Materials & Methods We used a matched food sample procedure to analyze the nutrient content of packaged GF foods available on the Austrian Market. We grouped packaged GF foods with matching gluten-containing foods from two nutrition databases, estimated their nutrient content by using a step-by-step estimation process and compared the nutritional quality. We defined primary (macronutrient and energy content) and secondary outcome parameter (micronutrients and cost of the products) for further analysis. Food products included Between fall 2014 and spring 2015, packaged GF foods from 19 brands were obtained from three main supermarket chains in Austria in 12 different branches across Vienna. We selected 162 packaged GF foods from seven different food categories representing the majority of consumed processed foods that are originally based on cereal formulations for celiac patients (Martin et al., 2013). We only analyzed packaged GF foods marked with the European gluten-free label (Commission of the European Communities, 2009), excluded foods with non-verified gluten-free labels and excluded foods from categories that are not gluten-based in their original formulation. Following food categories were analyzed: flour/bake mix, bread and bakery products, pasta and cereal-based food, cereals, cookies and cakes, snacks and convenience food (for detailed listing, see Table S1). Additionally, we assessed the cost for each product. Both food quality as well as cost ranged widely within individual food products. To minimize this within-product range and provide more homogenous data in both target variables (nutritional quality; product cost), we matched two gluten-containing foods differing in cost range (one budget and one pricier product) for each GF food. From originally 162 identified packaged GF foods, we excluded duplicates (86) and foods with incomplete nutrient information (13 foods) (see flow diagram and detailed list of exclusion in Supplemental Information 1). Our final sample consists of 63 GF and 126 similar gluten-containing foods for subsequent nutrient content matching procedure. Step-by-step estimation process for nutrient content The selected GF foods were matched with two similar gluten-containing foods available in two different databases used in the Austrian Nutrition Surveys (BLS 3.02 Max Rubner Institute, Germany; Austrian Nutrient Database: ÖNWT, dato denkwerkzeuge, Vienna, Austria). We used a Microsoft Excel worksheet to compile the composition in macro- and micronutrients of the GF foods per 100 g in its raw form. We imputed the quantity for each ingredient in a descending order. In a second step, we estimated the quantity of each ingredient for every product based on the percentage of the final recipe and its rank order reported on the label (theoretical nutrient composition). Furthermore we compared the theoretical macronutrient composition of the food with the given information on the food label. The process was reiterated by adjusting the percentage of the different ingredients until the final results reflected the values of energy content reported on the food label. To assess the precision of this procedure, we calculated the estimation precision (theoretical nutrient content/nutrient content on the food label in %). The precision for the estimation of all macronutrients was very good and within an overall variation range of 