20Q: Healthy Eating Makes for Healthy Hearing - The Recipe

1.  What prompted you to even think about diet and hearing? 

Variability. There is a great deal of inter-individual variability in the susceptibility to acquired hearing loss.  For example, why does one person have a greater threshold shift due to noise exposure than a co-worker with the same exposure? Genetics, sex, age, race/ethnicity, previous noise exposure, medications, other co-morbidities are all plausible modifying factors.  Understanding the factors that may mitigate or exacerbate risk for hearing loss is critical for the design and application of appropriate intervention.  We all are well aware that nutrition is relevant to many aspects of health.  It seems almost obvious that our diets likely influence susceptibility to hearing loss.  However, this seemingly obvious relationship remains mostly speculation. 

2.  Speculation? 

While studies in animal models have demonstrated relationships, only a handful of studies have examined the influence of diet on auditory function in humans.  It would take several hours to review the literature, but if you are interested in learning more, Colleen Le Prell and I have a book chapter in the upcoming Otology and Neurotology text from Thieme that gives a in-depth review. 

3.  How about a few highlights?

Sure.  It’s probably easiest to review if we divide the general topic into three main areas:  Caloric Restriction, Macronutrients, and Micronutrients.

4.  Hold on  . . . you’re starting to lose me already.  What’s a “macronutrient” and what’s a “micronutrient?”

Well, a nutrient is a chemical an organism needs to acquire from the environment or in this case food to maintain structural and functional integrity, to live and grow. 

A macronutrient has numerous definitions but in regards to diet refers to the nutrients that are consumed in the largest quantities and provide the bulk of energy; these include carbohydrates, proteins, and fats.

A micronutrient refers to the vitamins and minerals of diet required in minute amounts, these include vitamins (A, C, E, etc.).

5.  Thanks, that helps.  I’m pretty sure I know what caloric restriction is, so let’s talk about that first.

Caloric restriction (CR) refers to just that, limiting the normal caloric intake, and in this case we’re looking at animal research.  In most studies, a 20-30% reduction is used.  Studies using CR have demonstrated protection against age-related hearing loss in animal models, which mostly include mice and rats (Seidman, 2000; Someya, Yamasoba, Weindruch, Prolla, & Tanokura, 2007; for a review see Someya, Tanokura, Windruch, Prolla, & Yamasoba, 2010).  This area of research was based on an abundance of literature suggesting extended lifespan in animals with CR, again, from rodent models.  However, the evidence in species more closely related to humans is less supportive.  For example, the University of Wisconsin-Madison (UW-M) and the National Institute of Aging (NIA) are each conducting studies examining the effects of CR on longevity and age-related hearing loss in the rhesus monkey.  No reliable effect of CR has been found to date in regards to hearing loss susceptibility compared to normal diet in either study (Torre et al., 2004; Fowler, Torre, & Kemnitz, 2002). However, the monkeys have also not reached their maximum longevity, so the full benefits of CR may not yet have been realized. 

If you read major newspapers, you might have seen recent discussions of a study published in Nature (Mattison et al., 2012) on caloric restriction and longevity from the NIA.   The findings are consistent with no life-prolonging effects of CR and contrast with the UW-M study which did demonstrate extended life with CR (Colman et al., 2009). Differences in study design, diet composition, and source of the monkeys are being considered to gain insight into the discrepant findings. Interestingly, the CR in the UW-M and NIA studies are a general reduction of 20-30% caloric intake, but not targeted to a specific group of nutrients. 

6.  So, we’re ready to talk about nutrients?

Yes, but before leaving CR, I want to  comment on my previous point about a general versus targeted CR.  In a recent review, Lopez-Torres and Baria (2008) discuss targeted CR, where specific macronutrients:  protein, carbohydrates, or fat intake, are each restricted individually.  The only macronutrient restriction that increased longevity was reduced protein intake.

7.  What else do we know about hearing and macronutrients?

Several studies have manipulated intake of macronutrients to examine effects on hearing loss.  I just mentioned that too much protein may decrease longevity (in rodents).  However, reduced levels of protein may increase vulnerability to noise-induced hearing loss and ototoxicity, at least in guinea pigs (Ohinata, Yamasoba, Schacht, & Miller, 2000; Lautermann, McLaren, & Schacht, 1995). Several studies have also suggested that long-term (12 month) increased fat intake increases risk for hearing loss in rats (Du et al, 2012).  However, other animal studies have not demonstrated changes in auditory function in shorter term (14 week) high fat intake (Evans et al., 2006).  A high-cholesterol and high triglyceride diet has also been demonstrated to increase risk for noise-induced hearing loss (Pillsbury, 1986).  We do not have much animal data on the effects of carbohydrates intake on hearing loss susceptibility, but we do have some human data that maybe we can get to later.

8.  Sure.  Human data sounds good.  You were also going to talk about micronutrients?

Yes, this is the final major area and what has received the most study is examining specific micronutrients, such as vitamins and trace minerals. 

9. What exactly is a “trace mineral?”

A trace mineral is part of our micronutrients, so minerals that we consume in minute amounts, examples include iron, magnesium, zinc, and etc.

