What many men do not realize is that the size of their meals, especially certain macronutriet components, can strongly influence their testosterone levels for hours afterward. What the research shows is that you can turn your meals into an "anti-androgen" pretty easily. Here are just a few ways to do it:
1. Too Much Fat. One thing that has intrigued researchers is how and why a high fat meal lower testosterone levels for hours afterward. One study wrote that "postprandial [post-meal] testosterone concentrations have been shown to significantly decrease after a fat-rich meal, which may be due to inhibition of testosterone production by chylomicrons."  Chylomicrons are simply lipoproteins that include things like cholesterol and triglycerides. These same researchers looked at high fat meals in the 65-85% range and found that total and free testosterone were reduced by 22% and 23%, respectively. And, even more significantly, they remained lowered for 8 hours afterward!
The researchers already knew the results, because about 10 years earlier researchers found that a high fat meal very significantly lowered testosterone but a "mixed meal" had a negligible effect on testosterone.  The mixed meal was "mixed carbohydrate and protein with minimal fat." Therefore, carbs are kept to a reasonable quantity and fat is minimized. One interesting thing is that one study showed that when participants were given just fat, their testosterone did not change appreciably. 
2. High Glycemic Load Foods. There are a couple of studies now that show that consuming glucose significantly lowers testosterone post-meal. In fact, the above study that showed that straight fats do not lower testosterone also showed that dextrose, which is essentially glucose, lowers testosterone.  I cover another study on my Testosterone and Glucose page, where participants were actually given glucose and saw their testosterone drop by about 25%. From these studies, we can conclude that high glycemic foods likely have the potential to lower your T significantly.
3. Rapid Weight Loss. Lost of fat or high glycemic foods can really lower your testosterone, but the king is probably the exact opposite: food deprivation. One study on wrestlers who were attempting to drop weight rapidly before a match showed a testosterone drop of over 60%!  Ouch! These were healthy, fit young men and yet their testosterone was drastically dropped by dieting. One study suggests that a reduction of more than15% of your baseline calories will begin to lower your testosterone levels.
4. Caloric Restriction. And what happens when you lower calories significantly for long periods of time? Does the body recalibrate and testosterone levels readjust. The answer appears to be 'no'. Caloric restricion is an anti-aging method that has had rather lackluster results in primates. Basically, the idea is that you eat a lowered amount of calories such that you reduce your body weight and reset your body's basal metabolic rate. Researchers decide that his could be instructive in determining just what reducing calories in the long term can do to one's testosterone levels. One study looked at men who had been practicing CR an average of 7.5 years and matched them with controls eating a typical Western Diet.  The study showed that long termcaloric restriction resulted in a 32% drop in testosterone levels. (There could be other factors involved, since CR practitioners tend to eat more fiber, less fat, etc. than the typical Western Diet.)
LOW FAT DIET: Notice what diet is unlikely to decrease your testosterone post meal: the kind of diet that I eat on a regular basis. I eat a kind of hybrid Low Fat Diet that is based on supplemental protein, for muscle building purposes, coupled with a low glycemic load diet. The extra protein that I consume should put this in the category of the "Mixed Diet" above that preserved testosterone post-meal. This is because carbs are reasonable and low glycemic and fat is low. And remember: a low glycemic, Low Fat Diet has a good chance of regressing both your prediabetes/diabetes AND arterial plaque, something I cover in my page on Low Fat Diets and Diabetes. However, some studies show that an increase the protein/carbohydrate ratio leads to a significant decrease in testosterone levels. For example, one small study of normal, healthy men showed a 21% in testosterone levels as the protein-to-carbohydrate ratio was increased. 
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SELF-TEST BEFORE AND AFTER: The bottom line is it seems clear that the extremes in meal macronutrient selection will often lower your testosterone significantly. However, these changes are poorly understood and probably have considerable individual variation. For this reason, it is probably wise to monitor your testosterone before and after any major changes. For information on labs that have been brought up on the forum, see my page on Testosterone Testing Labs. (I have no affiliation with or personal knowledge of these labs, so do your own due diligence.)
1) Metabolism, 2001 Nov, 50(11):1351-5, "Effects of a high-fat diet on postabsorptive and postprandial testosterone responses to a fat-rich meal"
2) Metabolism, 1990 Sep, 39(9):943-6, "Effects of a fat-containing meal on sex hormones in men"
4) Aging Cell, 2010 April, 9(2):236 242, "Long-term effects of calorie restriction on serum sex hormone concentrations in men"
5) Life Sciences, May 4 1987, 40(18):1761 1768, "Diet-hormone interactions: Protein/carbohydrate ratio alters reciprocally the plasma levels of testosterone and cortisol and their respective binding globulins in man"
6) Karila, TA., et al. Rapid Weight Loss Decreases Serum Testosterone. (2008) Int. Journal of Sports Medicine. May 30