The more health claims made about a food, the worse it is for you.
In the case the dairy industry, the above statement is nothing less than spot on (shameless plug, I'll admit). Sick cows fed everything under the sun other than the grass they were designed to eat are not the ideal source for your dairy consumption. And if the producers opt to then pasteurize or homogenize or in some shape or form bastardize that dairy, then what was once an incredibly healthy source of nutrition soon becomes udderly unrecognizable as a food.
However, organic, grass-fed cows (and sheep, goats, etc) raised the way nature intended can produce quality dairy products which are extremely beneficial for health. Sufficient quantities of bio-available calcium (i.e. animal sources for those of us who aren't ruminant herbivores) keep parathyroid hormone low while also increasing the likelihood of tryptophan converting to niacin rather than serotonin (and that should make the health conscious happy). It also helps maintain a favorable calcium to phosphorous ratio in the diet, without which blood pressure, inflammation, and even tumor growth are often increased. Calcium also down regulates the production of adrenalin. Oh--and it's involved in muscle contraction and is an essential component in the electrical conduction system of the heart, too.
But does dairy cause mucus production?
Quite simply--yes. In those who are sensitive to it, dairy can create an immune response in the body...just like any and every food someone eats that isn't conducive to their specific digestive capabilities.
As one study from the journal Medical Hypotheses states:
Excessive milk consumption has a long association with increased respiratory tract mucus production and asthma. Such an association cannot be explained using a conventional allergic paradigm and there is limited medical evidence showing causality. In the human colon, β-casomorphin-7 (β-CM-7), an exorphin derived from the breakdown of A1 milk, stimulates mucus production from gut MUC5AC glands. In the presence of inflammation similar mucus overproduction from respiratory tract MUC5AC glands characterises many respiratory tract diseases. β-CM-7 from the blood stream could stimulate the production and secretion of mucus production from these respiratory glands. Such a hypothesis could be tested in vitro using quantitative RT-PCR to show that the addition of β-CM-7 into an incubation medium of respiratory goblet cells elicits an increase in MUC5AC mRNA and by identifying β-CM-7 in the blood of asthmatic patients. This association may not necessarily be simply cause and effect as the person has to be consuming A1 milk, β-CM-7 must pass into the systemic circulation and the tissues have to be actively inflamed. These prerequisites could explain why only a subgroup of the population, who have increased respiratory tract mucus production, find that many of their symptoms, including asthma, improve on a dairy elimination diet.
So I guess it depend on who you are and what you've done to your digestive capacity via nutrition and lifestyle practices. Let me 'splain:
Oftentimes an inability to digest/assimilate dairy begins secondary to another offending agent. Gluten is a prime suspect. Alcohol and medicinal drugs are also common culprits. Anything which is a stress to the biological system has the potential to inflame the gut wall (for more on this subject, see the Seesaw of Sickness in my book, Spot On: Nutrition found here: http://triumphtraining.com/collections/books/products/spot-on-nutrition). This microtrauma to the intestine causes tiny holes to form, allowing food particles to pass into the bloodstream undigested. The body then creates antibodies to that particular food, potentially causing you to have an immune response to whatever you’re eating. Additionally, the constant inflammation causes what's termed villous atrophy. Lining the wall of your intestines, you have little finger like projections called villi. These, in turn, have tiny little microvilli covering them--you have about 200 million per square millimeter. The job of the microvilli is to help you assimilate nutrition from your food by producing various enzymes. One of these enzymes, in the case of our current discussion, is lactase--the enzyme you need to do anything with the lactose in dairy. No micromilli equals limited lactase (and other digestive enzymes) which limits your ability to consume dairy without suffering ill effects.
Of course, raw dairy typically comes with the exact enzymes one needs to safely and effectively consume it. But pasteurization destroys all those enzymes along with most if not all of the heat-sensitive nutrients. This is one reason why folks who are "lactose intolerant" often do fine when eating raw dairy. These people also typically fare better with full fat dairy instead of skim or low fat versions which will have more lactose per serving than the unadulterated milk products. Sheep and goat dairy are often better tolerated than diary from cow (ask if you wanna know why); and still others are sensitive to the form--doing fine with hard cheeses or yogurt yet having trouble with milk. Lastly, quantity and frequency of exposure are also factors which need to be considered in regards to how a person reacts to dairy. While the healthy digestive system should be able to handle just about anything that's thrown at it or in it (up to a point), the sad truth is most of us have done such damage to ourselves that we need to do some serious rehab of the gut wall and our health in general before we're able to eat whatever we want.
And, personally, I think that level of consciousness--even if forced upon us due to digestive complaints or otherwise--is actually a blessing. It's a painful signal that we're moving in the wrong direction, and it's time to redirect.
For those who need their academic mind satisfied, I've included a couple of studies below. N = 1, however, so I suggest you experiment on you to find what works best for your specific biochemistry.
http://www.ncbi.nlm.nih.gov/pubmed/2154152 In the first of three studies investigating the widely held belief that "milk produces mucus," 60 volunteers were challenged with rhinovirus-2, and daily respiratory symptoms and milk and dairy product intake records were kept over a 10-day period. Nasal secretion weights were obtained by weighing tissues collected and sealed immediately after use. Information was obtained on 51 subjects, yielding 510 person-days of observation. Subjects consumed zero to 11 glasses of milk per day (mean, 2.7; SE, 0.08), and secretion weights ranged from zero to 30.4 g/day (mean, 1.1; SE, 0.1). In response to an initial questionnaire, 27.5% reported the practice of reducing intake of milk or dairy products with a cold or named milk or dairy products as bad for colds. Of the latter group, 80% stated the reason as "producing more mucus/phlegm." Milk and dairy product intake was not associated with an increase in upper or lower respiratory tract symptoms of congestion or nasal secretion weight. A trend was observed for cough, when present, to be loose with increasing milk and dairy product intake; however, this effect was not statistically significant at the 5% level. Those who believe "milk makes mucus" or reduce milk intake with colds reported significantly more cough and congestion symptoms, but they did not produce higher levels of nasal secretions. We conclude that no statistically significant overall association can be detected between milk and dairy product intake and symptoms of mucus production in healthy adults, either asymptomatic or symptomatic, with rhinovirus infection.
http://www.ncbi.nlm.nih.gov/pubmed/16373954 There is a belief among some members of the public that the consumption of milk and dairy products increases the production of mucus in the respiratory system. Therefore, some who believe in this effect renounce drinking milk. According to Australian studies, subjects perceived some parameters of mucus production to change after consumption of milk and soy-based beverages, but these effects were not specific to cows' milk because the soy-based milk drink with similar sensory characteristics produced the same changes. In individuals inoculated with the common cold virus, milk intake was not associated with increased nasal secretions, symptoms of cough, nose symptoms or congestion. Nevertheless, individuals who believe in the mucus and milk theory report more respiratory symptoms after drinking milk. In some types of alternative medicine, people with bronchial asthma, a chronic inflammatory disease of the lower respiratory tract, are advised not to eat so-called mucus-forming foods, especially all kinds of dairy products. According to different investigations the consumption of milk does not seem to exacerbate the symptoms of asthma and a relationship between milk consumption and the occurrence of asthma cannot be established. However, there are a few cases documented in which people with a cow's milk allergy presented with asthma-like symptoms.