LOREN CORDAIN AND JOE FRIEL
THE HOOK: The author of The Paleo Diet (eat like our Stone Age ancestors) teams with endurance-sports coach Joe Friel to tailor it for active types.
THE DIET: Lots of game meats, raw produce, no refined anything except during endurance sports, then bars, bagels, gels, etc.
NO. 1 LESSON LEARNED: It's tough to exercise when you're hungry all the time.

I understand now why Atkins adherents are so passionate about their meat-heavy lifestyles. I lost six pounds on this plan, and my body-fat percentage went from six to five. I also understand why Atkins defenders get so cranky: I spent all but the last two weeks of this diet feeling seriously underfueled—tired, spacey, and hungry.

What I did eat was delicious: elk, buffalo, salmon. But the recipes require a fairly knowledgeable hand in the kitchen—and a stomach for organ meat. (I skipped the heart, tongue, and testicles.) And unless I was exercising, I wasn't allowed much in the way of carbs. I grew so tired of the approach that boredom or upset stomachs would end meals before I had eaten enough. I found myself looking forward to exercise because I could use it to excuse a granola bar.

Co-author Joe Friel admits to feeling weak during his first two weeks on the diet but says he started coming around by week three. It took me three times as long to feel somewhat normal. Halfway through, I broke down and bought an egg-cheese-and-potato-laden breakfast burrito, writing in my food diary, "Woke up with a hunger way beyond what the fruit and vegetables in my house could cover."

Still, my ratio of good cholesterol to bad cholesterol—possibly more important than your total number—was close to the best I recorded all year. There's definitely something to this approach. But as an active guy who's not looking to lose weight, there's just no way I'm going to stick to a plan that leaves me hungry and tired all the time.