Human Study Arm
Forty adult volunteer subjects, 24 men and 16 women, were included in this study (). Ages ranged from 34 to 73 years with an average age of 54 years. All subjects were hypercholesterolemic by standards of the American Heart Association. All subjects had triglyceride levels less than 400 mg/dl, making the calculation of LDL levels reliable. Pure tone thresholds were measured and averaged for the right and left ears. These values were then averaged to yield a combined ear average over all frequencies tested. By analyzing the range of values, it can be observed that the combined ear average pure tone thresholds varied from normal hearing to a moderate sensorineural hearing loss (). The number of DPOAEs present at each frequency was also averaged for the right and left ears and then combined to yield a combined ear average DPOAE for each subject tested ().
Average values, standard error of the mean, and ranges for age in years, serum total cholesterol, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, and triglycerides in milligrams per deciliter for the human subjectsa
TABLE 2 Average values with standard error of the mean and ranges of the combined ear average pure tone thresholds (decibel sound pressure level) and the combined ear average distortion product otoacoustic emissions (number of positive responses over frequency (more ...)
Human Pure Tone Thresholds
For the statistical analysis, multiple linear regression was performed using the combined ear average pure tone threshold as the dependent variable with age, sex, HDL, LDL, and triglycerides as the predictor variables. Age, sex, and triglyceride level were all found to have a statistically significant effect on the pure tone threshold (). Higher triglycerides, older age, and male sex were all associated with higher thresholds. Importantly, LDL and HDL levels did not demonstrate statistically significant effects.
Calculated B coefficients, beta values, and p values of the individual explanatory variables on the dependent variables average pure tone thresholds and average distortion product otoacoustic emissionsa
To verify our findings and rule out other subtle associations, we broke down our data and analyzed various subgroups. Multiple linear regression analyses were then performed separately for the dependent variables right ear average pure tone threshold and left ear average pure tone threshold using the same predictor variables of age, sex, LDL, HDL, and triglycerides. Again, age, sex, and triglyceride levels were found to be statistically significant with p values < 0.05, whereas LDL and HDL levels were not statistically significant. We then studied each frequency separately for each ear. For the right ear, age was a statistically significant predictor at all frequencies tested, sex was a statistically significant predictor between the frequencies 2 and 8 kHz, and triglyceride level was a statistically significant predictor at all frequencies except 500 Hz. For the left ear, age was a statistically significant predictor for the frequencies 1 to 8 kHz, sex was a statistically significant predictor at 3 to 6 kHz, and triglyceride level was a statistically significant predictor at 4 to 8 kHz. LDL and HDL levels were not statistically significant predictors for pure tone thresholds for any frequency in either ear.
An estimate of the clinical significance of the association between triglycerides and pure tone thresholds was made by calculating age- and sex-adjusted audiograms for each of the 40 patients. This was done according to the United States Department of Labor Occupational Health and Safety guidelines (www.osha.gov
, 1910.95 App F, subpart G). On the basis of these correction tables, we adjusted each patient’s thresholds by subtracting median thresholds for the appropriate age, sex, and frequency from each subject’s threshold. We then added this value to the mean thresholds of a 40-year-old man to get representative “average” thresholds. We then plotted average audiograms from the top 20 patients with “very high” triglyceride values and the bottom 20 patients with “less high” triglyceride values ().
FIG. 1 Age- and sex-adjusted audiograms for the human study arm. The 20 patients with the highest triglycerides (very high: 272 ± 7 mg/dl) are compared with the 20 patients with the lower triglycerides (less high: 141 ± 11 mg/dl). Patients with (more ...)
These simulated audiograms demonstrated that patients with very high triglycerides had slightly worse pure tone thresholds than those with less high triglycerides, and that these differences were greater at high frequencies compared with low frequencies. At 4 kHz, the differences between the two groups were approximately 6 dB; and at 6 kHz, they were approximately 12 dB. It should be noted that although this figure can be used to visualize the clinical significance of the triglyceride effect, these curves only represent corrected means from two groups of patients. In contrast, the regression analysis performed earlier is more sensitive at detecting associations between variables because patients at the extremes are not averaged with those in the middle of the range.