10.  Okay thanks.  And do micronutrients have some effect on hearing?

The majority of these micronutrient studies involve either supplementing the animal’s diet or looking at nutrient deficiency and the effects of either age, noise, or an ototoxic drug.  A number of specific nutrients, such as vitamin A (Shim, Kang, Ahn, & Chung, 2009; Ahn, Kang, Kim, & Chung, 2005) B-vitamins (Quaranta, Scaringi, Bartoli, Margarito, & Quaranta, 2004; Durga, Verhoef, Anteunis, Schouten, & Kok, 2007), vitamin C (McFadden, Woo, Michalak, & Ding, 2005; Seidman, 2000), vitamin E (Kalkanis, Whitworth, & Rybak, 2004; Scholik, Lee, Chow, & Yan, 2004; Hou et al., 2003), and magnesium (Haupt & Scheibe, 2003; Sendowski, Raffin, & Braillon-Cros, 2006) have been demonstrated to provide protective effects when individually supplemented.  There are also benefits with combined dietary nutrient supplements (Heman-Ackah, Juhn, Huang, & Wiedmann, 2010; Le Prell, Hughes, & Miller, 2007).  Deficiencies can also have an impact, but in exacerbating risk for hearing loss [Biesalski, Wellner, & Weiser, 1990 (vitamin A); Kashio et al., 2009 (vitamin C); Ikeda, Kusakari, Kobayashi, & Saito, 1987 (vitamin D)] are some examples.

11.  Whew.  A lot of animal research.  How do we translate and study these findings in humans?

That is a good question.  The translation of these findings to human populations is a critical step.  It is one thing to show that supplementing an animal with vitamins will reduce susceptibility to hearing loss and another to say it will happen in humans; our biochemistry is obviously not the same as a mouse or a rat.  For example, virtually all mammals with the exception of fruit bats, guinea pigs, monkey, and man synthesize vitamin C endogenously (from within), thus do not require dietary vitamin C, whereas vitamin C is an essential vitamin for man (required from diet).  Also, in most cases these animals are fed some type of lab chow that is supplemented with a synthetic version of these vitamins, they are not out in the field eating vegetables, fruit, and insects.

Studies of diet and nutrition in human populations are not a simple undertaking.  You can imagine the cost as well as subject adherence in a prospective study (where we follow subjects over time) if the subject needed to maintain a specific diet over an extended period.  Therefore, our findings as related to hearing and diet are currently mostly limited to retrospective (where we look backward) epidemiological studies.  If you are unfamiliar with the term, epidemiology refers to the study of patterns, causes, and effects of health and disease in defined populations.  Here we look at relationships between the foods people eat and measures of auditory function.

12.  You said mostly limited to retrospective, has there been any prospective work?

Ahh, good pick up.  There is a classic study by Rosen and colleagues (1970) that manipulated fat intake among patients in two Finnish mental institutes.  The purpose of the study was to look at the relationship between heart disease, cholesterol, saturated fat intake and hearing loss.  The control diet was the “normal” institutional diet which was relatively high in saturated fats; the experimental diet was a low-fat diet.  One institute’s patients were placed on the experimental low-fat diet and the other institute kept its patients, the control subjects, on the standard high-fat menu.  Cardiovascular function and hearing were then followed over a 5 year period.  The experimental subjects not only showed reduced levels of cholesterol, but significantly better hearing thresholds.  In a twist, the researchers then switched the diets at the two institutions for a 3.5 year period and found a reverse in the results, where the patients moved to the experimental diet had reduced cholesterol and less progression in hearing loss compared to the group that was switched back on to the high saturated fat diet.

 13.  I bet it would be hard to get that study through a human subjects review board nowadays!  What are the keys to a good prospective study?   

Well, we have to start with measuring a person’s diet, and there are numerous ways we can do this, just like there are numerous ways we can measure auditory function, and of course they each have their own advantages and limitations. While the “gold standard” of auditory function is pure tone thresholds; the “gold standard” of dietary assessment is weighed food records.  This is where the subject maintains a weighed record of all food and drink consumed for some determined period of time.  However, the process of actually weighing everything a person consumes may influence their choices.  For example, if a person is weighing how much ice cream they eat they may not add that extra scoop.  Often we may not realize how much we really eat!  Similar to asking your patient to keep a journal of how often they have difficulty hearing to determine need for a hearing aid.

The other common dietary assessments are 24-hour recall, a food diary kept for several days (without weighing) and the food frequency questionnaire.   A 24-hour recall involves reporting the foods and drinks consumed over the previous 24 hours.  A food diary is a prospective diary of food intake similar to the weighed food record, but without actually weighing the foods.  The food frequency questionnaire is an estimate of usual diet over the past weeks or months.  These three types of studies have been validated using weighed food records to establish their validity and reliability.  In addition, we can look at biomarkers of nutrients from blood, plasma, and urine.  Weighed food records and food diaries are going to involve a much more intensive analysis (and will be much more expensive).  The 24 hour recalls and food frequency questionnaires may be more prone to recall error, but require a limited duration (and are relatively cheap).

