I'll have to hunt for it. It was a shock to me too, but I've seen it in multiple places. Numerous medical studies. Red meat apparently inhibits testosterone production. Some of the foods that increase it are surprising: shrimp,strawberries and coconut. I already knew about pumpkin seeds.
Not many athletes eat much red meat any more during their seasons- since it also takes about 48 hours to digest- more blood goes to the gut to help digestion instead of to muscles. Most athletes eat a lot more chicken and fish instead since it digests quicker. Pretty common knowledge for anyone who works out much or plays a sport.
Back in the 60's, marathon runners used to eat steak before they ran. Nothing wrong with a little red meat,in moderation. But if you're seriously training, you gotta be careful when you eat it.