7% (for detailed listing, see Supplemental Information 2). As a result of this process, we could estimate the amount of ingredients available in the GF products and extrapolate the nutritional components for following ingredients and nutrients: water (g/100 g), sugar (g/100 g), energy content (in kcal/100 g), macronutrients (carbohydrates, proteins, total fat, saturated fatty acids, monounsaturated fatty acids (MUFA), polyunsaturated fatty acids (PUFA), fibre; all in g/100 g), cholesterol (mg/100 g), minerals (i.e., Iron, Calcium, Sodium, Potassium, Phosphorous and Zinc; all in mg/100 g) and vitamins (Vitamin E, Thiamin, Riboflavin, Niacin, and Vitamin C; all in mg/100 g; Vitamin D, Retinol, β-carotene equivalents in µg/100 g). Statistical analyses and availability of the database Statistical analyses were conducted using IBM SPSS Statistics 22. Unpaired t-test was used to compare means; bivariate comparisons were tested by χ2 test. The Bonferroni post-hoc test was used to correct for multiple comparisons; p-values < 0.05 were classified as significant. Post-hoc power analysis was calculated by the difference between two independent means with G*Power 3.1.9 (Erdfelder et al., 2009). The Gluten-Free Food Database (Austria) can be accessed via the science collaboration platform: Open Science Framework. Contributions to the dataset can be made upon request and registration via the online platform (Open Science Framework).

Results The database provides quantitative information of macro- and micronutrients of the GF product. It contains nutrient data present in the traditional databases of gluten-containing foods (see Tables 1 and 2). Energy (kcal) Protein (g) Carbohydrates (g) Sugar (g) Total fat (g) Saturated fatty acids (g) MUFA (mg) PUFA (mg) Fiber (g) Flour/bake mix Flour a 345.5 3.7 77.6 1.0 1.7 0.2 0.3 1.0 3.8 Bake mix white (cake) 338.4 3.2 77.5 17.8 1.2 0.3 0.5 0.5 2.3 Bake mix brown (cake) 394.7 2.6 82.9 52.5 5.4 3.1 1.8 0.4 2.8 Bake mix (Pizza) a 322.7 5.9 70.3 1.5 1.9 0.4 0.8 0.7 3.9 Breadcrumbs a 350.8 5.9 70.1 0.4 4.8 1.3 1.9 1.2 6.0 Bread/Bakery products Rustic bread 238.4 3.7 51.0 0.7 1.9 0.3 0.8 0.6 1.4 Whole-grain bread 263.6 8.5 40.8 5.1 7.2 1.0 2.2 3.9 8.4 Toast a 224.8 4.8 39.0 2.1 5.3 1.4 2.0 1.5 6.3 Bun 259.2 1.4 48.7 5.2 6.2 3.0 1.6 1.2 1.6 Ciabatta 213.4 3.3 44.9 2.9 2.0 0.3 2.3 0.7 8.3 Raisin bread 261.0 4.0 49.1 18.6 4.2 1.5 0.8 0.8 1.6 Scone 293.2 3.7 52.2 13.8 7.5 2.2 1.4 1.1 2.7 Baguette 270.1 4.5 56.7 6.4 2.5 0.4 1.9 3.0 5.3 Lye Pretzel 343.8 4.8 59.2 7.8 9.4 4.7 1.0 0.9 2.2 Rusk 343.9 0.3 82.5 0.8 0.9 0.3 3.2 0.7 0.7 Crispbread 351.6 6.9 77.8 6.0 0.9 0.2 0.4 0.2 2.9 Wraps 228.7 3.1 38.8 0.4 7.5 2.2 0.3 0.4 2.8 Pasta and cereal-based products Fusilli 335.9 8.2 69.9 1.0 2.2 0.3 0.7 1.1 7.2 Spaghetti 329.0 8.7 66.3 1.3 2.8 0.4 0.9 1.4 9.4 Penne 338.4 6.9 72.4 4.3 1.9 0.3 0.5 0.9 4.9 Lasagne sheets 373.0 7.0 76.3 0.8 4.0 1.1 1.6 0.8 2.4 Vermicelli 371.2 12.5 71.6 2.0 3.4 0.4 1.0 1.7 13.5 Tagliatelli 370.9 12.1 72.0 1.9 3.3 0.4 1.0 1.7 13.1 Cous Cous 345.0 8.8 73.8 1.5 1.1 0.1 0.4 0.4 5.0 Cereals Granola (chocolate) 392.3 5.5 72.6 34.0 8.5 4.7 3.1 0.6 4.6 Granola (nuts) 478.0 7.1 64.9 16.7 21.0 7.7 10.3 2.5 4.8 Cornflakes 322.4 8.5 62.9 1.4 3.7 0.6 1.1 1.6 7.6 Cookie and Cakes Shortbread 385.3 3.3 73.6 13.8 8.3 2.7 3.9 1.5 1.2 Neapolitan wafers (original) 236.0 2.5 22.9 18.1 15.0 8.3 5.2 1.1 3.3 Cookie (chocolate) 479.2 2.0 