The present study is part of a larger study regarding taste and smell changes in testicular cancer patients. The taste and smell function and the palatability of ONS were assessed at the following time points: pre-chemotherapy (T0; baseline), during the first cycle (T1; day seven of the first cycle), before the second cycle (T2; day one of the second cycle prior to drug administration), during the second cycle (T3; day seven of the second cycle), and after chemotherapy (T4; 1 month after start of the last cycle). Measurements were performed at the same time of the day for all patients: late morning to early afternoon. The measurements were conducted in the same order for all patients: (1) smell test, (2) taste test, (3) palatability ONS. At baseline, data on height, smoking status, educational level, and sports level were collected during a structured interview. A digital scale was used to measure bodyweight in light clothing, without shoes. Body mass index (BMI) was calculated as weight (kg) divided by the square of height (m2). Data concerning disease and treatment were derived from medical records.
Oral nutritional supplements
To study the effect of chemotherapy on the pleasantness of ONS regarding type and flavour, a variety of ONS was selected. Six ONS (Nutricia Advanced Medical Nutrition—Danone) were tested: two high protein milk-based (vanilla and strawberry), two juice-based (apple and orange), and two yoghurt-based (vanilla-lemon and peach-orange) ONS. All ONS had a energy density of 150 kcal/100 ml. The nutrient content varied between the three ONS categories (Online Resource 1).
The ONS were served in 30 ml clear plastic tubs at cold temperature. The patients were asked to take at least one sip of each sample. Next, patients had to fill out a questionnaire regarding the palatability and sensory attributes of the ONS on a seven-point scale (Online Resource 2). The questionnaire comprised nine closed questions regarding the palatability, 16 attributes, and two open questions. The following two questions were used from this questionnaire regarding the palatability and metallic taste of ONS: “How much do you like the taste of this product?” (1 = dislike very much, 7 = like very much) and “Please, specify to which extent ‘metallic’ is applicable to the product” (1 = strongly disagree, 7 = strongly agree). The patients received a warming-up sample (semi-skimmed milk) and completed the questionnaire to get used to the procedure. The ONS were presented in randomized order among patients and test sessions. All samples were labelled with a three-digit-code, varying over test sessions to avoid recognition bias by numbers. Patients were blinded to which ONS was being served. Patients rinsed their mouth with water after each sample.
Taste and smell function
Filter-paper taste strips (Burghart, Wedel, Germany) were used to measure recognition thresholds for sweet, sour, salty, and bitter taste . The patients were requested not to smoke, brush teeth, use chewing gum or to eat or drink with the exception of water 1 h prior to the measurement. The following standard concentrations of each taste were used: sweet: 0.05, 0.1, 0.2, and 0.4 g/ml sucrose; sour: 0.05, 0.09, 0.165, and 0.3 g/ml citric acid; salty: 0.016, 0.04, 0.1, and 0.25 g/ml sodium chloride; bitter: 0.0004, 0.0009, 0.0024, and 0.006 g/ml quinine hydrochloride. The taste strips were placed in the middle of the tongue for whole mouth testing. The taste strips were presented in increasing concentrations in a randomized order. Patients had to choose one of five possible answers (sweet, sour, salty, bitter or no taste). Patients rinsed their mouth with water after each taste strip. Scores for each taste range from 0 (no concentrations correctly identified) to 4 (all concentrations correctly identified). A total taste score (range 0–16) was derived by summing the scores for all tastes.
