Effect of a Dairy Diet on Nasopharyngeal Mucus Secretion
To examine the effects of dairy versus nondairy diets on self-reported levels of nasopharyngeal mucus secretion.
Prospective, randomized, double-blinded controlled study.
Twenty-six men and 82 consecutive women over the age of 15 years attending the otolaryngology department at East and North Hertfordshire NHS Trust who reported experiencing increased levels of nasopharyngeal mucus secretions were selected for a double-blinded trial of dairy versus dairy-free dietary supplementation for the last 4 days of a 6-day dairy-free diet. Main outcome measures were comparisons of mean daily reporting of subjective levels of nasopharyngeal secretions by linear scoring (1-100) and by an ordinal scale of 1 to 4. On each day, t tests were used to compare differences.
There was a significant reduction in the reported linear secretion score seen from day 1 to 4 in nondairy (t = 4.39, P < .01) and in dairy (t = 3.94, P < .01) arms. There was a significant increase in secretion score days 4 to 7 in the dairy arm (t = -2.56, P = .01), and a continued but nonsignificant reduction in the nondiary arm (t = 1.54, P = .13, with an overall significant reduction between day 1 and 7 in the nondairy arm (t = 4.79, P < .00). In the ordinal secretion scale, both dairy arm (t = 2.754, P < .01) and nondiary arm (t = 5.52, P < .01) scores decreased significantly from days 1 to 4. There was a significant decrease in scores from days 1 to 7 in the nondairy group (t = 5.12, P < .01).
In this blinded trial, a dairy-free diet was associated with a significant reduction in self-reported levels of nasopharyngeal secretions in adults who previously complained of persistent nasopharyngeal mucus hypersecretion.
LEVEL OF EVIDENCE:
1b Laryngoscope, 129:13-17, 2019
We believe this to be the first study that has looked at
the effects of cow’s milk in the diet on subjects who report
excessive nasopharyngeal mucus hypersecretion. Although
belief in the MME is widespread amongst the public and
specialties allied to medicine, it has thus far been difficult
for ear, nose and throat practitioners and pediatricians to
advise their patients on dietary means to control symptoms
of excess nasopharyngeal secretions, especially given the
known nutritional benefits of dairy products in the diet.
There is so far no robust theory that would explain the
physiological basis of a MME. Mucus overproduction is a
recognized characteristic of chronic rhinosinusitis10 and
asthma.11 One theory indicates that cow’s milk is high in bcasein
A1, which breaks down to b-CM-7, which has been
shown to act on goblet cells to upregulate MUC5AC gene
expression, which is itself responsible for increase in mucus
secretion. This theory would, however, rely on a status of
increased permeability of b-CM-7 in the gut in sufferers of
excessive mucus so that it can pass in significant amounts
into the systemic circulation.
Anecdotally, the authors have heard some advocates
of the MME describe rhinitics, who have a dry mouth
and pharynx due to blocked nose and consequent mouth
breathing, perceive a stickiness of milk in their throat
that exacerbates their symptoms, creating the impression
of excess throat mucus.
