I had a reader write in with the following question having to do essentially with testosterone and nut consumption:
"I normally eat almonds, although I purchased Walnuts ...[and]...I noticed that they have a lot of Polyunsaturated fats! Do the benefits of eating the walnuts outweigh the poly fats! Or are the poly fats desired! I thought I read on your website that poly fats decreased testosterone!"
This is actually a very involved subject and one that would take a small book to answer fully. However, let me give a few key points on the subject:
1. Yes, polyunsaturated fat is in general associated with decreased testosterone as is total fat. Thus, eating a lot of polyunsaturated fat would probably not be a good idea for most guys not on HRT. However, eating a small amount of nuts is unlikely to cause a significant reduction in testosterone. This was born out in a study of senior men that found no significant reduction in bioavailable testosterone from eating 75 grams, a little over 1/2 a cup, of walnuts per day.  (A study on women with PCOS showed an increase in SHBG and corresponding decrease in free testosterone.  However, PCOS women tend to be overweight with overly high testosterone levels.)
2. Nuts are part of some of the most healthy diets in the world. The Abkhasians, one of the supercultures of the world with almost no heart disease or cancer, eat a steady supply of nuts. Nuts are also part of many Mediterrean Diets and the Mediterranean Diet is both pro-erection, pro-brain, pro-testosterone and pro-just-about-everything you hold dear as male.
3. Nuts boost Nitric Oxide and also can improve blood pressure and blood flow. All of this is pro-erection even without raising testosterone levels.
4. Nuts are not part of an Ornish-style Low Fat Diet. A Low Fat Diet can literally clear out your arteries and would be appropriate for many middle-aged and beyond guys. However, eating a few nuts is unlikely to skew results significantly in my opinion. A typical low fat diet is about 10% of calories as fat and adding a 1/4 cup of almonds, hazelnuts or walnuts to one's diet will only change one's diet to about 14% fat.
5. While it's true that a Low Fat Diet may in the short term diet lower testosterone a little, there is good evidence that this type of diet actually preserves testosterone decades beyond what someone eating a typical Western Diet achieves. See my link on How to Avoid Andropause for more details.
6. There are many nitric-oxide-promoting foods and drinks and so there is no need to consume nuts. Keep in mind, though, that nuts also have an abundance of certain hard-to-get nutrients, such as Vitamin E as well.
So the bottom line is that nuts are likely relatively neutral with regards to testosterone and free testosterone but have many nitric oxide-promoting properties, making them a valuable weapon for most middle aged and beyond guys. (For those on a strict Low Fat Diet, nuts will not be allowed.)
2) Nutrition Journal, 2008, 7:13, "The effect of walnut intake on factors related to prostate and vascular health in older men"
3) European Journal of Clinical Nutrition, 15 December 2010, "Differential effects of walnuts vs almonds on improving metabolic and endocrine parameters in PCOS"