64.3 5.7 23.8 11.9 8.5 2.5 2.3 Mignon wafers (hazelnut) 507.9 5.0 54.0 41.7 30.4 13.9 11.9 3.7 5.6 Marble cake 403.7 5.4 48.1 20.7 22.6 3.8 10.2 7.4 0.8 Ladyfinger 356.9 5.7 74.6 33.1 3.5 1.0 1.4 0.6 2.5 Cookie (whole-grain) 471.1 4.6 71.8 21.1 18.2 7.8 6.8 2.8 3.6 Granola bar 400.8 7.2 59.2 25.0 14.8 7.0 5.3 1.7 12.9 Cookie (orange)a 433.0 6.2 60.2 49.7 18.3 10.5 5.5 1.4 2.6 Apple strudel 270.9 4.2 43.2 18.1 8.7 3.4 2.9 2.2 1.6 Muffin 371.6 5.2 55.8 30.0 14.6 4.1 3.4 2.2 3.5 Snacks Cracker 340.8 10.1 43.5 0.0 13.8 9.3 3.3 0.4 0.3 Brezels 449.7 0.5 65.5 0.8 20.6 9.8 7.5 2.4 0.8 Grissini 392.5 2.0 67.4 2.6 12.6 3.9 6.1 2.4 0.7 Saltsticks 480.9 0.6 72.0 1.5 21.1 10.3 7.8 2.2 0.8 Wafers (plain) 329.7 8.6 63.8 1.4 4.1 0.6 1.2 1.8 7.7 Convenience foods Pizza (salami) 235.1 8.0 24.8 2.8 11.5 4.8 4.1 1.9 1.5 Pizza (margherita) 209.1 6.3 27.6 3.5 8.0 3.8 2.5 1.3 2.0 Lasagne 170.4 7.4 16.4 2.2 8.3 3.8 3.2 0.8 1.1 Chicken nuggets 251.2 15.3 22.5 0.5 11.0 1.5 4.2 4.8 3.0 Fish sticks 216.0 9.2 28.1 4.0 7.3 0.8 1.8 3.8 1.9 Tortellini (pork) 285.3 7.7 48.1 1.3 6.6 2.5 2.5 1.0 1.7 Soup (potato and leek) 355.9 16.2 47.7 7.9 10.4 2.2 4.3 2.6 8.3 Soup (mushrooms) 431.4 15.5 43.4 13.7 21.7 2.1 10.2 5.7 1.6 Wafer-cone (icecream filling) 278.0 5.2 27.8 24.2 16.2 5.9 7.0 2.9 3.5 Pudding (semolina) 362.5 6.2 82.0 14.8 0.6 0.1 0.2 0.2 1.8 Baked pastry case 483.9 4.3 43.3 21.6 32.9 15.7 12.0 3.6 4.4 Wafer (Oblate) 329.2 1.1 79.1 0.2 0.4 0.1 0.1 0.2 1.2 Rice drink (natural) 56.9 1.1 11.0 0.2 0.9 0.1 0.4 0.3 0.4 Flaky pastry 389.5 1.4 31.6 2.05 29.0 8.1 7.3 12.3 5.3 Frozen cake (almond, chocolate) 405.2 7.0 25.1 24.4 31.1 10.9 13.1 4.3 2.0 DOI: 10.7717/peerj.1337/table-1 Sodium (mg) Cholestorol (mg) Iron (mg) Calcium (mg) Potassium (mg) Zinc (mg) Phosphor (mg) Vitamin C (mg) Vitamin D (µg) Vitamin E (mg) Retinol (µg) ß-Carotin (µg) Thiamin (mg) Riboflavin (mg) Niacin (mg) Flour/bake mix Flour a 3.02 0.00 1.42 32.16 147.84 0.99 78.76 0.00 0.00 0.11 0.00 0.02 0.08 0.03 0.29 Bake mix white (cake) 39.67 0.00 1.17 54.73 240.21 1.08 204.11 0.12 0.00 0.04 0.00 0.00 0.03 0.02 0.29 Bake mix brown (cake) 41.71 9.47 1.23 17.52 252.71 0.79 118.76 0.32 0.00 0.06 0.00 0.00 0.02 0.02 0.29 Bake mix (Pizza) a 783.75 0.00 1.49 89.31 347.84 1.64 720.85 0.39 0.00 0.08 0.00 0.00 0.06 0.05 1.14 Breadcrumbs a 196.33 0.04 2.25 31.67 182.99 1.88 183.23 0.00 0.00 1.02 0.00 4.51 0.25 0.04 0.31 Bread/bakery products Rustic bread 120.53 0.00 0.38 10.58 75.45 0.48 35.90 0.00 0.00 1.01 0.00 0.00 0.06 0.02 0.48 Whole-grain bread 685.79 0.00 2.41 96.43 304.10 1.70 208.75 0.01 0.00 0.23 0.00 0.01 0.21 0.10 1.33 Toast a 394.29 0.02 1.58 34.01 273.02 1.24 135.35 0.03 0.00 2.05 0.00 0.37 0.17 0.11 1.92 Bun 402.11 4.02 0.60 17.67 52.68 0.39 36.52 0.00 0.00 0.48 0.00 1.28 0.03 0.06 0.48 Ciabatta 355.74 0.00 1.03 17.15 117.38 0.86 91.38 1.20 0.00 1.05 0.00 0.08 0.14 0.09 0.97 Raisin bread 299.48 0.37 1.01 59.31 157.72 0.73 80.86 0.43 0.00 0.08 0.02 0.04 0.07 0.09 0.51 Scone 314.90 32.64 0.82 30.76 91.38 0.50 73.71 0.01 0.00 0.19 0.06 0.05 0.07 0.14 1.09 Baguette 336.38 0.00 0.66 13.96 120.93 0.41 48.25 0.84 0.00 1.41 0.00 0.01 0.07 0.06 0.62 Lye Pretzel 790.76 7.06 0.40 124.30 137.68 0.57 78.43 13.10 0.00 0.49 0.02 1.29 0.04 0.15 0.15 Rusk 5.52 0.01 1.06 21.34 11.95 0.40 20.24 0.00 0.00 0.08 0.00 1.00 0.00 0.01 0.09 Crispbread 547.20 0.00 0.67 11.61 99.13 0.67 109.04 0.00 0.00 0.24 0.00 0.04 0.11 0.04 1.38 