This is a great question!

My opinion is that it goes both ways: Mental state <--> Physical state.

In my prior years of practicing Japanese Buddhism, the concept of "shiki shin funi" was emphasized. This is often thought of as the "oneness of body and mind". But actually, it literally translates as "Body, mind, not two". The meaning is that body and mind are two and not two at the same time. Reminds me of the wave/particle duality. The point is that the mind and the body cannot really be in completely different states - a disease of the body affects the mind and a disease of the mind affects the body.

if you eat badly, you're mental state is bound to deteriorate as well as your physical state. Just ask Dr. Emily Deans! A good mental state should lead to better actions and a better physical state as well.

Maybe Jack Kruse is right, your thoughts can change your gene expression! Actually, as loopy as he sounds, he is not incorrect on that point.

PS. There are two additional "funi's": self/environment and individual/society. I think these concepts are completely congruent with science (and paleo).

The mental aspect must not be underestimated, it is always a huge factor (as Korzybski's experiment has shown). Or just think about how often placebos work.

The physical aspect, on the other hand, must not be underestimated either; if you put trash into your body, thinking it will do you good, it will still destroy your body in the long term.

I actually advocate a less dualist approach; you can't separate the mental from the physical aspect and vice versa. What you eat influences your mental state, and your mental state influences how your body functions and react to what you put into it. Both aspects are just critically important for becoming (and being) a healthy person.

This question is, in fact, fairly similar to another one I saw here on PaleoHacks: "are psychologist needed". While you focus on the mental state, he was just focusing on the other aspect (diet).

I'm 17 and have been Paleo for a little over a year now, but I do cheat when I go out with friends because I'm still young and I want to enjoy myself. It's all a matter of what you want to get out of being Paleo.

Great question. There are lots of opinions about how much mind can influence matter.

The National Cancer Institute has this to say about how stress affects the body:

The body responds to stress by releasing stress hormones, such as epinephrine (also called adrenaline) and cortisol (also called hydrocortisone). The body produces these stress hormones to help a person react to a situation with more speed and strength. Stress hormones increase blood pressure, heart rate, and blood sugar levels. Small amounts of stress are believed to be beneficial, but chronic (persisting or progressing over a long period of time) high levels of stress are thought to be harmful (1).

Stress that is chronic can increase the risk of obesity, heart disease, depression, and various other illnesses. Stress also can lead to unhealthy behaviors, such as overeating, smoking, or abusing drugs or alcohol, that may affect cancer risk.

Science Daily reported back in 2007:

In a review of the scientific literature on the relationship between stress and disease, Carnegie Mellon University psychologist Sheldon Cohen has found that stress is a contributing factor in human disease, and in particular depression, cardiovascular disease and HIV/AIDS.

To me the answer is, of course your mental state affects your digestion since to me it's clear that mind and body aren't these two separate things.

I asked a related question a little while back: Has your state of mind affected your digestion? A few people reported their personal experience that speaks to your question.

I very much recommend the book "The Slow Down Diet" by Marc David. It's all about the relationship between how we eat and how we metabolize food.

Great question, my opinion on this is very simple:

everything is physical.

"Mental" is just our way of expressing physical realities that happen in our brain and "feel" like thoughts or moods to us.

In other words, "mental" things like thoughts and feelings are actually sensations caused by physical chemicals and impulses. I think Freud did a huge disservice by reinforcing artificial distinctions between mental and physical states.

You might say that that the mental can overcome the physical, but what is really happening is that the initial mental state is overcoming the new physical state.

I always wonder why more people don't accept this, given that we have proven without a doubt that "mental" states can be directly influenced by chemicals and other physical stimuli (i.e. ssris, other drugs, even visual stimuli!

So the feeling the students had after seeing that the cookies were intended for dogs was physical. A physical visual signal was converted into an electrical impulse that was directed at something that produced a physical feeling of illness.