As far as I'm aware there are no biological or physical benefits to 'cheating' on your paleo diet. So physically 100% is best. The 80/20 principal, or 90/10 is for your mental health. For fitting into society, taking a break, treating yourself.

Perhaps the real question is: Can you handle being 100% paleo? In this day and age with all the marketing and inundation of carb'd options, traveling and hectic work schedules, can you feasibly be 100% paleo. If Mark Sisson has planted the 80/20 in your mind, realize that his point is not to plan to be 80% paleo. Rather that you should cut yourself some slack if life's inevitable hand keeps you from being able to eat paleo you won't punish yourself with guilt.

It depends on what is in that 20%. Back when I started my 20% was things like candy...and of course that made me feel crappy. Luckily, since I was eating mostly paleo, I was able to recognize that feeling crappy really was caused by junk.

These days my 20% is properly fermented non-gluten grains and legumes- dosas, traditional grits, miso, buckwheat pasta, etc. These are traditional foods of healthy agrarian societies. They aren't as good as grass fed beef or wild salmon, but I sincerely doubt that they cause real problems.

I suspect that it has more to do with how you define 20%.

20% carbs in an otherwise fat-heavy paleo meal? The carbs will drive insulin production, and end up storing a lot of that meal as fat.

One non-paleo meal/day a week? Probably not as impactful. Plenty of people see health improvements from doing intermittent fasting without adjusting their macronutrient intake, and IF and Paleo low-carb have similar effects on insulin.

I also suspect it's easier to regulate an off-day than an off-percentage. I can see 20% creeping up pretty easily.

I feel that in going from 80% paleo to 100% you may face the "law of diminishing returns." The effort required to get to 100% paleo may yield less than a corresponding 20% health benefit. This is similar to what Kurt Harris describes in his PaNu 12 steps (see http://www.paleonu.com/get-started/ ). And of course the basis of the question comes from the "80/20 rule" of Mark Sisson. I've found that "bearing down" on my workouts or further refining my diet can sometimes get me over a plateau in my health or fitness goals, but I have a ways to go before I'm 100% paleo.

If you're talking about being 20% less paleo then yes, in abstraction you'll get 20% less of the benefits.

On the 20% more carb rather than, say, zero carb question, there are lots of arguments that cutting out that last bit of (non-paleo) carb could have positive effects. There are some arguments that there's a threshold effect, such that you need to lower carb a certain amount before you see any real benefits. (This is why a lot of scientific studies on "high fat" diets are rubbished- because they contain 20% sucrose!)

How much causes problems will vary. For most, a single cookie on a paleo diet won't make much difference, other people swear they stop losing weight if they so much as look at an apple. It will depend on your activity levels, whether your metabolism is broken etc.

Of course it goes without saying that although you frame this as 80:20 vs ZC, a lot of the downsides will come from the grains themselves, not the carb, making it impossible to quantify. If you're gluten intolerant then a surprisingly small amount of grain might be important.

One thing you may need to watch out for in that 20% is that you really still need to avoid gluten and legumes as much as possible. These have long lasting(weeks) negative effects on gut permeability. Antibodies to gluten have been shown to persist for weeks after exposures, and some say it can affect you for up to a year. Dairy, white rice, and corn seem to be less detrimental cheats, so you are going to go for that cheat meal, those are better choices than bread and other grain products.

Aim for the 100%, but don't obsess if you 'fail' (what a bad word-choice). If you notice that one particular fail/cheat causes a bad reaction, try to avoid it in the future.

I have repeatedly noticed that a slight amount of grains causes my psoriasis to get worse, so I really try to avoid it.

On the other hand, eating real dark chocolate in greater amounts doesn't give me the physical signs, but makes it harder to stay away from sweets and carbs in general the entire week afterwards. That's tough mentally ;-)

By my definition, I cheat very little. However, I do not consider wine, dark chocolate, potatoes, limited white rice, some corn, dairy nor traditionally prepared beans to be cheats. BTW, I don't call myself Paleo, Primal nor Archevore but I am closest to Primal.

I want very much for Paleo/Primal/Archevore to change conventional wisdom. That could have enormous benefits to society. IMHO, only a looser, 80/20 approach could ever take hold in society at large.

You're not going to find any academic research in this regard, if only because so little research has been done on paleo and none of it comparing controlled caloric intake of paleo vs. any other diet. Second, what does the other 20% consist of? If you're of the dogmatic paleo persuasion, you frown at legumes, dairy, and grains. However, the longest lived civilizations all include these in their diet so I'm not one to poo-poo complex/whole versions of these foods. That's a long way of saying: if you're 80% there, miring in minutia isn't going to (likely) make much of a difference.

Ultimately, it depends on you and how your body reacts to that "20%".

My own two cents would be that it prolly depends more on your current state of health. I mean, if youre already fit and lean im sure you can rock that 80/20 with dark chocolate, alcohol, grains, etc with no visible downside. I say visible. I happen to believe that those cheats will indeed affect your insides TO SOME EXTENT. I dont think youre going to get cancer, or that they would even show up on some bloodwork (like the OP mentioned), but i do believe that whatever you putin your mouth/body is going to affect the body to some extent. For the most part its dosedependent id say though so again, prolly not a big deal.

However, if youre coming at paleoprimalgrok-whatever from an overweight outofshape position than i think the 8020 is going to make it that extra 20% (or however much it actually ended up being) harder. I tend to be on the side of "be 100% disciplined in getting the fundamental basis that youre aiming to establish done. Once thats in place, you can play around; ie have some grains, have some yogurt, blahblah"

Potatoes are paleo! not a cheat food.

Eating glucose doesn't put you in glucose burning mode!!!!! You'll only burn off extra glucose you ate but the majority will be stored in glycogen reserves. No one is ever burning pure glucose or burn fat it is always a ratio. The ratio can't be changed by what you eat!

Nice facts/opinions by all. What intrigues me is the face that many people I talk to explain themselves as being 100% paleo therefore indulging in 0 carb intake which is very inaccurate to say the least. Naturally carbs are in many vegetables and green leaf that we take in every day. I feel that we are trying to construct the "perfect" diet. Your diet needs to be very specific to what you are trying to do, for example; before you post your opinion, explain where it is coming from. Are you a crossfitter? Body Builder? Marathon Runner? Or the every day average fitness guru. It will help the people asking the questions learn more from your insight. I perform many different types of cardio from fat burn, sustained long distance running, to performance base cardio such as sprints or crossfit WOD's. Depending on the focus for that week or that month I will base my diet, deciding that week if I'm going to be 80/20, 90/10 or 100%. You guys are awesome and you your motivation on this subject is amazing!! Motivated, Dedicated, rocfit

I didnt think potatoes were paleo