Wraps 402.35 0.63 0.21 18.09 83.57 0.19 39.96 0.25 0.23 0.07 0.05 0.06 0.03 0.01 0.58 Pasta and cereal-based products Fusilli 1.88 0.00 1.80 14.67 114.68 1.87 205.75 0.00 0.00 0.81 0.00 0.21 0.33 0.10 1.76 Spaghetti 1.00 0.00 2.40 18.00 120.00 2.50 256.00 0.00 0.00 1.11 0.00 0.30 0.44 0.13 1.93 Penne 1.90 0.00 3.15 22.73 198.37 1.43 211.40 0.00 0.00 0.48 0.00 0.12 0.28 0.12 1.61 Lasagne sheets 55.98 146.52 1.37 30.53 91.97 1.32 144.47 0.00 0.00 0.89 0.10 0.04 0.10 0.18 0.52 Vermicelli 6.71 0.00 3.81 45.15 297.12 3.05 327.38 0.93 0.00 1.68 0.00 0.31 0.51 0.16 2.14 Tagliatelli 5.94 0.00 3.65 41.70 274.68 3.02 321.78 0.80 0.00 1.62 0.00 0.32 0.51 0.16 2.14 Cous Cous 1.00 0.00 1.00 4.00 80.00 0.41 73.00 0.00 0.00 0.52 0.00 0.26 0.13 0.04 1.20 Cereals Granola (chocolate) 504.01 15.77 1.51 17.98 265.98 0.52 73.86 0.13 0.00 0.25 0.00 0.51 0.05 0.06 0.96 Granola (nuts) 393.96 0.16 1.72 44.88 314.76 1.60 174.62 0.11 0.00 3.54 0.00 1.78 0.20 0.09 1.40 Cornflakes 575.88 0.00 1.52 8.61 265.44 1.45 208.76 0.00 0.00 1.47 0.00 0.90 0.35 0.20 1.47 Cookie and cakes Shortbread 408.14 17.31 0.66 33.19 79.97 0.59 72.93 0.22 0.00 0.84 0.02 9.02 0.05 0.10 0.73 Neapolitan wafers (original) 16.20 21.70 0.71 8.86 259.18 0.59 61.06 0.01 0.00 0.12 0.00 0.01 0.03 0.03 0.37 Cookie (chocolate) 199.30 3.37 0.82 13.20 91.10 0.73 57.96 0.00 0.00 1.66 0.00 4.50 0.05 0.02 0.26 Mignon wafers (hazelnut) 417.91 28.84 2.70 28.12 476.92 1.16 124.82 0.02 0.00 1.31 0.00 0.03 0.09 0.06 0.68 Marble cake 54.88 138.60 1.32 22.11 103.26 0.76 98.91 0.00 0.00 12.54 0.10 0.02 0.05 0.15 0.24 Ladyfinger 98.46 141.37 1.00 37.26 79.88 0.80 100.96 0.00 0.00 0.72 0.10 0.00 0.05 0.15 0.28 Cookie (whole-grain) 295.19 5.16 1.17 15.24 68.83 0.78 89.30 0.13 0.00 0.37 0.00 0.12 0.14 0.05 0.87 Granola bar 237.91 4.24 2.14 96.16 292.51 1.37 170.55 0.45 0.00 1.00 0.01 0.16 0.19 0.19 1.23 Cookie (orange)a 190.66 23.28 1.74 103.46 389.10 0.83 120.50 4.78 0.00 0.39 0.06 0.27 0.08 0.13 0.45 Apple strudel 102.98 1.83 1.13 177.03 240.82 0.68 128.36 4.75 0.00 0.16 0.05 0.08 0.07 0.27 0.25 Muffin 247.84 79.20 2.66 29.50 219.98 0.74 102.31 0.00 0.00 3.54 0.06 0.01 0.06 0.11 0.30 Snacks Cracker 2416.36 47.71 1.40 434.99 83.08 2.05 305.78 0.43 0.00 0.41 0.13 0.06 0.03 0.12 0.48 Brezels 554.28 0.20 0.67 11.72 17.23 0.45 26.93 0.00 0.00 1.48 0.00 4.26 0.01 0.03 0.21 Grissini 226.87 20.01 0.63 52.59 64.73 0.62 62.82 0.06 0.00 1.31 0.03 15.02 0.02 0.09 0.07 Saltsticks 1007.36 0.21 0.93 15.82 25.22 0.47 30.69 0.00 0.00 1.56 0.00 4.47 0.02 0.04 0.34 Wafers (plain) 119.98 0.01 1.50 8.15 268.55 1.47 211.73 0.02 0.00 1.61 0.00 0.92 0.36 0.20 1.49 Convenience foods Pizza (salami) 614.15 22.66 0.97 103.75 275.57 1.13 115.28 7.43 0.00 3.23 0.04 0.24 0.11 0.11 1.68 Pizza (margherita) 437.51 14.95 0.68 110.73 266.87 0.85 106.10 6.97 0.00 2.97 0.05 0.26 0.05 0.10 1.02 Lasagne 450.21 38.88 0.79 56.08 211.47 1.38 99.60 5.51 0.02 1.14 0.04 0.24 0.11 0.13 1.51 Nuggets 396.19 35.00 1.34 16.80 158.47 1.37 179.53 0.05 0.00 5.46 0.00 0.16 0.14 0.09 4.71 Fish sticks 378.34 23.54 1.19 24.87 128.33 0.49 142.50 0.24 0.00 4.04 0.00 0.11 0.12 0.22 1.62 Tortellini (pork) 628.05 72.33 1.49 51.06 118.68 1.64 122.19 1.06 0.01 0.75 0.04 0.04 0.24 0.12 1.38 Soup (potato and leek) 3801.76 1.03 2.53 80.57 1014.25 0.88 260.47 11.70 0.00 1.61 0.00 0.42 0.17 0.28 4.63 Soup (mushrooms) 472.57 0.43 0.89 17.52 104.01 0.22 34.90 0.95 0.00 3.82 0.00 0.68 0.04 0.07 