‘Sniffin’ Sticks’ (Burghart, Wedel, Germany) were used to measure the smell function [18, 19]. In brief, this test consists of pen-like odour dispensing devices and includes three parts: a threshold (THR) test, a discrimination (DIS) test, and an identification (ID) test. The pens were presented approximately 2 cm under the middle of the nose. To measure the THR, a standard series of pens with 16 dilutions of n-butanol was used. Three pens were presented in a randomized order, one contained the odorant and two solvent. The patients had to identify the pen containing the odorant in two successive trials, which triggered a reversal of the staircase. The THR was defined as the mean of the last four reversals. For the DIS test, 16 triplets (two equal and one different odorant) were presented. The patients had to discriminate which of the three pens smelled differently. For the ID test, 16 common odours were presented and the patients had to identify the odour using a multiple choice task presented on a list of four different odorants. For the THR and DIS test, there was a 30-s interval between the presentation of the first pen of a triplet and the presentation of the first pen of the following triplet. The pens for the ID test and the taste strips were presented at a 30-s interval. The patients were requested not to smoke, brush teeth, use chewing gum or to eat or drink with the exception of water 15 min prior to the measurement. The THR score ranges from 1 to 16. The DIS and ID scores range from 0 to 16. A total smell score was derived by summing the THR, DIS and ID, resulting in a threshold, discrimination, identification (TDI) score (range 1–48). The extended version of the ‘Sniffin’ Sticks’ was used, containing 32 odour combinations for the DIS test and 32 odours for the ID test . Each patient received a unique combination of 16 out of 32 triplets for the DIS test, and a unique combination of 16 out of 32 pens for the ID test.
Two aspects of metallic taste were addressed. Patients had to report to which extent metallic was applicable to the ONS. Furthermore, patients had to report whether they experienced a metallic taste as a side effect of chemotherapy. To examine this second aspect, patients were asked to respond to the following open-ended questions regarding their subjective taste perception since the start of treatment: “Have you experienced a change in taste?” and “Have you experienced certain foods to taste differently?”. In addition, patients had to report how often they experienced a continuous bad taste in their mouth (never, rarely, sometimes, often or always) and patients had to describe the experienced taste with the following response options “sweet, sour, salty, bitter or other namely”. Patients were classified as experiencing a metallic taste, when they reported a metallic taste as a change in taste or as a bad taste in the mouth. By exploring both aspects of metallic taste, investigation whether especially the patients who experienced a metallic taste as a side effect of chemotherapy reported that metallic taste was applicable to ONS could be performed. Moreover, this enables investigation whether a metallic taste is applicable for specific types of ONS in cancer patients.
Descriptive statistics are presented as median with interquartile range (IQR) or percentage. Possible differences in palatability and metallic taste between the ONS per test session were investigated using the Friedman test, followed by the post-hoc related-samples Wilcoxon signed-rank test. A linear mixed model was used to investigate taste and smell function and the liking and metallic taste of each ONS separately, over time. An unstructured covariance type was used to model the covariance structure among repeated measures. For ONS showing a significant change or a trend towards significance in liking or metallic taste over time, possible differences in liking and metallic taste were compared to baseline and possible differences during the second cycle were explored. Test session was entered as fixed effect in the model (T0 as baseline). Contrast comparisons were carried out to explore possible differences during the second cycle (T2 versus T3). All models were estimated using maximum likelihood. Spearman’s rho correlation (r
s) was used to investigate the relation between taste and smell function and the liking of each ONS over all test sessions. For ONS showing a significant change in liking compared to baseline, spearman’s rho correlation was used to explore the relation between changes in taste and smell function and the change in liking compared to baseline. For taste and smell parameters without a change over time (i.e. all parameters, except salty taste), the mean of the taste and smell parameters over all test sessions was used for correlations (instead of the change over time of these parameters). Spearman’s rho correlation was used to explore the correlation between overall liking and metallic taste of ONS over all test sessions, across all ONS and per ONS separately. The Mann-Whitney U test was used to compare the scores to which extent ‘metallic’ was applicable for each ONS with respect to presence of metallic taste in patients. For this end, the highest rating reported by each patient during test sessions in which patients reported a metallic taste (T3 and T4) was used. Patients with missing data on a variable relevant for a specific analysis were excluded (indicated in tables). Given the exploratory nature of the study, no adjustments were made for multiple comparisons. A two-tailed p value of less than 0.05 was considered statistically significant. Statistical analyses were performed using SPSS, version 22 (SPSS Inc. Chicago, IL).