What if I told you that I knew of a cure for adult onset diabetes (in men)? Or what if I told you that there was a simple injection that would completely reverse the diabetes of many middle-aged and senior patients? Well, it's true and very few doctors know about it.
How did I find out about it? Well, I was told by one (pretty large) HRT clinic that every one of their type II diabetic patients was able to completely get off of insulin. Now a couple of them still had to take Metformin, but still it is incredible that all of these men were able to completely get off of insulin. This should be front page medical press news, but, unfortunately, no one is interested.
How does this HRT clinic do it? They simply take their men to fairly high testosterone levels - about an average of 1000 ng/dl via weekly injections and then make sure that estradiol is managed biweekly doses of Arimidex (if needed). So the typical man in this clinic is hypogonadal with testosterone less than 400 ng/dl and then gets boosted to the level of a 20 year old. Here is the key: in general, as testosterone rises, insulin levels fall. And, as insulin levels fall, insulin resistance is decreased and blood sugar levels fall. This is why increasing testosterone so significantly completely eliminates their need for insulin.
CAUTION: Can all low testosterone men be cured of their type II diabetes through HRT? Probably not. Nor am I saying that everyone should go on HRT. But I mention the above story, because it is definitely something to discuss with a knowledgeable physician if you are having insulin-related issues. Even if it halves your need for insulin, that would be an impressive accomplishment, right? Men with Metabolic Syndrome (prediabetes) should take note that testosterone therapy has actually been part of a very successful treatment protocol that completely reversed the condition in the great majority of men. See this link on Testosterone and Metabolic Syndrome for more information.
CAUTION #2: Some men with diabetes have weakened renal function. It turns out that testosterone can potentially be hard on kidneys, in men with existing kidney disease. Discuss with your doctor if you are diabetic and considering going on HRT.
NOTE: Diabetes is known to accelerate overall aging and it can have a profound affect on one's sex life. See this link on Diabetes and Erectile Dysfunction for more details.
So is the story above supported by the studies? Indirectly, yes, although studying the effects on diabetics of going to these higher testosterone levels has not been looked at as far as I know. But look below at the many ways that testosterone is tied into and can reverse or prevent adult onset diabetes:
1. Hypogonadism and Diabetes Risk. The studies show clearly that low testosterone increases the risk for diabetes. At what T level will the risk of diabetes increase significantly? A couple of studies give us some interesting insights. The first looked at several hundred community-dwelling seniors and found that men in the lowest quartile of low testosterone had a dramatically increased risk (2.7 times) of developing diabetes. The lowest quartile in this case was found to be total testosterone less than 256 ng/ml (8.6 nmol/l).  Other studies have found similar numbers, around 294 ng/dl (10 nmol/l) for example. 
2. Insulin and Insulin Resistance. As mentioned above, falling testosterone levels are correlated with rising insulin and insulin resistance. Of course, the question is if testosterone is causative. We know that adding body fat promotes insulin resistance and lowers testosterone. But can the reverse also be true? Can low testosterone actually increase insulin resistance?
The answer to that was clearly shown in some animal studies, where rats were castrated and the authors found that insulin resistance followed along with a decreased ability to utilize glucose. Over the years resarchers clearly concluded that even in humans "low testosterone levels and impaired mitochondrial function promote insulin resistance in men."  And , as further proof, researchers put low T men on Hormone Replacement Therapy and watched their insulin levels fall.. See my link on Testosterone and Insulin for more details, but a perfect example was a study that gave testosterone to hypogonadal men with type II diabetes. What happened? Every standard diabetes measure improved - that's what happened! And, yes, both fasting insulin and insulin sensitivity improved markedly. 
3. Blood Glucose Levels. Now, if testosterone really improves insulin sensitivity, wouldn't you expect testosterone therapy to generally result in lower blood glucose levels, at least in low T men? Of course, this has been verified both in studies and clinically, such as the story I told above. In fact, the same study mentioned in #2 also showed substantial reduction in blood glucose levels.  Both glycated hemoglobin (A1C) and fasting blood glucose levels went down. Glycated hemoglobin, for the unitiated, is essentially a way to measure a rolling (approximately) 90 day average of blood glucose levels. It is not a perfect measurement but, in this case, shows that average glucose levels were substantially reduced through testosterone therapy.
4. Eliminated Risk of Dying. Let's start with a profound study of type II (adult onset) diabetics that grouped men according to testosterone levels. They found that the hypogonadal men, defined as less than 306 ng/dl (10.4 nmol/l), had double the chance of dying. This is very large increase in mortality - usually studies are looking 20%, 30%, etc Yet in this case, a man has twice the risk of dying with both low testosterone and diabetes.
What was the solution? When the researchers gave the diabetic men with low T testosterone therapy, the results were impressive: a mortality rate equal to that of the diabetic men with normal T.  In other words, the researchers found that if you put them on HRT, it eliminated ALL the additional risk of dying, leaving them at the same risk on average as other diabetic men. This is good news, because it suggests that the ravages of low testosterone are largely reversible in men with glycemic issues such diabetes and possibly Metabolic Syndrome, a large cross section of any modern, industrialized society.
5. Visceral Fat. Belly fat and insulin resistance go hand in hand - something I disucss in my link on Visceral Fat. A lot of belly fat on a man (or woman for that matter) is a sure sign that they have lost or are losing their insulin sensitivity. Testosterone, or the lack thereof, can definitely play a strong role. A couple of studies have found, for example, that low testosterone is correlated with higher visceral fat levels. 
One of these studies found that "obesity is associated with low testosterone levels in diabetic men." Now we know that obesity can actually cause low testosterone by actually shutting off the signaling from the hypothalamus / pituitary. So is this all that is going on? Low T correlates with belly fat simply because obesity causes low T?
It turns out that that is not the whole story as evidenced by the fact that giving hypogonadal (diabetic) men testosterone lowers belly fat without doing anything else.  So, if you want to get rid of that belly fat, improving your testosterone levels will very likely help. Of course, it's not a miracle cure: diet and exercise are kings.
1) Asian Journal of Andrology, 2010, 12:136 151, "Androgens and male aging: current evidence of safety and efficacy"
2) Diabetes Care, Jan 2002, 25(1):55-60, "Endogenous Sex Hormones and the Development of Type 2 Diabetes in Older Men and Women: the Rancho Bernardo Study"
3) Eur J Endocrinol, June 1 2006, 154:899-906, "Testosterone replacement therapy improves insulin resistance, glycaemic control, visceral adiposity and hypercholesterolaemia in hypogonadal men with type 2 diabetes"
4) International Journal of Obesity and Related Metabolic Disorders : Journal of the International Association for the Study of Obesity, 2000, 24(4):485-491, "Low serum testosterone level as a predictor of increased visceral fat in Japanese-American men"
5) Diabetes Care, Apr 2007, 30(4):911-917, "Clinical and Biochemical Assessment of Hypogonadism in Men With Type 2 Diabetes Correlations with bioavailable testosterone and visceral adiposity"
6) Diabetes Care, July 2005, 28(7):1636-1642, "Relationship Between Testosterone Levels, Insulin Sensitivity, and Mitochondrial Function in Men"
7) The Aging Male, 2003, 6(1):1-7, "Testosterone supplementation in men with type 2 diabetes, visceral obesity and partial androgen deficiency"
8) Acta Physiologica Scandinavica, Dec 1992, 146(4):505 510, "The effects of testosterone on insulin sensitivity in male rats"