0.91 Wafer-cone (icecream filling) 101.42 22.61 1.39 97.43 358.09 0.78 127.37 0.81 0.12 3.66 0.02 0.04 0.04 0.16 0.37 Pudding (semolina) 117.48 0.00 0.54 9.64 91.27 0.50 79.69 0.10 0.00 0.09 0.00 0.00 0.06 0.02 1.18 Baked pastry case 108.30 0.32 1.61 30.08 138.24 1.30 202.23 0.70 0.00 2.87 0.00 6.82 0.18 0.09 1.91 Wafer (Oblate) 6.98 0.00 1.80 31.88 26.15 0.43 35.54 0.00 0.00 0.12 0.00 0.03 0.05 0.02 0.33 Rice drink (natural) 52.58 0.00 0.23 12.47 17.01 0.20 15.55 0.00 0.00 0.51 0.00 0.00 0.01 0.00 0.19 Flaky pastry 412.00 2.35 0.22 49.65 47.07 0.20 24.56 0.13 0.03 2.10 0.00 0.17 0.24 0.01 0.22 Frozen cake (almond, chocolate) 120.14 170.20 1.69 39.44 219.39 0.90 126.96 0.21 0.13 4.26 0.15 2.19 0.06 0.16 0.38 DOI: 10.7717/peerj.1337/table-2 Primary outcome parameter: macronutrient and energy content Across all food categories, energy content ranged between 270.5 ± 13.5 kcal/100 g (category: bread and bakery products) to 398.8 ± 25.4 kcal/100 g (category: snacks). Across all food categories, energy content, carbohydrate, total fat, saturated fatty acids, fiber and sugar did not differ between GF and products gluten-containing products (F < 1; p > 0.05). Protein content was significantly lower in GF foods (5.8 ± 3.7 g/100 g) than gluten-containing foods (8.6 ± 2.9 g/100 g); F = 31.9; p < 0.01 (see Fig. 1). Lower protein content was present in 4 out of 7 food categories (flour/bake mix, bread and bakery products, pasta and cereal-based products and snacks). In flour/bake mix products, the average protein content was 4.6 ± 3.4 g/100 g for GF and 9.9 ± 2 g/100 g for their gluten-containing counterparts (see Table 3). Figure 1: Nutrient content in g/100 g between gluten-free and gluten-containing foods across seven different food categories. Notes. Data displayed as mean values. * Significant differences (p < 0.05) between gluten-free and gluten containing foods. Flour/bake mix Bread/bakery products Pasta and cereal- based products Cereals Cookie and cakes Snacks Convenience foods Energy (kcal) Gluten free 346.4 ± 35.5 270.5 ± 46.7 351.9 ± 17.7 397.5 ± 63.6 395.8 ± 84.4 398.7 ± 59.2 297.3 ± 108.5 Gluten-containing 335.0 ± 23.0 280.9 ± 49.8 346.5 ± 10.9 397.4 ± 56.3 416.3 ± 78.1 371.6 ± 45.3 298.2 ± 102.0 P† 0.373 0.543 0.426 0.997 0.488 0.376 0.978 Protein (g) Gluten Free 4.6 ± 3.2 4.1 ± 2.2 9.2 ± 2.1 7.0 ± 1.2 4.8 ± 1.6 4.3 ± 4.1 7.4 ± 4.8 Gluten-containing 10.0 ± 2.0 8.3 ± 1.0 11.9 ± 0.9 9.2 ± 2.4 5.8 ± 1.9 10.5 ± 1.4 8.2 ± 3.9 P† <0.01 <0.01 <0.05 0.245 0.105 <0.01 0.605 Carbohydrates (g) Gluten Free 74.4 52.3 71.7 66.7 57.3 62.4 37.2 Gluten-containing 67.8 52.7 69.4 57.6 59.9 61.4 39.5 P† <0.05 0.912 <0.05 0.101 0.561 0.860 0.733 Sugar (g) Gluten Free 9.5 ± 17.2 5.5 ± 5.2 1.8 ± 1.1 17.4 ± 13.3 27.2 ± 14.6 1.3 ± 0.8 8.2 ± 8.8 Gluten-containing 6.0 ± 9.1 4.4 ± 5.4 0.8 ± 0.2 14.5 ± 6.4 33.7 ± 12.6 1.0 ± 0.2 9.8 ± 12.2 P† 0.543 0.912 <0.05 0.714 0.188 <0.860 0.655 Total fat (g) Gluten Free 2.9 ± 2.2 4.7 ± 2.7 2.7 ± 0.9 11.1 ± 7.3 16.4 ± 8.4 14.4 ± 6.2 13.1 ± 10.5 Gluten-containing 2.2 ± 2.3 3.8 ± 2.8 1.9 ± 1.0 14.3 ± 8.0 17.0 ± 8.4 9.0 ± 8.0 11.6 ± 9.0 P† 0.481 0.349 0.133 0.621 0.852 0.238 0.630 Saturated fatty acids (g) Gluten Free 0.9 ± 1.1 1.4 ± 1.3 0.4 ± 0.3 4.3 ± 2.9 7.1 ± 5.1 6.8 ± 3.8 4.1 ± 4.3 Gluten-containing 0.7 ± 1.2 0.9 ± 1.0 0.4 ± 0.3 4.4 ± 3.4 6.8 ± 4.5 4.2 ± 5.3 4.0 ± 4.1 P† 0.716 0.183 0.818 0.986 0.868 0.384 0.917 MUFA (mg) Gluten Free 