Sometimes I think Vitamin C should be called the Sex Vitamin. The reason is simple: it helps in so many ways with bedroom performance, including increasing oxytocin, lowering stress hormones and increasing mood just for starters. However, on this page I want to concentrate on how Vitamin C improves nitric oxide, or the ability of the arteries to dilate and allow more blood to flow.
As we'll show below, Vitamin C actually excels in these areas, which can best be termed as endothelial function. So what just what does "endothelial" refer to? If you've browsed through my site much, you already knows that this refers to damage to the delicate inner lining of your arteries. Classic endothelial dysfunction comes from the typical Western diet and lifestyle that leads to hardening of the arteries and arteriosclerosis. The reason that this is SO critical is that the lining of the arteries are what pumps out nitric oxide. This is critical for heart health, blood pressure control and, yes, erections.
Ascorbic acid improves the endothelial output through five different mechanisms according to one research summary:
"1) decreases in low-density lipoprotein (LDL) oxidation,
2) scavenging of intracellular superoxide,
3) release of NO from circulating or tissue S-nitrosothiols
4) direct reduction of nitrite to NO, and
5) activation of either endothelial NO synthase."  (A recent study, though, argued against option e. )
Now I know these are pretty heady items, but trust me, this is all a beautiful fantasy for us men. And, to make things even better, I would add a couple of other pro-Vitamin C blood flow boosting properties on top of these:
6. Vitamin C will likely help with maintaining and/or increasing acetylcholine levels, something I talk about in my link on Vitamin C and S-PS (Phosphatidylserine). Acetylcholine is also a vasodilator. Although it is not quite as potent as nitric oxide, it is important nonetheless for allowing your penile arteries to relax and let blood flow in.
7. Vitamin C (ascorbic acid) "stimulate endothelial nitric oxide (NO) synthesis in a time- and concentration-dependent fashion."  This basically means the more the Vitamin C, the more the nitric oxide. Woo hoo!
Could this all really be true? Could such an inexpensive molecule as Vitamin C actually be a significant partner in helping men with their erectile dysfunction by boosting nitric oxide output, lowering blood pressure and increaseing blood flow? The answer is a definitive 'yes'.
Now you may wonder why you have not heard about this? Well, this research is a relatively new development, coming mostly out of studies in the last 10-15 years (See below.) And there is little profit in Vitamin C, so supplement manufacturers are not spending money marketing this fact. Yet just look at how many categories of men that Vitamin C is likely to help:
1. Men with Heart Disease. One study of men with existing cardiovascular disease, specifically Coronary Artery Disease (CAD), showed a 53% in FMD (flow mediated dilation, the clinical measurement for blood flow) after a single 2 grams dose. Even better news followed in the second part of the study where they found that long term administration of a mere 500 mg/day led to a sustained 33% increased in blood flow.  Vitamin C likely does this by neutralizing the potent free radical superoxide anion that has been implicated in arterial disease.
Yet another study on CAD patients found similarly astonishing results after a 2 g oral dose of Vitamin C: "ascorbic acid produced marked improvement in dilation (2.0 0.6% to 9.7 2.0%), whereas placebo had no effect (1.1 1.5% to 1.7 1.5%, P=.003)."  Of course, 2 grams in one dose is quite high, but patients would very likely get substantial benefit in vasodilation, i.e. opening or relaxing fo the arteries, from even the more common 500 mg dose.
2. Men with Hypertension. One study on patients with hypertension (clinically high blood pressure) found that a relatively small dose of 500 mg daily lowered blood pressure significantly.  While it is true that the study found no improvement in blood flow, it is notable that Vitamin C helped these patients with what they needed help with - their high blood pressure. In other words, Vitamin C seems to help with exactly what is needed in men with artery disease and in men with hypertension.
3. Men and Arterial Stiffness. As the arteries build up arterial plaque, they become more "stiff" and thus less able to dilate. One study found that even healthy men can experience improvement in arterial stiffness through Vitamin C supplementation.  A study eight years later found both blood flow and arterial stiffness improved in men with hypertension. 
4. Men Eating High Fat. One of the rules on The Peak Testosterone Forum is no pushing of high fat diets. My reasons for this are many, but the primary ones are that high fat meals induce insulin resistance and endothelial dysfunction, the polar opposite of what men need who are coming to my site. One example of this is a study where researchers gave men a high fat meal (50 grams of fat) and then 2 grams of Vitamin C to try to compensate for the artery-numbing effects of all the blubber then men has just eaten. Amazingly enough, it worked!  Am I suggesting that you megadose Vitamin C in order to handle eating a bunch of fat? Of course not as the obvious solution is simply to not consume all that fat in the first place.
