Dr. M - "Glassman Is Right"

“…high-intensity interval training cuts down on your exercise time so dramatically. You're actually getting MORE benefits from high-intensity training than you do from aerobic/cardio, in a fraction of the time—all because you're utilizing your body as it was designed to be used. You can literally be done in about 20 minutes, compared to spending an hour running on the treadmill.

"… [T]he exercise physiology world has created an inextricable link between the aerobic metabolic system and the cardiovascular. But that's not true at all. There's no way that your heart and blood vessels are hooked up only to the mitochondria. The heart and blood vessels support the entire cellular metabolism," Dr. McGuff says. "The best way to get that benefit is with high-intensity intermittent exercise."

If you give it some thought, it's actually easy to see that your body was designed for high-intensity, short-interval exercise. As Dr. McGuff says:

"… the issue isn't necessarily the running for hours and hours and hours. It's the modality itself. You will never, in nature, see an animal jogging… What the steady-state activity does is it trains the plasticity out of your physiologic system—that ability to handle widely varying levels of exertion within a short span of time gets trained away. You actually make yourself less plastic and less adaptable to physical stress in general."

http://fitness.mercola.com/sites/fitness/archive/2012/01/06/dr-doug-mcguff-on-exercise.aspx?e_cid=20120106_FNL_art_1

It is interesting to see mainstream, relatively speaking, medical folks saying, in essence, "CrossFit makes perfect sense."  It works that way often - a practitioner finds something that works, and eventually, everyone else says "of course it does."

Another interesting nugget:

Your body's need for sugar is, biologically, very small. And when you consume more than you need, your body turns it into fat. As I've stated before, you do not get fat from eating fat—you get fat from eating too many carbs (sugar).

Dr. McGuff explains:

"Your skeletal muscle – if you're lucky – can hold maybe 250 grams of glucose, and your liver holds about 70. If you take 320 grams of glucose as what your storage capacity is, you can kill that with a single trip to Starbucks. Once you go beyond that, your body is going to find some sort of way to deal with those excess carbohydrates.

If your glycogen storage is full, your body has nowhere else to put it. So instead of going all the way through this metabolic pathway, it… produces body fat. That's called the novel glycogenosis. We are in the midst of a very bizarre, evil-scientist type experiment in the Western world, because we are dumping into our bodies an amount of carbohydrate and, in particular, refined sugars, that are way above the capacity of our metabolism to handle normally."

Simply put, intensity trumps duration.