1.0 ± 1.1 1.5 ± 0.8 0.9 ± 0.4 4.8 ± 4.0 5.9 ± 3.2 5.2 ± 2.6 4.8 ± 4.1 Gluten-containing 0.5 ± 0.9 1.3 ± 1.2 0.4 ± 0.4 6.4 ± 4.1 6.7 ± 3.6 2.7 ± 2.4 4.5 ± 3.7 P† 0.236 0.614 <0.05 0.627 0.509 0.117 0.783 PUFA (mg) Gluten Free 0.8 ± 0.6 1.4 ± 1.1 1.1 ± 0.4 1.5 ± 0.8 2.4 ± 1.7 1.8 ± 0.7 3.0 ± 3.0 Gluten-containing 0.6 ± 0.4 1.0 ± 0.8 0.8 ± 0.3 3.2 ± 0.9 2.6 ± 1.8 1.4 ± 1.0 1.9 ± 1.4 P† 0.220 0.195 0.100 0.055 0.723 0.412 0.101 Fiber (g) Gluten Free 4.1 ± 2.9 3.9 ± 2.7 7.9 ± 4.0 5.6 ± 1.4 3.5 ± 3.1 2.0 ± 2.8 2.6 ± 2.0 Gluten-containing 4.0 ± 2.3 3.3 ± 1.7 3.7 ± 0.7 7.4 ± 1.9 3.5 ± 3.8 4.6 ± 3.2 2.4 ± 2.0 P† 0.944 0.429 <0.05 0.247 0.979 0.188 0.669 Sodium (mg) Gluten Free 255.9 ± 326.9 388.4 ± 198.3 10.6 ± 18.6 491.3 ± 74.8 205.0 ± 122.6 856.0 ± 835.0 539.8 ± 894.2 Gluten-containing 281.6 ± 294.6 581.9 ± 284.6 15.8 ± 18.0 160.7 ± 127.2 247.8 ± 394.0 832.1 ± 626.7 715.8 ± 1186.1 P† 0.855 0.039 0.564 <0.05 0.724 0.938 0.623 Cholestorol (mg) Gluten Free 1.2 ± 3.1 3.4 ± 8.7 20.9 ± 51.3 5.3 ± 7.4 40.7 ± 49.0 13.6 ± 18.7 27.0 ± 43.1 Gluten-containing 2.6 ± 7.0 5.5 ± 12.1 21.5 ± 42.2 1.4 ± 2.0 32.4 ± 38.4 1.5 ± 2.6 28.5 ± 41.5 P† 0.617 0.602 0.980 0.318 0.595 0.083 0.908 Iron (mg) Gluten Free 1.6 ± 0.8 1.0 ± 0.7 2.5 ± 1.0 1.6 ± 0.1 1.5 ± 0.7 1.0 ± 0.4 1.2 ± 0.6 Gluten-containing 1.6 ± 1.1 1.2 ± 0.6 2.5 ± 0.3 6.1 ± 7.3 1.6 ± 1.3 2.2 ± 1.8 1.4 ± 1.3 P† 0.964 0.216 0.944 0.375 0.840 0.188 0.496 Calcium (mg) Gluten Free 47.3 ± 27.2 37.6 ± 33.9 25.3 ± 13.7 23.8 ± 15.4 55.6 ± 55.0 104.7 ± 165.9 48.8 ± 33.1 Gluten-containing 24.3 ± 19.1 18.9 ± 10.8 33.6 ± 7.6 75.4 ± 117.0 37.2 ± 27.5 50.3 ± 45.1 46.4 ± 40.0 P† <0.05 <0.05 0.105 0.524 0.202 0.384 0.842 Potassium (mg) Gluten Free 231.3 ± 127.5 138.3 ± 118.9 168.1 ± 82.5 282.1 ± 23.1 224.2 ± 136.7 91.8 ± 91.7 211.7 ± 234.1 Gluten-containing 222.3 ± 86.8 201.3 ± 61.0 295.4 ± 121.2 378.3 ± 152.7 235.1 ± 124.3 309.7 ± 179.8 241.8 ± 145.4 P† 0.848 <0.05 <0.05 0.370 0.818 0.352 0.607 Zinc (mg) Gluten Free 1.4 ± 0.8 0.7 ± 0.5 1.9 ± 0.9 1.2 ± 0.5 0.8 ± 0.2 1.0 ± 0.6 0.8 ± 0.5 Gluten-containing 1.2 ± 1.1 0.8 ± 0.4 4.6 ± 0.4 2.6 ± 1.0 0.8 ± 0.6 1.6 ± 1.1 1.0 ± 0.8 P† 0.734 0.766 <0.01 0.078 0.930 0.370 0.346 Phosphor (mg) Gluten Free 286.1 ± 267.0 84.1 ± 61.3 220.0 ± 85.2 152.4 ± 57.3 104.0 ± 33.5 127.6 ± 111.8 111.5 ± 66.4 Gluten-containing 194.7 ± 177.1 104.9 ± 49.2 301.3 ± 178.4 321.0 ± 222.1 109.2 ± 73.7 207.2 ± 152.4 130.7 ± 84.6 P† 0.350 0.247 0.290 0.292 0.823 0.352 0.456 Vitamin C (mg) Gluten Free 0.2 ± 0.2 1.2 ± 3.4 0.2 ± 0.4 0.1 ± 0.1 1.3 ± 2.4 0.1 ± 0.2 2.4 ± 3.5 Gluten-containing 0.0 ± 0.0 1.6 ± 7.6 1.0 ± 3.5 22.4 ± 49.9 2.0 ± 3.4 0.0 ± 0.1 16.7 ± 56.1 P† <0.05 0.885 0.612 0.516 0.512 0.404 0.341 Vitamin E (mg) Gluten Free 0.3 ± 0.4 0.7 ± 0.8 1.0 ± 0.4 1.8 ± 1.4 1.9 ± 3.3 1.3 ± 0.4 2.4 ± 1.6 Gluten-containing 0.3 ± 0.4 0.8 ± 1.0 1.0 ± 0.2 2.1 ± 1.2 2.3 ± 3.2 0.9 ± 0.5 1.7 ± 1.1 P† 0.952 0.841 0.972 0.728 0.724 0.178 0.065 ß-Carotin (mg) Gluten Free 1.1 ± 3.0 0.4 ± 0.5 0.2 ± 0.1 1.1 ± 0.5 1.2 ± 2.7 4.9 ± 5.3 0.8 ± 1.7 Gluten-containing 0.0 ± 0.0 0.2 ± 0.5 0.1 ± 0.3 1.2 ± 1.2 1.1 ± 1.6 0.4 ± 0.7 0.9 ± 1.5 P† 0.160 0.347 0.320 0.838 0.887 <0.05 0.786 Niacin (mg) Gluten Free 0.6 ± 0.4 0.9 ± 0.7 1.6 ± 0.5 1.3 ± 0.2 0.5 ± 0.3 0.5 ± 0.5 1.5 ± 1.4 Gluten-containing 1.5 ± 1.6 1.6 ± 0.8 3.6 ± 2.7 11.0 ± 22.0 0.7 ± 0.8 3.3 ± 3.1 1.8 ± 1.6 P† 0.128 <0.05 0.077 0.523 0.552 0.084 0.488 DOI: 10.7717/peerj.1337/table-3 Secondary