Effect of a Dairy Diet on Nasopharyngeal Mucus Secretion

OBJECTIVES/HYPOTHESIS:

To examine the effects of dairy versus nondairy diets on self-reported levels of nasopharyngeal mucus secretion.

STUDY DESIGN:

Prospective, randomized, double-blinded controlled study.

METHODS:

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.

RESULTS:

There was a significant reduction in the reported linear secretion score seen from day 1 to 4 in nondairy (t[53] = 4.39, P < .01) and in dairy (t[53] = 3.94, P < .01) arms. There was a significant increase in secretion score days 4 to 7 in the dairy arm (t[53] = -2.56, P = .01), and a continued but nonsignificant reduction in the nondiary arm (t[53] = 1.54, P = .13, with an overall significant reduction between day 1 and 7 in the nondairy arm (t[53] = 4.79, P < .00). In the ordinal secretion scale, both dairy arm (t[53] = 2.754, P < .01) and nondiary arm (t[53] = 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[53] = 5.12, P < .01).

CONCLUSIONS:

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

DISCUSSION

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

diet.

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.

CONCLUSION

We believe this is the first study to demonstrate the

possible existence of the MME in adults reporting nasopharyngeal

mucus hypersecretion.