Brouwer-Brolsma et al. Genes & Nutrition (2020) 15:7 https://doi.org/10.1186/s12263-020-00667-z REVIEW Open Access Food intake biomarkers for green leafy vegetables, bulb vegetables, and stem vegetables: a review Elske M. Brouwer-Brolsma1*†, Beate Brandl2†, Marion E. C. Buso1, Thomas Skurk2,3 and Claudine Manach4 Abstract: Background: Numerous studies acknowledged the importance of an adequate vegetable consumption for human health. However, current methods to estimate vegetable intake are often prone to measurement errors due to self-reporting and/or insufficient detail. More objective intake biomarkers for vegetables, using biological specimens, are preferred. The only concentration biomarkers currently available are blood carotenoids and vitamin C, covering total fruit and vegetable intake. Identification of biomarkers for specific vegetables is needed for a better understanding of their relative importance for human health. Within the FoodBAll Project under the Joint Programming Initiative “A Healthy Diet for a Healthy Life”, an ambitious action was undertaken to identify candidate intake biomarkers for all major food groups consumed in Europe by systematically reviewing the existent literature. This study describes the review on candidate biomarkers of food intake (BFIs) for leafy, bulb, and stem vegetables, which was conducted within PubMed, Scopus and Web of Science for studies published through March 2019. Results: In total, 65 full-text articles were assessed for eligibility for leafy vegetables, and 6 full-text articles were screened for bulb and stem vegetables. Putative BFIs were identified for spinach, lettuce, endive, asparagus, artichoke, and celery, but not for rocket salad. However, after critical evaluation through a validation scheme developed by the FoodBAll consortium, none of the putative biomarkers appeared to be a promising BFI. The food chemistry data indicate that some candidate BFIs may be revealed by further studies. Conclusion: Future randomized controlled feeding studies combined with observational studies, applying a non- targeted metabolomics approach, are needed in order to identify valuable BFIs for the intake of leafy, bulb, and stem vegetables. Keywords: Lettuce, Spinach, Endive, Garden rocket, Asparagus, Artichoke, Celery Background associations [1–3]. Despite these suggested health effects, The importance of an adequate vegetable intake to promote vegetable consumption rates among European citizens vary health has been recognized for years. Vegetables are substantially, ranging from 34.4% of the population not nutrient-dense foods containing high concentrations of consuming fruits and vegetables on a daily basis to 14.1% dietary fibers as well as various vitamins, minerals, and phy- eating ≥ 5 portions [4]. Obviously, the wide variety of avail- tochemicals. Accordingly, the potential impact of vegetable able vegetables substantially differs with respect to their nu- intake on human health has been extensively explored in trient content and associated benefits for health. Therefore, nutritional epidemiological studies, showing beneficial it is crucial to distinguish between the different vegetable groups, e.g., cruciferous vegetables, root vegetables, tubers, * Correspondence: [email protected] fruit vegetables, bulb and stem vegetables, and leafy vegeta- †Elske M. Brouwer-Brolsma and Beate Brandl contributed equally to this bles, when addressing potential health effects. study. Epidemiological data suggest that leafy vegetables de- 1Division of Human Nutrition and Health, Wageningen University, PO Box 17, 6700 AA Wageningen, The Netherlands serve special attention. A large meta-analysis including Full list of author information is available at the end of the article © The Author(s). 