"What is saturated fat bad for you?" One reader wrote in with essentially this question.
Many men have heard from their doctors and others that saturated fat is the root of all dietary evil. However, then they find many sites that claim the exact opposite: saturated fat is not only good for you but will boost testosterone and improve your health. When all is said and done, most guys just walk away from the subject figuring that there is no resolution. Now, before I go on, let me say that I am actually talking about the excess saturated fat that is in the typical American Diet. So keep that in mind as you read the detailed response I gave tto a reader, but the bottom line is this:
While saturated fat is slightly pro-testosterone - see my page on - it has many studies showing that it will lower nitric oxide levels, clump together cells in the blood and lower blood flow.
Of course, it DOES matter how much saturated fat that you have. But, if you have too much, it will hamper the ability of your arteries to relax unless you diligiently exercise post meal, and, yes, that will be bad for erections for men over about 45+. So this is the primary reason that you will see me opposed to higher saturated fat levels in the diet for most men in modern, urban settings. I am not saying saturated fat is consummate evil, but it can definitely be problematic for middle-aged and senior men, and this is the reason I flag it on this site.
Again, For those interested, read below my response to this reader:
Hi thanks for the quick reply. I've read through alot of articles lately that saturated fat is actually good for you, and that it was a "myth" that it was bad. Just curious... have you seen these articles?
This is a great question and one that I actually cover in depth on my site. However, Peak Testosterone has grown so large over time that it is probably difficult for guys to find out my coverage on the subject.
Below is a bullet point summary of the position that I take on the rather involved issues of saturated fat, cholesterol, diet, testosterone and the like:
CAUTION: Please talk to your doctor before making any changes, especially if you have any medications or medical conditions.
1. Constant Exercise Required. Some cultures do very well on a high saturated fat diet. These cultures, however, have very high activity and exercise levels and low stress levels. Studies have shown that post-meal exercise can overcome the negative arterial effects of high saturated fat diets. However, most men in Western societies do not exercise after every meal, so this is a moot point, except to those extremely athletic.
2. Decreased Blood Flow and Nitric Oxide. I'll let the research speak for itself:
a) "Consumption of a meal containing saturated fat reduced the antiinflammatory potential of HDL and impaired arterial endothelial function, whereas the antiinflammatory potential of HDL improved after consumption of polyunsaturated fat."  NOTE: Endothelial function is what you need for good erectile, because it refers to the ability of arteries to relax and allow lowered blood pressure and improved blood flow.
b) "Serum lipoproteins and glucose were determined before eating and 2 and 4 hours postprandially. Serum triglycerides increased from 94 55 mg/dl preprandially to 147 80 mg/dl 2 hours after the high-fat meal (p = 0.05). Flow-dependent vasoactivity decreased from 21 5% preprandially to 11 4%, 11 6%, and 10 3% at 2, 3, and 4 hours after the high-fat meal, respectively (all p <0.05 compared with low-fat meal data). No changes in lipoproteins or flow-mediated vasoactivity were observed after the low-fat meal."  NOTE: Again, vasoactivity refers to the ability of the arteries to relax or dilate and allow increased blood flow.
c) "The saturated fat diet was associated with enhanced noradrenaline sensitivity in small mesenteric arteries from VHF rats (VHF vs. VC, P < 0 05) and blunted endotheliumdependent relaxation in VHF and PHF rats (VHF vs. VC, P < 0 001; PHF vs. PC, P < 0 05). Endothelial dysfunction was attributable to a reduced nitric oxide component of relaxation in VHF rats, and blunted prostacyclin and endothelium-derived hyperpolarizing factor components in PHF rats." 
Again, if you're a young guy, you'll probably never notice. However, as you age, you'll probably see that these higher fat meals simply do not work for you.
3. High LDL-P and apoB In Some Men. As many of you know, LDL-P is the most important cholesterol number and NOT the standard LDL-C that your doctor pulls during your annual physicial. (Alternatively, you can use apoB, which is cheaper.) However, often high LDL-C and high LDL-P go hand in hand. And what often happens with men consuming a lot of saturated fat is their LDL-C and their LDL-P jump to overly high levels. High LDL-P can lead to deadly arterial plaque buildup (atherosclerosis) and so this is another reason I oppose high saturated fat levels, unless you are going to carefully monitor LDL-P (or apoB). I have many pages on the subject here: How to Regress Arterial Plaque and Reverse Atherosclerosis. (NOTE: I am not saying that all men on high fat diets have this issue.) Remember that plaque in your penile arteries means less nitric oxide down there and you know what that means.
One myth is that only small LDL particles will cause plaque. This is simply not true and abundant research shows that large LDL particles also cause hardening of the arteries: Large Particle LDL and Arterial Plaque Buildup.
By the way, some of you may protest and say, "What about the Masai?" The poster children for high fat diets are the Masai, who eat copious amounts of saturated fat. However, their cholesterol is around 150. The Masai have very low body fat levels and each pound of fat lost leads to a drop in cholesterol of about a point. One third of Americans are overweight and another third obese, so eating this much saturated fat will often lead to artificially elevated cholesterol levels - saturated fat does boost cholesterol and LDL-C. There are studies that show that saturated fat does not boost LDL by much. However, all I can tell you is that men on Low Fat Diets like myself see their LDL and cholesterol drop like a rock.
And virtually all primal societies had cholesterol of 150 or below and they had no heart disease. In contrast, in the U.S. and most modern societies, heart disease is, by far, the number one killer. Again, it's not actually not the total cholesterol number that is most important. It is your LDL-P that really counts and, to date, almost all studies showing plaque reversal involve very low LDL numbers. I give some thresholds in my link the LDL Levels Required to Lower Arteriosclerosis. And, by the way, the Masai actually were found to have significant atherosclerosis and are not known for tremendous longevity like the cultures that eat lower saturated fat levels, such as the Tarahumara, Hunza, Abkhasians, Vilcabambans, etc.
4. Insulin Resistance. If you consume enough carbs, consuming a lot of saturated (and total) fat will lead to massive insulin resistance. Researchers use high fat and high saturated fat diets to induce insulin resistance in research animals all the time! This link has been known for awhile and has been verified in humans on (nondiabetic) participants.  Insulin resistance is a male curse and is associated with Metabolic Syndrome, which in turn is associated with decreased testosterone, heart disease and erectile dysfunction. Again, if you exercise like a wild man, you can probably overcome insulin resistance. But how many of us can commit to walking four hours per day or spending two hours in the gym?
Also, guys who eat Low Carb may or may not have insulin resistance, due to their extremely low carb levels, and should test their post-prandial (post meal) blood sugar levels according to this page: Normal Postprandial Blood Glucose Levels.
1) Lipids And Cardiovascular Disease, August 2006, "Consumption of Saturated Fat Impairs the Anti-Inflammatory Properties of High-Density Lipoproteins and Endothelial Function"
2) Amer J of Cardiology, Feb 1 1997, 79(3):350 354, "Effect of a Single High-Fat Meal on Endothelial Function in Healthy Subjects"
3) The Journal of Physiology, 8 SEP 2004, 517(2), "Cholesterol-independent endothelial dysfunction in virgin and pregnant rats fed a diet high in saturated fat"