Human Distortion Product Otoacoustic Emissions
Multiple linear regression analysis was performed for the combined ear average DPOAE as the dependent variable with age, sex, HDL, LDL, and triglyceride level as the predictor variables. Age, sex, and triglyceride level were all found to have statistically significant effects on DPOAEs (). Higher triglycerides, older age, and male sex were all associated with a reduced number of DPOAE responses. LDL and HDL levels did not demonstrate statistically significant effects on DPOAEs.
Further analysis was undertaken using the right ear average DPOAE and the left ear average DPOAE separately using the same predictor variables: age, sex, HDL, LDL, and triglyceride level. For the right ear, age, sex, and triglyceride level were statistically significant for predicting DPOAEs. For the left ear, only age was a statistically significant predictor for predicting DPOAEs. LDL and HDL were not statistically significant predictors for the number of DPOAE responses in any of the analyses.
Guinea Pig Study Arm
All guinea pigs were healthy throughout the 14-week study period. All three groups experienced significant weight gain over the study period, which was statistically significant when analyzed by the paired Student’s t test (). The animals in Group 1 (control) had stable cholesterol levels throughout the study period. This is in contrast to the two study groups, which experienced a large increase in their serum cholesterol.
Average weights and cholesterol levels with standard error of the mean and range of valuesa
Guinea Pig Distortion Product Otoacoustic Emissions
For all three groups, the average DPOAE magnitude for each intensity level (45, 55, 65, and 75 dB SPL) were compared at Weeks 0 and 14 using the paired Student’s t test. DPOAEs that were not at least 3 dB above the noise floor at each individual frequency and stimulus level were not included for comparison. For the control group (Group 1), there was no statistically significant change in the magnitude of the DPOAE at any of the intensity levels tested. Likewise, for the male and female groups (Groups 2 and 3, respectively), there were also no statistically significant changes in the magnitude of the DPOAE between Weeks 0 and 14 at any of the intensity levels tested ().
Average distortion product otoacoustic emissions at each intensity level for the three groups of animals at Weeks 0 and 14 with associated p values as determined by a paired Student’s t testa
Further analysis was undertaken by comparing the DPOAE magnitudes at Weeks 0 and 14 for each individual frequency and intensity level. Again, a paired Student’s t test was used for statistical analysis. For the control group, there were statistically significant decreases in the magnitude of the DPOAE at 12 and 14 kHz when presented at a stimulus level of 45 dB SPL (p < 0.05). In contrast, there was a significant increase in the magnitude of the DPOAE at 14 kHz when delivered at a stimulus level of 55 dB SPL (p < 0.05). For the experimental group of male guinea pigs (Group 2), there was a significant decrease in the DPOAE magnitude at 6 kHz when tested at a stimulus level of 45 dB SPL (p < 0.05). In addition, there was also a significant decrease in the DPOAE magnitude at 12 kHz when tested at stimulus frequencies of 45, 55, and 65 dB SPL (p < 0.05). All other frequencies and stimulus levels tested showed no significant change in the DPOAE magnitude. For the experimental group of female guinea pigs (Group 3), there was a significant decrease in the DPOAE magnitude at 12 kHz when tested at a stimulus of 65 dB SPL. Other than these presumably spurious results, there were no significant changes in the DPOAE magnitudes at any other frequency or stimulus level tested.
The data were also analyzed by averaging the DPOAE magnitudes at all frequencies together. The male and female experimental groups were also combined into one large experimental group. When the average DPOAEs were compared between Weeks 0 and 14 at each of the presented intensity levels for the combined groups, there was found to be a statistically significant change in the DPOAE magnitude for each of the intensity levels (4.8, 5.2, 5.6, and 4.7 dB for 45, 55, 65, and 75 dB SPL, respectively). However, similar reductions in DPOAE magnitudes were found in our control group (3.5, 5.3, 5.8, and 6.1 dB for 45, 55, 65, and 75 dB SPL, respectively). The nonpaired Student’s t test demonstrated that there was no statistically significant difference between the change in DPOAE magnitude in the controls and the combined experimental group (p > 0.6). Thus, the high-fat diet was not associated with any change in DPOAE magnitude.