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Brouwer-Brolsma et al. Genes & Nutrition (2020) 15:7 Page 2 of 12 95 prospective studies recently revealed that a high in- observational cohort studies. However, compared to diar- take of leafy green vegetables was associated with a re- ies and 24 h recalls, FFQs are usually more restrained with duced risk of coronary heart disease (10 studies, RR = respect to the variety of foods queried as well as portion 0.83, 95% CI = 0.75–0.91), stroke (4 studies, RR = 0.88, size estimations. Moreover, both FFQs and recalls are 95% CI = 0.81–0.95), and a 24% reduction in all-cause prone to measurement error resulting from socially desir- mortality for each 100 g/day increment in intake [1]. In able answers and errors in food composition tables [17– another meta-analysis focusing on associations between 20]. all fruit and vegetable subtypes and type 2 diabetes, only Therefore, the use of candidate biomarkers of food in- green leafy vegetables (highest vs lowest intake categor- take (BFIs), i.e., objective food consumption markers in ies) were associated with a significant risk reduction (RR biological specimens such as urine or blood, is gaining = 0.84, 0.74–0.94) [2]. Spinach (Spinacia oleracea, interest, even though there are still very few well- Amaranthaceae family) represents a main member of validated dietary intake biomarkers [21]. The intake of leafy vegetables and has been shown to decrease post- total fruit and vegetable consumption is frequently esti- prandial arterial stiffness and systolic blood pressure in mated using blood vitamin C or carotenoids (i.e., plasma randomized controlled trials [5–7]. Moreover, garden α- and β-carotene, β-cryptoxanthin, and lutein) [22, 23]. rocket (Eruca sativa), a green-leafy vegetable from the However, the use of these biomarkers does not allow to Brassicaceae, has been proposed to possess anti-oxidative, distinguish between the types of fruit and vegetables anti-inflammatory, and anti-ulcer properties [8, 9]. Astera- consumed. Integrating the data as obtained by one or ceae is another botanical family of leafy vegetables repre- more of the traditional dietary assessment methods with senting amongst others endive and lettuce. Bulb and stem validated BFIs data may eventually facilitate nutritional vegetables have been less extensively studied than leafy research by resulting in more precise intake estimates vegetables and constitute a rather heterogenous group that have the ability to distinguish between the specific including in particular celery, artichoke and asparagus. varieties of fruits and vegetables consumed [24]. Celery (Apium graveolens L.) has been proposed to pos- To facilitate the identification and evaluation of new sess anti-oxidative and anti-inflammatory potency [10, candidate BFIs, the Food Biomarker Alliance (FoodBAll) 11], whereas artichoke (Cynara scolymus) received atten- [21], a project funded by the Joint Programming Initiative tion through its potential beneficial impact on LDL- a Healthy Diet for a Healthy Life, established guidelines to cholesterol, endothelial NOS expression, and hepato- conduct a literature search dedicated to food intake bio- digestive function [12]. Asparagus (Asparagus officinialis markers [25] and to evaluate their level of validation using with Asparagales as the botanical family) on its turn is a a set of consensus criteria [26]. The guidelines were ap- rich source of asparagine, potassium, vitamin C, and anti- plied for all major food groups: fruit and vegetables, meats, oxidative glycolipids [13]. fish, and other marine foods, dairy products, cereals and Up to now, vegetable intake in the epidemiological whole grains, alcoholic and non-alcoholic beverages, vege- studies was generally assessed using traditional self- table oils, nuts, and spices and herbs (http://foodmetabo- reported dietary assessment methods, such as food diar- lome.org/wp3)[27–36]. The present article presents the ies, 24 h recalls and food frequency questionnaires results of the in depth exploration of possible biomarkers (FFQs). Although these self-report dietary assessment of intake for green leafy vegetables (lettuce, spinach, en- methods provide valuable information, they are prone to dive, and garden rocket) as well as bulb and stem vegeta- measurement error and burdensome for both partici- bles (artichoke, asparagus, celery). pants and researchers. Bingham and colleagues observed that the intake of fruits and vegetables assessed by FFQ was almost 2-fold higher compared with a food diary [14]. Material and methods At the same time, the association between plasma vitamin Selection of vegetable varieties C and the intake of fruits and vegetables was stronger In order to cover the major varieties of vegetables con- when assessed by food diary than by FFQ, suggesting a sumed in Europe, FoodBAll aimed to cover cruciferous higher degree of measurement error with the FFQ than vegetables, root vegetables,