard Mediterranean Diet, about 30-35%. Low Fat Diets, in spite of the fact that they are higher carbohydrate are better yet for many men.
4. High Glycemic Loads. Refined carbohydrates, sugars and corn syrup will lead to nasty cycling of blood glucose and insulin levels. Researchers now believe that high post-meal glucose spikes kill off your beta cells (in the pancreas) leading, ultimately, to type II diabetes when enough of these cells die off. Regardless, eating low sugar carbs is absolutely critical. Also, many men do not know that high glycemic meals can hammer their testosterone for several hours as well. See this link on Testosterone and Glucose for more information.
Of course, the bottom line message is this: get moving and, of course, don't overeat!
1) Androgens, Insulin Resistance and Vascular Disease in Men: Testosterone and Insulin Resistance", http://www.medscape.com/viewarticle/512077_4; Table 1
2) Obes Res, 1993, 1:245 251, "Androgen treatment of abdominally obese men"
3) Diabetes Care, 2001, 24:2149 2151, "Androgen therapy improves insulin sensitivity and decreases leptin level in healthy adult men with low plasma total testosterone: a 3-month randomized placebo-controlled trial"
4) Seattle Pi, SHAYA TAYEFE MOHAJER, Associated Press, Updated 07:06 p.m., Sunday, May 29, 2011, "Man celebrates 85 years of living with diabetes"
5) Arterioscler Thromb Vasc Biol, 2004, 24:816-823, "Is Oxidative Stress the Pathogenic Mechanism Underlying Insulin Resistance, Diabetes, and Cardiovascular Disease? The Common Soil Hypothesis Revisited"
6) Diabetes Care, July 2005, 28(7):1636-1642, "Relationship Between Testosterone Levels, Insulin Sensitivity, and Mitochondrial Function in Men"