It may seem a little odd that Paleo dieters might need to avoid meat. Many Paleo diets scoff at the idea that you should be careful about eating meat. Some will offer the retort that sure, grass-fed organic meat is best, but surely the human body was designed for eating meat, and eating meat, if you follow the basic parameters Paleo Diet plan (that is, don't eat cured meats, processed meats, or high-fat meats), is fundamentally healthy.

Well, yes and no. Eating meat poses some of the same issues as eating wheat. And many kinds of meat contain neuroactive chemicals known as mu-opioids that literally can change the brain in response to their consumption.

This Is Your Brain on Hamburger

These brain-changing chemicals are known as the mu-opioids. Digested from meat, especially from beef blood, mu-opioids circulate through the bloodstream and eventually land in the brain, where they lock onto some of the same molecule-sized docking ports that respond to opioids from drugs such as the eponymous opium and also morphine, codeine, Vicodin, oxycodone, and several dozen other drugs.

When these receptor sites in the brain are activated, the production of pain chemicals is blocked and the production of pleasure chemicals is activated. That's why chowing down on a cheeseburger feels good. The mu-opioids relieve pain, they enhance a feeling of well being, and they have a number of interesting effects on the health of the digestive tract. Eating beef, for people who are especially sensitive to mu-opioids, triggers constipation. It slightly slows down the rate of breathing. In especially sensitive people, eating meat can even result—no kidding—in an altered state of consciousness, in which the beef eater “zones out” and indulges in a happy dream-like state

Mu-opioids make beef addictive. We feel so good after we consume beef blood that our brains make a note of where to find it. You know what they sell where there are golden arches, don't you?

Different people react to mu-opioids from beef blood with varying intensity. Some people crave beef, especially rare beef, and some people aren't especially affected. But if you are going to eat beef while you are on the Paleo Diet diet, be especially sure you don't eat a combination of beef and wheat: No hamburgers or cheeseburgers when you stray from your diet plan.

The combination of beef and wheat is especially addictive (and, as you'll see in a later article, adding cheese makes the problem even worse). Surprisingly, scientists at the Nutritional Physiology Research Centre at the University of South Australia tell us, eating lean pork on a regular basis doesn't present these problems. Simply switching from beef to lean pork without any other dietary changes, their six-month clinical study found, resulted in:

• Lowered appetite, enough to reduce total calorie consumption from 8690 to 8180 kJ/per day, which is about 125 calories per day,
• Slightly lower total consumption saturated fat, even though half of the pork consumed was in the form of sausage,
• Slightly lower systolic blood pressure (about 2 mm Hg), without introduction of new medications,
• Slightly lower diastolic blood pressure (also about 2 mm Hg), also without introduction of new medications,
• Drastically improved heart rate, about 12 beats per minute slower (the average pulse down to 60 beats per minute),
• Slightly lower LDL cholesterol, about 0.1 mmol/L (4 mg/dl),
• Unchanged HDL cholesterol,
• Significantly lower triglycerides, about 0.2 mmol/L (5 mg/dl), and
• About a 4-pound (2-kilo) average weight loss, lean body mass actually increasing, all of the weight loss coming from fat loss.

None of the 72 participants in the study reported any new digestive problems, and the very significant decline in average heart rate suggests that all kinds of inflammation, including digestive inflammation, were likely to have been reduced.

The amount of pork consumed by the participants in the study was just 1 kilo (2.2 pounds), about 7 servings per week. The sausage eaten in the study was fresh sausage, the kind you make from minced pork in your own kitchen, not processed sausage, and the diet specifically excluded highly processed pork products such as hot dogs and SPAM. But what if you just don't eat pork–or beef blood, either?

Paleo Pitfalls for People Who Keep Hallal or Kosher

Even if you don't eat blood and your don't eat pork, you can run into real problems with eating too much meat. Certain meats and protein foods prime your brain to want more. The most addicting foods are those that are especially high in the amino acid phenylalanine:

• Egg whites (although egg yolks don't have this effect),
• Seal and whale meat (certainly “paleo,” although probably not on your shopping list, but problem foods nonetheless),
• Sesame seeds,
• Sunflower seeds,
• Salted cod, and
• Whitefish, the kind of fish that Americans get at Long John Silver's and many other people in the English-speaking world consume with “chips.”

These foods are surprisingly addictive. Limit your consumption of bloody beef, egg whites (yolks are OK), sesame seeds (mmm, sesame seed bun), sunflower seeds, salted cod, and (especially battered and fried) fish (especially with fries), and it will be a lot easier for you to stick to your Paleo Diet diet plan.