Many other conditions from Metabolic Syndrome to intermittant claudication to homocystinuria also show improvements in endothelial dysfunction and/or some related parameters, testifying to Vitamin C's miraculous ability to "help just where needed." For all of the above reasons - endothelial function, lowered blood pressure, decreased arterial stiffness and vasodilation, Vitamin C shoud be a significant help in improving erectile dysfunction as well. Of course, the question is this: is there an actual study showing Vitamin C as improving erectile dysfunction similar to the Well-Documented Erectile Supplements I discuss elsewhere on my site?
So now comes the all-important question: has Vitamin C been shown in the studies to improve erectile dysfunction directly? The answer is 'no' - at least as far as I know. So the hard evidence is not there. However, there are some good signs and research that point vigorously to such a conclusion;
a) Heart Disease Reduction. One study found a 25% reduction in heart disease when comparing those taking greater than 700 mg/day versus those who took no supplemental Vitamin C. This is a significant difference for just one vitamin and is more remarkable considering the fact that the authors used a pool of 7 studies to come up with results.  Again, anything that improves heart disease is very likely to improve one's erections and hardness factor.
b) Frequency of Sexual Intercourse. One study of healthy young males found that Vitamin C increased the intercourse frequency. The dosage was definitely in the megadose range (3 grams/day), but who cares?!?
Finally, Vitamin C very likely protects your arteries not just with shorter term endothelial dysfunction but in the long term as well. Examples of this abound, including the recent discovery that Vitamin C can lower inflammation (C-Reactive Protein) in at risk men. See my link on How to Reduce Inflammation for more information. There are many other properties of Vitamin C that will do the same, including its ability to lower cortisol and boost mood, all documented in The Many Benefits of Vitamin C.
CAUTIONS: Are there any cautions associated with Vitamin C use? Well, any supplement as studies as Vitamin C is bound to have a few black eyes along the way and discuss those in my link on Are There Risks with Vitamin C?
DOSAGE: As of this writing, I take 1.5 grams of Vitamin C divided in three doses of 500 mg via Ester-C. (There are now "generic" versions of Ester-C that are less expensive, but Vitamin C is inexpensive to begin with. This dosage is definitely megadosing, something I normally do not like to do. However, my justification for this is that humans are one of the only animals that does not make its own Vitamin C and, therefore, it looks like this was a "bad" mutation that severely decreased our levels of this all-important molecule. Again, though, read my discussion of Vitamin C - Angel or Devil? for the counterargument.
2) Circulation, 1999, 99:3234-3240, "Long-Term Ascorbic Acid Administration Reverses Endothelial Vasomotor Dysfunction in Patients With Coronary Artery Disease"
3) Circulation, 1996, 93: 1107-1113, "Ascorbic Acid Reverses Endothelial Vasomotor Dysfunction in Patients With Coronary Artery Disease"
4) Free Radic Biol Med, 2000 May 1, 28(9):1421-9, "How does ascorbic acid prevent endothelial dysfunction?"
5) Free Radic Biol Med, 2004 Jan 1;36(1):123-30, "Coronary endothelial dysfunction is not rapidly reversible with ascorbic acid"
6) Am J Physiol Heart Circ Physiol, 2001 Feb, 280(2):H528-34, "Effect of ascorbic acid treatment on conduit vessel endothelial dysfunction in patients with hypertension"
7) J Cardiovasc Pharmacol, 1999 Nov, 34(5):690-3, "Oral vitamin C reduces arterial stiffness and platelet aggregation in humans"
8) American Journal of Hypertension, Apr 2007, 20(4):392 397, "Supplementation With Vitamins C and E Improves Arterial Stiffness and Endothelial Function in Essential Hypertensive Patients"
9) Clin Cardiol, 2002 May, 25(5):219-24, "Vitamin C preserves endothelial function in patients with coronary heart disease after a high-fat meal"
10) Am J Clin Nutr, 2004 Dec, 80(6):1508-20, "Antioxidant vitamins and coronary heart disease risk: a pooled analysis of 9 cohorts"
11) Biological Psychiatry, 15 August 2002, 52(4)371-374, "High-dose ascorbic acid increases intercourse frequency and improves mood: a randomized controlled clinical trial"
12) Journal of Biological Chemistry, 276, 40-47, "L-Ascorbic Acid Potentiates Endothelial Nitric Oxide Synthesis via a Chemical Stabilization of Tetrahydrobiopterin"