Obesity has become a major health concern worldwide due to its increasing prevalence and its detrimental health impact. The overarching aim of the present thesis was to study the aetiology of obesity from anthropological, psychological, environmental and genetic perspectives in a population residing in the Basque Country (Spain). First, the relationship between body morphology and composition (assessed through the somatotype components) and different nutritional categories was studied. The results of these analyses showed that not only fat mass but also fat free mass increase in obesity development.Changes in body composition and morphology also entail changes in body image, which may influence the perception of the own image, self-esteem and attitudes towards it. The present thesis shows that body image perception is strongly related to nutritional status both in men and women. However, in general women desired to be thinner while men desired bigger body frames.Lifestyle changes in modern society (increase consumption of energy dense food and decrease physical activity) have been proposed as the main factors responsible for the increasing prevalence of obesity in westernized countries during last decades. Results in this thesis indicated that physical activity, sleep quality and dietary habits are predominantly associated with general fat mass while tobacco and alcohol consumption among others are predominantly associated with fat distribution.Genetic background is also considered a significant contributor to obesity risk. Although genetics could not be responsible of obesity epidemic in the last decades, as genetic background could not be significantly altered in that time frame, genetic factors strongly influence whether a person becomes obese or stays slim within a population, at an individual level. This thesis confirms a modest contribution to obesity of 12 SNPs in or near NEGR1, TMEM18, GNPDA2, BDNF, UCP2, NRXN3, FTO, KCTD15 and MAP2K5.

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