Supplements are interesting to the HIT bodybuilder, because most are designed for people involved in high-volume training. It′s important for them because the body is subjected to far more than usual stresses, especially on the recovery systems. For example, studies show that after 40 minutes of vigorous exercise, the body begins to produce cortisol, and the longer the exercise lasts, the more cortisol is produced. This is not a concern for HIT trainees, because the routines are designed to be completed in far less than 40 minutes.
MM indicated in his Heavy Duty Nutrition book that a balance diet is by far the best way to a good physique. Even the largest bodybuilder can progress with good food and rest. However, there are some supplements that might benefit a HIT athlete. Based on the cortisol example, you may not need to worry about it after workouts, but every human has high levels in the morning when they first wake up, due to an all night fast. Therefore, some people advocate taking BCAAs right away with breakfast to give the body some other chemicals to feed on. BCAAs are claimed to be good because they are the most abundant amino acids in skeletal muscles, and will therefore be available for protein synthesis, and muscle growth.
Whether this is true or not is a great matter of debate.
However, MM did advocate taking a daily multi-vitamin if there was any question about the content of your diet.
Personally, I think that is a good idea, because N. American crops were recently revealed to be 40% lower in nutrients than they were 50 years ago. This is mostly due to rushed harvesting, and longer shipping routes, making for less rich foods.
There are only two other supplements I would ever advise taking - l-glutamine, and creatine. Glutamine is great for the joints, as well as brain and colon function. And creatine is good for strength and ′biovolumizing′, meaning it draws water into the muscles, making them look bigger.
There are thousands of products out there that you could spend you money on. But you probably are not being payed to take them like the big pros are, so use your cash wisely, and realize that there is no substitute for healthy eating.