Pinnock et al. found no correlation between milk
intake and upper respiratory tract mucus production in
an uncontrolled, open trial of 60 healthy volunteers inoculated
with the common cold virus (rhinovirus-2).12 This
study looked at relative dietary intake, and therefore, the
subject numbers on a dairy-free diet were minimal. This
trial was therefore unable to study the effects of a
dairy-free diet. The focus was not on people who had daily
mucus overproduction symptoms, but short-term symptoms
in the presence of the common cold. A second study,
by Pinnock and Arney, interviewed 169 healthy volunteers
(70 believers and 99 nonbelievers in the MME). The
believers described the MME as a phenomenon in which
cough and/or sensations relating to the thickness of saliva
or mucus experienced in the throat for a period of up to 24
hours after the ingestion of a small volume of milk. They
reported more chronic respiratory symptoms and a 39.5%
lower dairy product intake than nonbelievers.13 Out of
these respondents, 130 participated in a randomized,
double-blinded trial comparing the same-day effects of a
disguised cow’s milk drink to a disguised nonmilk (soy)
drink, which were added to their usual diet. The effectiveness
of the disguising of the test drink was validated in
the article and showed the participants were unable to
identify which drink they had taken. Their study demonstrated
an overall increase in mucus sensation in both
groups and no statistical difference between the reported
sensory responses between the groups.9
Wijga et al. demonstrated reduced risk of asthma
symptoms with frequent consumption of products containing
milk fat in a cohort study of 2,978 preschool children.14
Haas et al. found no bronchoconstrictive effects when subjects
were exposed to 300mL of ultra–high temperature
processed milk compared to rice milk.15Woods et al. placed
20 patients on a 2-week dairy-free diet in a randomized,
crossover, double-blind, placebo controlled trial. The active
challenge group were given a single-dose drink equivalent
of 300mL of milk. They found no definitive link between
milk consumption and the prevalence of asthma-related
symptoms.16 Yusoff found improved lung function in a
single-blind prospective study of 13 asthmatic children on
an 8-week egg- and milk-free diet.17
Our results have shown that perceived mucus production
improves on the first day of a dairy-free diet in
both groups, with a decreasing day-on-day trend in days 1
to 4. This trend continues in the blinded nondiary group.
In the blinded diary group, however, perceived mucus
production worsens from the first day of addition of dairy
products. The perceived worsening in mucus production
on a dairy diet increases day on day for the 4 days on supplements,
suggesting a cumulative effect, with the mean
score on day 7 approaching but not yet reaching the mean
baseline score within the study timeframe.
Given the relatively low power of the study, the respective
effect sizes of 0.40 and 0.55 (Cohen’s d) for the linear
and ordinal scores indicate that this intervention has
significant potential to impact considerably on perceived
mucus production in patients who report hypersecretion.
We designed our study so that our subjects were
placed on a short-duration (6 days), dairy-free diet because
we considered this would optimize compliance and provide
a reasonable timeframe for a MME to manifest. Further
studies could be set up with longer periods on a dairy-free
We feel our blinding through disguise of the supplement
was sufficiently effective for the purposes of the study.
Although 60% of subjects thought they knew which group
they were in were correct, this was no different to chance,
and in a relatively small number of subjects within the
study as a whole (30/108).
The relatively high predominance of females in the
study reflected in part a higher number in the recruitment
group of consecutive patients, and partly a higher
willingness to participate, which may reflect social circumstances.
There was no difference in the proportions
of male and female between the two randomized groups.
Our analysis showed that a range of other variables
had no demonstrable effect on the intervention. Patients
who had previously undergone nasal surgery did report
lower linear scores at baseline; however, there was no
indication that previous nasal surgery modified the
effect of the intervention. There were no other variables
correlated to outcome scores.
This study included only subjective measures of
levels of nasopharyngeal secretions. This was for two
reasons; patients’ subjective symptoms when it comes to
mucus production are the most important measure when
considering outcomes of an intervention, and we considered
that objective measurements of secretions from the
nose, mouth, and pharynx would not be necessary for an
initial study. The collection of mucus in each patient for
viscoelasticity and viscosity analysis would be difficult,
time consuming, and costly, and would reduce the likelihood
of recruitment into the study. Further studies could
include analysis of nasopharyngeal mucus secretion and
nasal mucosal biopsies to assess the level of mucosal
inflammation. This study suggests the benefits of a dairyfree
diet in adults reporting nasopharyngeal hypersecretion
could be realized within just a few days. We would
recommend a future larger, blinded study that repeats
this analysis and assesses the difference between the
groups over a longer period of time.
Although these results may empower clinicians to
guide their rhinitic patients to avoid dairy products, this
study is the first of its kind, and the MME should be
evaluated with further, larger studies. We would advise
caution when removing dairy products from the diet,
and patients should have adequate professional dietetic
advice before contemplating a diary-free diet.
We believe this is the first study to demonstrate the
possible existence of the MME in adults reporting nasopharyngeal