14.  How is all this information used?

That brings up the second component of dietary assessment, the analysis.  The vast majority of nutritional epidemiology studies use a single nutrient analysis method.  Meaning, the dietary information is derived from one of the methods above, and those foods are separated into their respective nutrient content.  This provides an estimate of the amount of caloric and individual nutrient intake of the subject.  The individual nutrient relationship with the health outcome of interest is then analyzed.  For example, we could look at the relationship between vitamin C and otoacoustic emissions.  Although this type of analysis has been quite valuable, it has several limitations.  (1) People do not eat isolated nutrients, we eat meals consisting of a variety of complex combinations of nutrients and interactions between nutrients; (2) there is also a high intercorrelation among some nutrients, so if entered into the same statistical model they would reduce each other’s relationship with the outcome measure; (3) the effect of a single nutrient may be too small to detect; and (4) you increase your odds of finding a significant relationship by chance if you are looking at 25 separate nutrients (Hu, 2002).  This is the type of analysis that has been used for looking at hearing and diet up to this point.    

15.  Sounds like there could be some design problems, but what have these studies shown so far in humans?

It will actually not take too long to discuss this topic because there have only been a few studies and they are fairly consistent with the animal literature.  Most of this work has been conducted in the past few years.  For example, some work that I did (Spankovich et al., 2011) demonstrated a relationship between vitamin C, vitamin E, magnesium, and lycopene with better auditory function and higher cholesterol and fat with poorer auditory function.  Gopinath et al. (2011a) demonstrated a relationship of vitamin A and vitamin E with prevalence of hearing loss, but not incidence over a 5 year period.  In related research, Gopinath et al. (2011b) found a significant relationship between increased odds of hearing loss with higher dietary cholesterol intake.  There also is research data showing increased odds of hearing loss with higher carbohydrate and sugar intake (Gopinath et al., 2010).  On the other hand, Shargorodsky and colleagues (2010) found no significant relationship between vitamin C, vitamin E, beta carotene, B12 with hearing loss in adult males and only a small benefit of higher folate intake in men over 60 years of age. 

16.  Why do you think there has been an inconsistency in the research findings?

The discrepancy was likely related to methodology differences.  The studies we did and the work by Gopinath and her colleagues were analyses performed in the same cohort of persons in Australia, called the Blue Mountains Hearing Study. The study by Shargorodsky and colleagues analyzed data from a study in the United States called the Health Professionals Follow-up study.  It is of course possible that regional dietary differences and genetic differences influenced study outcomes in the two populations.  However, a critical design difference is that the audiologic data collected in the Blue Mountains Hearing Study, included OAEs, pure tone thresholds, and tympanometry, while the Health Professionals Follow-up study was a self-report of previous professionally diagnosed hearing loss.  There are likely participants in the Health Professionals Follow-up study who have some hearing loss, but have not sought professional diagnosis, and thus they are classified as normal using the hearing criteria set by that group.

17.  Is there maybe a better way to design these studies?

Well, all of these studies use a single nutrient analysis approach and are therefore susceptible to the limitations that we discussed earlier.   I don’t know if it’s better, but another approach is looking at dietary patterns rather than considering individual nutrients or trying to enter individual nutrients simultaneously into a statistical model.  One approach for analyzing dietary patterns is to look at indices of dietary quality.  These indices are typically constructed based on dietary recommendations.  An example is the Healthy Eating Index (HEI).

In case you’re not familiar with the HEI, it is a measure of how well American diets conform to the recommended dietary guidelines of the U.S. Department of Agriculture (USDA).  The overall Index has a total possible score ranging from zero to 100, with 100 being the maximum score.  The score is a sum of ten components each worth 10 points.  Components 1-5 measure the degree to which the person’s diet conforms to USDA serving recommendations for grains, vegetables, fruits, milk, and meat.  Component 6 measures total fat as a percentage of total food energy intake, component 7 saturated fat as a percentage of total food energy intake, component 8 a measure of total cholesterol intake, component 9 total sodium intake, and 10 variety in a person’s diet.  Components 1-5 are given the maximum score of 10 if the minimal recommended servings for each component are met or exceeded, while components 6-10 were given a maximum score of 10 if intake is less than a certain percentage of their intake or level (USDA, 1995).   

18.  I think I already know the answer to this, but how healthy is the average American diet?

The average American diet HEI score is approximately a 64 out of a possible 100. A score greater than 80 is considered good, 51-80 needs improvement, and less than 51 poor.  Only 12% of Americans fall in the good range.  People were mostly likely to underconsume fruit, vegetables, and grains, while intakes of fat and saturated fat tend to be well above recommended levels (USDA, 1995).

19.   Is the HEI score related to hearing loss?

We have a preliminary analysis that suggests there may be a relationship to hearing sensitivity.  I, along with my colleague Colleen Le Prell, have been exploring the National Health Examination and Nutrition Survey, also known as the NHANES to look at this relationship.

20.  That sounds interesting.  I’m out of questions but can we talk about this more at a later date?  I promise I’ll be careful with what I eat in the meantime.

No problem.  Sounds like fun.  And in fact, maybe I can bring Colleen along with me!

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