outcome parameter: micronutrients and product cost Overall, sodium content in gluten-containing foods (448.9 ± 704.6 mg/100 g) did not differ compared to GF foods (373.5 ± 569.2 mg/100 g; F < 1, p > 0.05). In one category (cereal products), sodium content was higher in GF foods. Across all three analyzed GF cereal products, sodium content was 491.3 ± 91.6 mg/100 g while in gluten-containing foods, sodium content was 160.7 ± 139.3 mg/100 g (F = 13.4; p < 0.01). For bread and bakery products, sodium content was lower in GF products (388.4 ± 206.4 mg/100 g) compared to gluten-containing foods (581.9 ± 290.3 mg/100 g; F = 4.5; p < 0.05). Across all other categories, sodium content did not differ significantly. 27% of all products showed high sodium content (defined as >500 mg/100 g, Commission of the European Communities (2006)), this did not differ between GF and gluten-containing foods (χ2 [1] = 1.94; p > 0.05). In contrast, 65% of GF and 61% of gluten-containing foods showed low sodium content (defined as <120 mg/100 g, Commission of the European Communities (2006)). Potassium content was significantly lower in GF products (190.4 ± 160 mg/100 g) than in products containing gluten (247.5 ± 130 mg/100 g; F = 6.9; p < 0.05). This difference was present in 2 out of 7 food categories (pasta and cereal-based products; snacks), while all other categories did no show significant differences. Moreover, zinc content was significantly lower in GF pasta products (1.9 ± 0.9 mg/100 g vs. 4.6 ± 0.4 mg/100 g; F = 82.1; p < 0.01). GF Pasta products showed higher fiber content (7.9 ± 4.2 g/100 g) when compared to gluten-containing products (3.7 ± 0.7 g/100 g; F = 13.6; p < 0.01). Across all GF products, 19% can be classified as source high in fiber (defined as >6 g/100 g, Commission of the European Communities (2006)). Substantial cost disparities were present between GF products and gluten-containing products. On average, the cost for GF products ranged from 2.95€ (white flour) to 80.80€ per kg (Wafer, Oblaten) and was significantly higher in GF products (11.58 ± 11.43€) compared to gluten-containing products (6.62 ± 5.36€; F = 53.1; p < 0.01) across all product categories. Within bread and bakery products, GF food were +267% more expensive than similar gluten-containing products. The lowest cost disparity (+205% higher cost for GF products) was observed for cereal products (Fig. 2). Figure 2: Cost in €/kg between gluten-free and gluten-containing foods across seven different food categories. Notes. Data displayed as mean values. Within all food categories differences in mean cost between GF and and gluten-containing foods were significant (p < 0.05).

Discussion The present study is the first attempt to present a large dataset comparing GF foods and gluten-containing products available in one German-speaking country (Austria). The data showed that there is great variability between GF foods and gluten-containing products for specific nutrients. One key finding of this study is that protein content was significantly lower in GF foods across all staple foods. In flour/bake mix products, the average protein content was >2 fold lower compared to their gluten-containing counterparts. This finding is in line with previous findings (Wu et al., 2015), except that we did not observe significant differences in total fat, saturated fat, PUFA and MUFA in our products (Kulai & Rashid, 2014; Matos Segura & Rosell, 2011; Miranda et al., 2014). Only in pasta and cereal-based products was MUFA content significantly higher in GF foods. The low amount of proteins in GF foods can be explained by their formulation. In GF formulations, carbohydrate-rich but protein-poor ingredients are used, such as white rice flour, tapioca or potato starch (Mezaize et al., 2009). This can lead to lower protein content in GF foods, which may be a reasonable explanation for the observed differences in our data. Reports about protein intake and its clinical relevance for celiac patients is conflicting. In a prospective study comparing dietary intake from 88 celiac patients (7-day dietary record) with data from non-celiac individuals from the German National Diet and Nutrition Survey (NVS II), no differences in protein intake for males or females were observed (Martin et al., 2013). On the other hand, Miranda et al. (2014) analyzed 58 adults with CD and showed that protein intake was lower in women who were on a GF diet compared to a diet containing gluten. In this study, the protein content of breads was almost one third lower than their equivalent foods with gluten. In our dataset, GF breads contained half of the proteins compared to regular breads with gluten. Addtionally, in a cross-sectional study, Van Hees et al. (2015) compared dietary intake of amino acids in 77 CD patients. Compared to 33 healthy controls, celiac patients with good adherence to a GF diet showed significantly lower amino acid concentrations in blood (tyrosine, phenylalanine and tryptophan). The authors argue that both, a reduced intake of vegetable protein and malabsorption as a results of CD may be responsible for this result. The findings of our study suggest that reduced protein content in GF products may facilitate problematic protein intake in CD patients and should be considered in dietary counseling. In 65% of all analyzed GF foods, low sodium content (defined as <120 mg/100 g, Commission of the European Communities (2006)) was observed. Interestingly, in bread and bakery products, sodium content was lower compared to gluten-containing foods. The lower amount of sodium in GF bread may be accounted for the joint initiative “Weniger Salz is g’sünder” with the aim to reduce salt in bread and bakery products by 15% by 2015 initiated by the Austrian Ministry of Health (2011) and the Industrial Bakers of Austria (Lloyd-Williams et al., 2014). Foods from the datasets used in this study contained nutrient information that were assessed prior to this initiative (started 2011), which may be a possible explanation for this discrepancy.

Conclusions In conclusion, this study presents the first findings for a thorough analysis of GF products in a German-speaking country. There are some marked differences between GF and gluten-containing foods. Based on the nutrient composition of GF foods, our results indicate that GF foods are not aligned with particular health benefits, but rather show critical nutrients which should be considered in future formulations. The findings of our study indicate that re-thinking the health aspects ascribed to GF products, at least based on nutrient content of GF foods, should be considered and publicly communicated. Especially in the face of a growing market share, common health misconceptions should be kept in mind when discussing GF products.