It's good to know that there are top athletes who are vegetarians. One of them is Dave Scott who is 6 times triathlon champion (though, I also heard that now in his retirement, he started eating fish)
I recommend all to use webs as your knowledgebase. Go to
http://www.wholehealthmd.com/reflib/1,1529,,00.html and you can check the nutrition value of every food (almost). You will be surprised that some of nuts and veggie have as much protein as red meat without the negative effect. For instance: 3 ounce broiled sirloin has 24g of Protein, but comes with 66mg of Cholestrol. 1 ounce of sunflower seed has 7g of protein with zero Cholestrol. Please note that plant based protein usually do not give a complete Amino acid and therefore it's good to have variety of plant based food to get the 8 essential Amino acid.
For this past 12 months, I have been learning alot from websites and books about nutritions and discover that there are many theory out there. Lots of them are similar and some are contraversial. I do not think that there is one absolute diet theory as everyone is guessing about how our body work. Even the pros are guessing and the one who has much funding can do scientific experiments. Most experiments are done in animals and it is important to take into account that animals (i.e. rats, mouse) need different nutrient than human. It's only our creator that knows exactly the right diet. So it's best to be open minded and see the improvement yourself whether it works or not.
I have been fighting skin condition (psoriasis) for the past three years. After my 1.5 years with 2 different top notch dermatologist without any result (getting worst), I gave up and went to look for natural way via diet. I found several programs from books and websites. Last year, I started with whole food diet, supplement and cleansing - no red meat -only fish and chicken - no processed sugar and flour. It improved my skin by 10% and then stop improving and reached plateu. Meanwhile, I lost 20lb without wanting to loose weight (since I only weigh 130lb to begin with -5"2')
I began to have more energy and started running. In search for the right cure for my psoriasis, this past thanksgiving week, I started a new diet/lifestyle: Not only vegetarian, but I also eat everything raw. This is a biblical based diet (hacres.com), so it is totally based on your faith whether you believe this or not.
I eat raw of these:
-mostly vegetable (vegetable juice recommended)
-15% fruit
-Mix my own trailmix with raw nuts and unsulphured dried fruit
-With critical supplements: barley grass powder juice and carrot powder juice (I don't have time to juice carrot everyday)
After 3 weeks on vegetarian, I still do not have clear result on my psoriasis yet, but the scaled skin is getting much softer.
Now, with my raw food life, I also started bike to work (even in the winter) 9 miles each way. I still have energy to go to gym at night for my upper body strength for my bad back. Since I stopped eating processed flour and sugar over a year ago, I have not gotten any germ/virus borne illness. This is due to my healty diet and therefore healthy body does not tolerate virus or bad bacteria living comfortably in my body.
Though, I am still new in vegetarian - I discovered that what I believed before: cannot survive without meat as source of protein was not true.
Hope this help. Wish the best for your vegetarian lifestyle.