I just had someone post on The Peak Testosterone Forum that he had recently quit a high fat diet. And it's no wonder: high fat diets often do the OPPOSITE of what is expected and many men have men have no idea why.
First of all, what I am talking about when I say a "high fat meal," I mean a meal where the fat calories are approximately 45-55% and above of the total calories and there are some carbs present. Let's take one of the typical ingredients in a low carb/high fat meal: ground round. Let's say a man grabs some 90/10 ground beef. The package says that it is 10% fat, but this is misleading, because 85 of its 182 calories are actually fat calories, which is right at 46%. And, typically, other butters and oils are added to the meal on one form or another and the percent fat goes well over 50%. And, once you get near this 50% mark, things can get ugly really fast.
Don't believe me? Here are 7 Reasons You May Want to Rethink that High Fat Meal:
1. Testosterone (Free and Total). One found a prolonged loss of testosterone (about 8 hours) after a high fat meal.  Furthermore, the study found the loss to be significant, 22% and 23%, in total and free testosterone, respectively. Ouch! An earlier study found that it was likely that a high fat diet affected the testes as opposed to leutinizing hormone. 
2. Insulin Resistance. High fat meals have long been used by researchers to induce insulin resistance in laboratory animals! For example, one study wrote "chronic high-fat feeding in rats induces profound whole-body insulin resistance, mainly due to effects in oxidative skeletal muscle."  Many men on higher fat diets assume that by lowering carbs, they are improving their MetS (prediabetes) or avoiding it. However, this is not necessarily the case and in a signficant block of men the opposite will happen.
NOTE: The same study above also noted that "prior exercise, a single low-fat meal, or fasting all significantly increased insulin-stimulated glucose utilization."  In other words, exercise, fasting or a Low Fat Diet can all somewhat counteract a High Fat meal. But one can't help but ask the question, why do it in the first place?
3. Endothelial Dysfunction. High fat meals actually increase endothelial dysfunction. Saturated fat, in particular, sludges the blood and lower nitric oxide levels. Of course, if you've read much on this site, you realize that endothelial dysfunction is virtually synonymous with erectile dysfunction.  Now, admittedly, eating vegetables was found to partially overcome this in the study. But, again, why risk it? The high fat meal was found to reduce the ability of the arteries to expand and platelets were found to clump together more easily.
4. Blood Flow! High fat meals hammer endothelial function, as mentioned above, and correspondingly blood flow. Less nitric oxide, less expandable arteries and less fluid plasma all translate to decreased blood flow. And, yes, erections are all about blood flow, so this will not be good for your sex life. Knowledgeable researchers have known that this is a significant problem with high fat diets and have actually studied ways to try to cirdumvent the issues. For example, one study found that massive doses of Vitamin C helped heart patients retain their endothelial function post-meal.  Yet another study found that exercise can do the same in healthy adults who ate a high fat meal.  And, as I mention elsewhere on the site, walnuts have been found to partially do the same.
NOTE: For additional reasons, to dial back the fat, see my pages on The Potential Risks of a High Fat Diet and The Potential Research-Backed Dangers of a Low Carb Diet.
Doesn't the fact that you have to so diligently protect yourself from the effects of a high fat diet tell you something? How do you know you can exercise after a high fat meal? It is much more safe to just not eat so much fat in my opinion!
5. CCK Resistance. Researcher discovered that that rats fed a high fat diet developed CCK resistance.  Cholecystokinin, or CCK, is the messenger to the brain which tells the liver to loeert blood sugar output.
6. BPH (Prostate Enlargement). As men age, they often struggle with an enlarged prostate and difficulty with urination. What causes this? Well, researchers have recently discovered that one of the key factors is insulin resistance and - you guessed it - a high fat diet is a risk factor because it induces insulin resistance. These authors state that "a western diet, characterized by high fat consumption, predisposes men to BPH, while a diet rich in flavonoids and lignanes, containing phyto-estrogens, lowers this risk."  Check out my link on BPH Prevention for more information.
7. Endotoxemia. Endotoxemia is a condition where harmful chemicals (antigens) released by bacteria in the gut raise inflammation levels in the body. What causes endotoxemia? Researchers found that a single high fat meal raised endotoxin levels by 50% and postulated that "low-grade endotoxemia may contribute to the postprandial inflammatory state and could represent a novel potential contributor to endothelial activation and the development of atherosclerosis."  That's right: high fat could potentially harden your arteries, including the ones in your penis.
NOTE: I am NOT attacking meat eating here, but rather a high fat diet based on modern, industrial meats that are nothing like wild game. Almost all of the problems in this article vanish if one eats range fed meats. I discuss many options in my page on Low Fat Meats.
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8) Cell Metabolism, Aug 6 2009, 10(2):99-109, "Intestinal Cholecystokinin Controls Glucose Production through a Neuronal Network", Grace W.C. Cheung, et. al.
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