BMC Public Health 2009, 9:340 http://www.biomedcentral.com/1471-2458/9/340 Background Acne is a very common skin condition of the face and upper trunk affecting millions of adolescents everyday [1]. It is of great interest and importance to explore it further to elucidate possible associated factors which may provide clues to its aetiology. The distribution of acne in populations has been shown to vary across gender [2-8], ethnicity [1,9], socio-demo- graphic variables [8,10], cigarette smoking [11,12], diet [13,14], mental health problems [15-17] and may even be associated with suicidal thoughts [15,18]. These studies have helped us identify risk factors for the development of acne and made us understand the burden this condition represents for young people. The importance acne may have on the daily life of adolescents must not be trivial- ized, and it has been demonstrated that the quality of life of acne patients is at the same level as patients with other chronic conditions such as asthma, epilepsy, diabetes, back pain and arthritis [19] Since acne is a visual disease and starts in adolescents, it is particularly relevant to explore the way in which this skin condition is associated with psycho-social factors. Adoles- cence is an important and vulnerable period in most peo- ple's lives as it is the time of transition between the dependence of childhood to the independence of adult- hood [20]. There is, however, conflicting evidence about a coexistence of acne and mental health problems: some studies have found an association [15-17], while others have not been able to identify any [21,22]. Another field of controversy in the study of acne is the way in which lifestyle factors and diet in particular contribute to the distribution of acne in populations. After decades of a lack of studies examining a possible link between diet and acne, there have now been observations that consum- ing milk can increase the prevalence of acne [13,14] and that a low glycaemic diet can decrease the prevalence of acne [23]. In yet another study of two non-Westernized societies, no cases of active acne among young people was observed: it was discussed whether this finding could be related to lifestyle [24]. This also points to ethnic differ- ences as a field of interest in the study of acne. Interestingly, in a great majority of cases both acne and mental health problems start in adolescents. Depression and anxiety are common among adolescents, and the increase in prevalence begins at puberty [25]. There is an increasing evidence for a link between mental health problems and diet, e.g. sugar [26], consuming fish [27,28] and other lifestyle factors such as smoking [29]. It has been hypothesized that the same nutritional factors may influence both acne and mental problems [30,31]. In a publication in 2007 by Katzman and Logan they espe- cially mention zinc, folic acid, selenium, chromium and w-3 fatty acids as examples of nutrients that may cause depression, anger and/or anxiety on one side and acne on the other side. In a report of five cases from 2008 both acne and mental health improved after 2-month time period of omega-3-based dietary supplement [31]. In light of this knowledge it is of interest to study acne in a general adolescent population. The aims of this paper are thus first to investigate the association between acne and mental distress and thereafter the possible confound- ing effect of diet on this relationship. Methods Participants and study design The study population consisted of non-healthcare seeking late adolescents in the Norwegian capital of Oslo, a mul- ticultural Western city of about 1/2 million inhabitants. The design of the study was questionnaire-based and cross-sectional. A four-page questionnaire provided infor- mation on somatic, mental and lifestyle variables. The study itself (Youth 2004) was conducted by the University of Oslo, the Norwegian Institute of Public Health and the Regional Centre for Child and Adolescent Mental Health, Eastern and Southern Norway. The data were collected at schools and by post [32]. In 2004, all pupils (mainly born in 1986 and thus 18 or 19 years old) in Oslo in their final year of compulsory schooling were invited to participate. This part of the study was conducted in class. Of the 3659 pupils invited, 3308 (90%) participated. In addition, 1085 adolescents in Oslo aged 18 or 19 who were not in their final year were invited by post to participate and 467 (43%) returned the questionnaire. These 1085 adolescents had participated earlier in a population-based study in Oslo [33]. In total, 3775 (80%) of the 4744 adolescents invited participated. Measures Acne This was collected by the question. "In the last week, have you had pimples?" and four possible categories was dichotomized into "No" and "Yes, a little" versus "Yes, quite a lot" and "Yes, very much". The question regarding acne have been assessed in a validation study and also been used in a population study regarding self-esteem and body satisfaction among late adolescents with acne [34,35]. In the validation study the adolescents' answers have been compared to a dermatologist's assessment and the overall agreement, sensitivity and specificity regarding acne was 74%, 93% and 43%. Mental distress This was measured using Hopkins Symptom Checklist 10 (HSCL-10) which is a ten-item, shortened version of the Page 2 of 8 (page number not for citation purposes)