Alpha lipoic acid is one of our most powerful antioxidants and is known for its ability to protect at the cellular level in both water and lipid mediums. We can actually make lipoic acid, but we lose that ability with aging. Therefore, we can replace some of that from food, but the amount is quite small relatively speaking. As you'll see below, the sweet spot for alpha lipoic acid is probably for diabetic, prediabetic and 40+ year old men. And what also stands out is that this supplement can likely help some of these men with their erectile strength and general bedroom performance. Here are 5 Ways That Alpha Lipoic Acid Can Help with Erectile Dysfunction:
1. Increase Nitric Oxide Through Decreased Phosphorylation (Due to Aging). There are many natural processes of aging working against men as they hit middle age and one of them is in a completely different pathway than is benetitted by the standard PDE5 inhibitors, such as Cialis and Viagra. Basically, what happens is that, as your body ages, your levels of one of its master antioxidants, glutathione, declines. And the decrease in glutathione causes an increase in ceramide and this, in turn, causes a loss of phosphorylation - the addition of a phosphate group - to eNOS. That's a lot of chemistry I realize, but it has the net effect of basically stunting the relaxation of your arteries, including penile.  Of course, this means less blood flow and worse erections.
Researchers decided to show just how important this is by giving young and old rats acetylcholine. Basically, acetylcholine works by stimulating the phosphorylation mentioned above and, in young rats, this worked as expected and vasodilation (relaxing of the arteries) occurred. However, in older animals, there was very little vasodilation that resulted from acetylcholine, supporting the theory that ceramide accumulation leads to arterial unresponsiveness. 
I have noticed this on the Peak Testosterone Forum before. Some young guys have sworn that whole eggs improve their erections. And the reason is likely the high dose of choline that is present in egg yolks. However, it is only the younger men who seem to benefit from this.
By the way, the results of these studies and a few others that we will mention below caused one set of authors to create a "rallying cry" for using ALA more in the prevention and treatment of cardiovascular disease. They stated, for example, that "in summary, the favourable antioxidant, anti-inflammatory, metabolic and endothelial effects of lipoic acid in rodents warrant further assessment of its potential role for prevention and treatment of cardiovascular diseases."  And, if it improves heart disease, it likely improves erectile function.
NOTE: See also my page on How to Raise Your Glutatione Levels Naturally.
2. Increase Nitric Oxide Due to Diabetes and Prediabetes / Metabolic Syndrome. All of the above studies on nitric oxide, phosphorylation and aging were done on animals. Have there been any studies on nitric oxide and alpha lipoic acid on humans? Yes, there have been such studies and they involved diabetic patients. Basically, researchers found that alpha lipoic acid increases NO in diabetic patients but not in controls. 
More importantly, ALA greatly increases vasodilation - the relaxing of arteries - in prediabetics. Typical symptoms of prediabetes: an A1C > 5.7 but not diabetic; prehypertension or hypertension; high triglycerides; low SHBG; a "pattern B" lipid profile from a VAP or NMR. And I would guess that about half the men reading this would qualify for prediabetes and their NO levels and blood flow are impaired because of it. ALA can help these men greatly as well according to multiple studies. In fact, one study found that "oral supplementation with 300 mg/day of LA for four weeks improved flow-mediated vasodilation by 44% compared to placebo" in prediabetics 
Therefore, alpha lipoic acid is not necessarily going to improve endothelial function or vasodilation in everyone that takes it. However, if you are middle-aged, senior, prediabetic or diabetic, then it will probably have a significant effect. (Always talk with your doctor first of course.)
CAUTION: Generally speaking, supplements should be considered a short term solution and ALA is no exception. Personally, I am going to cycle ALA as a method of regularly detoxing. However, ALA can a) lower thyroid hormone levels  and b) there is some concern that, in the long term, it could lower nutrient status, since it is so good at binding to things. Now there is no evidence of the latter that I know, but no one has been taking it for decades.
