How should vegetarians (sedentary, active, and athletes) use supplements?
First, start with the very best diet. Make it a priority to eat the very best foods and avoid processed foods such as wheat flour, sugar, etc. Eat often enough: for many active people, this means every 3-4 hours (more often if stress levels are high). And, get adequate high quality protein. This is more difficult if you're vegetarian. Eggs are perfect, if you eat them. Dairy may be good too, with raw and organic being best.
As discussed in the book, balancing fats is one of the most important issues for optimal health and fitness. For most people, this means obtaining additional EPA (from omega-3 fat) from supplements. The best choice is fish oil because it already contains EPA. If you don't consume fish or fish oil capsules, it's difficult to obtain sufficient EPA from flax because the body does not convert much of it.