I have a vague recollection that Berg believed vegetarians were depriving themselves of much needed nutrients that could only be gotten by eating meat. If I remember correctly, he described them disparagingly as feeble and weak-minded. As with so many other issues, Berg the lunatic (literally!)and so-called prophet with a direct link to God was completely wrong.
Here's a couple excerpts from a great article answering ten of the most common misconceptions about vegetarianism.
"As for having strength and energy on a vegetarian diet, some of the world's top athletes are vegetarian. A few examples: Carl Lewis (perhaps the greatest Olympian of all time), Robert Parish (one of the "50 Greatest Players in NBA History"), Desmond Howard (Heisman Trophy winner and Super Bowl MVP), Bill Pearl (professional bodybuilder and four-time Mr. Universe), Jack La Lanne (Mr. Fitness himself) and Chris Evert (tennis champion). Vegetarian athletes have the advantage of getting all the plant protein, complex carbohydrates and fiber they need without all the artery-clogging cholesterol and saturated animal fats found in meat that would slow them down. In fact, Carl Lewis says that "my best year of track competition was the first year I ate a vegan diet."
"I highly doubt that anyone is going to suggest that vegetarians Steve Jobs, Leonardo da Vinci, Pythagoras, Albert Einstein, Leo Tolstoy or Mohandas Gandhi were especially brilliant because they were vegetarians, and I also don't think one can make the argument that meat eaters attained their great heights as a result of their diet. Interestingly, studies show that vegetarians are smarter than meat eaters, but there is probably not causality there -- it's probably just that thoughtful people tend to question things more deeply, hence the decision to become vegetarian. Here's a 2006 study from the British Medical Journal about vegetarians being smarter than meat eaters."