3. Arterial Plaque (Atherosclerosis). There is mounting evidence that alpha lipoic acid may be able to help maintain or reduce atherosclerosis (arterial plaque). First of all, it tends to improve lipid profiles by lowering LDL and triglycerides and raising HDL.  However, it also inhibits vascular adhesion molecules, which are ground zero for plaque formation. Interestingly, an article in the Linus Pauling Institute showed that alpha lipoic acid did this not by raising glutatione but rather somehow with its chelating abiilites - more on that later - as described here:
"We found that alpha-lipoic acid significantly inhibits both the formation of adhesion molecules and the adherence of monocytes to endothelial cells in culture. However, to our surprise these processes were not influenced by vitamin C or glutathione, suggesting that general oxidative stress does not play a significant role in the activation of human endothelial cells to produce adhesion molecules. Because alpha-lipoic acid also is a good metal chelator, we hypothesized that metals may be involved in the production of adhesion molecules. To test this hypothesis, we added compounds to the cell culture that specifically chelate iron or copper. Our results showed that treatment with metal chelators also inhibits the production of adhesion molecules and monocyte adherence to cultured endothelial cells, thus supporting our hypothesis that the metal-chelating activity of alpha-lipoic acid may be responsible for its salubrious effects on endothelial cell function." 
Clearing out plaque can greatly help men improve their blood flow and also avoid more severe erectile dysfunction and heart disease. See my page on How to Clear Your Arteries of Plaque for other key strategies as well.
4. Neuropathy. One of the big complaints that I get from many men is that of "penile insensitivity." Guys say that they almost feel dead down there. Of course, there can be many reasons for this, but one of them at times is probably that they have developed the beginnings of neuropathy. As prediabetes and insulin resistance set in, nerve function can be mildly impaired. One example of this is retrograde ejaculation, somethng I discuss in my page on Semen Volume. Alpha lipoic acid has some solid research behind it showing that it can heal or help neuropathy in diabetics. And it can undoubtedly help men with prediabetes as well. Look at what the authors of this study stated:
"Oral treatment with ALA for 5 weeks improved neuropathic symptoms and deficits in patients with DSP. An oral dose of 600 mg once daily appears to provide the optimum risk-to-benefit ratio."
5. Detoxification of Heavy Metals. One thing that probably is an issue more often than we realize is the buildup of heavy metals in certain tissues. Of course, lead and mercury and copper have all been implicated in various neurological, vascular and other chronic diseases and disorders. Because of this, a fair amount of alternative practitioners will now use alpha lipoic acid as a metal chelator. Chelation refers to any protocol that removes toxic substances from the body. I have seen some alternative doctors suggesting that ALA be cycled, for example, i.e. detoxifying the body every so often with a "cleanse."
Is there any research support for this view? First of all, alpha lipoic is known for it's ability to bind with many metals. For example, there is a study out there that discusses its ability to bind with the neurotoxin copper. (We tend to get overly high doses of copper and it has been implicated in Parkinson's.) The authors stated that "we found that TA had a profound dose-dependent inhibitory effect upon Cu(2+)-catalysed ascorbic acid oxidation (monitored by O2 uptake and spectrophotometrically at 265 nm) and also increased the partition of Cu2+ into n-octanol from an aqueous solution suggesting that TA forms a lipophilic complex with Cu2+." 
1) Brit J of Pharmacology, online 29 Jan 2009, 153(8), "Lipoic acid supplementation and endothelial function"
2) Biochemical Society Transactions, 2003, 31(6), "Vascular endothelial dysfunction in aging: loss of Akt-dependent endothelial nitric oxide synthase phosphorylation and partial restoration by R-alpha-lipoic acid"
3) Diabetes Care, 2006 Nov, 29(11):2365-70, "Oral treatment with alpha-lipoic acid improves symptomatic diabetic polyneuropathy: the SYDNEY 2 trial"
4) Biochem Pharmacol, 1995 Jun 29, 50(1):123-6, "Thioctic (lipoic) acid: a therapeutic metal-chelating antioxidant?"
5) British Journal of Pharmacology, 2008, 153:1615 1622, "Lipoic acid significantly restores, in rats, the age-related decline in vasomotion"
6) Free Radical Biology and Medicine, Jul 2001, 31(1):53-61, "Beneficial effects of a-lipoic acid and ascorbic acid on endothelium-dependent, nitric oxide-mediated vasodilation in diabetic patients: relation to parameters of oxidative stress"
7) J Diet Suppl, 2012 Jun, 9(2):116-27, "Evidence for using alpha-lipoic acid in reducing lipoprotein and inflammatory related atherosclerotic risk."
10) Circulation, 2005 Jan 25; 111(3):343-8, ."Irbesartan and lipoic acid improve endothelial function and reduce markers of inflammation in the metabolic syndrome: results of the Irbesartan and Lipoic Acid in Endothelial Dysfunction (ISLAND) study"