In our Cheese show we considered the suggestion from the President of the Physicians Committee for Responsible Medicine (PCRM), Neal Barnard, that dairy cheese is addictive. In this 2003 article entitled “Breaking the Food Seduction”, introducing his book of the same name, Dr. Barnard writes:
Cow’s milk—or the milk of any other species, for that matter—contains a protein called casein that breaks apart during digestion to release a whole host of opiates called casomorphins. A cup of cow’s milk contains about six grams of casein. Skim milk contains a bit more, and casein is concentrated in the production of cheese.
I looked into the references from Dr. Barnard’s book that PCRM was kind enough to send. But the analysis I offered in the show was that the idea that cheese is addictive is at best overstated and at worst wrong.
This is why.
First off, a quick biology lesson. We have receptors throughout the cells of our body including our brains. Receptors are like locks that only specific keys can open and cause effects in the cell. We have natural opiates produced by our bodies (e.g. endorphin) . An opiate agonist is a chemical that binds with the opiate receptor and causes effects like analgesia ( decrease in pain sensitivity), feelings of euphoria and can create tolerance and addiction. It is triggered when an opiate molecule, of a specific size and shape, fits like a key into that receptor’s lock. However, there are some molecules that fit into the receptor but don’t activate it. Opioid antagonists block opioid receptors from activation. As Dr. Barnard’s essay suggests, opioids might regulate preferences for foods high in fat and sugar not because these foods contain opioids but because eating them releases natural opioids (Drewnowski et al 1992).
So Dr. Barard states proteins in dairy are broken down into opoid molecules called casomorphins. This is definitely true. However, in the breakdown of dairy, opioid antagonistsare also produced which would have the opposite effect and lead cheese to not be addictive. This differs from cheese to cheese (Sienkiewicz-Szłapka et al. 2009):
I would conclude that if these agonistic casomorphins are really important for causing an addiction to cheese we would see a big difference in the popularity of cheeses depending on the ratio of agonistic to antagonistic casomorphins. Unfortunately the authors of this study did not test cheddar – one of the most popular cheeses in the world – but if it’s anything like the other semi-hard cheeses in the study we would expect it to have a greater percentage of opioid antagonists.
The next problem with the idea of cheese addiction is the question of whether or not the opioid peptides (protein fragments) could pass through the gut into the bloodstream and the brain.
There is evidence that these peptides pass through the guts of infants (casomorphins are also found in human breastmilk) and through their immature blood-brain barrier (which keeps lots of stuff out of the brain that circulates through the blood) but has there been any evidence that they cause morphinelike effects?
There has been a case study linking cow’s milk consumption and sleep apnea but in a larger study of infants who were breast or formula fed they found no difference in average psychomotor development between the two groups.
Another study found markers of human beta casomorphin 8 in the brainstems of human infants. (To correct something I said in the show, these casomorphins are from human breast milk and not dairy).
- formula fed infants are much more sensitive to peptides
- formula fed infants eat a larger proportion of dairy in their diet than most adults (unless someone subsists entirely on dairy)
- these peptides would be working on a much smaller body mass than adults
- there still isn’t definitive evidence that they are having morphinelike effects
This leads me to think that it is improbable that dairy has addictive properties in adult humans.
There is evidence that casomorphins pass through the gut of adults but, as far as I could find, no evidence that dairy has opiatelike effects on adult humans. The only thing that came close was this study showing a lowering of blood pressure in those who consumed “sour” milk as opposed to regular milk – however the difference between those milks was not attributed to casomorphins but to other peptides.
One main effect that opiates like morphine have is analgesia (decreased sensitivity to pain). If dairy has enough opiate peptides to create addiction one would expect that dairy would also decrease pain sensitivity. However, I was unable to find any studies relating dairy consumption and pain sensitivity and during my postdoctoral research at the University of North Carolina working with pain data I never saw any questions about dairy in the protocol.
This blog has been pretty far afield from my scientific speciality and I’m grateful for any criticism of my methods or conclusions. But three main pieces of evidence suggest to me that Dr Barnard is in error about the possible addictive properties of dairy.
- Dairy has both agonistic and antagonistic opiate peptides and there is no evidence that products with more agonistic peptides are more popular
- The effects of bovine casomorphins in infants, those who should be most sensitive to them, are small
- Dairy hasn’t been shown to have opiatelike effects in adult humans
So, why do people have such a hard time giving up cheese? Because fat is high in calories and throughout our evolutionary history, when calories were much scarcer than they are today, fat was the most efficient way to get your calories for the day. Because of this, these foods, and some others (e.g. sweet foods, salty foods), may have a reward system that makes us feel good when we eat them promoting seeking out and eating these foods which throughout our ancestral past led to reproductive success. Nowadays sugar and fat are no longer as rare as they once were but our reward system still amps up when we eat them (see mismatch hypothesis)
I think this is a simpler explanation in addition to explaining my “addiction” to almond butter, dark chocolate and avocadoes. mmm