'The wisdom of crowds': a survey on the rating of nutritional values of meals in digital pictures.

Background: Photo-based nutrition diaries might be useful to assess dietary intake without much effort and maybe even without nutrition expertise. This proof-of-concept study aimed to investigate the principle of 'the wisdom of crowds' by examining how accurately both nutrition experts and non-experts can rate nutritional values of meals presented in digital pictures.

Methods: An online survey was conducted among adults in Germany from 2016 to 2017. Participants rated a random selection of six meal pictures according to their nutritional values (energy, carbohydrates, sugar and fat content) and their healthiness. Rating results were compared with the nutritional value calculated by the German Nutrient Database or according to the manufacturer's information ('truth'). Descriptive statistical analysis, Mann-Whitney-U test and multiple linear regression analysis were performed using RStudio.

Results: In total, 110 (92.7 % women, mean age: 38.7±14.0 years) nutrition experts and 233 (31.3 % women, mean age: 21.2±2.6 years) non-experts participated. Overall meal pictures, experts overestimated the average content of all nutritional values (sugar: 3.8 %, energy: 4.9 %, carbohydrates: 4.9 % and fat: 10.4 %). Non-experts overestimated the average energy content by 10.4 %, fat content by 17.1 % and sugar content by 27.5%. The average carbohydrate content was underestimated by 9.0%. A statistically significant difference between the two crowds' ratings was found for energy (p=0.03), carbohydrates (p<0.001) and sugar (p<0.001), but not for fat (p=0.44). An increasing deviation of nutritional value ratings from the truth towards overrating was associated with decreasing rating of healthiness (all p<0.001).

Conclusion: This study suggests that both experts and non-experts rate nutritional values of meals in digital pictures in an appropriate manner, although both crowds occasionally deviated significantly from the truth, especially over-rating occurred with decreasing rating of healthiness. Due to the proof-of-concept approach and the limited generalisability of the results, the principle of 'the wisdom of crowds' is not fully supported. Studies with a large representative population are necessary. However, findings suggest that crowd-based meal picture ratings could be a method of digital dietary self-monitoring in combination with gamification elements.